The eco–biological–behavioural perfect storm that follows heavy snowfall

Abstract

Snow shovelling has long been known to be an activity that puts particular strain on the cardiovascular system. It incorporates repetitive dynamic aerobic and resistance activity, which induces a Valsalva manoeuvre and places disproportionate workloads on arm rather than leg muscles, resulting in raised heart rate, blood pressure and myocardial oxygen demand.1 Such effects are exacerbated by the vasoconstricting and sympathovagal effects of cold-air inhalation, 2 which together may conspire to precipitate myocardial ischemia, plaque rupture, coronary artery thrombosis or arrhythmias. 3 Other patient factors such as underlying cardiopulmonary fitness, self-pacing, 4 circadian variation and the time of snow clearance, 5 and the use of automated or semi-automated snow removal devices, can further modify cardiovascular responsiveness. 1In a linked research study, Auger and colleagues have used routinely collected data to determine an association between the quantity and duration of snowfall and the risks of acute myocardial infarction (MI) in Quebec, Canada, between 1981 and 2014. 6 Their findings add weight to our understanding that the act of snow shovelling in cold temperatures sets the stage for an eco-biological-behavioural "perfect storm," particularly among those physically deconditioned who have or who are at risk of heart disease. 3,7 Auger and colleagues' elegant study incorporates detailed region-specific weather conditions, including daily snowfall and temperature, with individual-level data on hospital admissions and vital statistics. The authors have determined that a heavy snowfall (20 cm v. 0 cm) is associated with a 16% relative increase in the odds of MI-related hospital admission and a 34% relative increase in the odds of death due to MI on the day after a snowfall among men. The adverse cardiovascular effects associated with snowfall were not apparent among women. This finding supports their a priori hypothesis that such sex-related differences were largely attributable to snow-shovelling activities, which would more likely be undertaken by men than by women. This is not the first study to have explored the effects of weather on cardiovascular events. A large body of evidence has established a higher incidence of MI during winter months than during the summer. 5,8 However, epidemiologic evidence associating snowfall and MI has been less consistent.9,10 Unlike most other epidemiologic studies, which incorporated aggregate ecologic data, Auger and colleagues used individual-level data and a time-stratified case-crossover study design, whereby each individual was used as his or her own control. Accordingly, weather conditions were the primary factor that differed between cases and controls. For the most part, the authors' use of the casecrossover design eliminates or substantially reduces the effects of confounders -traditional factors known to be associated with an increased risk of nonfatal and fatal MI, such as socioeconomic status, comorbidity and seasonality. Another strength of their study ...