Background Cuff pressure in endotracheal (ET) tubes should be in the range of 20–30 cm H 2 O. We tested the hypothesis that the tube cuff is inadequately inflated when manometers are not used.
Background
Cuff pressure in endotracheal (ET) tubes should be in the range of 20–30 cm H 2 O. We tested the hypothesis that the tube cuff is inadequately inflated when manometers are not used.Methods With IRB approval, we studied 93 patients under general anesthesia with an ET tube in place in one teaching and two private hospitals. Anesthetists were blinded to study purpose. Cuff pressure in tube sizes 7.0 to 8.5 mm was evaluated 60 min after induction of general anesthesia using a manometer connected to the cuff pilot balloon. Nitrous oxide was disallowed. After deflating the cuff, we reinflated it in 0.5-ml increments until pressure was 20 cmH 2 O.
Methods
With IRB approval, we studied 93 patients under general anesthesia with an ET tube in place in one teaching and two private hospitals. Anesthetists were blinded to study purpose. Cuff pressure in tube sizes 7.0 to 8.5 mm was evaluated 60 min after induction of general anesthesia using a manometer connected to the cuff pilot balloon. Nitrous oxide was disallowed. After deflating the cuff, we reinflated it in 0.5-ml increments until pressure was 20 cmH 2 O.Results Neither patient morphometrics, institution, experience of anesthesia provider, nor tube size influenced measured cuff pressure (35.3 ± 21.6 cmH 2 O). Only 27% of pressures were within 20–30 cmH 2 O; 27% exceeded 40 cmH 2 O. Although it varied considerably, the amount of air required to achieve a cuff pressure of 20 cmH 2 O was similar with each tube size.
Results
Neither patient morphometrics, institution, experience of anesthesia provider, nor tube size influenced measured cuff pressure (35.3 ± 21.6 cmH 2 O). Only 27% of pressures were within 20–30 cmH 2 O; 27% exceeded 40 cmH 2 O. Although it varied considerably, the amount of air required to achieve a cuff pressure of 20 cmH 2 O was similar with each tube size.Conclusion We recommend that ET cuff pressure be set and monitored with a manometer.
Conclusion
We recommend that ET cuff pressure be set and monitored with a manometer.

Background A critical function of the endotracheal tube cuff is to seal the airway, thus preventing aspiration of pharyngeal contents into the trachea and to ensure that there are no leaks past the cuff during positive pressure ventilation. However, complications have been associated with insufficient cuff inflation. Consequences of micro-aspiration of oropharyngeal secretions include nosocomial pulmonary infections [ 1 ]. Conventional high-volume, low-pressure cuffs may not prevent micro-aspiration even at cuff pressures up to 60 cm H 2 O [ 2 ], although some studies suggest that only 25 cm H 2 O is sufficient [ 3 ]. In contrast, newer ultra-thin cuff membranes made from polyurethane effectively prevent liquid flow around cuffs inflated only to 15 cm H 2 O [ 2 ]. In the absence of clear guidelines, many clinicians consider 20 cm H 2 O a reasonable lower limit for cuff pressure in adults. Catastrophic consequences of endotracheal tube cuff over-inflation such as rupture of the trachea [ 4 - 6 ], tracheo-carotid artery erosion [ 7 ], and tracheal innominate artery fistulas are rare now that low-pressure, high-volume cuffs are used routinely. However, post-intubation sore throat is a common side effect of general anesthetic and may partly result from ischemia of the oropharyngeal and tracheal mucosa [ 8 - 10 ], and the most common etiology of non-malignant tracheoesophageal fistula remains cuff-related tracheal injury [ 11 , 12 ]. In addition, acquired laryngeal stenosis may be caused by mechanical abrasion or pressure necrosis of the laryngeal mucosa secondary to high cuff pressure [ 13 , 14 ]. Animal data indicate that a cuff pressure of only 20 cm H 2 O may significantly reduce tracheal blood flow with normal blood pressure and critically reduces it during severe hypotension [ 15 ]. Similarly, inflation of endotracheal tube cuffs to 20 cm H 2 O for just four hours produces serious ciliary damage that persists for at least three days [ 16 ]. Thus, appropriate inflation of endotracheal tube cuff is obviously important. Lomholt et al. recommended selecting a cuff pressure of 25 cmH 2 O as a safe minimum cuff pressure to prevent aspiration and leaks past the cuff [ 17 ]; Bernhard et al. supported this recommendation [ 18 ]. On the other hand, Nordin et al. studied the relationship between cuff pressure and capillary perfusion of the rabbit tracheal mucosa and recommended that cuff pressure be kept below 27 cm H 2 O (20 mmHg) [ 19 ]. Seegobin and Hasselt reached similar conclusions in an  in vitro  study and recommended cuff inflation pressure not exceed 30 cm H 2 O [ 20 ]. It is thus essential to maintain cuff pressures in the range of 20–30 cm of H 2 O Adequacy of cuff inflation is conventionally determined by palpation of the external balloon. Previous studies suggest that this approach is unreliable [ 21 , 22 ]. One study, for instance, found that cuff pressure exceeded 40 cm H 2 O in 40-to-90% of tested patients [ 22 ]. However, increased awareness of over-inflation risks may have improved recent clinical practice. Our first goal was thus to determine if cuff pressure was within the recommended range of 20–30 cmH 2 O, when inflated using the palpation method. Because cuff inflation practices are likely to differ among clinical environments, we evaluated cuff pressure in three different practice settings: an academic university hospital and two private hospitals. It is also likely that cuff inflation practices differ among providers. We therefore also evaluated cuff pressure during anesthesia provided by certified registered nurse anesthetists (CRNAs), anesthesia residents, and anesthesia faculty. Cuff pressure can be easily measured with a small aneroid manometer [ 23 ], but this device is not widely available in the United States. It would thus be helpful for clinicians to know how much air must be injected into the cuff to produce the minimum adequate pressure. We designed this study to observe the practices of anesthesia providers and then determine the volume of air required to optimize the cuff pressure to 20 cmH 2 O for various sizes of endotracheal tubes. Taking another approach to the same question, we also determined compliance of the cuff-trachea system  in vivo  by plotting measured cuff pressure against cuff volume.
Background
A critical function of the endotracheal tube cuff is to seal the airway, thus preventing aspiration of pharyngeal contents into the trachea and to ensure that there are no leaks past the cuff during positive pressure ventilation. However, complications have been associated with insufficient cuff inflation. Consequences of micro-aspiration of oropharyngeal secretions include nosocomial pulmonary infections [ 1 ]. Conventional high-volume, low-pressure cuffs may not prevent micro-aspiration even at cuff pressures up to 60 cm H 2 O [ 2 ], although some studies suggest that only 25 cm H 2 O is sufficient [ 3 ]. In contrast, newer ultra-thin cuff membranes made from polyurethane effectively prevent liquid flow around cuffs inflated only to 15 cm H 2 O [ 2 ]. In the absence of clear guidelines, many clinicians consider 20 cm H 2 O a reasonable lower limit for cuff pressure in adults.Catastrophic consequences of endotracheal tube cuff over-inflation such as rupture of the trachea [ 4 - 6 ], tracheo-carotid artery erosion [ 7 ], and tracheal innominate artery fistulas are rare now that low-pressure, high-volume cuffs are used routinely. However, post-intubation sore throat is a common side effect of general anesthetic and may partly result from ischemia of the oropharyngeal and tracheal mucosa [ 8 - 10 ], and the most common etiology of non-malignant tracheoesophageal fistula remains cuff-related tracheal injury [ 11 , 12 ]. In addition, acquired laryngeal stenosis may be caused by mechanical abrasion or pressure necrosis of the laryngeal mucosa secondary to high cuff pressure [ 13 , 14 ]. Animal data indicate that a cuff pressure of only 20 cm H 2 O may significantly reduce tracheal blood flow with normal blood pressure and critically reduces it during severe hypotension [ 15 ]. Similarly, inflation of endotracheal tube cuffs to 20 cm H 2 O for just four hours produces serious ciliary damage that persists for at least three days [ 16 ]. Thus, appropriate inflation of endotracheal tube cuff is obviously important.Lomholt et al. recommended selecting a cuff pressure of 25 cmH 2 O as a safe minimum cuff pressure to prevent aspiration and leaks past the cuff [ 17 ]; Bernhard et al. supported this recommendation [ 18 ]. On the other hand, Nordin et al. studied the relationship between cuff pressure and capillary perfusion of the rabbit tracheal mucosa and recommended that cuff pressure be kept below 27 cm H 2 O (20 mmHg) [ 19 ]. Seegobin and Hasselt reached similar conclusions in an  in vitro  study and recommended cuff inflation pressure not exceed 30 cm H 2 O [ 20 ]. It is thus essential to maintain cuff pressures in the range of 20–30 cm of H 2 Oin vitroAdequacy of cuff inflation is conventionally determined by palpation of the external balloon. Previous studies suggest that this approach is unreliable [ 21 , 22 ]. One study, for instance, found that cuff pressure exceeded 40 cm H 2 O in 40-to-90% of tested patients [ 22 ]. However, increased awareness of over-inflation risks may have improved recent clinical practice. Our first goal was thus to determine if cuff pressure was within the recommended range of 20–30 cmH 2 O, when inflated using the palpation method. Because cuff inflation practices are likely to differ among clinical environments, we evaluated cuff pressure in three different practice settings: an academic university hospital and two private hospitals. It is also likely that cuff inflation practices differ among providers. We therefore also evaluated cuff pressure during anesthesia provided by certified registered nurse anesthetists (CRNAs), anesthesia residents, and anesthesia faculty.Cuff pressure can be easily measured with a small aneroid manometer [ 23 ], but this device is not widely available in the United States. It would thus be helpful for clinicians to know how much air must be injected into the cuff to produce the minimum adequate pressure. We designed this study to observe the practices of anesthesia providers and then determine the volume of air required to optimize the cuff pressure to 20 cmH 2 O for various sizes of endotracheal tubes. Taking another approach to the same question, we also determined compliance of the cuff-trachea system  in vivo  by plotting measured cuff pressure against cuff volume.in vivoMethods With approval of the University of Louisville Human Studies Committee and informed consent, we recruited 93 patients (42 men and 51 women) undergoing elective surgery with general endotracheal anesthesia from three hospitals in Louisville, Kentucky: 41 patients from University Hospital (an academic centre), 32 from Jewish Hospital (a private hospital), and 20 from Norton Hospital (also a private hospital). Patients with emergency intubations, difficult intubations, or intubation performed by non-anesthesiology staff; pregnant women; patients with higher risk for aspiration (e.g., full stomach, history of reflux, etc.); and patients with known anatomical laryngeo-tracheal abnormalities were excluded from this study. The Human Studies Committee did not require consent from participating anesthesia providers. However, no data were recorded that would link the study results to specific providers. Protocol Independent anesthesia groups at the three participating hospitals provided anesthesia to the participating patients. Because one purpose of our study was to measure pressure in the endotracheal tube cuff during routine practice, anesthesia providers were blinded to the nature of the study. They were only informed about the second purpose of the study: determining the relationship between cuff volume and pressure. Ninety-three patients were randomly assigned to the study. The groups were not equal for the three different types of practitioners; however, determining differences of practice between different anesthesia providers was not the primary purpose of our study. General anesthesia was induced by intravenous bolus of induction agents, and paralysis was achieved with succinylcholine or a non-depolarizing muscle relaxant. Male patients were intubated with an 8 or 8.5 mm internal diameter endotracheal tube, and female patients were intubated with a 7 or 7.5 mm internal diameter endotracheal tube. This is a standard practice at these hospitals. Patients who were intubated with sizes other than these were excluded from the study. Anesthesia was maintained with a volatile aesthetic in a combination of air and oxygen; nitrous oxide was not used during the study period. At the University of Louisville Hospital, at least 10 patients were evaluated with each endotracheal tube size (7, 7.5, 8, or 8.5 mm inner diameter [Intermediate Hi-Lo ®  Tracheal Tube, Mallinckrodt, St. Louis, MO]); at Jewish Hospital, at least 10 patients each were evaluated with size 7, 7.5, and 8 mm Mallinckrodt Intermediate Hi-Lo ®  Tracheal Tubes; and at Norton Hospital, 10 patients each were evaluated with size 7 and 8-mm Mallinckrodt Intermediate Hi-Lo ®  Tracheal Tubes. Consecutive available patients were enrolled until we had recruited at least 10 patients for each endotracheal tube size at each participating hospital. All tubes had high-volume, low-pressure cuffs. Measurements We recorded endotracheal tube size and morphometric characteristics including age, sex, height, and weight. An anesthesia provider inserted the endotracheal tubes, and the intubator or the circulating registered nurse inflated the cuff. This is the routine practice in all three hospitals. Adequacy is generally checked by palpation of the pilot balloon and sometimes readjusted by the intubator by inflating just enough to stop an audible leak. Investigators measured the cuff pressure at 60 minutes after induction of anesthesia using a manometer (VBM, Sulz, Germany) that was connected to the pilot balloon of the endotracheal tube cuff via a three-way stopcock. This type of aneroid manometer is nearly as accurate as a mercury manometer, but easier to use [ 23 ]. Pressure was recorded at end-expiration after ensuring that the patient was paralyzed. The cuff pressure was measured once in each patient at 60 minutes after intubation. We did not collect data on the readjustment by the providers after intubation during this hour. A syringe attached to the third limb of the stopcock was then used to completely deflate the cuff, and the volume of air removed was recorded. The cuff was considered empty when no more air could be removed on aspiration with a syringe. The cuff was then progressively inflated by injecting air in 0.5-ml increments until a cuff pressure of 20 cmH 2 O was achieved. The entire process required about a minute. Data analysis Our primary outcomes were 1) measured endotracheal tube cuff pressures as a function of tube size, provider, and hospital; and 2) the volume of air required to produce a cuff pressure of 20 cmH 2 O as a function of tube size. Outcomes were compared by tube size, provider, and hospital with either an ANOVA (if the values were normally distributed) or the Kruskal-Wallis statistic (if the values were skewed). Compliance of the cuff system was evaluated by linear regression of measured cuff pressure  vs . measured cuff volume. Data are presented as means (SD) or medians [interquartile ranges] unless otherwise noted;  P  < 0.05 was considered statistically significant.
Methods
With approval of the University of Louisville Human Studies Committee and informed consent, we recruited 93 patients (42 men and 51 women) undergoing elective surgery with general endotracheal anesthesia from three hospitals in Louisville, Kentucky: 41 patients from University Hospital (an academic centre), 32 from Jewish Hospital (a private hospital), and 20 from Norton Hospital (also a private hospital).Patients with emergency intubations, difficult intubations, or intubation performed by non-anesthesiology staff; pregnant women; patients with higher risk for aspiration (e.g., full stomach, history of reflux, etc.); and patients with known anatomical laryngeo-tracheal abnormalities were excluded from this study. The Human Studies Committee did not require consent from participating anesthesia providers. However, no data were recorded that would link the study results to specific providers.Protocol Independent anesthesia groups at the three participating hospitals provided anesthesia to the participating patients. Because one purpose of our study was to measure pressure in the endotracheal tube cuff during routine practice, anesthesia providers were blinded to the nature of the study. They were only informed about the second purpose of the study: determining the relationship between cuff volume and pressure. Ninety-three patients were randomly assigned to the study. The groups were not equal for the three different types of practitioners; however, determining differences of practice between different anesthesia providers was not the primary purpose of our study. General anesthesia was induced by intravenous bolus of induction agents, and paralysis was achieved with succinylcholine or a non-depolarizing muscle relaxant. Male patients were intubated with an 8 or 8.5 mm internal diameter endotracheal tube, and female patients were intubated with a 7 or 7.5 mm internal diameter endotracheal tube. This is a standard practice at these hospitals. Patients who were intubated with sizes other than these were excluded from the study. Anesthesia was maintained with a volatile aesthetic in a combination of air and oxygen; nitrous oxide was not used during the study period. At the University of Louisville Hospital, at least 10 patients were evaluated with each endotracheal tube size (7, 7.5, 8, or 8.5 mm inner diameter [Intermediate Hi-Lo ®  Tracheal Tube, Mallinckrodt, St. Louis, MO]); at Jewish Hospital, at least 10 patients each were evaluated with size 7, 7.5, and 8 mm Mallinckrodt Intermediate Hi-Lo ®  Tracheal Tubes; and at Norton Hospital, 10 patients each were evaluated with size 7 and 8-mm Mallinckrodt Intermediate Hi-Lo ®  Tracheal Tubes. Consecutive available patients were enrolled until we had recruited at least 10 patients for each endotracheal tube size at each participating hospital. All tubes had high-volume, low-pressure cuffs.
