Hemodynamic changes during spinal surgery in the prone position

Background: Hypotension and fluctuations in blood pressure (BP) during lumbar spinal surgery in the prone position under anesthesia are not unusual. The purpose of this study was to investigate the causes of the decrease in BP during lumbar spinal surgery in the prone position using a noninvasive monitor of cardiac output. Methods: Twenty ASA Class I or II patients, scheduled for elective lumbar spinal surgery in the prone position, had their hemodynamic status monitored by a BioZ.com system Impedance Cardiograph during anesthesia. Hemodynamic data (heart rate [HR], mean BP, cardiac index [CI], stroke volume [SV] and systemic vascular resistance [SVR]) were registered at baseline, post-induction of general anesthesia, 10 minutes after the patient was turned from the supine to the prone position and 1 hour after the start of surgery. Friedman's test and the paired t test were used to compare the collected data on hemodynamic parameters. Results: The mean BP, SV, CI and HR were found to have significant differences (p<0.05) at the designated time points as analyzed by Friedman's test, while the SVR and central venous pressure showed no significant changes. CI and SV were found to be markedly decreased from 2.4±0.3 to 2.0±0.3L/minute/m² and from 45.8±9.7 to 36.7+9.2 mL, respectively, after patients assumed the prone position. Mean BP also decreased significantly. After 1 hour of surgery, the mean BP decreased further with a fall in HR but the SV remained unchanged. Conclusion: Decreases in SV and CI are the main causes of a decrease in BP in the prone position during lumbar spinal surgery.