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 8.8 μg norepinephrine.