Relative potency of norepinephrine and mephentermine bolus for the treatment of spinal hypotension during elective caesarean delivery: a randomized, blinded up-down sequential allocation study.

BACKGROUND: Vasopressors are the mainstay of management of spinal hypotension during caesarean delivery. Norepinephrine, an α- and β-adrenergic agonist, has a lower incidence of bradycardia and is an alternative to phenylephrine. Mephentermine is another such vasopressor and is used in many developing countries. Earlier studies compared norepinephrine and mephentermine using fixed or indirectly calculated doses. The purpose of the current study was to estimate their equipotent doses for the treatment of hypotension.

METHODS: One hundred healthy parturients with term, uncomplicated, singleton pregnancy undergoing elective caesarean delivery, who developed spinal hypotension, were studied. Hypotension was defined as >20% fall from baseline systolic blood pressure and was treated with a blinded, predetermined dose of norepinephrine or mephentermine. The dose for each patient was determined according to the response in the previous patient using the Narayana rule for up-down sequential allocation. The ED95 and ED50 of norepinephrine and mephentermine boluses and their relative potency ratio were calculated.

RESULTS: The ED95 was 7.2 µg (95% confidence interval [CI]: 4.9 to 9.5) for norepinephrine and 1.59 mg (95% CI: 1.43 to 1.75) for mephentermine. The ED50 values were 6.9 µg (95% CI: 6.2 to 7.7) and 1.39 mg (95% CI: 1.33 to 1.46), respectively. The relative ED95 and ED50 potency ratios of norepinephrine to mephentermine were 222 (95% CI: 201 to 242) and 201 (95% CI: 193 to 210), respectively.

CONCLUSION: When used as bolus for treatment of spinal hypotension during elective caesarean delivery, norepinephrine is approximately 220 times more potent than mephentermine; therefore, norepinephrine 8 μg is approximately equivalent to mephentermine 1.7 mg.