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BACKGROUND

Intravenous patient-controlled analgesia (PCA) with fentanyl or remifentanil is an option for labor analgesia in patients for whom neuraxial anesthesia is contraindicated (e.g., platelet disorders, coagulopathy, or patient’s refusal).1 Both remifentanil and fentanyl PCA have significantly lower visual analog scale pain scores 1 hour after treatment initiation compared to baseline, with pain score reduction being significantly greater with remifentanil compared to fentanyl.1,2 Patients’ pain scores with fentanyl and remifentanil PCA return to baseline by 3 hours after treatment initiation, likely reflecting increased pain levels with advancing labor.1

METHOD OF ADMINISTRATION

Fentanyl

A starting dose for fentanyl intravenous PCA is 12.5 µg with a lockout interval of 6 minutes and can be increased to 25 µg every 6 minutes, if needed. Maximum hourly dose is 250 µg.1,3 To minimize maternal and fetal opioid exposure and related side effects, there should not be a basal infusion rate.1,3 Fentanyl has a rapid onset of action, because its high lipophilicity permits rapid distribution from plasma into highly vascularized compartments, muscle, and fat. The transfer half-life into the central nervous system is 4.7 to 6.6 minutes.4 Fentanyl is metabolized by the 3A4 substrate of the CYP450 system in the liver with an elimination half-life of 3 to 8 hours.4

Remifentanil

The intravenous PCA dose of remifentanil for labor analgesia is 0.2 to 0.8 µg/kg (with an increment of 0.1 µg/kg) with a lockout interval of 2 to 3 minutes.5 Remifentanil has a more rapid onset of action than fentanyl with onset within 1 minute and peak effect in 2 minutes.6 Remifentanil is rapidly hydrolyzed by plasma esterase leading to a context-sensitive half-life of 3 to 4 minutes and an elimination half-time of 10 to 20 minutes.1

SIDE EFFECTS

With both fentanyl and remifentanil PCA, patient-reported pain scores are significantly higher and patient satisfaction is lower than with neuraxial epidural analgesia.2 Both remifentanil and fentanyl are associated with maternal respiratory depression and episodic desaturation (oxygen saturation <95%), and thus necessitate continuous patient monitoring with respiratory rate and pulse oximetry.1,5,6 Remifentanil is associated with significantly increased maternal sedation compared to fentanyl or epidural analgesia.1,5,6 Remifentanil also has a greater incidence of pruritus than fentanyl.1

Reduced fetal heart rate beat-to-beat variability has been associated with remifentanil.5 However, remifentanil is not associated with an increased rate of newborns having Apgar scores less than 7 at 5 minutes compared to epidural analgesia.2 There is no significant difference in Apgar scores or umbilical cord blood gas in babies born to mothers who received fentanyl or remifentanil when they are compared to each other or to epidural analgesia.1,2,7 The rate of cesarean delivery is similar in mothers using fentanyl, remifentanil, and epidural labor ...

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