Comparison of the potency of phenylephrine and norepinephrine bolus doses used to treat post-spinal hypotension during elective caesarean section.
BACKGROUND
Phenylephrine, although considered the vasopressor of choice, can cause reflex bradycardia and a fall in cardiac output. Norepinephrine, due to its direct positive chronotropic and reflex negative chronotropic actions, is expected to overcome this problem. However, limited information about its effective dose for management of post-spinal hypotension, and its potency compared to phenylephrine, is available.


METHODS
One hundred consecutive patients who developed post-spinal hypotension were treated with a predetermined dose of either phenylephrine or norepinephrine. Correction of hypotension after one minute was considered 'success'. The starting dose for the first patient and testing interval (the incremental or decremental dosing) were 100 μg and 10 μg in the phenylephrine group, and 6 μg and 0.5 μg in the norepinephrine group. Doses for subsequent patients were determined by the responses of previous patients according to the Narayana rule for up-down sequential allocation. ED95 and ED50 of phenylephrine and norepinephrine boluses and their potency ratio were calculated.


RESULTS
Using Probit analysis, ED95 and ED50 values were 43.1 µg (95% CI 39.5 to 65.0 µg) and 33.2 µg (95% CI 5.1 to 37.0 µg) for phenylephrine, and 3.7 µg (95% CI 3.5 to 4.7 µg) and 3.2 µg (95% CI 1.8 to 3.4 µg) for norepinephrine. The relative potency ratio of norepinephrine and phenylephrine was 11.3 (95% CI 8.1 to 16.9).


CONCLUSION
Based on the results of this study, norepinephrine is about 11 times more potent than phenylephrine. When used as bolus doses for treatment of hypotension, 100 μg phenylephrine should be approximately equivalent to 9 μg norepinephrine.
['Mohta|M|M|', 'Dubey|M|M|', 'Malhotra|R K|RK|', 'Tyagi|A|A|']
D058646:Adrenergic alpha-1 Receptor Agonists D000316:Adrenergic alpha-Agonists D000328:Adult D000773:Anesthesia, Obstetrical D000775:Anesthesia, Spinal D002585:Cesarean Section D004311:Double-Blind Method D017558:Elective Surgical Procedures D005260:Female D006801:Humans D007022:Hypotension D009638:Norepinephrine D010656:Phenylephrine D011247:Pregnancy D016896:Treatment Outcome
Potency Phenylephrine Norepinephrine Hypotension Caesarean section Spinal anaesthesia