Dosing for phenylephrine
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Optimal Dosing for Phenylephrine in Cesarean Delivery Under Spinal Anesthesia
Introduction to Phenylephrine Dosing
Phenylephrine is a vasopressor commonly used to prevent and treat hypotension during spinal anesthesia for cesarean delivery. Determining the optimal dosing regimen is crucial for maintaining maternal hemodynamic stability and ensuring favorable neonatal outcomes.
Weight-Based Infusion Regimens
A study investigated four weight-based phenylephrine infusion regimens (0.25, 0.375, 0.5, and 0.625 µg/kg/min) to prevent hypotension during cesarean delivery under combined spinal-epidural anesthesia. The effective doses for 50% (ED50) and 90% (ED90) of patients were found to be 0.31 µg/kg/min and 0.54 µg/kg/min, respectively1. This suggests that a prophylactic infusion rate of around 0.54 µg/kg/min is effective for most patients.
Fixed-Rate Infusion Regimens
Another study compared fixed-rate phenylephrine infusions (25, 50, 75, and 100 µg/min) and found that lower doses (25 and 50 µg/min) required fewer interventions to maintain maternal systolic blood pressure within 20% of baseline compared to higher doses (75 and 100 µg/min)2. This indicates that lower fixed-rate infusions may provide better hemodynamic stability.
Bolus Dosing for Hypotension Treatment
For treating spinal anesthesia-induced hypotension, a study determined the ED50 and ED90 of phenylephrine boluses in normotensive and preeclamptic parturients. Normotensive parturients required higher doses (ED50: 72.1 µg, ED90: 107 µg) compared to preeclamptic parturients (ED50: 47.6 µg, ED90: 70.7 µg)4. This highlights the need for dose adjustments based on the patient's condition.
Ondansetron's Impact on Phenylephrine Requirement
The administration of intravenous ondansetron before spinal anesthesia was shown to reduce the ED50 of phenylephrine infusion by approximately 26%, from 0.32 µg/kg/min to 0.24 µg/kg/min5. This suggests that ondansetron can lower the required dose of phenylephrine for preventing hypotension.
Comparative Studies with Other Vasopressors
Comparative studies have shown that phenylephrine and norepinephrine have different potency ratios. For instance, the ED90 for phenylephrine was 90.9 µg compared to 8.0 µg for norepinephrine, with a potency ratio of 11.4:16. This information is useful for clinicians considering alternative vasopressors.
Safety and Cardiovascular Tolerability
A study on the pharmacokinetics and safety of oral phenylephrine (10, 20, and 30 mg) found that the drug was well-tolerated with no significant cardiovascular adverse effects7. This supports the safety profile of phenylephrine in clinical use.
Conclusion
Optimal dosing of phenylephrine for preventing and treating hypotension during cesarean delivery under spinal anesthesia varies based on the administration method and patient condition. Weight-based infusions around 0.54 µg/kg/min, fixed-rate infusions of 25-50 µg/min, and bolus doses adjusted for normotensive and preeclamptic patients are effective strategies. Additionally, the use of ondansetron can reduce the required dose of phenylephrine. Clinicians should tailor phenylephrine dosing to individual patient needs to ensure maternal and neonatal safety.
Sources and full results
Most relevant research papers on this topic
Dose-Response Study of 4 Weight-Based Phenylephrine Infusion Regimens for Preventing Hypotension During Cesarean Delivery Under Combined Spinal-Epidural Anesthesia.
The optimal dose of phenylephrine for preventing hypotension during spinal anesthesia for cesarean delivery is 0.31 g/kg/min, with no significant difference in adverse effects or neonatal outcomes.
RCT
Rigorous JournalA Double-Blind, Placebo-Controlled Trial of Four Fixed Rate Infusion Regimens of Phenylephrine for Hemodynamic Support During Spinal Anesthesia for Cesarean Delivery
Prophylactic phenylephrine infusions at 25 and 50 &mgr;g/min reduced the incidence and severity of maternal predelivery hypotension, but did not significantly reduce the number of physician interventions needed to maintain maternal blood pressure within 20% of baseline.
RCT
Highly CitedEquivalent dose of ephedrine and phenylephrine in the prevention of post-spinal hypotension in Caesarean section.
Phenylephrine and ephedrine have an 81.2 potency ratio for preventing hypotension after spinal anesthesia for Caesarean section.
RCTHighly CitedComparative Dose-Response Study of Phenylephrine Bolus for the Treatment of the First Episode of Spinal Anesthesia-Induced Hypotension for Cesarean Delivery in Severe Preeclamptic versus Normotensive Parturients
Severe preeclamptic parturients require a 34% reduction in phenylephrine dose for treating spinal anesthesia-induced hypotension compared to normotensive parturients.
RCTRigorous JournalA Prospective, Randomized, Double-Blinded Study of the Effect of Intravenous Ondansetron on the Effective Dose in 50% of Subjects of Prophylactic Phenylephrine Infusions for Preventing Spinal Anesthesia–Induced Hypotension During Cesarean Delivery
Intravenous ondansetron reduces the effective dose of prophylactic phenylephrine infusions for preventing hypotension during spinal anesthesia in 50% of patients compared to saline control.
RCTRigorous JournalProphylactic Norepinephrine and Phenylephrine Boluses to Prevent Postspinal Anesthesia Hypotension During Cesarean Section: A Randomized Sequential Allocation Dose-Finding Study
An 8-g prophylactic bolus of norepinephrine and a 90-g prophylactic bolus of phenylephrine effectively prevent postspinal anesthesia hypotension in patients during cesarean section.
RCTPharmacokinetics, Safety, and Cardiovascular Tolerability of Phenylephrine HCl 10, 20, and 30 mg After a Single Oral Administration in Healthy Volunteers
Phenylephrine HCl 10, 20, and 30 mg have similar pharmacokinetics and cardiovascular tolerability after a single oral administration in healthy volunteers.
RCTRigorous JournalDetermination of the 90% Effective Dose of Phenylephrine Boluses to Treat Spinal Anesthesia-Induced Hypotension in Patients with Severe Preeclampsia during Cesarean Delivery: A Pilot Study
Phenylephrine 60 g may be effective and safe for treating spinal anesthesia-induced hypotension in patients with severe preeclampsia during cesarean delivery.
Non-RCTRigorous JournalUp-Down Determination of the 90% Effective Dose of Phenylephrine for the Treatment of Spinal Anesthesia-Induced Hypotension in Parturients Undergoing Cesarean Delivery
The 90% effective dose of phenylephrine for treating spinal anesthesia-induced hypotension in cesarean delivery is approximately 150 g.
Non-RCTThe Dose-Dependent Effects of Phenylephrine for Elective Cesarean Delivery Under Spinal Anesthesia
A higher dose of phenylephrine (100 &mgr;g/min) can significantly reduce maternal heart rate and cardiac output, potentially impacting fetal outcome.
RCTHighly Cited