When conducting spinal anesthesia (SA) in a position on the abdomen, arterial hypotension may occur, which is due to anesthesia and the effect of the position itself on the blood flow. The aim of the work was the development of a prognostic model for changes in hemodynamics in vertebrologic operations in the abdomen on the background of spinal anesthesia. Materials and methods. Postural blood circulation reactions were studied in 144 patients of the orthopedic profile operated in the abdominal position under spinal anesthesia: in the position on the back, after 5 and 20 minutes after turning on the stomach. Previously (one day before surgery) the same reactions were studied without anesthesia. Results. In 16.0±3.1 % of patients during SA, hemodynamic correction with α 1 -adrenomimetics was required after rotation on the stomach. A randomized analysis of changes in hemodynamics before and during anesthesia revealed in these patients the stress of blood flow compensation, which was manifested when rotated to the abdomen without anesthesia in arterial hypertension and increased general peripheral vascular resistance. Under the influence of SA this compensation was suppressed, which led to instability of blood circulation. Significant risk of such a complication occurred in younger patients and with increased BMI. According to the results of the retrospective analysis, a mathematical model has been developed that allows calculating the prognostic index of hemodynamic instability (PING) and predicting hemodynamic instability during SA in the presence of abdominal patient. The PING value can range from infinitesimal to 1. Conclusions: before conducting surgical interventions in the abdominal position under SA it is expedient to preoperatively study postural blood flow reactions with the calculation of PING. With PING>0.5 you should refrain from conducting CA and choose another kind of anesthesia.