Quantification of Autonomic Response to Passive Change of Posture in Healthy Individuals

Abstract

Spectral heart rate variability analysis is commonly used as a non-invasive measure of cardiac autonomic regulation. These indices are traditionally based on power spectral density estimation of short data segments (e.g. 5 min), since stationarity of the heart rate variability signal is required for traditional spectral estimation. To study autonomic regulation of cardiac function as a result of time-varying interventions, a time-frequency analysis, which generates a time-varying spectrum and, thus, time-varying spectral indices, is usually a more suitable approach. To investigate the influence of the application of stationary or time-varying methods on heart rate variability before and after an autonomic challenge, this paper compares static and time-varying indices before and after slow and fast passive changes in posture. The results show a significant decrease in the high frequency index and an increase in the low-to-high frequency ratio after slow tilt only, compared to baseline, using either approach, indicating a shift to sympathetic dominance after tilt, as would be expected. The results also show a high correlation between each index (static vs. dynamic), suggesting that the autonomic adaptations to slow passive tilt in healthy subjects are fast enough to be measured by either approach.