Univariate and bivariate symbolic analyses of cardiovascular variability differentiate general anesthesia procedures

General anesthesia attenuates autonomic function and baroreflex control. This side effect should be prevented as much as possible because it limits the subject's ability in responding to physiological challenges during surgery (e.g. arterial pressure and ventricular contractility drops). This study is designed to rank two of the most commonly exploited general anesthesia treatments, i.e. intravenous anesthesia (IA) based on a propofol-opioid combination and volatile anesthesia (VA) based on a sevoflurane-opioid combination, according to their ability to maintain autonomic nervous system activity and baroreflex control. Univariate and bivariate symbolic techniques were applied to spontaneous heart period (HP) and systolic arterial pressure (SAP) variability series recorded during IA and VA procedures in 19 and 18 patients undergoing elective intracranial neurosurgery. Traditional linear univariate and bivariate frequency domain markers of the autonomic nervous system state and baroreflex control were evaluated as well. We found that: (i) univariate symbolic analysis of HP series suggests a better preservation of vagal modulation in VA than in IA; (ii) bivariate symbolic markers assessing the degree of HP-SAP association differentiate IA from VA, while baroreflex sensitivity and squared coherence function cannot; (iii) bivariate symbolic analysis indicates a better preservation of the HP-SAP association at slow frequencies in IA than in VA, thus suggesting a more active baroreflex control in IA. We conclude that symbolic indexes can be fruitfully exploited to rank general anesthesia treatments, and their performance appears to be superior to that of more traditional linear markers.