Effects of sympathetic activation and regular periodic breathing on complexity of heart period variability

Summary Background. Heart rate exhibits different levels of complexity which are probably related to the neural autonomic modulation. Aim of the study. The aim is to analyze the effect of the sympathetic activation and of a regular periodical external input on the complexity of heart period. Methods. Fifteen healthy young subjects were recorded at rest during spontaneous breathing, during head-up tilt, and at rest during controlled breathing at different rates (10, 15, and 20 breaths/min). The minimum of corrected conditional entropy (CCE) was used to measure complexity (the higher the minimum, the larger the complexity). Spectral indexes in low and high frequency (LF and HF) bands were calculated as well. Results. CCE minimum was significantly lower during tilt and controlled respiration at 10 breaths/min compared to rest during spontaneous breathing, while LF/HF ratio was larger during tilt but smaller during controlled respiration at 10 breaths/min. During faster respiratory rates CCE minimum and the LF/HF were unchanged. No significant correlation was detected between the CCE minimum and the mean or variance of the heart period. CCE minimum was correlated with LF/HF ratio only at rest and during tilt. Conclusions. These results indicate that complexity of heart period is not related to its mean or variance, does not depend only on regularity of respiration, and is affected by interventions capable of entraining different regulatory mechanisms and reducing the number of temporal scales involved on heart period regulation.