Protocol
Independent anesthesia groups at the three participating hospitals provided anesthesia to the participating patients. Because one purpose of our study was to measure pressure in the endotracheal tube cuff during routine practice, anesthesia providers were blinded to the nature of the study. They were only informed about the second purpose of the study: determining the relationship between cuff volume and pressure. Ninety-three patients were randomly assigned to the study. The groups were not equal for the three different types of practitioners; however, determining differences of practice between different anesthesia providers was not the primary purpose of our study.General anesthesia was induced by intravenous bolus of induction agents, and paralysis was achieved with succinylcholine or a non-depolarizing muscle relaxant. Male patients were intubated with an 8 or 8.5 mm internal diameter endotracheal tube, and female patients were intubated with a 7 or 7.5 mm internal diameter endotracheal tube. This is a standard practice at these hospitals. Patients who were intubated with sizes other than these were excluded from the study. Anesthesia was maintained with a volatile aesthetic in a combination of air and oxygen; nitrous oxide was not used during the study period.At the University of Louisville Hospital, at least 10 patients were evaluated with each endotracheal tube size (7, 7.5, 8, or 8.5 mm inner diameter [Intermediate Hi-Lo ®  Tracheal Tube, Mallinckrodt, St. Louis, MO]); at Jewish Hospital, at least 10 patients each were evaluated with size 7, 7.5, and 8 mm Mallinckrodt Intermediate Hi-Lo ®  Tracheal Tubes; and at Norton Hospital, 10 patients each were evaluated with size 7 and 8-mm Mallinckrodt Intermediate Hi-Lo ®  Tracheal Tubes. Consecutive available patients were enrolled until we had recruited at least 10 patients for each endotracheal tube size at each participating hospital. All tubes had high-volume, low-pressure cuffs.Measurements We recorded endotracheal tube size and morphometric characteristics including age, sex, height, and weight. An anesthesia provider inserted the endotracheal tubes, and the intubator or the circulating registered nurse inflated the cuff. This is the routine practice in all three hospitals. Adequacy is generally checked by palpation of the pilot balloon and sometimes readjusted by the intubator by inflating just enough to stop an audible leak. Investigators measured the cuff pressure at 60 minutes after induction of anesthesia using a manometer (VBM, Sulz, Germany) that was connected to the pilot balloon of the endotracheal tube cuff via a three-way stopcock. This type of aneroid manometer is nearly as accurate as a mercury manometer, but easier to use [ 23 ]. Pressure was recorded at end-expiration after ensuring that the patient was paralyzed. The cuff pressure was measured once in each patient at 60 minutes after intubation. We did not collect data on the readjustment by the providers after intubation during this hour. A syringe attached to the third limb of the stopcock was then used to completely deflate the cuff, and the volume of air removed was recorded. The cuff was considered empty when no more air could be removed on aspiration with a syringe. The cuff was then progressively inflated by injecting air in 0.5-ml increments until a cuff pressure of 20 cmH 2 O was achieved. The entire process required about a minute.
Measurements
We recorded endotracheal tube size and morphometric characteristics including age, sex, height, and weight.An anesthesia provider inserted the endotracheal tubes, and the intubator or the circulating registered nurse inflated the cuff. This is the routine practice in all three hospitals. Adequacy is generally checked by palpation of the pilot balloon and sometimes readjusted by the intubator by inflating just enough to stop an audible leak. Investigators measured the cuff pressure at 60 minutes after induction of anesthesia using a manometer (VBM, Sulz, Germany) that was connected to the pilot balloon of the endotracheal tube cuff via a three-way stopcock. This type of aneroid manometer is nearly as accurate as a mercury manometer, but easier to use [ 23 ]. Pressure was recorded at end-expiration after ensuring that the patient was paralyzed. The cuff pressure was measured once in each patient at 60 minutes after intubation. We did not collect data on the readjustment by the providers after intubation during this hour.A syringe attached to the third limb of the stopcock was then used to completely deflate the cuff, and the volume of air removed was recorded. The cuff was considered empty when no more air could be removed on aspiration with a syringe. The cuff was then progressively inflated by injecting air in 0.5-ml increments until a cuff pressure of 20 cmH 2 O was achieved. The entire process required about a minute.Data analysis Our primary outcomes were 1) measured endotracheal tube cuff pressures as a function of tube size, provider, and hospital; and 2) the volume of air required to produce a cuff pressure of 20 cmH 2 O as a function of tube size. Outcomes were compared by tube size, provider, and hospital with either an ANOVA (if the values were normally distributed) or the Kruskal-Wallis statistic (if the values were skewed). Compliance of the cuff system was evaluated by linear regression of measured cuff pressure  vs . measured cuff volume. Data are presented as means (SD) or medians [interquartile ranges] unless otherwise noted;  P  < 0.05 was considered statistically significant.
Data analysis
Our primary outcomes were 1) measured endotracheal tube cuff pressures as a function of tube size, provider, and hospital; and 2) the volume of air required to produce a cuff pressure of 20 cmH 2 O as a function of tube size. Outcomes were compared by tube size, provider, and hospital with either an ANOVA (if the values were normally distributed) or the Kruskal-Wallis statistic (if the values were skewed). Compliance of the cuff system was evaluated by linear regression of measured cuff pressure  vs . measured cuff volume. Data are presented as means (SD) or medians [interquartile ranges] unless otherwise noted;  P  < 0.05 was considered statistically significant.vsPResults Morphometric and demographic characteristics of the patients were similar at each participating hospital (Table  1 ). Measured cuff pressures averaged 35.3(21.6)cmH 2 O; only 27% of the patients had measured pressures within the recommended range of 20–30 cmH 2 O. Fifty percent of the values exceeded 30 cmH 2 O, and 27% of the measured pressures exceeded 40 cmH 2 O. Thus, 23% of the measured cuff pressures were less than 20 mmHg. Measured cuff volume averaged 4.4 ± 1.8 ml. Neither measured cuff pressure nor measured cuff volume differed among the hospitals (Table  2 ). Table 1 Demographic and Morphometric Characteristics as a Function of Hospital. University of Louisville Hospital Norton Hospital Jewish Hospital P N 41 20 32 --- Tube size (N) 7 / 7.5 / 8 / 8.5 10 / 11 / 10 / 10 10 / 0 / 10 / 0 10 / 10 / 12 / 0 --- Age (yr) 41 (14) 50 (14) 51 (16) 0.006 Weight (kg) 88 (27) 100 (40) 78 (17) 0.020 Height (m) 1.7 (0.1) 1.7 (0.1) 1.7 (0.1) 0.669 Results presented as number or mean (SD). Table 2 Principal Results as a Function Hospital University of Louisville Hospital Norton Hospital Jewish Hospital P Measured Cuff Pressure (cmH 2 O) 26 [18, 38] 34.5 [20, 50.5] 33 [22.5, 48.5] 0.469 Measured Cuff Volume (ml) 4.0 [3.0, 5.0] 4.3 [3.0, 6.0] 4.5 [3.2, 5.5] 0.646 Volume Required for 20 cmH 2 O (ml) 2.7 [2.1, 3.8] 2.5 [2.3, 3.3] 2.9 [2.2, 3.7] 0.792 Results presented as median [interquartile range]. There was no correlation between the measured cuff pressure and the age, sex, height, or weight of the patients. Nor did measured cuff pressure differ as a function of endotracheal tube size. Measured cuff volumes were also similar with each tube size. Interestingly, the amount of air required to achieve a cuff pressure of 20 cmH 2 O was similar with each tube size (Table  3 ). However, there was considerable variability in the amount of air required. Table 3 Principal Results as a Function of Tube Size. 7.0 mm 7.5 mm 8.0 mm 8.5 mm P Patients with measured cuff pressure >30 cmH 2 O (%) 57 57 47 30 0.444 Measured Cuff Pressure (mmHg) 32 [22, 52] 38 [24, 49] 30 [16, 38] 24 [21, 40] 0.467 Measured Cuff Volume (ml) 3.9 [3.0, 5.0] 4.5 [2.7, 5.0] 4.6 [3.5, 6.1] 3.8 [3.0, 5.0] 0.616 Volume Required for 20 cmH 2 O (ml) 2.6 [2.0, 3.1] 2.5 [1.8, 3.0] 3.0 [2.5. 4.1] 3.3 [2.0, 3.9] 0.143 Results presented as medians [interquartile range] or percent. CRNAs (n = 72), anesthesia residents (n = 15), and anesthesia faculty (n = 6) performed the intubations. There were no statistically significant differences in measured cuff pressures among these three practitioner groups ( P  = 0.847). The compliance of the tube was determined from the measured cuff pressure (cmH 2 O) and the volume of air (ml) retrieved at complete deflation of the cuff; this showed a linear pressure-volume relationship: Pressure= 7.5. Volume+2.7, r 2  = 0.39 (Fig.  1 ). Figure 1 The relationship between measured cuff pressure and volume of air in the cuff.  There was a linear relationship between measured cuff pressure (cmH 2 O) and volume (ml) of air removed from the cuff: Pressure = 7.5. Volume + 2.7, r 2  = 0.39.
