Nonlinear measures of heart rate variability after fentanyl-based induction of anesthesia.

Abstract

thesia (2). Also, spectral analysis of HRV suggests autonomic nervous system impairment after cardiac surgery (3). Newer HRV measures are model independent, suitable for nonlinear processes, and measure aspects of HRV different from the traditional methods such as standard deviation or spectral analysis (4). One of these nonlinear measures, approximate entropy (ApEn), correlates with postoperative ventricular dysfunction (5). However, the new nonlinear methods of HRV measurement have not been applied in the operating room. We wanted to determine whether these methods could be used as tools to detect changes in HRV which occur during the anesthetic management of patients. We compared the effect of the induction of anesthesia on HRV as measured by two nonlinear methods, ApEn and point correlation dimension (PD2). ApEn is a modification of traditional informational entropy measures designed to measure “regularity” in small amounts of noisy data, even in chaotic systems (4-6). In essence, it is calculated by identifying patterns within a time series (e.g., groups of two consecutive R-R intervals) and determining the effect of increasing pattern size (e.g., from two to three consecutive R-R