Effects of exercise training intensity on cardiac autonomic regulation in habitual smokers.

BACKGROUND: The sympathoexcitatory effects of cigarette smoking cause an autonomic imbalance that may lead to cardiovascular disease. Aerobic training improves autonomic function by developing cardiorespiratory fitness; however, it is still uncertain whether aerobic training ameliorates the compromised autonomic modulation in smokers. This study aimed to investigate the effects of 8 weeks' aerobic training at different exercise intensities on autonomic regulation in habitual smokers.

METHODS: Healthy males (n = 34) were randomly assigned to a moderate-intensity aerobic training (MAT, 60% of heart rate reserve [HRR]), a high-intensity training group (HAT, 75% HRR), or a control group (CG). Training groups performed 8 weeks' aerobic training on a treadmill (3 times/week), but all subjects continued to smoke cigarettes as usual. Heart rate variability was monitored to evaluate the effect of aerobic training on autonomic regulation.

RESULTS: Aerobic training improved autonomic balance despite the continued smoking. In the time domain, rMSSD and pNN50 were significantly increased in HAT than in CG. On spectral analysis, the absolute and normalized units of high frequency (HF) were significantly increased in HAT, whereas the LF/HF ratio and the normalized unit of LF were significantly decreased compared to that in CG. SD1 and the SD1/SD2 ratio of the Poincaré plot analysis were significantly increased compared to CG. Although MAT showed a similar tendency to HAT in nonlinear indexes, there were no significant differences compared to CG.

CONCLUSION: Aerobic training, particularly high-intensity training, increases the parasympathetic contribution to the sympathovagal system, leading to an improvement in autonomic balance despite continued cigarette smoking.