Association between quantity and duration of snowfall and risk of myocardial infarction

Similar to our results, Liu et al found an association between snowfall and hospitalization for myocardial infarction (MI) with ST- segment elevation (1). However, they also found that cold temperature was associated with hospitalization for MI, whereas we did not. In detailed sensitivity analyses adjusting for snowfall, we in fact found no evidence of an association between temperature and MI (Appendix 5) (2). While comparability of diagnostic categories and possibly climate may explain the results, a difference in study design seems a strong possibility. Liu et al analysed count data using an ecologic method. As outlined in our paper (2), the results from such approaches should be interpreted with caution because of the risk of ecologic bias owing to an inability to account for temporal trends (3). Consider for instance the possibility that both temperature and incidence of MI hospitalization changed over time for unrelated reasons (4,5). Negative binomial regression - an ecologic method - would suggest an association, and be unable to distinguish between unrelated temporal trends and a true association. For this reason, we would be very interested to see a reanalysis of the St. Boniface data that capitalizes on individual rather than aggregate data, and better adjusts for temporal trends in weather or incidence of MI over time. We agree with Liu et al that increased public awareness and possibly reorganization of health services for predictable seasonal changes in MI events are worthy of consideration.

(1) Liu S, Ducas R, Hiebert B, et al. How cold is too cold: the effect of seasonal temperature variation on risk of STEMI. Eur Heart J 2015 Aug;36(S1)

(2) Auger N, Potter BJ, Smargiassi A, et al. Association between quantity and duration of snowfall and risk of myocardial infarction. CMAJ 2017;189:E235-E242.

(3) Webster TF. Bias magnification in ecologic studies: a methodological investigation. Environ Health 2007;6:17.

(4) Pachauri RK, Meyer LA, Barros VR, et al., editors. Climate change 2014: synthesis report. Contribution of Working Groups I, II and III to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change. Geneva: Intergovernmental Panel on Climate Change; 2014:1-151.

(5) Moran AE, Forouzanfar MH, Roth GA, et al. The global burden of ischemic heart disease in 1990 and 2010: the Global Burden of Disease 2010 study. Circulation 2014;129:1493-501.

Conflict of Interest:

None declared

This study by Auger and colleagues showed that the amount and duration of snowfall was associated with an increased risk of hospitalization and death due to myocardial infarction (MI) among men from Quebec, Canada.1 The study apparently did not adjust by barometric pressure (BP), if the data is available, does the inclusion of BP have a confounding effect or effect modification on the estimates? Some studies have shown that a decrease in BP is associated with MI and coronary deaths.2-4 In the historic snowstorm that hit the eastern United States in January of 1978, BP in some cities dropped as low as 40 millibars in 24 hours. A blizzard is distinctive because of the fast drop in BP in a 24-hour period. Three days after this storm of 1978, numbers of admissions for MI, total mortality and ischemic heart disease deaths increased for the five- day period following the blizzard in Rhode Island, however, the effect of BP was not assessed in that study and the blizzard was considered the only culprit.5 A recent Asiatic study found that extreme ambient temperatures were associated with cardiorespiratory mortality. An interesting finding was the decrease of cold effect on mortality by 2.36% associated with an increase in BP indicating that it may have an impact on the effect of other weather variables on mortality.6 Furthermore, there is evidence of a positive correlation between BP and blood oxygen saturation.7 Patients with MI or ischemic heart disease are particularly susceptible to changes in blood oxygen saturation.

References

1. Auger N, Potter BJ, Smargiassi A, et al. Association between quantity and duration of snowfall and risk of myocardial infarction CMAJ 2017;189:E235-42

2. Danet S, Richard F, Montaye M, et al. Unhealthy effects of atmospheric temperature and pressure on the occurrence of myocardial infarction and coronary deaths. A 10-year survey: the Lille-World Health Organization MONICA project (Monitoring trends and determinants in cardiovascular disease). Circulation 1999;100:E1-7

3. Goerre S, Egli C, Gerber S, et al. Impact of weather and climate on the incidence of acute coronary syndromes. Int J Cardiol 2007;118:36-40.

4. Ohlson CG, Bodin L, Bryngelsson IL, Helsing M, Malmberg L. Winter weather conditions and myocardial infarctions. Scand J Soc Med 1991;19:20- 5.

