Approximate entropy of heart rate as a correlate of postoperative ventricular dysfunction

Background: Instantaneous changes in the heart rate, i.e., heart rate variation, traditionally have been quantified by the standard deviation of a series of intervals between successive heart beats. Approximate entropy provides another measure of variability by calculating the logarithmic likelihood that patterns that are similar remain similar on the next incremental comparisons. Approximate entropy is a nonnegative number that will distinguish data sets by their amount of regularity, with larger numbers indicating more randomness. We hypothesized that a decrease in the approximate entropy of heart rate would be associated with postoperative ventricular dysfunction (e.g., myocardial infarction, unstable angina, congestive heart failure, prolonged inotropic support). Methods: Twenty- three high-risk noncardiac patients were continuously monitored by ambulatory electrocardiographic recorders from the evening before surgery up to 80 h during the postoperative period: 9 demonstrated postoperative ventricular dysfunction, and 14 had an uncomplicated postoperative course. Hourly approximate entropy average values were calculated. Results: Approximate entropy was high (>0.7) in all but two patients preoperatively. Postoperative approximate entropy <0.55 had a sensitivity of 88% and a specificity of 71% for being associated with postoperative ventricular dysfunction; preoperative approximate entropy values were not significantly different between the two groups. Conclusions: These results suggest that changes in approximate entropy can distinguish between patients who sustained poor outcome and those who had an uncomplicated course.