Results
Morphometric and demographic characteristics of the patients were similar at each participating hospital (Table  1 ). Measured cuff pressures averaged 35.3(21.6)cmH 2 O; only 27% of the patients had measured pressures within the recommended range of 20–30 cmH 2 O. Fifty percent of the values exceeded 30 cmH 2 O, and 27% of the measured pressures exceeded 40 cmH 2 O. Thus, 23% of the measured cuff pressures were less than 20 mmHg. Measured cuff volume averaged 4.4 ± 1.8 ml. Neither measured cuff pressure nor measured cuff volume differed among the hospitals (Table  2 ).Demographic and Morphometric Characteristics as a Function of Hospital.University of Louisville HospitalNorton HospitalJewish HospitalPPNTube size (N) 7 / 7.5 / 8 / 8.5Age (yr)Weight (kg)Height (m)Results presented as number or mean (SD).Principal Results as a Function HospitalUniversity of Louisville HospitalNorton HospitalJewish HospitalPPMeasured Cuff Pressure (cmH 2 O)Measured Cuff Volume (ml)Volume Required for 20 cmH 2 O (ml)Results presented as median [interquartile range].There was no correlation between the measured cuff pressure and the age, sex, height, or weight of the patients. Nor did measured cuff pressure differ as a function of endotracheal tube size. Measured cuff volumes were also similar with each tube size. Interestingly, the amount of air required to achieve a cuff pressure of 20 cmH 2 O was similar with each tube size (Table  3 ). However, there was considerable variability in the amount of air required.Principal Results as a Function of Tube Size.7.0 mm7.5 mm8.0 mm8.5 mmPPPatients with measured cuff pressure >30 cmH 2 O (%)Measured Cuff Pressure (mmHg)Measured Cuff Volume (ml)Volume Required for 20 cmH 2 O (ml)Results presented as medians [interquartile range] or percent.CRNAs (n = 72), anesthesia residents (n = 15), and anesthesia faculty (n = 6) performed the intubations. There were no statistically significant differences in measured cuff pressures among these three practitioner groups ( P  = 0.847).PThe compliance of the tube was determined from the measured cuff pressure (cmH 2 O) and the volume of air (ml) retrieved at complete deflation of the cuff; this showed a linear pressure-volume relationship: Pressure= 7.5. Volume+2.7, r 2  = 0.39 (Fig.  1 ).Discussion Previous studies suggest that the cuff pressure is usually under-estimated by manual palpation. For example, Braz  et al . [ 22 ] observed cuff pressure exceeding 40 cm H 2 O in 91% of PACU patients after anesthesia with nitrous oxide, 55% of ICU patients, and 45% of PACU patients after anesthesia without nitrous oxide. In an experimental study, Fernandez  et al . [ 21 ] observed that when the cuff was inflated randomly to 10, 20, or 30 cmH 2 O, participating physicians and ICU nurses were able to identify the group in 69% of the high-pressure cases, 58% of the normal pressure cases, and 73% of the low pressure cases. Our results are consistent in that measured cuff pressure exceeded 30 cmH 2 O in 50% of patients and were less than 20 cmH 2 O in 23% of patients. Cuff pressures were thus less likely to be within the recommended range (20–30 cmH 2 O) than outside the range. It was nonetheless encouraging that we observed relatively few extremely high values, at least many fewer than reported in previous studies [ 22 ]. This result suggests that clinicians are now making reasonable efforts to avoid grossly excessive cuff inflation. Measured cuff inflation pressures were virtually identical at the three study sites: one academic center and two private hospitals. These data suggest that management of cuff pressure was similar in these two disparate settings. Interestingly, there was also no significant or important difference as a function of provider – measured cuff pressures were virtually identical whether filled by CRNAs, residents, or attending anesthesiologists. Our results thus fail to support the theory that increased training improves cuff management. We evaluated three different types of anesthesia provider in three different practice settings. Although we were unable to identify any statistically significant or clinically important differences among the sites or providers, our results apply only to the specific sites and providers we evaluated. While it is likely that these results are fairly representative, it is obvious that results would not be identical elsewhere because of regional practice differences. Fernandez  et al . [ 21 ] found that the volume of air required to inflate the endotracheal tube cuff varies as a function of tube size and type. But interestingly, the volume required to inflate the cuff to a particular pressure was much smaller when the cuff was inflated inside an artificial trachea; furthermore, the difference among tube sizes was minimal under those conditions. We similarly found that the volume of air required to inflate the cuffs to 20 cmH 2 O did not differ significantly as a function of endotracheal tube size. These data suggest that tube size is not an important determinant of appropriate cuff inflation volume. A caveat, though, is that tube sizes were chosen by clinicians in our study and presumably matched patient size; results may well have differed if tube size had been randomly assigned. We intentionally avoided this approach since our purpose was to evaluate cuff pressures and associated volumes in three routine clinical settings. A limitation of this study is that cuff pressure was evaluated just once 60 minutes after induction of anesthesia. Because nitrous oxide was not used, it is unlikely that the cuff pressures varied much during the first hour of the study cases. We recognize that people other than the anesthesia provider who actually conducted the case often inflated the cuffs. However, a full hour was plenty of time for the provider to have checked and adjusted cuff pressure to a suitable level. We observed a linear relationship between the measured cuff pressure and the volume of air retrieved from the cuff. The regression equation indicated that injected volumes between 2 and 4 ml usually produce cuff pressures between 20 and 30 cmH 2 O independent of tube size for the same type of tube. However, there was considerable patient-to-patient variability in the required air volume. Measuring actual cuff pressure thus appears preferable to injecting a given volume of air.
Discussion
Previous studies suggest that the cuff pressure is usually under-estimated by manual palpation. For example, Braz  et al . [ 22 ] observed cuff pressure exceeding 40 cm H 2 O in 91% of PACU patients after anesthesia with nitrous oxide, 55% of ICU patients, and 45% of PACU patients after anesthesia without nitrous oxide. In an experimental study, Fernandez  et al . [ 21 ] observed that when the cuff was inflated randomly to 10, 20, or 30 cmH 2 O, participating physicians and ICU nurses were able to identify the group in 69% of the high-pressure cases, 58% of the normal pressure cases, and 73% of the low pressure cases. Our results are consistent in that measured cuff pressure exceeded 30 cmH 2 O in 50% of patients and were less than 20 cmH 2 O in 23% of patients. Cuff pressures were thus less likely to be within the recommended range (20–30 cmH 2 O) than outside the range. It was nonetheless encouraging that we observed relatively few extremely high values, at least many fewer than reported in previous studies [ 22 ]. This result suggests that clinicians are now making reasonable efforts to avoid grossly excessive cuff inflation.et alet alMeasured cuff inflation pressures were virtually identical at the three study sites: one academic center and two private hospitals. These data suggest that management of cuff pressure was similar in these two disparate settings. Interestingly, there was also no significant or important difference as a function of provider – measured cuff pressures were virtually identical whether filled by CRNAs, residents, or attending anesthesiologists. Our results thus fail to support the theory that increased training improves cuff management.We evaluated three different types of anesthesia provider in three different practice settings. Although we were unable to identify any statistically significant or clinically important differences among the sites or providers, our results apply only to the specific sites and providers we evaluated. While it is likely that these results are fairly representative, it is obvious that results would not be identical elsewhere because of regional practice differences.Fernandez  et al . [ 21 ] found that the volume of air required to inflate the endotracheal tube cuff varies as a function of tube size and type. But interestingly, the volume required to inflate the cuff to a particular pressure was much smaller when the cuff was inflated inside an artificial trachea; furthermore, the difference among tube sizes was minimal under those conditions. We similarly found that the volume of air required to inflate the cuffs to 20 cmH 2 O did not differ significantly as a function of endotracheal tube size. These data suggest that tube size is not an important determinant of appropriate cuff inflation volume. A caveat, though, is that tube sizes were chosen by clinicians in our study and presumably matched patient size; results may well have differed if tube size had been randomly assigned. We intentionally avoided this approach since our purpose was to evaluate cuff pressures and associated volumes in three routine clinical settings.et alA limitation of this study is that cuff pressure was evaluated just once 60 minutes after induction of anesthesia. Because nitrous oxide was not used, it is unlikely that the cuff pressures varied much during the first hour of the study cases. We recognize that people other than the anesthesia provider who actually conducted the case often inflated the cuffs. However, a full hour was plenty of time for the provider to have checked and adjusted cuff pressure to a suitable level.We observed a linear relationship between the measured cuff pressure and the volume of air retrieved from the cuff. The regression equation indicated that injected volumes between 2 and 4 ml usually produce cuff pressures between 20 and 30 cmH 2 O independent of tube size for the same type of tube. However, there was considerable patient-to-patient variability in the required air volume. Measuring actual cuff pressure thus appears preferable to injecting a given volume of air.Conclusions Cuff pressure should be measured with a manometer and, if necessary, corrected.
Conclusions
Cuff pressure should be measured with a manometer and, if necessary, corrected.Competing interests The author(s) declare that they have no competing interests.
Competing interests
The author(s) declare that they have no competing interests.Authors' contributions SP oversaw day-to-day study mechanics, collected data on many of the patients, and wrote an initial draft of manuscript. DIS contributed to study design, data analysis, and manuscript preparation. PM, SW, and AV recruited patients and performed many of the measurements. JD conceived of the study and participated in its design. AW contributed to protocol development, patient recruitment, and manuscript preparation. All authors read and approved the final manuscript.
