OBJECTIVE: Our purpose was to evaluate the predictive value of the baseline systemic vascular resistance index for the development of maternal hypotension during regional anesthesia for cesarean delivery. STUDY DESIGN: Patients receiving a standardized spinal or epidural anesthetic for nonemergency cesarean delivery were studied prospectively. Hemodynamic data were obtained noninvasively with an NCCOM-3 cardiac output monitor (Bomed Medical Manufacturing, Irvine, Calif.), which uses thoracic electrical bioimpedance to estimate stroke volume and cardiac output. Measurements obtained were indexed to body surface area. The systemic vascular resistance index was calculated from mean arterial pressure and thoracic electrical bioimpedance - derived cardiac index. Hemodynamic data obtained were analyzed to identify statistically significant predictors of maternal hypotension. RESULTS: Maternal hypotension occurred in 24 of 42 (57%) patients studied. The incidence of hypotension did not differ between the types of anesthesia: spinal 17 of 27 (62%) versus epidural 7 of 15 (47%, p = 0.48). The mean interval to the onset of hypotension was 12.2 minutes (SD 2.2 minutes, range 2 to 24 minutes). Mean (SD) baseline maternal systolic blood pressure was higher in patients who had hypotension (145 torr [4]) than those who did not (129 torr [4], p = 0.01). The mean (SD) baseline systemic vascular resistance index was higher in patients who had hypotension (633 [SD 36] dyne · cm · sec^-5/m²) than those who did not (454 [SD 29] dyne · cm · sec^-5/m²; p = 0.001). With receiver-operator characteristic curves, a baseline systemic vascular resistance index of 500 had a sensitivity of 83%, a specificity of 78%, a positive predictive value of 83%, and a negative predictive value of 78% for maternal hypotension (odds ratio 17.5, 95% confidence interval 3.1 to 109.4). A baseline systolic blood pressure of 140 torr had a sensitivity and specificity of 42% and 72%, respectively (odds ratio 1.9, 95% confidence interval 0.4 to 8.8). CONCLUSIONS: Baseline systemic vascular resistance index obtained by noninvasive cardiac output monitoring with thoracic electrical bioimpedance and systolic blood pressure are useful to predict the risk for maternal hypotension with regional anesthesia. Patients with increased baseline systemic vascular resistance index or systolic blood pressure are at increased risk for hypotension. (AM J OBSTET GYNECOL 1996;174:1019-25.)