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ED90 and ED95 of Phenylephrine for Treatment of Hypotension after Intrathecal Isobaric Plain Bupivacaine in Elective Cesarean Delivery
Abstract Number: T-17
Abstract Type: Original Research
INTRODUCTION: Clinical empirical bolus dose of phenylephrine for treatment of maternal hypotension after spinal anesthesia is usually less than 100μg. Studies have shown that effective doses (ED90 and ED95) of phenylephrine required to treat hypotension induced by spinal anesthesia during cesarean delivery (CD) were 147μg and 159μg respectively, when 0.75% w/v hyperbaric bupivacaine was co-administrated with opioids. We routinely use combined spinal epidural (CSE) anesthesia with intrathecal administration of 0.5% w/v plain bupivacaine for CD. Decreased baricity of bupivacaine may potentially generate a higher level of sensory block and increased hypotension incidence. The purpose of this study was to determine ED90/ED95 of intravenous bolus phenylephrine as a sole vasopressor for treatment of hypotension after intrathecal isobaric bupivacaine in CSE setting.
METHODS: Continual reassessment method (CRM) was used. Healthy parturients with term singleton who were scheduled for elective primary CD were enrolled. Preoperatively, 500 mL of lactated Ringer’s solution was preloaded. CSE was performed in the left lateral decubitus position with intrathecal administration of 0.5% w/v isobaric plain bupivacaine 2 mL. Non-invasive blood pressure (BP) was measured every minute until delivery. One of the six predetermined doses of phenylephrine, ranging from 60μg to 160μg with 20μg increments, was given as an intravenous bolus to parturient if systolic BP (SBP) dropped at least 20% off the baseline value. An increase of SBP of at least 20% over the baseline SBP value within 2 minutes was considered a successful treatment. Except for the starting treatment dose of 60μg, all treatment doses of phenylephrine were determined by Bayesian Phase dose-ranging Clinical Trials (BPCT) software (Paris, France).
RESULTS: Twenty of the twenty-four treatments were successful. Probabilities of success associated with each of 6 bolus doses (from 60μg to 160μg with 20μg increments) were 58.9%, 80.3%, 90.7% (95% CI: 74.1-99.5%), 95.5%, 98.3%, 99.2%, respectively. There were 8 cases (33.3%) of bradycardia recorded in the study.
DISCUSSION: ED90 and ED95 of phenylephrine bolus doses as a sole agent to treat hypotension induced by intrathecal administration of 0.5% w/v isobaric plain bupivacaine were approximately 100μg and 120μg, respectively, which were lower than those reported earlier. Although the mass of local anesthetics is thought to influence the spread of blockade, hypotension from less denser blockade with lower concentration of plain bupivacaine is likely easier to treat with lower dose of phenylephrine. The incidence of bradycardia was high with the treatment of phenylephrine as sole agent for hypotension. CRM can be an efficient method for ED study.
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