5. Faich G, Rose R. Blizzard morbidity and mortality: Rhode Island, 1978. Am J Public Health 1979 Oct;69(10):1050-2

6. Chung Y, Lim YH, Honda Y, et al. Mortality related to extreme temperature for 15 cities in Northeast Asia. Epidemiology 2015;26:255-262

7. Pope CA, Dockery DW, Kanner RE, et al. Oxygen saturation, pulse rate, and particulate air pollution: a daily time-series panel study. Am J Respir Crit Care Med 1999;159:365-372

Conflict of Interest:

None declared

To the editor,

Our group found the manuscript by Auger et al on "Association between quantity and duration of snowfall and risk of myocardial infarction" very interesting.

We performed a retrospective analysis of all ST-segment elevation myocardial infarction (STEMI) patients between 2009-2014 who presented to St. Boniface Hospital in Winnipeg, Manitoba, Canada. Temperature and snowfall were collected from Environment Canada. Negative binomial regression was used to identify the relationship between weather and STEMI. We examined daily high (DH), low, average temperature on the same day, previous day and two days prior. Daily snowfall was analyzed similarly.

Over the 6 year study period, there were 1817 STEMI. DH was the strongest predictor of STEMI The average STEMI rate has a statistically significant linear trend across temperature (p<0.001). Extreme high temperatures (>30oC) were not associated with higher rates of STEMI. With every 10oC decrease in DH, the risk of STEMI increased by 5%. With multivariable analysis, daily high (RR 1.05, 95% CI 1.02-1.09, p=0.005) and any snowfall occurring the day prior (RR 1.14, 95% CI 0.99-1.32, p=0.075) remained predictive of STEMI rate.

We found that daily high temperature and previous day snowfall were associated with increased risk of STEMI, the most serious type of myocardial infarction. The authors studied the effect of snowfall on risk of hospital admission and death related to MI, while we studied the risk of STEMI. Despite a different climate and study population, both studies showed increased risk of myocardial infarction with previous day snowfall. Increased public awareness and/or reallocation of health care resources should be considered to respond to predictable seasonal increased incidence of STEMI.

Reference: Liu S, Ducas R, Hiebert B, Koley L, Phillip R, Tam JW. How cold is too cold: the effect of seasonal temperature variation on risk of STEMI. Eur Heart J. 2015 Aug;36(S1)

Conflict of Interest:

None declared

To the editor

I read with interest the manuscript of Auger et al entitled "Association between quantity and duration of snowfall and risk of myocardial infarction" recently published in the CMAJ.(1)

The major findings of their work using a large database from Quebec (administrative data) found that the quantity and duration of snowfall were associated with subsequent risk of hospital admission or death due to MI, driven primarily by an effect in men.

About five years ago, our group published a retrospective analysis of 500 acute coronary syndromes during two consecutive winters (2). The mean age of our population in Kingston, Ontario was 65.7?13.4 years (range 31- 94) and 66.7 % of the events occurred in males. A total of 35 (7 %) events were documented to have occurred following snow-shoveling. Between patients with snow-shoveling related and non-related events there were no significant differences in the prevalence of diabetes, hypertension, hypercholesterolemia or sleep apnea. Logistic regression did not show any significant group differences in age and known coronary artery disease; however, those suffering a snow-shoveling related event were 3.6 times more likely to have a family history of premature cardiovascular disease (p=0.001) and were 4.8 times more likely to be male (p=0.01) (Table 1); the latter in concordance with Auger's study. Those taking more than 4 cardiovascular medications seemed to be protected, maybe because a recommendation to avoid this physical activity was already made.

This new manuscript about the effects of snow-shoveling and its association with cardiovascular health is welcomed; and we hope it helps in advancing clear recommendations on who should avoid this type of physical activity.

References

1. Auger N, Potter BJ, Smargiassi A, Bilodeau-Bertrand M, Paris C, Kosatsky T. Association between quantity and duration of snowfall and risk of myocardial infarction. CMAJ 2017; 189:E235-42.

2. Nichols RB, McIntyre WF, Chan S, Scogstad-Stubbs D, Hopman WM, Baranchuk A. Snow-Shoveling and the Risk of Acute Coronary Syndromes. Clin Res Cardiol 2012;101(1):11-15.

Note re Table 1: Reproduced from Nichols RB, McIntyre WF, Chan S, et al. Snow-shoveling and the risk of acute coronary syndromes. Clin Res Cardiol 2012;101:11-5. © 2012 with permission from Springer.

Conflict of Interest:

None declared