Cardiac Autonomic Impacts of Bushfire Smoke-A Prospective Panel Study

Air pollution is associated with cardiovascular disease and mortality. Most studies have focussed on urban or traffic-related pollution, and less is known about the impacts from bushfire smoke on cardiovascular autonomic function, although it is associated with increased sudden cardiac death and mortality. We sought to investigate its instantaneous and short-term impacts on heart rate variability (HRV).

Twenty-four (24)-hour Holter electrocardiography (ECG) was repeated twice (during bushfire [Phase 1] and then clean air [Phase 2]) in 32 participants from two Australian towns (Warburton and Traralgon, Victoria) surrounding planned burning areas. This was compared with 10 control participants in another town (Maffra, Victoria) with two clean air assessments during the same periods. The primary HRV parameters assessed were those assessing overall HRV (Standard Deviation of Normal-to-Normal intervals [SDNN]), long-term HRV (Standard Deviation of the Average of Normal Sinus-to-Normal Sinus intervals for each 5-minutes [SDANN]), low frequency [LF]) and short-term HRV (Root Mean Square of Successive Differences between N-N intervals [RMSSD], High Frequency [HF], LF:HF ratio). Average concentrations of particulate matter <2.5 μm in diameter (PM_2.5) were measured at fixed site monitors in each location.

Mean PM_2.5 levels were significantly elevated during bushfire exposure in Warburton (96.5±57.7 μg/m³ vs 4.0±1.9 μg/m³, p<0.001) and Traralgon (12.6±4.9 μg/m³ vs 3.4±3.1 μg/m³, p<0.001), while it remained low in the control town, Maffra, in each phase (4.3±3.2 μg/m³ and 3.9±3.6 μg/m³, p=0.70). Although SDANN remained stable in controls, the exposed cohort showed significant worsening in SDANN during bushfire smoke exposure by 9.6±25.7ms (p=0.039). In univariable analysis, smoke exposure was significantly associated with higher ΔSDNN and ΔSDANN (p=0.03, p=0.01 exposed vs control). The association remained significant in ΔSDANN after adjusting for age, sex and cigarette smoking (p=0.02) and of borderline significance in ΔSDNN (p=0.06).

Exposure to the bushfire smoke was independently associated with reduced overall and long-term HRV. Our findings suggest that imbalance in cardiac autonomic function is a key mechanism of adverse cardiovascular effects of bushfire smoke.