Before the Pressure Falls: Preoperative Autonomic Reserve as a Physiological Predictor of Hemodynamic Instability After General Anesthesia Induction

Abstract

Background: Post-induction hypotension is a frequent complication of general anesthesia and may reflect inadequate cardiovascular compensation to anesthetic-induced vasodilation. Preoperative assessment of autonomic reserve may identify patients at increased risk. Methods: This prospective observational study included 111 adults undergoing elective surgery under general anesthesia at Sri Lakshmi Narayana Institute of Medical Sciences, Puducherry from May 2024 to May 2025. Preoperative autonomic reserve was assessed using short-term heart rate variability and standardized cardiovascular autonomic tests. Hemodynamic variables were recorded for 10 minutes following induction. Post-induction instability was defined by a reduction in mean arterial pressure of at least 20% from baseline or MAP below 65 mmHg. Results: Hemodynamic instability occurred in 39 patients (35.1%). These patients demonstrated significantly lower SDNN, RMSSD, high-frequency power, deep-breathing E:I ratio, and 30:15 ratio, with higher LF/HF ratio and greater orthostatic systolic pressure reduction. Vasopressor requirement was also higher. Lower SDNN, two or more abnormal autonomic tests, and lower baseline MAP independently predicted instability. The combined predictive model achieved an AUROC of 0.82. Conclusion: Reduced preoperative autonomic reserve was strongly associated with post-induction hemodynamic instability. Simple autonomic assessment may improve perioperative risk stratification and facilitate individualized hemodynamic preparedness before general anesthesia, particularly in patients with otherwise apparently acceptable baseline cardiovascular parameters. Keywords: Autonomic reserve; General anesthesia; Heart rate variability; Hemodynamic instability; Post-induction hypotension.