The effect of different phenylephrine infusion rates on uteroplacental blood flow during cesarean delivery under spinal anesthesia

Introduction
Hypotension associated with spinal anesthesia is more common and profound in the pregnant population, resulting in adverse effects to both the mother and the fetus. It is now widely accepted that the vasopressor of choice during cesarean delivery is phenylephrine. However, an overdose of phenylephrine may cause reflex bradycardia and decreased maternal and fetal cardiac output. In contrast, lower phenylephrine doses may not be adequate to avoid or control hypotension. The optimal phenylephrine dose and its direct effect on uteroplacental blood flow are yet to be determined.
Aim of the work
This study aimed to examine the direct effect of different phenylephrine infusion rates on uterine blood flow during cesarean delivery spinal anesthesia. Assessment of uteroplacental blood flow was performed using Doppler ultrasound of the uterine artery from which uterine blood flow indices were obtained, namely, peak systolic velocity (PSV) and pulsatility index (PI).
Materials and methods
This is a prospective, randomized double-blind study. We included 90 age-matched American Society of Anesthesiologists (ASA) I or II parturients with term singleton pregnancies admitted for elective cesarean delivery under spinal anesthesia. We excluded candidates with hypertension, cardiovascular or cerebrovascular disease, type 1 diabetes mellitus, allergy or hypersenstivity to phenylephrine, known fetal abnormalities, intrauterine growth retardation, and any contraindication to spinal anesthesia. The patients were distributed randomly into three equal groups ( = 30 each). Groups 25, 50, and 75 received 25, 50, and 75 μg/min phenylephrine infusion, respectively, after spinal anesthesia was administered. The maternal uterine artery was identified by colored Doppler ultrasound and pulsed-wave Doppler was used to measure PSV and calculate PI before spinal anesthesia and at 5 and 15 min after the block was performed. Maternal hemodynamics and measures of fetal well-being (Apgar score and umbilical venous pH) were also recorded.
Results
PI at 15 min after spinal anesthesia was significantly higher in group 75 in comparison with the baseline value ( < 0.05) and also in comparison with groups 50 and 25 ( < 0.05). Furthermore, the percentage of decrease in PSV, compared with the baseline, was also significantly higher in group 75 compared with the other two groups at both 5 and 15 min ( < 0.05). Group 75 also showed a significantly higher incidence of hypertension and bradycardia in comparison with both the other groups. However, the number of hypotensive episodes as well as nausea and vomiting was significantly higher in group 25 compared with the other two groups ( < 0.05). There was no significant difference in fetal outcome among the different groups.
Conclusion and recommendations
At a dose of 75 μg/min, phenylephrine induced a significant reduction in uteroplacental blood flow as evidenced by decreased PSV compared with baseline values and an increase in PI compared with the other two groups. This decrease in uteroplacental blood flow was not associated, however, with poor fetal outcome. Further studies are needed to address the correlation between uteroplacental blood flow and fetal outcome with different phenylephrine doses in patients with uteroplacental insufficiency.