Objective ·To explore the optimal dose of ropivacaine for elective cesarean section under spinal anesthesia combined with a single low-dose of intravenous S-ketamine, and observe the rates of hypotension associated with this method. Methods ·Forty eligible women were included in the study for spinal anesthesia block combined with intravenous low-dose S-ketamine (0.15 mg/kg). The first patient received a 12 mg spinal anesthesia dose of ropivacaine. If the upper sensory block level of the patient was not lower than the sixth thoracic vertebra (T6) and visual analog scale (VAS) score was ≤2 before delivery, the next patient had 1/9 chance of receiving a lower dose or 8/9 chance of receiving the same dose of ropivacaine as the previous patient. If the patient had VAS score>2 or needed an extra epidural analgesics before delivery, a higher dose of ropivacaine was used for the next patient. The primary outcome was whether the effect of spina anesthesia was accurate, that is, the amount of ropivacaine to maintain patients' VAS score ≤2 before delivery, and the incidence of post-spinal anesthesia hypotension. Secondary outcomes mainly included the rate of hypotension-related symptoms and drug interventions, upper sensory block level of anesthesia, level of sedation (Ramsay sedation scale was used as the evaluation standard) and neonatal outcomes. 90% effective dose (ED₉₀) and 95% confidence interval (95% CI) of ropivacaine were estimated by Isotonic regression. Results ·In combination with low-dose S-ketamine, the ED₉₀ of ropivacaine for cesarean section spinal anesthesia was 11.8 mg (95%CI 11.7－12.7). The incidence of hypotension and hypoxemia of forty parturients was 5.0% (2 cases) and 2.5% (1 case), respectively, and there was no bradycardia occurring. All parturients did not use phenylephrine injection as a remedy, and there was no adverse reactions related to hypotension occurring (dyspnea, nausea, vomiting, shiver and blurred vision). There was no reflux and aspiration occurring. Only one parturient before delivery had a Ramsay sedation score of 2 (awake, quiet and cooperative), and the rest scored 3 (sleepy but respond quickly to instruction). At 5 min after delivery, according to the Ramsay sedation scale, 32 parturients were 2 scores, and 8 parturients were 3 scores. At the end of the operation, the Ramsay sedation scale of all parturients was 2 scores. During the cesarean section, the incidence of visceral traction pain, nightmare and dizziness of patients was 7.5%, 62.5% and 52.5%, respectively. In addition, the Apgar scores of 40 newborns at 1 min and 5 min after birth were all 10, and the umbilical vein blood gas analysis showed that the pH was (7.37±0.03). Conclusion ·Combined with a intravenous single low-dose of S-ketamine during spinal anesthesia for elective cesarean section, the ED₉₀ of ropivacaine is 11.8 mg, and this method can effectively reduce the incidence of hypotension.