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Heart disease mortality risk escalated by climate change

Scientists from 35 different research institutions around the world have put together one of the largest multinational datasets ever assembled, to investigate the impact of very hot and very cold weather on the risk of dying from heart disease. This study has relevance in terms of the effects of climate change on the health of people, particularly those who suffer from cardiovascular conditions. 

Healthy people have the capacity to withstand exposure to a hot or cold environment and can tolerate some degree of increase and decrease in their internal temperatures. But clinical reports indicate that people with cardiovascular diseases are less able to withstand such changes. The cardiovascular system is intimately involved in thermoregulatory processes, and blood vessels dilate during hot conditions and constrict during the cold. In patients with a compromised cardiovascular system, these functions may be impaired.

The analysis of these global data, published in the American Heart Association’s journal Circulation, showed that more deaths from heart failure occurred on days when temperatures were at their highest, or lowest, compared to days with more moderate temperatures. The researchers defined extreme temperature as the top 1 percent or bottom 1 percent of the “minimum mortality temperature,” which is the temperature at which the lowest death rate was observed.

Although several different categories of heart disease were considered in this study, people with heart failure were the ones most likely to be negatively impacted by very cold and very hot weather. Heart failure patients experienced a 12 percent greater risk of dying on days with extremely hot weather (above 86oF or 30oC) in the city of Baltimore, and this risk increased by 37 percent on days with extreme cold (below 20oF or -7oC).

“It underscores the urgent need to develop measures that will help our society mitigate the impact of climate change on cardiovascular disease,” said study co-author Dr. Haitham Khraishah, a cardiovascular disease fellow at the University of Maryland School of Medicine (UMSOM) and University of Maryland Medical Center (UMMC).

The researchers found that, for every 1,000 cardiovascular deaths:

  • Extreme hot days (above 86°F/30oC in Baltimore) accounted for 2.2 additional deaths.
  • Extreme cold days (below 20°F/-6.7oC in Baltimore) accounted for 9.1 additional deaths.
  • Of the different types of heart diseases, the greatest number of additional deaths was found for people with heart failure (2.6 additional deaths on extreme hot days and 12.8 on extreme cold days).

“While we do not know the reason why temperature effects were more pronounced with heart failure patients it could be due to the progressive nature of heart failure as a disease,” said Dr. Khraishah. “One out of four people with heart failure are readmitted to the hospital within 30 days of discharge, and only 20 percent of patients with heart failure survive 10 years after diagnosis.”

Dr. Barrak Alahmad, research fellow at the Harvard T.H. Chan School of Public Health, led the effort to build the heart disease mortality database for this study over the past four years. In addition, colleagues from more than 35 scientific institutions worldwide collaborated. 

The team developed and expanded the heart disease mortality database as part of the Multi-Country Multi-City (MCC) Collaborative Research Network. This is a consortium of epidemiologists, biostatisticians and climate scientists studying the health impacts of climate and related environmental stressors on death rates.

“This study provides an indisputable link between extreme temperatures and heart disease mortality from one of the largest multinational datasets ever assembled,” said Dr. Mark T. Gladwin of UMSOM. “The data can be more deeply mined to learn more about the role of health disparities and genetic predispositions that make some populations more vulnerable to climate change.”

The researchers took into account levels of humidity and air pollution, which could have accounted for excess deaths in places where temperatures were extreme. They also controlled for the delayed effect of temperature on human health (lag effect) and climate zone. They do acknowledge that data on heart disease deaths from South Asia, the Middle East and Africa were underrepresented in their study, implying that the impact of extreme heat may, in fact, be even greater than their findings suggest. 

“This landmark paper is a call to view climate change as a growing public health concern and highlights the need to investigate it as a potential cause of health disparities,” said Stephen N. Davis, MBBS, Chair of the Department of Medicine at UMSOM.

This analysis was funded by the Kuwait Foundation for the Advancement of Science.

By Alison Bosman, Staff Writer

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