Effects of cigarette smoking on the circadian rhythm of heart rate variability

Background - The effects of cigarette smoking on the circadian rhythm of he art rate variability (HRV) are not known. Methods - We studied the effects of cigarette smelting on the circadian rhy thm of HRV in 24 smoking and 21 non-smelting healthy subjects. Twenty-four hour ambulatory electrocardiograms were recorded and ti me domain parameter s of HRV (SDNN [standard deviation of all R-R intervals], SDANN [standard d eviation of the averages of R-R intervals in all 5-minute segments of the e ntire recording], RMSSD [the square root of the mean of the sum of the squa res of differences between adjacent R-R intervals]) were determined for the entire 24-hour period and for each 3-hour period. Results - In total, SDNN and SDANN were significantly lower in smokers than non-smokers (116 +/- 26 vs 136 +/- 27, P < 0.05 for SDNN, 109 +/- 25 vs 12 1 +/- 24, p < 0.05 for SDANN). However, there were no statistical differenc es between smokers and non-smokers in heart rate (81+/-9 vs 76 +/- 10, P > 0.05) and RMSSD (32 +/- 12 vs 37 +/- 18, P > 0.05). These HRV parameters sh owed a circadian variation: they increased at night and decreased during th e day in both groups. The parameters were lower in smokers than non-smokers during daytime (especially, between 8-14 hours). However, no differences w ere detected during night-time. Conclusions - Time domain parameters of HRV (SDNN, SDANN and RMSSD) in both smelting and non-smoking healthy subjects have a circadian rhythm. SDNN an d SDANN were lower in smokers than non-smelters during daytime.