Colder air and reduced sunshine increases risk of myocardial infarction

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A new study indicates that low air temperature, low atmospheric air pressure, high-wind velocity, and shorter sunshine duration are associated with an increased risk of myocardial infarction, with the strongest association being with air temperature. These findings help to further knowledge regarding the role of weather as a potential trigger for myocardial infarction.

Moman A Mohammad (Department of Cardiology, Clinical Sciences, Lund University, Lund, Sweden) and others write in JAMA Cardiology that “in the last decades” studies have linked colder temperatures and snow to the incidence of myocardial infarction. However, they add that these studies have mostly used “surrogate variables” for myocardial infarction and only looked at air temperature and snow. “The objective of this study was to investigate day-to-day weather and the occurrence of myocardial infarction in Sweden using data from the Swedish Meteorological and Hydrological Institute (SMHI) and SWEDEHEART,” the authors comment.

Using data from both SMHI and SWEDEHEART, Mohammad et al identified 274,029 patients who had a myocardial infarction between January 1998 and December 2013 and for whom data were available for the average weather on the day of the onset of their symptoms. They report that the highest incidence of myocardial infarction was on days when the temperature was 0 degrees Celsius and rates of myocardial infarction started to decline when temperatures rose to greater than 3 to 4 degrees Celsius. Additionally, a 1-SD increase in minimum air temperature was associated with a 28% reduction in myocardial infarction. These results were consistent after adjustment for other weather parameters and stratifying patients into non-ST-segment elevation myocardial infarction (STEMI) and non-STEMI (NSTEMI). Sunshine duration was negatively associated with myocardial infarction but no association was found in the multivariate analysis of STEMI; higher precipitation and air pressure were also negatively associated with STEMI and with a minor risk increase in myocardial infarction.

Mohammed et al observe that the results were consistent across a large range of subgroups and healthcare regions. They note: “Seasonal analyses showed more pronounced association of air temperature during warmer seasons, and regional analyses showed a pronounced and positive association of myocardial infarction and snow and wind in the northernmost region.”

As to why colder air appears to increase the risk of a myocardial infarction, the authors comment that coronary vasoconstriction “is the most probable cause”. They add that studies indicate that the cold significantly alters coronary vascular resistance in patients with existing heart disease but appears not to affect healthy individuals in such a way. “Coronary vasoconstriction owing to cold could alter the arterial wall shear stress and possibly induce plaque fracture,” Mohammed et al state. Although they add that the cold could also be linked to type 2 infarctions (because of ischaemia due to reduced oxygen supply), they note the strongest association between the cold and myocardial infarction was for STEMI and “type 2 infarctions generally do not present as STEMI”.

Regarding the finding that sunshine duration was negatively associated with myocardial infarction, they say: “Vitamin D has been proposed to play an important role in the seasonality of cardiovascular disease and mortality, and this could explain the associated risk of myocardial infarction and sunshine duration.” However, the authors observe that previous studies have “failed to show any effect of vitamin D supplementation on myocardial infarction risk”.

“In this large, nationwide study, low air temperature, low atmospheric pressure, high wind velocity, and shorter sunshine duration were associated with risk of myocardial infarction with the most evident association observed for air temperature. This study adds to the knowledge on the role of weather as a potential trigger of myocardial infarction,” Mohammed et al conclude.

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