Heart rate variability during sleep in patients with vasovagal syncope

Background: There are a few studies showing no significant heart rate variability (HRV) over a 24-hour period in vasovagal syncope (VVS) patients, but no research has examined HRV and its sympathetic and parasympathetic components during rapid eye movement (REM) and non-REM sleep. The authors hypothesized that REM sleep might be a critical state in which VVS patients would show abnormal responses. Objectives: To analyze the sympathetic and parasympathetic components of HRV during REM and SWS in patients with VVS compared to normal subjects, and in patients with positive HUTT compared to negative ones. Methods: Thirty-seven VVS patients and 20 normal age-matched controls were submitted to polysomnography with 24-hour Holter monitoring to assess HRV Time and frequency domain techniques were carefully performed for 24 hours and during Stages 3 and 4 of REM and non-REM sleep. Variation of sympathetic activity index (VSAI) was defined as the difference in the low frequency (LF) component of HRV between REM and Stages 3 and 4 of non-REM sleep. An analysis of variance was performed to compare patients and controls; patients with positive and negative head-up tilt testing. Results: The LF component was lower in syncope compared to normal patients (1,769.54 +/- 1,738.17, 3,225.37 +/- 2,585.05, respectively, P = 0.03). There was a significant decrease in VSAI in the syncope group compared to the control group (-539.39 +/- 1,930.78, 1,268.10 +/- 2,420.20, respectively, P = 0.01). The other sleep variables analyzed including very LF, high frequency, low frequency/high frequency and time domain parameters did not reach statistical significance. Syncope patients also showed an increase in slow wave sleep (28.2 +/- 10.5, 19.7 +/- 7.8, P = 0.01). Conclusions: VVS patients exhibited sympathetic suppression during REM sleep. Possible mechanisms are discussed in this article.