Authors' contributions
SP oversaw day-to-day study mechanics, collected data on many of the patients, and wrote an initial draft of manuscript.DIS contributed to study design, data analysis, and manuscript preparation.PM, SW, and AV recruited patients and performed many of the measurements.JD conceived of the study and participated in its design.AW contributed to protocol development, patient recruitment, and manuscript preparation.All authors read and approved the final manuscript.Pre-publication history The pre-publication history for this paper can be accessed here:
Pre-publication history
The pre-publication history for this paper can be accessed here:


Background To reach and grasp an object in space on the basis of its image cast on the retina requires different coordinate transformations that take into account gaze and limb positioning. Eye position in the orbit influences the image's conversion from retinotopic (eye-centered) coordinates to an egocentric frame necessary for guiding action. Neuroimaging studies have revealed eye position-dependent activity in extrastriate visual, parietal and frontal areas that is along the visuo-motor pathway. At the earliest vision stage, the role of the primary visual area (V1) in this process remains unclear. We used an experimental design based on pattern-onset visual evoked potentials (VEP) recordings to study the effect of eye position on V1 activity in humans.
Background
To reach and grasp an object in space on the basis of its image cast on the retina requires different coordinate transformations that take into account gaze and limb positioning. Eye position in the orbit influences the image's conversion from retinotopic (eye-centered) coordinates to an egocentric frame necessary for guiding action. Neuroimaging studies have revealed eye position-dependent activity in extrastriate visual, parietal and frontal areas that is along the visuo-motor pathway. At the earliest vision stage, the role of the primary visual area (V1) in this process remains unclear. We used an experimental design based on pattern-onset visual evoked potentials (VEP) recordings to study the effect of eye position on V1 activity in humans.Results We showed that the amplitude of the initial C1 component of VEP, acknowledged to originate in V1, was modulated by the eye position. We also established that putative spontaneous small saccades related to eccentric fixation, as well as retinal disparity cannot explain the effects of changing C1 amplitude of VEP in the present study.
Results
We showed that the amplitude of the initial C1 component of VEP, acknowledged to originate in V1, was modulated by the eye position. We also established that putative spontaneous small saccades related to eccentric fixation, as well as retinal disparity cannot explain the effects of changing C1 amplitude of VEP in the present study.Conclusions The present modulation of the early component of VEP suggests an eye position-dependent activity of the human primary visual area. Our findings also evidence that cortical processes combine information about the position of the stimulus on the retinae with information about the location of the eyes in their orbit as early as the stage of primary visual area.
Conclusions
The present modulation of the early component of VEP suggests an eye position-dependent activity of the human primary visual area. Our findings also evidence that cortical processes combine information about the position of the stimulus on the retinae with information about the location of the eyes in their orbit as early as the stage of primary visual area.

Background In humans, goal-directed movements to an object in space are improved by foveal vision, namely by gaze anchoring on the object [ 1 ]. Since motor and visual information are encoded in different reference frames, accurate reaching and grasping movements in space require ongoing registration and coordinate transformation of visual percepts with gaze and limb positioning. One essential transformation is to convert the retinal image from eye-centered coordinates in a target location with respect to the head by taking into account the position of the eyes in the orbit [ 2 ]. Single-cell recordings in monkeys revealed that the neural substrate of the visual-to-motor coordinate transformations is a change in the visual or motor response properties according to gaze position in extrastriate visual, parietal, oculomotor, premotor and motor areas [for review see [ 3 , 4 ]]. Functional magnetic resonance imaging (fMRI) studies have localized human homologues of such monkey areas and have showed that eye position signals modulate activity of extrastriate visual areas [ 5 ] and the parieto-frontal network related to hand-arm movements [ 6 , 7 ]. Less is known at the earliest stage of visual processing, namely in the primary visual area (V1). Previous electrophysiological [ 8 - 10 ] and modeling [ 11 ] studies described eye position dependent activity in V1 neurons to a lesser extent than that reported in parietal and premotor cortex but consistent with the idea that both retinal and eye position signals may also converge at early vision stage. However, the direct influence of eye position on V1-related activity in humans has not been investigated. Consequently, we used a specific experimental design based on pattern-onset visual evoked potentials (VEP) recordings to study the effect of eye position exclusively on V1 activity in humans. Thus, we focused our investigation on the first major VEP component C1, obtained using pattern-onset stimulation, because its distribution over the scalp and its retinotopic properties indicate an origin from the calcarine fissure that is V1. This issue has been the conclusion of all previous studies over the past 10 years regarding the cortical visual areas that generate the early components of pattern-onset VEP [for review see [ 12 ]]. The most recent reports even demonstrated in individual subjects a close anatomical correspondence between modelled dipoles for the C1 component and sites of activation in the calcarine fissure obtained in fMRI in response to the same visual stimuli [ 12 - 14 ]. The purpose of the present report was therefore to investigate the eye position-dependent activity of V1 in humans by characterizing the early C1 component of VEP and testing its properties at different eye positions.
Background
In humans, goal-directed movements to an object in space are improved by foveal vision, namely by gaze anchoring on the object [ 1 ]. Since motor and visual information are encoded in different reference frames, accurate reaching and grasping movements in space require ongoing registration and coordinate transformation of visual percepts with gaze and limb positioning. One essential transformation is to convert the retinal image from eye-centered coordinates in a target location with respect to the head by taking into account the position of the eyes in the orbit [ 2 ]. Single-cell recordings in monkeys revealed that the neural substrate of the visual-to-motor coordinate transformations is a change in the visual or motor response properties according to gaze position in extrastriate visual, parietal, oculomotor, premotor and motor areas [for review see [ 3 , 4 ]]. Functional magnetic resonance imaging (fMRI) studies have localized human homologues of such monkey areas and have showed that eye position signals modulate activity of extrastriate visual areas [ 5 ] and the parieto-frontal network related to hand-arm movements [ 6 , 7 ]. Less is known at the earliest stage of visual processing, namely in the primary visual area (V1). Previous electrophysiological [ 8 - 10 ] and modeling [ 11 ] studies described eye position dependent activity in V1 neurons to a lesser extent than that reported in parietal and premotor cortex but consistent with the idea that both retinal and eye position signals may also converge at early vision stage. However, the direct influence of eye position on V1-related activity in humans has not been investigated. Consequently, we used a specific experimental design based on pattern-onset visual evoked potentials (VEP) recordings to study the effect of eye position exclusively on V1 activity in humans. Thus, we focused our investigation on the first major VEP component C1, obtained using pattern-onset stimulation, because its distribution over the scalp and its retinotopic properties indicate an origin from the calcarine fissure that is V1. This issue has been the conclusion of all previous studies over the past 10 years regarding the cortical visual areas that generate the early components of pattern-onset VEP [for review see [ 12 ]]. The most recent reports even demonstrated in individual subjects a close anatomical correspondence between modelled dipoles for the C1 component and sites of activation in the calcarine fissure obtained in fMRI in response to the same visual stimuli [ 12 - 14 ]. The purpose of the present report was therefore to investigate the eye position-dependent activity of V1 in humans by characterizing the early C1 component of VEP and testing its properties at different eye positions.Results and discussion The C1 component reverses classically in polarity for upper  vs.  lower visual field stimulation [for review see [ 12 ]]. Consequently, we first characterized the C1 component over the 20 subjects by observing its polarity inversion for a central eye position. Figure  1B  illustrates the representative polarity inversion of the C1 component on the grand averaged of VEP over the 20 subjects and in response to stimuli in the upper and lower quadrants of the right visual fields. Equivalent VEP and C1 polarity inversion were obtained in response to stimuli in the upper and lower quadrants of the left visual fields. The polarity inversion of the C1 components on the grand average VEPs in response to upper and lower hemifield stimuli were described previously for occipito-parietal sites using a 10–20 system montage with 62 scalp sites (see box included in Figure  1 ). It is noteworthy that a similar polarity inversion was measured in our study using only two occipital intermediate sites (IN3 and IN4) of the modified 10–20-system montage (see Methods for details). Once the C1 component was characterized for both upper and lower quadrant visual fields, the effect of eye position on C1 amplitude was measured only for both left and right lower quadrants visual field that is for the most salient C1 component which we observed. Mean peak latencies of C1 was calculated in each subject in response to both left and right lower visual quadrants and for both IN3 and IN4 recording sites. They ranged between 94.8 ms and 98.0 ms that are consistent with the C1 latency range previously observed in numerous studies [ 12 , 15 ]. For each subject, the C1 amplitude for five different eye positions (0°, 10° and 20° both left- and