Phenylephrine dosing
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Phenylephrine Dosing for Preventing Hypotension During Cesarean Delivery
Introduction to Phenylephrine Dosing
Phenylephrine is a vasopressor commonly used to prevent and treat hypotension during spinal anesthesia for cesarean delivery. Despite its widespread use, the optimal dosing regimen remains a topic of research and debate. This article synthesizes findings from multiple studies to provide a comprehensive overview of phenylephrine dosing strategies.
Weight-Based Phenylephrine Infusion Regimens
A study investigated the effectiveness of different weight-based phenylephrine infusion regimens in preventing hypotension during cesarean delivery under combined spinal-epidural anesthesia. The study found that the effective dose for 50% (ED50) and 90% (ED90) of patients were 0.31 µg/kg/min and 0.54 µg/kg/min, respectively. These doses were effective in preventing hypotension without significant adverse effects or differences in neonatal outcomes.
Fixed-Rate Infusion Regimens
Another study compared four fixed-rate infusion regimens of phenylephrine (25, 50, 75, and 100 µg/min) for hemodynamic support during spinal anesthesia. The study concluded that doses of 25 and 50 µg/min were associated with fewer interventions to maintain maternal systolic blood pressure (SBP) within 20% of baseline compared to higher doses. Higher doses (75 and 100 µg/min) were linked to an increased incidence of predelivery hypertension.
Comparative Studies with Ephedrine
A comparative study aimed to determine the dose equivalence between phenylephrine and ephedrine for preventing post-spinal hypotension. The study found that the minimum effective dose for phenylephrine was 532.9 µg, while for ephedrine, it was 43.3 mg. The potency ratio for equivalence was determined to be 81.2, indicating that phenylephrine is significantly more potent than ephedrine.
Dose-Response in Preeclamptic vs. Normotensive Parturients
Research comparing the dose-response of phenylephrine in severe preeclamptic versus normotensive parturients found that preeclamptic patients required a 34% lower dose. The ED50 and ED90 for normotensive parturients were 72.1 µg and 107 µg, respectively, while for preeclamptic parturients, these values were 47.6 µg and 70.7 µg.
Dose-Dependent Effects on Maternal and Fetal Outcomes
A study examining the dose-dependent effects of phenylephrine on maternal cardiovascular stability and fetal outcomes found that higher doses (100 µg/min) significantly reduced maternal heart rate (HR) and cardiac output (CO) by up to 20%. Despite these reductions, there were no significant differences in Apgar scores or umbilical cord blood gases among different dosing groups.
Impact of Ondansetron on Phenylephrine Dosing
A study investigated the effect of intravenous ondansetron on the effective dose of phenylephrine. It was found that ondansetron reduced the ED50 of phenylephrine by approximately 26%, suggesting that ondansetron can lower the required dose of phenylephrine for preventing hypotension during cesarean delivery.
Prophylactic Infusion Strategies
Prophylactic phenylephrine infusion at 100 µg/min was shown to significantly reduce the incidence, frequency, and magnitude of hypotension compared to bolus administration. This method was found to be simple, safe, and effective for maintaining arterial blood pressure during spinal anesthesia.
Effective Dose for Treating Hypotension
The 90% effective dose (ED90) of phenylephrine for treating spinal anesthesia-induced hypotension was estimated to be approximately 150 µg. This dose was determined using an up-down sequential allocation method and was found to be effective in 90.5% of cases.
Conclusion
Phenylephrine is a potent vasopressor for preventing and treating hypotension during cesarean delivery under spinal anesthesia. Optimal dosing varies based on individual patient characteristics, including weight and the presence of conditions like preeclampsia. Both weight-based and fixed-rate infusion regimens have been shown to be effective, with lower doses generally associated with fewer adverse effects. Future research should continue to refine these dosing strategies to enhance maternal and fetal outcomes.
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