RÉSUMÉ
BACKGROUND AND OBJECTIVES: Spinal anesthesia-associated maternal hypotension in Cesarean delivery is the most frequent and troublesome complication, posing serious risks to mothers and compromising neonatal well-being. The effective volume of intravenous crystalloid as the preventive strategy in this context has not been estimated. METHODS: Eighty-five parturients with ASA physical status I/II undergoing elective Cesarean delivery were screened and 67 eligible women were assigned to receive pre-spinal crystalloid loading. Hyperbaric 0.5% bupivacaine 2mL (10mg) plus morphine 50µg was given to all patients. The volume of crystalloid was determined by an up-and-down sequential method. The crystalloid was infused at a rate of 100-150mL.min(-1) prior to the spinal anesthetic injection. The initial volume of crystalloid was 5mL.kg(-1). Volume-effect data were fitted to a sigmoidal maximum efficacy model and the median effective volume (EV(50)) and corresponding 95% confidence interval (95% CI) were estimated using maximum likelihood estimation and logistic regression with Firth's correction. RESULTS: A total of 67 subjects completed the study and were analyzed. Twenty-eight (41.8%) patients developed hypotension with their systolic blood pressure (SBP) decreasing > 20% of baseline. The EV(50) of crystalloid were 12.6mL.kg(-1) (95% CI, 11.6 to 14.8mL.kg(-1)). With Firth's correction, the pooled probability of an effective preventive volume of crystalloid at 13mL.kg(-1) was 50.2% (95% CI, 30% to 83.1%). CONCLUSIONS: The estimated EV(50) of the preloaded crystalloid required to prevent spinal anesthesia-induced hypotension in a Cesarean section is, approximately, 13mL.kg(-1). However, prophylactic or therapeutic vasoconstrictors should also be prepared and administered at an appropriate time.