Short-term effects of ultrafine particles on heart rate variability: A systematic review and meta-analysis

An increasing number of epidemiological studies have examined the association between ultrafine particles (UFP) and imbalanced autonomic control of the heart, a potential mechanism linking particulate matter air pollution to cardiovascular disease. This study systematically reviews and meta-analyzes studies on short-term effects of UFP on autonomic function, as assessed by heart rate variability (HRV). We searched PubMed and Web of Science for articles published until June 30, 2022. We extracted quantitative measures of UFP effects on HRV with a maximum lag of 15 days from single-pollutant models. We assessed the risk of bias in the included studies regarding confounding, selection bias, exposure assessment, outcome measurement, missing data, and selective reporting. Random-effects models were applied to synthesize effect estimates on HRV of various time courses. Twelve studies with altogether 1,337 subjects were included in the meta-analysis. For an increase of 10,000 particles/cm³ in UFP assessed by central outdoor measurements, our meta-analysis showed immediate decreases in the standard deviation of the normal-to-normal intervals (SDNN) by 4.0% [95% confidence interval (CI): 7.1%, -0.9%] and root mean square of successive R-R interval differences (RMSSD) by 4.7% (95% CI: 9.1%, 0.0%) within 6 h after exposure. The immediate decreases in SDNN and RMSSD associated with UFP assessed by personal measurements were smaller and borderline significant. Elevated UFP were also associated with decreases in SDNN, low-frequency power, and the ratio of low-frequency to high-frequency power when pooling estimates of lags across hours to days. We did not find associations between HRV and concurrent-day UFP exposure (daily average of at least 18 h) or exposure at lags ≥ one day. Our study indicates that short-term exposure to ambient UFP is associated with decreased HRV, predominantly as an immediate response within hours. This finding highlights that UFP may contribute to the onset of cardiovascular events through autonomic dysregulation.