Heart rate variability are needed to address the LF component (with a lower bound of 0.04 Hz). Although time domain methods, especially the SDNN and RMSSD methods, can be used to investigate recordings of long durations, a substantial part of the long-term variability is day–night differences. Thus, long-term recordings analyzed by time domain methods should contain at least 18 hours of analyzable ECG data that include the whole night. Although cardiac automaticity is intrinsic to various pacemaker tissues, heart rate and rhythm are largely under the control of the autonomic nervous system. The parasympathetic influence on heart rate is mediated via release of
Heart rate turbulence normal cardiac cycle, so the ventricles of the heart haven't had time to fill up to their normal level, before contracting and pumping their contents out. This results in a pulse (blood pressure) weaker than expected and triggers normal homeostatic mechanisms that try to compensate by constricting arteries and increasing heart rate (the turbulence onset part of HRT). This is accomplished by the brain reflexively withdrawing the parasympathetic nerve signals and increasing the sympathetic nerve signals it sends to the heart. The compensatory constriction of the arteries and increased heart rate frequently cause blood pressure to oveshoot normal values (overcompensates),