Decreased entropy of symbolic heart rate dynamics during daily activity as a predictor of positive head-up tilt test in patients with alleged neurocardiogenic syncope.

Entropy measures of RR interval variability during daily activity over a 24h period were compared in 30 patients with a positive head-up tilt (HUT) test and 30 patients with a negative HUT test who had a history of alleged neurocardiogenic syncope. Two different entropies, approximate entropy (ApEn) and entropy of symbolic dynamics (SymEn), were employed. In patients showing a positive HUT test, the entropies were significantly decreased when compared with the patients with a negative HUT test. In addition, SymEn in the patients with a negative HUT test was significantly lower than in the normal controls. Discriminant analysis using SymEn could correctly identify 89.3% (520/582) of the 1 h RR interval data of the patients with a positive HUT test regardless of the time of day. Baseline entropies of heart rate dynamics during daily activity were found to be significantly lower in patients with alleged neurocardiogenic syncope and a positive HUT test than in those with the same history but with a negative HUT test. The decreased entropy of symbolic heart rate dynamics may be of predictive value of a positive HUT test in patients with alleged neurocardiogenic syncope.