To study the ED50 value of ropivacaine for unilateral spinal anesthesia in elderly patients with different heights undergoing total knee arthroplasty

Objective: To investigate the median effective dose (ED50) of ropivacaine for unilateral spinal anesthesia in total knee arthroplasty (TKA) in elderly patients with different heights. Methods: Sixty ASA ⅱ-ⅲ patients, aged ≥ 60 yr, BMI 20.0–29.9 kg/m2, undergoing total knee arthroplasty under unilateral spinal anesthesia, were enrolled in this study. The patients were divided into three groups according to their height. The height of the patients was 151–155 cm, which was recorded as S group. Group M (height 156–160 cm); Group H (height 161–165 cm), puncture was performed in the L3-4 space, and 0.25% ropivacaine was used (such as 1% ropivacaine hydrochloride 1 mL, plus sterile water for injection 3 mL). According to the results of the preliminary experiment and the principle of the sequential method, the first patient in group S was given a dose of 0.25% ropivacaine of 6.0 mg, the first patient in group M was given a dose of 7.0 mg, and the first patient in group H was given a dose of 8.5 mg. If the dose of local anesthetic used in the previous patient met the criteria for efficacy, the dose of local anesthetic was reduced by 0.5 mg for the next patient. Otherwise, it was upregulated by 0.5 mg. The study was completed when 7 inflection points were obtained. The median effective dose (ED50) of ropivacaine was estimated by means of the turning point method, and then Probit regression analysis was used to calculate the more precise ED50, ED95 and 95% confidence interval (CI) of ropivacaine. CI) was calculated. Vital signs, level of sensory block and motor block were recorded at each time point after administration. Results: ED50 of group S, group M and group H was 6.04 mg, 7.11 mg and 7.96 mg, respectively. Probit regression analysis showed that ED50 and ED95 in group S were 6.02 mg (95% CI: 5.29–6.74 mg) and 6.24 mg (95% CI: 5.52–6.97 mg), and ED50 and ED95 in group M were 7.05 mg (95%CI: 5.29–6.74 mg) and 7.05 mg (95% CI: 5.52–6.97 mg), respectively. The ED50 and ED95 of group H were 7.97 mg (95%CI: 7.26–8.68 mg) and 8.18 mg (95% CI: 7.47–8.90 mg), respectively (P < 0.05). No adverse reactions such as hypotension and bradycardia occurred in all patients during the operation. There was no significant difference in the level of sensory block on the affected side among the three groups (P > 0.05). Conclusions: The ED50 of hypogravity ropivacaine for unilateral spinal anesthesia in total knee arthroplasty increases with the increase of body height. The median effective dose of ropivacaine for unilateral spinal anesthesia in elderly patients with different body height groups is 6.02 mg, 7.05 mg and the 95% effective drug doses were 6.24 mg, 7.24 mg and 8.18 mg, respectively.