Power Spectral Heart Rate Analysis Demonstrates Decreased Activity of the Sympathetic Nervous System During Low Bupivacaine Spinal Anesthesia

Abstract

Study Objective: To evaluate the onset of spinal anesthesia with power spectral heart rate analysis to determine the influence of the block on the autonomic nervous system. Design: Prospective, clinical evaluation. Setting: Tertiary-care teaching hospital. Patients: 27 ASA physical status I and II patients scheduled for lower extremity orthopedic surgery and free of major cardiac disease or cardiac drugs with direct influence of heart rate (HR) or blood pressure (BP). Interventions: Prior to anesthesia, a baseline power spectral heart rate reading was taken in the supine position. Spinal anesthesia was established in the sitting position with 15 mg of bupivacaine and 0.2 mg epinephrine introduced at the L 3–L 4 interspace with a 22-gauge Quincke needle. The patient was returned supine, and power spectral heart rate data were again collected at 5-minute intervals throughout the procedure. Level of the spinal block was checked at 5-minute intervals until 30 minutes and considered complete when two consecutive readings were unchanged. Measurements and Main Results: Heart rate and BP were recorded at baseline and at five-minute intervals after injection. Power spectral heart rate data included low-frequency activity (LFa), high-frequency activity (HFa), and the ratio (LFa/HFa). Spinal level achieved was recorded by thoracic dermatome at complete onset. Heart rate and BP remained within 20% of control in all cases. Complete onset of the spinal block was present by 30 minutes in all cases. The average level of spinal anesthesia was T 8. Compared with baseline, LFa activity decreased, HFa activity remained unchanged, and the ratio was decreased. During endoprosthesis insertion, 9 of 14 total hip patients had a transient ten-fold increase in LFa activity, without HFa change, and a corresponding increase in the ratio. Conclusions: Power spectral heart rate analysis during low thoracic bupivacaine spinal anesthesia is compatible with decreased sympathetic activity during stable hemodynamic intervals. Insertion of hip endoprosthesis resulted in a dramatic, transient increase in sympathetic activity, indicating that sympathetic activation was still possible despite the presence of surgical anesthesia from the subarachnoid block.