NYCTHEMERAL PROFILE OF NONSPECTRAL HEART-RATE-VARIABILITY MEASURES INWOMEN AND MEN - DESCRIPTION OF A NORMAL SAMPLE AND 2 SUDDEN CARDIAC-ARREST SUBSAMPLES

Heart rate variability (HRV), a noninvasive systemic index of the cent ral autonomic nervous system, demonstrates considerable within-subject variability, including a strong systematic 24-hour nycthemeral (or le ss precisely, circadian) component. Recent interest in the timing of s udden cardiac arrest (SCA), especially the pronounced morning rise in sudden deaths, has motivated research into coincident dynamic phenomen a in HRV indices of central autonomic nervous system activity. In this study, statistical (nonspectral) HRV measures (SD and %RR50) were sum marized for consecutive 15-minute blocks from 24-hour Holter electroca rdiogram tapes. Six subgroups were scrutinized: women and men respecti vely in three clinical strata (normal subjects [n = 85 women and 40 me n], SCA with no current or prior myocardial infarction [MI] [n = 9 wom en and 31 men], SCA with old MI [n = 7 women and 48 men]). Significant nycthemeral effects were observable for all HRV measures in five of t he six groups, with a dramatic fall in HRV during the hours of the mor ning with the highest phenomenologic incidence of SCA. Both strata of SCA subjects had much lower HRV than the normal subjects. This effect was strongest during the night-time hours, particularly for a purporte d index of vagal tone (%RR50). For reasons that are not known, the nin e female SCA survivors who had no current or previous MI presented ver y distinct 24-hour patterns for the HRV measures studied. Twenty-four- hour profiles of shea-term statistical HRV provide a rich field for th e observation of within-subject adaptations of the central autonomic n ervous system inputs to the heart in both normal and clinical subgroup s.