Effects of particle size fractions on reducing heart rate variability in cardiac and hypertensive patients

It is still unknown whether the associations between particulate matter (PM) and heart rate variability (HRV) differ by particle sizes with aerodynamic diameters between 0.3 μm and 1.0 μm (PM_0.3-1.0), between 1.0 μm and 2.5 μm (PM_1.0-2.5), and between 2.5 μm and 10 μm (PM_2.5-10). We measured electrocardiographics and PM exposures in 10 patients with coronary heart disease and 16 patients with either prehypertension or hypertension. The outcome variables were standard deviation of all normal-to-normal (NN) intervals (SDNN), the square root of the mean of the sum of the squares of differences between adjacent NN intervals (r-MSSD), low frequency (LF; 0.04-0.15 Hz), high frequency (HF; 0.15-0.40 Hz), and LF:HF ratio for HRV. The pollution variables were mass concentrations of PM_0.3-1.0, PM_1.0-2.5, and PM_2.5-10. We used linear mixed-effects models to examine the association between PM exposures and log₁₀-transformed HRV indices, adjusting for key personal and environmental attributes. We found that PM_0.3-1.0 exposures at 1- to 4-hr moving averages were associated with SDNN and r-MSSD in both cardiac and hypertensive patients. For an interquartile increase in PM_0.3-1.0, there were 1.49-4.88% decreases in SDNN and 2.73-8.25% decreases in r-MSSD. PM_0.3-1.0 exposures were also associated with decreases in LF and HF for hypertensive patients at 1- to 3-hr moving averages except for cardiac patients at moving averages of 2 or 3 hr. By contrast, we found that HRV was not associated with either PM_1.0-2.5 or PM_2.5-10. HRV reduction in susceptible population was associated with PM_0.3-1.0 but was not associated with either PM_1.0-2.5 or PM_2.5-10.