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243 CIRCADIAN VARIATION OF HEART RATE VARIABILITY FOLLOWING METAL-RICH FINE PARTICULATE EXPOSURES IN BOILERMAKER CONSTRUCTION WORKERS
Although recent research demonstrates associations between particulate exposures and decreased heart rate variability (HRV), it is unclear whether exposures alter the natural circadian rhythm of HRV. In a cohort of boilermaker welders, we compared the circadian variation of hourly HRV on a workday during exposure to high levels of metal-rich particulates to a non-workday when particulate exposures were low.
We conducted a panel study among 36 males who were monitored by 24 h ambulatory electrocardiograms (ECG) over both a workday (exposure period) and non-workday (control period). ECGs were analysed and the standard deviation of normal-to-normal beats index (SDNNi) was calculated from 5 min data and summarised hourly over the 24 h measurement. Work and non-work periods were compared using a paired t-test.
Welders were exposed to a mean (SD) workday PM2.5 concentration of 1.14 (0.76) mg/m3. There was conservation of the circadian rhythm of HRV on workdays, with an increase in HRV during the sleeping hours, peaking at 5 am, and a decrease in the afternoon, reaching a nadir at 1 pm. However, we observed lower hourly SDNNi on workdays as compared to non-workdays. Furthermore, the largest difference between work and non-work HRV was during the time that most participants reported active working. For example, at 1 pm the mean (SD) hourly SDNNi was 52.47 (20.21) ms on a non-workday and 40.67 (15.13) ms on a workday. The difference between work and non-work hourly SDNNi was statistically significant (p<0.05) between 8 am and 3 pm. Lower SDNNi was also observed in the hours following work and in the early morning; at 5 and 6 am the difference was statistically significant.
Although the circadian variation in HRV was preserved following workday exposure, HRV was decreased as compared to a non-workday. While the largest differences between work and non-work HRV …