Focused exposures to airborne traffic particles and heart rate variability in the elderly

Background: Exposure to airborne particles may increase cardiac risk by altering autonomic balance. Because these risks may be particularly great for traffic-related particles, we examined associations between particles and heart rate variability as 44 subjects participated in 4 repeated trips aboard a diesel bus. Methods: Twenty-four hour electrocardiograms were correlated with continuous particle concentrations using generalized additive. models controlling for subject, weekday, time, apparent temperature, trip type.. activity, medications, and autoregressive terms. Associations were assessed for short- and medium-term moving averages of measured concentrations. Results: Heart rate variability was negatively associated with fine particulate matter. Positive associations were demonstrated with heart rate and the low-to-high frequency power ratio. Associations were strongest with 24-hour mean concentrations, although strong short-term associations also were found during bus periods, independent of daily exposures. Overall, associations were greatest for high-frequency power with the following effects per interquartile change in the 24-hour mean concentrations: -15% (95% confidence interval = - 17% to - 14%) for PM2.5 (4.6 mu g/m(3)); -19% (-21% to -17%) for black carbon (459 ng/m(3)); and -14% (-15% to -12%) for fine particle counts (39 pt/cm(3)). For each interquartile change in the 5-minute PM2.5 concentration (10 mu g/m(3)) aboard the bus, an 11% (95% confidence interval = -14% to -8%) decrease in high-frequency power was observed. Conclusions: This investigation indicates that fine particles are negatively associated with heart rate variability, with an overall trend towards reduced parasympathetic tone. Although daily associations were evident for all particles, short-term associations were predominantly limited to traffic-related particles.