Measurements of heart rate variability in patients with unexplained syncope

Syncope is a common clinical issue with complex and heterogenous etiologies. Research has shown a large proportion of syncope remained unexplained, even after serial investigations. Head-up tilt test with or without isoproterenol infusion could elicit a recurrence in most patients with unexplained syncope. Therefore, autonomically neural-mediated bradycardia and hypotension have been cited as the mainstays of the mechanism. Since the autonomic function can be reflected by heart rate rariability, this study proposed to determine 24-hour heart rate variability in patients with unexplained syncope.

Fifteen patients, 14 males and 1 female, who were defined as showing symptoms of unexplained syncope, were the subjects of this study. Their ages ranged from 38 years to 79 years. Seventeen healthy volunteers matching in age and sex served as the control group. For each patient and volunteer, 24-hour ambulatory electrocardiographic monitoring was done. Heart rate variability was defined as SDNN index, SDNN, SDANN, rMSSD, and pNN50 in time domain measures; and low, high, and total frequency in frequency domain measures.

There was a significantly lower value at low frequency, high frequency, total frequency, rMSSD, and pNN50 in the syncope group than in the control group (p < 0.05). There was no significant difference in SDNN, SDNN index, SDANN and LF/HF ratio between these two groups.

Patients with unexplained syncope had autonomic withdrawal; this was the case either in both sympathetic and parasympathetic components without changing the sympatho-vagal balance, or in purely parasympathetic component.