Cardiac autonomic imbalance in female nurses with shift work

Abstract

The pathophysiology underlying the shift work-related cardiovascular disease is still poorly understood. The chronic effects of shift work on cardiac autonomic functions were assessed in 47 hospital nurses working under a rotating three-shift system (shift nurses) and 36 public health nurses without shift work (non-shift nurses). The heart rate variability, %LF and %HF (i.e., proportions of sympathetic and vagal activities, respectively), and LF / HF ratio were calculated from the electrocardiographic RR intervals by using autoregressive spectral analysis, and heart rate-corrected QT interval (QTc and QT index) was also measured. The LF / HF ratio, %LF, and QT index were significantly larger in the shift nurses than in the non-shift nurses; also, the power spectral density of HF (PSD_HF) was significantly decreased in the shift nurses. There was a significant, inverse correlation between the corrected QT interval and PSD_HF in the non-shift nurses, but not in the shift nurses. It is suggested that shift work in female nurses may cause a sympathodominant state due to depressed vagal tones. Also, a pathophysiology of shift work-related cardiovascular disease, derived from the present and previous findings, may be characterized by the attenuation of the inverse association between the corrected QT interval and vagal activity observed in non-shift workers.

Introduction

Rotating shift work is known to be disruptive for sleep, wakefulness, eating patterns, and social activities (Moore-Ede and Richardson, 1985), and some articles have suggested the causal relation between shift work and cardiovascular disease (CVD) in male workers (Åkerstedt et al., 1984, Knutsson et al., 1986, Harrington, 1994) and in female nurses (Kawachi et al., 1995). Also, the relative risk of CVD due to shift work has been estimated to be approximately 1.4 (Tenkanen et al., 1997). Concerning the mechanism of shift work-related CVD, Murata et al. (1999) demonstrated the prolongation of the heart rate-corrected QT (QTc) interval on the electrocardiogram (ECG) in male shift workers. Thereafter, they have speculated that QTc prolongation involved in cardiovascular events may result from imbalance of cardiac autonomic control in the shift workers (Murata et al., 2005). Nonetheless, since there is no consistent evidence for increased risks of cardiovascular mortality or of sudden death due to prolonged QTc interval (Montanez et al., 2004), the exact pathophysiology underlying the phenomena is still poorly understood.

In light of gender-specific medicine, autonomic nervous effects of shift work may differ between both sexes. To assess the chronic effects of shift work on the cardiac autonomic function in women, we explored the QTc interval and heart rate variability (HRV) in Japanese hospital nurses with and without shift work (Ishii et al., 2004). Then, we observed significantly elevated LF / HF ratio and %LF in the former, but failed to find a significant difference in the QTc interval between the two groups, possibly because most of the nurses without shift work (i.e., control subjects) were those who had changed from rotating shift work to day work. In this time, we could find female control subjects to minimize such a selection bias. Therefore, comparisons of cardiac autonomic functions were again attempted between the hospital nurses with shift work and public health nurses without shift work, and whether the cardiac autonomic findings obtained from female workers were similar to those from male workers (Murata et al., 2005) was discussed. In addition to the QTc interval, we also employed the QT index (QTI), which is thought to be one of the formulas designed to overcome the shortcomings inherent in Bazett's formula (Rautaharju et al., 1990, Whitsel et al., 2001), as one of endpoint parameters in this study.