Autonomic Nervous System Function and Depth of Sedation in Adults Receiving Mechanical Ventilation

BACKGROUND: The effect of the depth of sedation on the function of the autonomic nervous system is not well known. OBJECTIVES: To describe the effect of level of sedation on heart rate variability as a marker of the function of the autonomic nervous system in patients receiving mechanical ventilation. METHODS: This pilot study was part of a larger study in which sedation level was measured continuously for up to 24 hours. The sample consisted of 14 patients receiving mechanical ventilation. The R-R interval was measured continuously via electrocardiography. Sedation level was determined by using the Patient State Index and was categorized as deep (<60) or light (=60). Continuous heart rate data of 5 to 10 minutes for each sedation level for each patient were analyzed. RESULTS: Parasympathetic activity as indicated by root mean square of successive difference of the R-R interval, the high-frequency component, and the percentage of differences of successive N-N intervals (intervals due to normal sinus depolarization) that differed more than 50 milliseconds was significantly lower for deep sedation than for light sedation. The markers indicating sympathetic activity, including the low-frequency component and the ratio of the low-frequency component to the high-frequency component, did not differ significantly between the 2 levels of sedation. Most patients were receiving benzodiazepines. CONCLUSIONS: Deep sedation may be associated with depression of parasympathetic function in patients receiving mechanical ventilation. Use of benzodiazepines most likely contributed to this finding.