Analysis of heart rate variability: a useful tool to evaluate autonomic tone in the anesthetized patient?

Abstract

HE discovery of general anesthesia is one of the most important advancements in medicine within the last century. Unfortunately, it is impossible to perform anesthesia adequately , without altering autonomic nervous system tone. Coping with altered autonomic responses is made possible by continuous monitoring of changes in vital signs, including blood pressure and heart rate, and by fine-tuning anesthetic depth, and correcting imbalances in autonomic tone with pharmacological agents. Because anesthesia blunts autonomic reflexes without abolishing them, most interventions to correct autonomic imbalance are followed by reflex responses, leading to fluctuations in vital signs. Intraoperative variations in blood pressure and heart rate have taught experienced anesthesiologists to practice patience and moderation in the correction of vital signs, and to avoid unnecessary pharmacological interventions to “correct the correction”. The lag time between step changes in drug concentration, and changes in vital signs, reflects the degree of hyster esis between changes in autonomic balance at the end-organ level, and the delays in pharmacological responses to correct the imbalance. Measurements of changes in autonomic tone, therefore, offer the possibility of correcting the dysfunction more rapidly, before the end-organ changes occur in higher-risk patient populations. The issue of clinical relevance for anesthesiologists is, whether or not this can in fact, be achieved without invasive monitoring of autonomic nerve impulses. It was first observed by Hales, 1 that the time interval between successive heart beats can be extracted in a series of numbers, whose variability can be studied; hence the origin of the misleading term “heart rate variability”. This term is misleading, because it is not the variability in heart rate per se, but rather, variability of the time intervals between cardiac contractions, that