Analysis of Autonomic Function During Anesthesia Using Response Surface Model

Spectral analysis of heart rate variability (HRV) is a widely used, noninvasive technique to assess autonomic indexes of neural cardiac control. The presence of low-frequency (LF) and high-frequency (HF) oscillatory rhythms in the variability of the R-R interval (RRI) is well established. To date, it is believed that LF is mediated by the parasympathetic and sympathetic systems, whereas HF is mediated primarily by the parasympathetic system. The current study used HRV in order to characterize autonomic nerve system (ANS) activity in patients before and during stable hypnosis, nociceptive surgical stimulation and (adequate) or light (inadequate) analgesia. These effects may vary with the anesthetic technique used. Despite these previous studies, attempts to derive the relation between anesthetic concentrations and HRV have so far remained inconclusive. Such characterization would provide additional information on the behavior of the ANS during different anesthesic drug level and may lay the basis for the development of new clinical application.

Propofol is now widely used in clinical practice because of its favorable recovery profile and low incidence of side effects. However, induction of anesthesia with propofol is often associated with a significant decrease in arterial blood pressure and heart rate (HR). The hypotensive effect of propofol has been attributed to a decrease in systemic vascular resistance or in cardiac output caused by a combination of venous and arterial vasodilation, impaired baroreflex mechanisms and depression of myocardial contractility. Besides, there are some conflicting data regarding the effects of propofol on cardiac sympathetic or parasympathetic tone.

The investigators hypothesis was that at different propofol anesthetic drug level would have reproducible effects on HRV. In addition, these results should be reliably characterized in anesthetized patients provided that HRV uses analysis that can assess transient and rapid changes in ANS activity. Therefore, the first goal of this study was to test the hypothesis that propofol anesthesia would affect HRV depending on the concentrations, and so do opioid and midazolam. Second, the investigators will evaluate the effect of anesthetic combination of autonomic system.