Heavy Snowfalls Create Perfect Storm for Fatal, Nonfatal MIs

MONTREAL, QC — When faced with mounting snow, Canadian researchers have given some an excuse—nay, even a reason—not to take on the increased cardiac demands of snow shoveling[1].

Compared with no snow, they found that a heavy snowfall (20 cm/7.87 in) increased the risk of hospital admission due to MI by 16% (odds ratio 1.16, 95% CI 1.11–1.21) and the risk of dying of an MI by 34% (OR 1.34, 95% CI 1.26–1.42) among men the day after the white stuff hit.

For women, the same heavy snowfall was associated with odds ratios of 1.01 (95% CI 0.95–1.07) and 1.04 (95% CI 0.96–1.13), respectively.

Of special note, the associations with snowfall were present regardless of age or existing cardiovascular comorbidities or risk factors.

"These results call for caution after snowfalls, particularly for men who may be at higher risk, and may have increasing relevance over time, considering climate change and predictions that extreme weather will increase," lead author Dr Nathalie Auger (University of Montreal Health Centre, Quebec) and colleagues write.

The study was published online today in CMAJ.

In an accompanying editorial[2], Dr David A Alter (University of Toronto, ON) writes that the findings "add weight to our understanding that the act of snow shoveling in cold temperatures sets the stage for an eco-biological-behavioral 'perfect storm,' particularly among those physically deconditioned who have or who are at risk of heart disease."

As for who should get a pass on shoveling snow, he added, "Perhaps it would be wise for men over 50 years of age who have cardiovascular risk factors or established heart disease and who are physically unfit or habitually sedentary to avoid the activity.

"At the very least, individuals should be made aware of the risks, particularly with heavy snowfall. Such awareness necessitates broader public-health attention."

The researchers note that snowfall has been flagged as a risk factor for MI, but that prior evidence is based on aggregate patient data, which can weaken statistical power and carries a risk of ecological bias. Still, daily death counts due to MI have been associated with snowfalls in Pennsylvania, Rhode Island, and the Twin Cities.

For the present analysis, Auger and colleagues obtained individual hospital admission records for MI from a provincial hospital discharge database from 1989 to 2014 and death registration certificates for Quebec residents from 1981 to 2013. Region-specific data on weather conditions, including daily snowfall and temperature, were obtained from Environment Canada and matched directly to the patient's region of residence.

In all, 128,073 individual hospital admissions and 68,155 deaths due to MI were analyzed. About 60% of admissions and deaths involved men. A third of MIs occurred the day after a snowfall and about 10% after snowfalls of 5 cm (1.96 in) or more.

Compared with 0 days, 2 to 3 consecutive days of >5 cm snow increased the likelihood among men of hospital admission due to MI (OR 1.09, 95% CI 1.04–1.14) and death due to MI (OR 1.20, 95% CI 1.12–1.28). A dose-response trend was also present for 2 to 3 consecutive days of snowfall lasting 6 hours or more.

Among women, however, 2 to 3 consecutive days of 5 cm or more snowfall was not associated with hospital admission (OR 0.98, 95% CI 0.92–1.04) or death (OR 1.08, 95% CI 0.99–1.17) due to MI.

The "findings are most likely related to the increased cardiac demands of snow shoveling," Auger writes. Snow shoveling requires more than 75% of the maximum heart rate, particularly with heavy loads; and the intense arm effort, repetitive motion, peripheral vasoconstriction, and greater myocardial afterload increase the risk of plaque rupture and clot formation.

Although exposure to cold temperatures is known to reduce coronary blood flow, temperature was not associated with hospital admission or death due to MI in models adjusted for snowfall.

The researchers suggest that men are potentially more likely than women to shovel, although information on sex-specific shoveling habits was unavailable. Other potential limitations to the study are the lack of information on the size of the areas shoveled or whether snow removal was with snow blowers vs manual shoveling.

"Although these are potentially important considerations, the hypothesis that shoveling is associated with an increased risk of MI events among men remains plausible. Some researchers have even reported that cardiac demands are elevated with automated snow removal," Auger added.

The researchers conclude that "clinicians and public-health practitioners should consider awareness campaigns before the winter season to inform individuals of the risk of MI after a snowfall and possibly advise against shoveling for the most at-risk patients."