The role of the autonomic nervous system in the patterns of heart rate fragmentation

Abstract Although rapid oscillations in heart rate (HR) are commonly attributed to vagal modulation of the heart, an ultra-rapid pattern of HR variation has recently been studied and named heart rate fragmentation (HRF). Objective Considering that the underlying mechanisms of HRF are unknown, this study aims to evaluate the influence of the autonomic nervous system on HRF in rats. Methods Electrocardiogram recordings were performed in awake rats at baseline conditions and after selective or combined pharmacological autonomic blockade. Series of RR intervals were calculated and HRF indices were computed, namely percentage of inflection points (PIP), percentage of patterns with zero (W0), one (W1), two (W2) or three (W3) inflection points, and percentage of patterns with only hard (WH), soft (WS) or mixed (WM) inflection points. Moreover, the correlations between HRF indices and several important heart rate variability (HRV) indices were estimated. Results PIP and W3 increased while W1 decreased after autonomic blockade (sympathetic, parasympathetic or combined). In contrast, the parasympathetic blockade noticeably decreased hard (WH) and increased soft (WS) HR transitions. Except for W0, HRF patterns grouped by the number of inflection points were not correlated to any studied HRV index. In contrast, patterns grouped by the type of inflection points are highly correlated to magnitude-dependent HRV indices. Conclusion Cardiac sympathetic and parasympathetic influences similarly decrease HRF, while the parasympathetic control markedly increases the hard HR inflection points. Further studies should be conducted to verify whether the autonomic nervous system plays a different role in diseases marked by autonomic imbalance.