rightward) was then measured at these mean latencies for each lower quadrant and each recording site and, with respect to a 80 ms pre-stimulus baseline. We observed that checkerboard presented in the lower visual field elicited VEPs with modulated amplitudes of the C1 component in respect of eye position. Grand averaged VEPs over the 20 subjects in response to flashed stimuli in the right lower quadrant of the visual field for three different eye positions (0° and 20° both left- and rightward) are shown in Figure  2A . It is noteworthy that the eye position-dependent modulation of C1 amplitude was observed at the IN3 recording site for a 20° rightward eye position (Figure  2A , red trace) and at the IN4 recording site with a 20° leftward eye position (Figure  2A , green trace). In other words, the eye position effect was observed at the occipital recording site contralateral to the direction of the eye deviation. Equivalent VEPs but reverse eye-position effects were obtained in response to left field stimuli. A one-way Friedman repeated measures analysis of variance was conducted for each recording site and for each visual stimulation revealing a significant main effect of eye position on the amplitude of the C1 component. Complete data obtained at both recording sites, for each lower visual field are shown in Figure  2B . We chose to represent these data using relative amplitude measures that are amplitude for deviated eye position subtracted by the amplitude for central eye position. It means that amplitude zero corresponds to the central eye position situation. A post-hoc Dunnett's test (p < 0.05) using 0° as reference showed that the amplitude of the C1 component measured contralaterally to the deviated eye position was significantly different from 0° except for one situation (Figure  2B , white histograms). Conversely, the amplitude of the C1 component recorded ipsilaterally to the deviated eye position was not significantly different from 0° excepted for one situation (Figure  2B , grey histograms). Note that no parametric effect was observed for the C1 amplitude between 10° and 20° of eye eccentricity (Student's t-test, p < 0.05). The overall result of the present study revealed that the amplitude of first major component of VEP elicited by checkerboard (C1) is modulated by the eye position. The previous data linking the C1 component to a striate cortex generator, namely V1 activity (see introduction), led us to suggest that eye position influences the earliest cortical stage of visual processing. One may first suggest that the present results may be explained by the difficulty of maintaining eccentric fixation, which may have altered the pattern of fixational eye movements, such as microsaccades [fast, conjugate jerks, smaller than 1/3°, see [ 16 ]]. Recent studies have shown that microsaccades modulated neural activity in V1 [ 17 , 18 ], but to our knowledge no study has examined the effect of maintaining eccentric fixation on the occurrence of microsaccades. In the present study, we were able to track spontaneous saccadic eye movements superior to 1°. A one-way Friedman repeated measures analysis of variance for each visual stimulation revealed no significant main effect of eye position on the amplitude (p = 0.35) and on the frequency (p = 0.45) of saccades superior to 1°. We cannot rule out quantitatively a possible role for microsaccades in the eye position-dependent V1 activity. Regardless of such putative effects, however, one may argue that in case of an increase of the number of microsaccades related to eccentric fixation, a similar effect in terms of magnitude would be observed for both left- and rightward deviation. This was not the case in the present study. The question also arises if the effects of changing C1 amplitude may be due to oculomotor signals and/or retinal disparity that is to the difference in the position of the visual stimulus on each retina related to relative monitor distance. The absence of any parametric effect for the C1 amplitude between 10° and 20° of eye eccentricity may indicate that eye position effects are not due to the difference in the horizontal retinal disparity, but one may argue that a putative relationship between the horizontal disparity and the C1 amplitude is not linear. We evaluated the difference of both horizontal and vertical disparity between the different eye's deviations in our study (see Methods for details and Figure  3 ). Figure  3B  shows that the difference between the magnitude of disparity for the central eye position and each deviated eye condition depends on both eye deviation and the distance of the visual stimulus from the fixation point. Interestingly, such a difference in terms of horizontal and vertical disparity does not depend on the stimulated quadrant visual field in the range of the present checkerboard's width. In other words, the magnitude of the relative disparity for a given distance from the fixation point and for a given deviated eye condition is similar for both left and right visual field stimulation. Since we observed that the C1 amplitude varied inversely in function of the hemifield stimulation (Figure  2 ), the eye-position effect observed on the C1 amplitude cannot be simply related to a horizontal and/or vertical disparity effect. Both direct and indirect arguments also suggest that variations in the C1 amplitude are not due to attention. Firstly, the subjects were instructed to keep firmly visual attention on the fixation point suggesting that the potential degree of attention required fixating binocularly the red fixation dot was similar across the different deviated eye positions. Secondly, numerous recent studies gave impetus to an emerging view that V1 activity may be modulated by attention through delayed feedback signals (160–260 ms) from extrastriate and/or oculomotor structures while the initial C1 sensory response (50–90 ms) was not modulated by attention [ 13 - 15 , 19 - 22 ]. Our present findings, together with those aforementioned, allow considering that both eye position and attention-related signals may affect the early stage of visual processing in different manner. The former may comes from extraocular muscle afferents and/or corollary discharges while the later is considered as a late top-down process [ 14 , 20 ]. Finally, Trotter et al. (1999) have shown that an eye position signal (extraretinal signal) is involved in the neural modulation process dealing with the eye position-dependent visual response observed in area V1 of behaving monkeys. Oculomotor signals coming from extraocular muscle afferents and/or corollary discharges are considered as the substrate of such an eye position signal and have been previously described in V1 [ 23 , 24 ].
Results and discussion
The C1 component reverses classically in polarity for upper  vs.  lower visual field stimulation [for review see [ 12 ]]. Consequently, we first characterized the C1 component over the 20 subjects by observing its polarity inversion for a central eye position. Figure  1B  illustrates the representative polarity inversion of the C1 component on the grand averaged of VEP over the 20 subjects and in response to stimuli in the upper and lower quadrants of the right visual fields. Equivalent VEP and C1 polarity inversion were obtained in response to stimuli in the upper and lower quadrants of the left visual fields. The polarity inversion of the C1 components on the grand average VEPs in response to upper and lower hemifield stimuli were described previously for occipito-parietal sites using a 10–20 system montage with 62 scalp sites (see box included in Figure  1 ). It is noteworthy that a similar polarity inversion was measured in our study using only two occipital intermediate sites (IN3 and IN4) of the modified 10–20-system montage (see Methods for details).vs.Once the C1 component was characterized for both upper and lower quadrant visual fields, the effect of eye position on C1 amplitude was measured only for both left and right lower quadrants visual field that is for the most salient C1 component which we observed.Mean peak latencies of C1 was calculated in each subject in response to both left and right lower visual quadrants and for both IN3 and IN4 recording sites. They ranged between 94.8 ms and 98.0 ms that are consistent with the C1 latency range previously observed in numerous studies [ 12 , 15 ]. For each subject, the C1 amplitude for five different eye positions (0°, 10° and 20° both left- and rightward) was then measured at these mean latencies for each lower quadrant and each recording site and, with respect to a 80 ms pre-stimulus baseline.We observed that checkerboard presented in the lower visual field elicited VEPs with modulated amplitudes of the C1 component in respect of eye position. Grand averaged VEPs over the 20 subjects in response to flashed stimuli in the right lower quadrant of the visual field for three different eye positions (0° and 20° both left- and rightward) are shown in Figure  2A . It is noteworthy that the eye position-dependent modulation of C1 amplitude was observed at the IN3 recording site for a 20° rightward eye position (Figure  2A , red trace) and at the IN4 recording site with a 20° leftward eye position (Figure  2A , green trace).In other words, the eye position effect was observed at the occipital recording site contralateral to the direction of the eye deviation. Equivalent VEPs but reverse eye-position effects were obtained in response to left field stimuli. A one-way Friedman repeated measures analysis of variance was conducted for each recording site and for each visual stimulation revealing a significant main effect of eye position on the amplitude of the C1 component. Complete data obtained at both recording sites, for each lower visual field are shown in Figure  2B . We chose to represent these data using relative amplitude measures that are amplitude for deviated eye position subtracted by the amplitude for central eye position. It means that amplitude zero corresponds to the central eye position situation. A post-hoc Dunnett's test (p < 0.05) using 0° as reference showed that the amplitude of the C1 component measured contralaterally to the deviated eye position was significantly different from 0° except for one situation (Figure  2B , white histograms). Conversely, the amplitude of the C1 component recorded ipsilaterally to the deviated eye position was not significantly different from 0° excepted for one situation (Figure  2B , grey histograms). Note that no parametric effect was observed for the C1 amplitude between 10° and 20° of eye eccentricity (Student's t-test, p < 0.05).The overall result of the present study revealed that the amplitude of first major component of VEP elicited by checkerboard (C1) is modulated by the eye position. The previous data linking the C1 component to a striate cortex generator, namely