The effect of parturient height on the median effective dose of intrathecally administered ropivacaine

BACKGROUND: Pain during cesarean delivery is one of the more common reasons for a successful medico-legal claim. However, creating an extensive block area can result in hypotension, so determining the precise dose of local anesthetic is critical. OBJECTIVES: Investigate effects of parturient height on the median effective dose (ED50) of intrathecally-administered ropivacaine. DESIGN: Prospective cross-sectional analytic study. SETTING: Anesthesiology department in a provinicial hospital in China. METHODS: Parturients undergoing cesarean delivery under combined spinal and epidural anesthesia were stratified according to height as follows: 150 cm to 155 cm, 156 cm to 160 cm, 161 cm to 165 cm and 166 cm to 170 cm. The spinal component of the anesthetic was established by bolus administration of up-and-down doses of 0.75% plain ropivacaine as determined by the Dixon method. The initial dose of ropivacaine was 5.79 mg and the testing interval dose change was set at 0.75 mg. The block height for the first cold feeling at T5 was considered satisfactory anesthesia. MAIN OUTCOME MEASURES: ED50 values and vasopressor requirements, nausea, vomiting and shivering. RESULTS: In 120 parturients, the ED50 for satisfactory block height using intrathecal ropivacaine was 5.92 mg (95% confidence interval[CI] 5.02–6.86 mg) patients of 150 to 155 cm in height; 6.52 mg (95% CI 5.45–7.65 mg) in 156 cm to 160 cm; 7.49 mg (95%CI 6.83–8.25 mg) in 161 cm to 165 cm; 8.35 mg ( 95%CI 7.55–9.23 mg) in 166 to 170 cm. The ED50 of ropivacaine increased with increasing height of the subject. There were no significant differences in incidence of hypotension, vasopressor requirements, nausea, vomiting and shivering. CONCLUSION: The ED50 of intrathecal ropivacaine using sensitivity to cold sensation increased with parturient height, indicating that dose may be determined in part by height. LIMITATION: The ED95 rather than the ED50 for spinal anesthesia is more useful clinically. We did not control for the effect of weight on the dose of local anesthetic. Factors such as baricity, volume, concentration injected, temperature of the solution, and viscosity can affect intrathecal spread of the local anesthetics and block quality.