Autonomic control of heart rate variability in vasovagal syncope: a study of the nighttime period in 24-hour recordings

Patients experiencing vasovagal syncope have been claimed to show reduced v agal tone over 24-hour electrocardiography recordings. Assessment of sympat hovagal balance in the absence of external stimuli, ie, nighttime electroca rdiography monitoring, might help to clarify if increased sympathetic activ ity is present in these patients. Heart rate variability was examined at ni ghttime in 40 patients with recurrent episodes of vasovagal syncope within the last 2 years (22 men; mean age, 37 years) and 20 comparable healthy vol unteers. Time domain parameters (pNN50 [proportion of successive RR interva ls difference > 50 ms in %] and rMSSD [root-mean-square successive differen ce of RR intervals in ms]), indexes of vagal tone, and frequency domain par ameters, expressing the overall heart rate variability, vagal thigh frequen cy [HF]) and sympathetic (low frequency [LF]) activity, and autonomic balan ce (LF/HF ratio) were compared between groups by Mann-Whitney test. Signifi cant (p < 0.05) reduction of heart rate variability and vagal tone (pNN50 a nd rMSSD) were found for patients with vasovagal syncope, together with inc reased sympathetic activity (increased LF/HF ratio). These findings could o pen new insights in the pathogenesis of vasovagal syncope because of the sh ift of the autonomic balance toward sympathetic activation near the syncopa l episode.