V1 activity (see introduction), led us to suggest that eye position influences the earliest cortical stage of visual processing.One may first suggest that the present results may be explained by the difficulty of maintaining eccentric fixation, which may have altered the pattern of fixational eye movements, such as microsaccades [fast, conjugate jerks, smaller than 1/3°, see [ 16 ]]. Recent studies have shown that microsaccades modulated neural activity in V1 [ 17 , 18 ], but to our knowledge no study has examined the effect of maintaining eccentric fixation on the occurrence of microsaccades. In the present study, we were able to track spontaneous saccadic eye movements superior to 1°. A one-way Friedman repeated measures analysis of variance for each visual stimulation revealed no significant main effect of eye position on the amplitude (p = 0.35) and on the frequency (p = 0.45) of saccades superior to 1°. We cannot rule out quantitatively a possible role for microsaccades in the eye position-dependent V1 activity. Regardless of such putative effects, however, one may argue that in case of an increase of the number of microsaccades related to eccentric fixation, a similar effect in terms of magnitude would be observed for both left- and rightward deviation. This was not the case in the present study.The question also arises if the effects of changing C1 amplitude may be due to oculomotor signals and/or retinal disparity that is to the difference in the position of the visual stimulus on each retina related to relative monitor distance. The absence of any parametric effect for the C1 amplitude between 10° and 20° of eye eccentricity may indicate that eye position effects are not due to the difference in the horizontal retinal disparity, but one may argue that a putative relationship between the horizontal disparity and the C1 amplitude is not linear. We evaluated the difference of both horizontal and vertical disparity between the different eye's deviations in our study (see Methods for details and Figure  3 ). Figure  3B  shows that the difference between the magnitude of disparity for the central eye position and each deviated eye condition depends on both eye deviation and the distance of the visual stimulus from the fixation point. Interestingly, such a difference in terms of horizontal and vertical disparity does not depend on the stimulated quadrant visual field in the range of the present checkerboard's width. In other words, the magnitude of the relative disparity for a given distance from the fixation point and for a given deviated eye condition is similar for both left and right visual field stimulation. Since we observed that the C1 amplitude varied inversely in function of the hemifield stimulation (Figure  2 ), the eye-position effect observed on the C1 amplitude cannot be simply related to a horizontal and/or vertical disparity effect.Both direct and indirect arguments also suggest that variations in the C1 amplitude are not due to attention. Firstly, the subjects were instructed to keep firmly visual attention on the fixation point suggesting that the potential degree of attention required fixating binocularly the red fixation dot was similar across the different deviated eye positions. Secondly, numerous recent studies gave impetus to an emerging view that V1 activity may be modulated by attention through delayed feedback signals (160–260 ms) from extrastriate and/or oculomotor structures while the initial C1 sensory response (50–90 ms) was not modulated by attention [ 13 - 15 , 19 - 22 ]. Our present findings, together with those aforementioned, allow considering that both eye position and attention-related signals may affect the early stage of visual processing in different manner. The former may comes from extraocular muscle afferents and/or corollary discharges while the later is considered as a late top-down process [ 14 , 20 ].Finally, Trotter et al. (1999) have shown that an eye position signal (extraretinal signal) is involved in the neural modulation process dealing with the eye position-dependent visual response observed in area V1 of behaving monkeys. Oculomotor signals coming from extraocular muscle afferents and/or corollary discharges are considered as the substrate of such an eye position signal and have been previously described in V1 [ 23 , 24 ].Conclusions The present results and previous works obtained from neural recordings in monkeys indicate that changes in eye position can modify response properties in V1 that is at the earliest cortical stage of visual processing. Among the visuo-motor processing allowing accurate reaching and grasping movements in space on the basis of the image seen by the retina, the primary visual area may be therefore one of the first cortical relay to convert the image in eye-centered coordinates into a target location by taking into account the position of the eyes in the orbit. It endorses some recent arguments pointing out that V1 could no longer be considered only in relation to the pattern of light falling on the retina but appears to be a cortical area in which contextual influences take place too [ 9 ].
Conclusions
The present results and previous works obtained from neural recordings in monkeys indicate that changes in eye position can modify response properties in V1 that is at the earliest cortical stage of visual processing. Among the visuo-motor processing allowing accurate reaching and grasping movements in space on the basis of the image seen by the retina, the primary visual area may be therefore one of the first cortical relay to convert the image in eye-centered coordinates into a target location by taking into account the position of the eyes in the orbit. It endorses some recent arguments pointing out that V1 could no longer be considered only in relation to the pattern of light falling on the retina but appears to be a cortical area in which contextual influences take place too [ 9 ].Methods Subjects Twenty right-handed healthy volunteers with normal visual acuity (range age 19–29 years, 9 males) participated in VEP recordings after they provided their written informed consent. The study was approved by the Basse-Normandie ethics committee (Caen, France). Experimental design The subjects were seated on a swivel armchair with their head stabilized with a headrest. An experimenter slowly rotated the armchair and locked it in one of the five different angles: 0°, 10° and 20° both left- and rightward from the center of the monitor leading to five different eye positions (Figure  1A ). The vertical swivel axis passed through the base of the nose between both eyes. Such a passive vertical movement of rotation of the whole body stimulated only the horizontal semicircular canals of the vestibular system with a time constant around 15 sec [ 25 ]. The VEP recording was started at least 1 minute after the rotational movement in order to stay away from the influence of the passive whole body rotational movement of the subjects. The subjects had to fixate binocularly a red fixation dot continuously visible in the center of the display as stimulus was flashed in 1 of the 4 quadrants of the visual field. Upper quadrants were used only in the case of central eye position in order to observe the inversion of polarity of the C1 component that allowed its detection on VEP recordings. The stimulus consisted of a black and white rectangular checkerboard (12 × 9° of visual angle, 0.6 cycle.deg -1  of spatial frequency) flashed against a black background (ISI = 500–1000 ms) and delivered by a visual stimulator (Nicolet, Madison, USA). The subjects were instructed to fixate continuously the fixation dot and to keep their attention on it, for each quadrant of the visual field so that the projection of the stimulus on the retina was equivalent whatever was the deviated eye position. The edges of the monitor and the space up to 1 meter around the monitor were masked with an opaque black sheet preventing any cue perception in the room except the flashing checkerboard. VEP recordings With respect to the purpose of the present study, we recorded VEP from the scalp using the two occipital intermediate sites (IN3 and IN4) of the modified 10–20-system montage [ 26 ] with both left and right mastoids serving as reference. The VEP from each site was recorded (Vicking, Nicolet, Madison, USA) at a sampling rate of 2500 Hz (0.1–100 Hz of band-pass filter with a 50 Hz notch filter). Prior to averaging, artifact rejection was performed to discard epochs with eye blinks. A total of 200 non-rejected epochs was averaged for each recording. Both horizontal and vertical eye movements were also monitored in each subject and during all VEP recordings, with EOG electrodes placed around the orbit of the right eye. The EOG system had a resolution superior to 1° of visual angle. All EOG records were analyzed by computer, using a dedicated software (SAMO) [ 27 ] which detects saccadic components and quantifies the amplitude and frequency of the spontaneous saccadic eye movements. Measurement of the disparity We evaluated the difference of disparity between the different eye's deviations (Figure  3 ). The difference in retinal angles (β - α) defines the magnitude of disparity, classically designated (η) [ 28 ]. Such a disparity depends on the position of the head's subject relative to the screen, designated (θ), on the interocular distance (a), and on the distance (d) between the middle of the cyclopean axis (c) and the fixation point (e1). We evaluated the magnitude of the disparity for any point (e2) of the checkerboard (Figure  3A ). Using the dot product, for the left eye, α can be expressed as following: a1e1.a1e2 = |a1e1| |a1e2| cos(α)     (1) Expressing of the coordinates of the vectors a1e1 and a1e2 in the canonical reference ( x,y,z ) centred on the cyclopean axis (c): a1e1.a1e2 = a1e1 x  a1e2 x  + a1e1 y  a1e2 y  + a1e1 z  a1e2 z      (2) and combining both (1) and (2) led to: cos(α) = (a/2 cos(θ))(e2 x +a/2 cos(θ)) + (d +a/2sin(θ)) 2  / |a1e1| |a1e2|     (3) Similarly, for the right eye, we obtained: cos (β) = (-a/2cos(θ))(e2 x -a/2cos(θ)) + (d -a/2sin(θ)) 2  / |a1e1| |a1e2|     (4) Applying the numeric values defined in our study (d = 1 m, a = 8 cm), and combining both (3) and (4) allows to estimate the magnitude of the disparity (η = β-α) in function of the distance from the fixation point for each given eye's deviation. Therefore, we plotted the magnitude of disparity with each distance (e) and for each eye's deviation [-20°, -10°, 10°, 20°] normalized by the disparity calculated for the central eye position (θ = 0°, Figure  3B ).
Methods
Subjects Twenty right-handed healthy volunteers with normal visual acuity (range age 19–29 years, 9 males) participated in VEP recordings after they provided their written informed consent. The study was approved by the Basse-Normandie ethics committee (Caen, France).
