Cardiovascular risk profile in shift workers : cardiac control, biological and lifestyle risk factors

Background: Evidence available so far indicates a 40% excess cardiovascular disease risk among shift workers. As, in the Netherlands alone, about one million people are working in shifts, this might have a considerable public health impact. Factors responsible for this elevated risk have not yet been elucidated. Both changes in biological and lifestyle risk factors and disturbance of the cardiac control, as reflected by an increased frequency of premature ventricular complexes and decreased heart rate variability, might be involved in this excess risk. The purpose of this study was to investigate whether shift work related changes occur in these factors that might explain the elevated CVD risk among shift workers.Methods: A cohort study was carried out in 1997 to 1999 among 227 shift workers and 150 controls working in daytime, all nurses and industrial workers. One-year changes in cardiac control (premature ventricular complexes and heart rate variability), biological risk factors (blood pressure, body mass index, waist to hip ratio and blood cholesterol) and lifestyle risk factors (dietary habits, smoking, and decreased physical activity) were investigated.Results: We observed a significantly greater one-year increase in the frequency of premature ventricular complexes in the shift workers compared with the day workers. The frequency of ventricular extrasystoles went up in 48.9 % of the shift workers, and in 27.3 % of the day workers. The Spearman correlation coefficient between the number of nights worked and the change in frequency of PVC's was 0.33; P = 0.004. The one-year change in the HRV parameters measured (SDNNi, Low and High frequency power and %LF) was similar between the shift and day workers. However, among the shift workers the low frequency power component of the total heart rate variability (%LF) was stronger during sleep after a night shift than after a day shift (%LF + 3.04, P < 0.01). This suggests an increased sympathetic activity during a sleep after night shift. The magnitude of the reported effects was related to the shift schedule. Backward rotating schedules (three to five shifts of night work, evening work, day work, respectively) appeared to be the most unfavourable. Smoking was the only variable among the other biological and lifestyle risk factors that showed an unfavourable one-year change in the shift workers compared with the day workers (difference in change: 2.5 cigarettes per day; p < 0.05).