Pathophysiological study on the effects of shift work on cardiovascular diseases

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The purpose of this study was to clarify the pathophysiological effects of shift work on cardiovascular diseases. The study population consisted of 156 male shift workers and 90 male day workers who had been employed at a copper-smelter plant. The heart rate-adjusted QT interval (QTc) on ECG, total power spectral density (t-PSD) of 100 RR intervals, PSDs with frequencies of 0.04-0.15 Hz and of 0.15-0.4 Hz (PSD_<LF> and PSD_<HF>), RR interval variability (CV_<RR>), LF- and HF-component variability (C-CV_<LF> and C-CV_<HF>) and %LF(PSD_<LF>/(PSD_<LF>+PSD_<HF>) 100) were measured (LF and HF components are thought to reflect the sympathetic and parasympathetic activities, respectively). There were no significant differences in age, smoking habit, biochemical data or blood pressure between the shift and day workers. The QTc in the shift workers was significantly prolonged as compared to the day workers. By contrast, the C-CV_<LF> and log-transformed PSD_<LF> were significantly depressed in the shift workers. In the day workers, the QTc was significantly correlated with the log-transformed t-PSD and PSD_<HF>, C-CV_<HF> and %LF. These findings suggest that QTc prolongation in healthy men reflects depression of the cardiac autonomic function, especially parasympathetic hypoactivity. The prolonged QTc in shift workers appears to be associated with the cardiac autonomic hypofunction, partly due to sympathetic hypoactivity. For the prevention of cardiovascular mortality in shift workers, minimizing further occupational stress that can reduce the parasympathetic activity, except shift work, may be important.