Power spectral analysis of arterial blood pressure after spinal anesthesia

The continuous, on-line and real-time analysis of the power spectrum (CORAPS) of systemic arterial pressure (SAP) and heart rate (HR) signals is one of the current progresses in the development of non-invasive indexes for autonomic nervous system. It might be a useful clinical tool to monitor the progress of preganglionic block of sympathetic nervous system and the homeostatsis of cardiac neuroregulation after spinal anesthesia. The purpose of this study is to examine the relationship between the hypotensive response and the changes of components in CORAPS throughout the course of spinal anesthesia.

In 10 ASA class I-II patients (aged from 17 to 75 years) scheduled for elective surgery, we analyzed the changes of spectrum of systemic arterial blood pressure signals during spinal anesthesia. Spinal anesthesia was performed with 0.5% bupivacaine (dosage at 13-15 mg). For CORAPS, SAP signals were simultaneously relayed to an analog-digital converter connected to a computer. Power spectral density of the data was computed using a fast Fourier transform. The areas of the spectral peaks within each measurement were calculated as the follows: very low (BVLF, 0.01-0.08 Hz), low (BLF, 0.09-0.15 Hz), and high (BHF, 0.16-0.25 Hz).

During the time interval (S2 period) when spinal anesthesia was initiated and 15 min thereafter the power density of low frequency (BLF), (1.2 vs. 2.3), and very low frequency (BVLF) components decreased, (3.4 vs. 7.8), in comparison with that at the baseline level (S1 period). We also found the shift of dominance between low frequency (BLF) and high frequency (BHF) in S2 period. The effects of intrathoracic pressure on the venous return may contribute to the increase of power density of BHF components (1.7 vs. 1.1). We also found that even in patients with stable hemodynamic variability, the decrease of BLF and BVLF components, and the shift of dominance were the same as in patients with unstable hemodynamic variability.

It might prove that the cephalic spread of spinal block abolished the sympathetic tone of the autonomic nervous system. These results validated previous speculations of a sympathetic block during spinal anesthesia. More importantly, patients with stable hemodynamic variability still have profoundly sympathetic block with decreased power density of BLF and BVLF. The CORAPS is a good clinical monitoring tool to evaluate the homeostatsis of cardiac neuroregulation after spinal anesthesia.