Volume mediano efetivo de cristaloides na preven？？o de hipotens？o arterial em pacientes submetidas à cesariana com raquianestesia

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 2 ml (10 mg) plus morphine 50 μg 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 100-150 ml.min-1 prior to the spinal anesthetic injection. the initial volume of crystalloid was 5 ml.kg-1. volume-effect data were fitted to a sigmoidal maximum efficacy model and the median effective volume (ev50) 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 ev50 of crystalloid were 12.6 ml.kg-1 (95% ci, 11.6 to 14.8 ml.kg-1). with firth's correction, the pooled probability of an effective preventive volume of crystalloid at 13 ml.kg-1 was 50.2% (95% ci, 30% to 83.1%). conclusions: the estimated ev50 of the preloaded crystalloid required to prevent spinal anesthesia-induced hypotension in a cesarean section is, approximately, 13 ml.kg-1. however, prophylactic or therapeutic vasoconstrictors should also be prepared and administered at an appropriate time.