A randomised controlled trial comparing weight adjusted dose versus fixed dose  prophylactic phenylephrine infusion on maintaining systolic blood pressure during  caesarean section under spinal anaesthesia.

Lucy Mwaura, Vitalis Mung’ayi, Jimmie Kabugi, Samina Mir
Department of Anaesthesia, Aga Khan University, East Africa.

Abstract
Background: Spinal anaesthesia is the standard of care for elective caesarean delivery. It has advantages over general anaesthesia. However the sympathetic blockade induced by spinal anaesthesia results in an 80 percent incidence of hypotension without prophylactic management. Current evidence supports co-loading with intravenous fluids in conjunction with the use of vasopressors as the most effective way to prevent and treat the hypotension. Phenylephrine is the accepted vasopressor of choice in  the parturient. A prophylactic phenylephrine infusion combined with a fluid co-load is proven to be an effective and safe method  of maintaining maternal hemodynamic stability. While most published studies have assessed the effectiveness of a prophylactic  phenylephrine fixed dose infusion, few studies have assessed the effect of a prophylactic phenylephrine weight adjusted dose  infusion on maintaining maternal hemodynamic stability following spinal anesthesia for a cesarean delivery.

Objective: To compare the incidence of hypotension between women undergoing elective caesarean section under spinal anaesthesia, receiving prophylactic phenylephrine infusion at a fixed dose of 37.5 micrograms per minute versus a weight adjusted  dose of 0.5 micrograms per kilogram per minute.

Methods: One hundred and eight patients scheduled for non-urgent caesarean section under spinal anaesthesia were randomized into 2 groups; control group and intervention group using a computer generated table of numbers.  Control group; Received prophylactic phenylephrine fixed dose infusion at 37.5 micrograms per minute.  Intervention group; Received prophylactic phenylephrine weight adjusted dose infusion at 0.5 micrograms per kilogram per  minute.

Results: The two groups had similar baseline characteristics in terms of ;Age, sex, weight and height. There was a 35.2% incidence of hypotension in the fixed dose group and an 18.6% incidence of hypotension in the weight adjusted dose group. This  difference was found to be of borderline statistical significance p-value 0.05, and the difference in the incidence rates between  the two groups was found to be statistically significant p= 0.03. The difference in the incidence of reactive hypertension and  bradycardia between the two groups was not statistically significant: p-value of 0.19 for reactive hypertension and p-value of  0.42 for the incidence of bradycardia. There was also no statistically significant difference in the use of phenylephrine boluses,  use of atropine, intravenous fluid used and the number of times the infusion was stopped.

Conclusion: Among this population, the incidence of hypotension was significantly less in the weight adjusted dose group than  in the fixed dose group. There was no difference in the number of physician interventions required to keep the blood pressure  within 20% of baseline, and no difference in the proportion of reactive hypertension or bradycardia between the two groups.  Administering prophylactic phenylephrine infusion at a weight adjusted dose of 0.5 micrograms per kilogram per minute results  in a lower incidence of hypotension compared to its administration at a fixed dose of 37.5 micrograms per minute.

Keywords:Weight adjusted dose, fixed dose prophylactic phenylephrine infusion, systolic blood pressure, caesarean section,
spinal anaesthesia.