POWER SPECTRAL HEART-RATE ANALYSIS DEMONSTRATES DECREASED ACTIVITY OFTHE SYMPATHETIC NERVOUS-SYSTEM DURING LOW BUPIVACAINE SPINAL-ANESTHESIA

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 evaluati on. Setting: Tertiary-care teaching hospital. Patients: 27 ASA physica l status I and II patients scheduled for lower extremity orthopedic su rgery and free of major cardiac disease or cardiac drugs with direct i nfluence of head 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 sitti ng position with 15 mg of bupivacaine and 0.2 mg epinephrine introduce d at the L-3-L-4 interspace with a 22-gauge Quincke needle. The patien t was returned supine, and power spectral heart rate data were again c ollected at 5-minute intervals throughout the procedure. Level of the spinal block was checked at 5-minute intervals until 30 minutes and co nsidered complete when two consecutive readings were unchanged. Measur ements and Main Results: Heart rate and BP were recorded at baseline a nd 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 th oracic dermatome at complete onset. Heart rate and BP remained within 20% of control in all cases. Complete onset of the spinal block was pr esent by 30 minutes in all cases. The average level of spinal anesthes ia 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 increa se in LFa activity, without HFa change, and a corresponding increase i n the ratio. Conclusions: Power spectral heart rate analysis during lo w 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 symp athetic activity indicating that sympathetic activation was still poss ible despite the presence of surgical anesthesia from the subarachnoid block. (C) 1998 by Elsevier Science Inc.