Objective - To clarify the mechanisms of excessive risk of cardiovascular disease among taxi drivers, the effects of taxi driving with long work hours on taxi drivers' blood pressure and heart rate variability were examined. Methods - We measured the physical activity level, ambulatory blood pressure, and Holter ECG in 34 healthy middle-aged male taxi drivers over one work day and the two following non-work days. The subjects worked for a 24-h period starting at 07:00, then rested from work for 48 h. Ambulatory blood pressure was recorded automatically at 30-min intervals over the 72-h period. Holter ECG records were also kept for the 72 h, and temporal changes in the R-R interval were analyzed by the complex demodulation method (CDM). The amplitude of the high frequency (HF) components (central frequency 0.30 Hz), and the ratio of low-frequency components (central frequency 0.09 Hz) to high-frequency components (L/H) was calculated. Results - Blood pressure during the work day was significantly higher than that during the non-work days, with a stronger effect in the hypertensive subjects. There were no significant overall differences in heart rate variability between the work day and the non-work days, but the temporal patterns of HF and L/H on the work day revealed a potent effect of reversed-phased circadian rhythm for work at night. Conclusions - Long duty taxi driving raises blood pressure and may increase cardiovascular risk as a result. The conflict between the circadian rhythms of autonomic nervous activity and work activity at night may also result in increased risk.
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