Volumetric capnography in XXXX . Influence of growth on the alveolar plateau slope.

@entity1 @entity544
BACKGROUND: Lung growth in @entity1 is associated with dramatic increases in the number and surface area of alveolated airways. Modelling studies have shown the slope of the alveolar plateau (phase III) is sensitive to the total cross-sectional area of these airways. Therefore, the influence of age and body size on the phase III slope of the volumetric capnogram was investigated. METHODS: Phase III slope (alveolar dcCO2/dv) and airway deadspace (VDaw) were derived from repeated single-breath @entity544 expirograms collected on 44 healthy mechanically ventilated @entity1 (aged 5 months-18 yr) undergoing minor surgery. Ventilatory support was standardized (VT = 8.5 and 12.5 ml/kg, f = 8-15 breaths/min, inspiratory time = 1 s, end-tidal partial pressure of @entity544 = 30-45 mmHg), and measurements were recorded by computerized integration of output from a heated pneumotachometer and mainstream infrared @entity544 analyzer inserted between the endotracheal tube and anesthesia circuit. Experimental data were compared to simulated breath data generated from a numeric pediatric lung model. RESULTS: An increased VDaw, a smaller VDaw/VT, and flatter phase III slope were found at the larger tidal volume (P < 0.01). Strong relationships were seen at VT = 12.5 ml/kg between airway deadspace and age (R2 = 0.77), weight (R2 = 0.93), height (R2 = 0.78), and body surface area (R2 = 0.89). The normalized phase III slopes of @entity1 were markedly steeper than that of adolescents and were reduced at both tidal volumes with increasing age, weight, height, and body surface area. Phase III slopes and VDaw generated from modelled @entity544 washout simulations closely matched the experimental data collected in @entity1 . CONCLUSIONS: Morphometric increases in the alveolated airway cross-section with lung growth is associated with a decrease of the phase III slope. During adolescence, normalized phase III slopes approximate those of healthy adults. The change in slope with lung growth may reflect a decrease in diffusional resistance for @entity544 transport within the alveolated airway resulting in diminished acinar @entity544 gradients.