Subjects
Twenty right-handed healthy volunteers with normal visual acuity (range age 19–29 years, 9 males) participated in VEP recordings after they provided their written informed consent. The study was approved by the Basse-Normandie ethics committee (Caen, France).Experimental design The subjects were seated on a swivel armchair with their head stabilized with a headrest. An experimenter slowly rotated the armchair and locked it in one of the five different angles: 0°, 10° and 20° both left- and rightward from the center of the monitor leading to five different eye positions (Figure  1A ). The vertical swivel axis passed through the base of the nose between both eyes. Such a passive vertical movement of rotation of the whole body stimulated only the horizontal semicircular canals of the vestibular system with a time constant around 15 sec [ 25 ]. The VEP recording was started at least 1 minute after the rotational movement in order to stay away from the influence of the passive whole body rotational movement of the subjects. The subjects had to fixate binocularly a red fixation dot continuously visible in the center of the display as stimulus was flashed in 1 of the 4 quadrants of the visual field. Upper quadrants were used only in the case of central eye position in order to observe the inversion of polarity of the C1 component that allowed its detection on VEP recordings. The stimulus consisted of a black and white rectangular checkerboard (12 × 9° of visual angle, 0.6 cycle.deg -1  of spatial frequency) flashed against a black background (ISI = 500–1000 ms) and delivered by a visual stimulator (Nicolet, Madison, USA). The subjects were instructed to fixate continuously the fixation dot and to keep their attention on it, for each quadrant of the visual field so that the projection of the stimulus on the retina was equivalent whatever was the deviated eye position. The edges of the monitor and the space up to 1 meter around the monitor were masked with an opaque black sheet preventing any cue perception in the room except the flashing checkerboard.
Experimental design
The subjects were seated on a swivel armchair with their head stabilized with a headrest. An experimenter slowly rotated the armchair and locked it in one of the five different angles: 0°, 10° and 20° both left- and rightward from the center of the monitor leading to five different eye positions (Figure  1A ). The vertical swivel axis passed through the base of the nose between both eyes. Such a passive vertical movement of rotation of the whole body stimulated only the horizontal semicircular canals of the vestibular system with a time constant around 15 sec [ 25 ]. The VEP recording was started at least 1 minute after the rotational movement in order to stay away from the influence of the passive whole body rotational movement of the subjects. The subjects had to fixate binocularly a red fixation dot continuously visible in the center of the display as stimulus was flashed in 1 of the 4 quadrants of the visual field. Upper quadrants were used only in the case of central eye position in order to observe the inversion of polarity of the C1 component that allowed its detection on VEP recordings. The stimulus consisted of a black and white rectangular checkerboard (12 × 9° of visual angle, 0.6 cycle.deg -1  of spatial frequency) flashed against a black background (ISI = 500–1000 ms) and delivered by a visual stimulator (Nicolet, Madison, USA). The subjects were instructed to fixate continuously the fixation dot and to keep their attention on it, for each quadrant of the visual field so that the projection of the stimulus on the retina was equivalent whatever was the deviated eye position. The edges of the monitor and the space up to 1 meter around the monitor were masked with an opaque black sheet preventing any cue perception in the room except the flashing checkerboard.VEP recordings With respect to the purpose of the present study, we recorded VEP from the scalp using the two occipital intermediate sites (IN3 and IN4) of the modified 10–20-system montage [ 26 ] with both left and right mastoids serving as reference. The VEP from each site was recorded (Vicking, Nicolet, Madison, USA) at a sampling rate of 2500 Hz (0.1–100 Hz of band-pass filter with a 50 Hz notch filter). Prior to averaging, artifact rejection was performed to discard epochs with eye blinks. A total of 200 non-rejected epochs was averaged for each recording. Both horizontal and vertical eye movements were also monitored in each subject and during all VEP recordings, with EOG electrodes placed around the orbit of the right eye. The EOG system had a resolution superior to 1° of visual angle. All EOG records were analyzed by computer, using a dedicated software (SAMO) [ 27 ] which detects saccadic components and quantifies the amplitude and frequency of the spontaneous saccadic eye movements.
VEP recordings
With respect to the purpose of the present study, we recorded VEP from the scalp using the two occipital intermediate sites (IN3 and IN4) of the modified 10–20-system montage [ 26 ] with both left and right mastoids serving as reference. The VEP from each site was recorded (Vicking, Nicolet, Madison, USA) at a sampling rate of 2500 Hz (0.1–100 Hz of band-pass filter with a 50 Hz notch filter). Prior to averaging, artifact rejection was performed to discard epochs with eye blinks. A total of 200 non-rejected epochs was averaged for each recording. Both horizontal and vertical eye movements were also monitored in each subject and during all VEP recordings, with EOG electrodes placed around the orbit of the right eye. The EOG system had a resolution superior to 1° of visual angle. All EOG records were analyzed by computer, using a dedicated software (SAMO) [ 27 ] which detects saccadic components and quantifies the amplitude and frequency of the spontaneous saccadic eye movements.Measurement of the disparity We evaluated the difference of disparity between the different eye's deviations (Figure  3 ). The difference in retinal angles (β - α) defines the magnitude of disparity, classically designated (η) [ 28 ]. Such a disparity depends on the position of the head's subject relative to the screen, designated (θ), on the interocular distance (a), and on the distance (d) between the middle of the cyclopean axis (c) and the fixation point (e1). We evaluated the magnitude of the disparity for any point (e2) of the checkerboard (Figure  3A ). Using the dot product, for the left eye, α can be expressed as following: a1e1.a1e2 = |a1e1| |a1e2| cos(α)     (1) Expressing of the coordinates of the vectors a1e1 and a1e2 in the canonical reference ( x,y,z ) centred on the cyclopean axis (c): a1e1.a1e2 = a1e1 x  a1e2 x  + a1e1 y  a1e2 y  + a1e1 z  a1e2 z      (2) and combining both (1) and (2) led to: cos(α) = (a/2 cos(θ))(e2 x +a/2 cos(θ)) + (d +a/2sin(θ)) 2  / |a1e1| |a1e2|     (3) Similarly, for the right eye, we obtained: cos (β) = (-a/2cos(θ))(e2 x -a/2cos(θ)) + (d -a/2sin(θ)) 2  / |a1e1| |a1e2|     (4) Applying the numeric values defined in our study (d = 1 m, a = 8 cm), and combining both (3) and (4) allows to estimate the magnitude of the disparity (η = β-α) in function of the distance from the fixation point for each given eye's deviation. Therefore, we plotted the magnitude of disparity with each distance (e) and for each eye's deviation [-20°, -10°, 10°, 20°] normalized by the disparity calculated for the central eye position (θ = 0°, Figure  3B ).
Measurement of the disparity
We evaluated the difference of disparity between the different eye's deviations (Figure  3 ). The difference in retinal angles (β - α) defines the magnitude of disparity, classically designated (η) [ 28 ]. Such a disparity depends on the position of the head's subject relative to the screen, designated (θ), on the interocular distance (a), and on the distance (d) between the middle of the cyclopean axis (c) and the fixation point (e1). We evaluated the magnitude of the disparity for any point (e2) of the checkerboard (Figure  3A ).Using the dot product, for the left eye, α can be expressed as following:a1e1.a1e2 = |a1e1| |a1e2| cos(α)     (1)Expressing of the coordinates of the vectors a1e1 and a1e2 in the canonical reference ( x,y,z ) centred on the cyclopean axis (c):x,y,za1e1.a1e2 = a1e1 x  a1e2 x  + a1e1 y  a1e2 y  + a1e1 z  a1e2 z      (2)and combining both (1) and (2) led to:cos(α) = (a/2 cos(θ))(e2 x +a/2 cos(θ)) + (d +a/2sin(θ)) 2  / |a1e1| |a1e2|     (3)Similarly, for the right eye, we obtained:cos (β) = (-a/2cos(θ))(e2 x -a/2cos(θ)) + (d -a/2sin(θ)) 2  / |a1e1| |a1e2|     (4)Applying the numeric values defined in our study (d = 1 m, a = 8 cm), and combining both (3) and (4) allows to estimate the magnitude of the disparity (η = β-α) in function of the distance from the fixation point for each given eye's deviation. Therefore, we plotted the magnitude of disparity with each distance (e) and for each eye's deviation [-20°, -10°, 10°, 20°] normalized by the disparity calculated for the central eye position (θ = 0°, Figure  3B ).List of abbreviations used C1 : early component of visual evoked potential fMRI : functional magnetic resonance imaging IN3 and IN4 : occipital intermediate recording sites ISI : inter-stimulus interval V1 : primary visual area VEP : visual evoked potential
List of abbreviations used
C1 : early component of visual evoked potentialfMRI : functional magnetic resonance imagingIN3 and IN4 : occipital intermediate recording sitesISI : inter-stimulus intervalV1 : primary visual areaVEP : visual evoked potentialAuthors' contributions All authors designed the study. FA and OE carried out the experiments and generated the method for data analysis. OE performed the statistical analysis and generated the method for estimation of the magnitude of disparity. FA and LP wrote the first versions of the manuscript. All authors read, discussed and approved the final version of the manuscript.
Authors' contributions
All authors designed the study. FA and OE carried out the experiments and generated the method for data analysis. OE performed the statistical analysis and generated the method for estimation of the magnitude of disparity. FA and LP wrote the first versions of the manuscript. All authors read, discussed and approved the final version of the manuscript.