Scientists have pointed out the danger of extreme temperatures in case of heart problems

Sudden temperature changes can be dangerous for people with heart failure: a new study has shown that both cold and high fever are associated with an increased risk of hospitalization. Moreover, the effects of cold may not appear immediately, whereas with an increase in temperature, the danger arises within a few days. Why heat and cold create additional stress on the heart, how quickly this effect manifests itself and to whom it is especially important to monitor the temperature outside the window — in the Izvestia article.

Cold acts with a delay, heat acts almost immediately.

The researchers analyzed 482,000 cases of hospitalization due to heart failure in Sweden from 2006 to 2021. The data was taken from the national patient registry and then compared with temperature information from different municipalities. The results of the study are published in the Journal of the American College of Cardiology (JACC).

The authors separately considered cold periods, heat, and simply suboptimal body temperature. A cold period was considered to be at least two consecutive days when the temperature dropped to the 5th percentile or lower for a particular municipality during the cold season. The 95th percentile threshold was used for heat in the warmer months.

It turned out that the body's reaction to cold and heat is different. After cold periods, an increased risk of hospitalization was observed for two to six days, with the most pronounced statistical relationship observed after three to five days. A similar pattern was observed when exposed to lower temperatures in general.

High temperatures, on the contrary, were associated with an increased risk of hospitalization for three days, starting from the first day — that is, almost immediately after exposure. At the same time, no statistically significant association with hospitalization was found in this study directly for heat periods defined as heat waves.

Why is it hard for the heart to tolerate heat and cold

The ambient temperature affects the functioning of the cardiovascular system, as the body constantly has to maintain a relatively stable internal temperature. In the heat, the body dilates blood vessels and increases sweating to release heat. Due to fluid loss and changes in blood circulation, the heart has to work harder.

In cold weather, the opposite processes occur: blood vessels constrict to reduce heat loss. This increases blood flow resistance and can increase the strain on the heart. For a person with a healthy cardiovascular system, such changes are usually part of the body's normal adaptation, but with heart failure, the range of possibilities for compensation may be limited.

That is why temperature stress can become an additional factor that worsens the condition of a person with an existing disease. In the study, it is especially important that the reaction to different temperatures turned out to be uneven in time: after the cold, the deterioration could occur after a few days, whereas at a higher temperature the risk appeared faster.

The authors of the paper believe that this difference is of practical importance. After warnings of severe cold weather, it may be necessary to monitor the condition of patients for a longer time, whereas in case of heat, a faster reaction is required.

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Why the results are especially important for the Nordic countries

The study was conducted in Sweden, a country with a cold climate where exposure to low temperatures is a common part of life. However, researchers are also paying attention to the other side of the problem: as global warming continues, populations in regions with cooler climates may become more vulnerable to extreme heat.

The authors note that this is especially true for the Nordic countries: people regularly encounter cold, but they may not be sufficiently adapted to prolonged periods of high temperatures. Therefore, climate change can create new pressures on health systems, even where extreme heat has not historically been a major meteorological risk.

However, the study does not show that any heat or cold snap will necessarily lead to hospitalization. We are talking about a statistically identified increased risk among people with heart failure. In addition, the work was carried out on data from the Swedish population, so the results cannot be automatically transferred to all countries and climatic zones.

Nevertheless, the data obtained complement the broader picture of the impact of extreme weather on the cardiovascular system. A scientific statement from the American Heart Association released earlier also noted that extreme heat and cold can trigger heart attacks, strokes, heart failure, and sudden cardiac death.

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The temperature is suggested to be taken into account when monitoring patients.

The authors of the new study believe that meteorological data can be used together with medical information to respond more promptly to the worsening condition of patients with heart failure. It may be especially important to take into account the time lag after exposure to cold. If the risk of hospitalization increases after a few days after a cold snap, warning the patient about adverse weather can be useful not only on the day of extreme temperature, but also after it. In case of heat, on the contrary, a more rapid reaction is potentially required due to the faster development of the adverse effect.

The researchers emphasize that healthcare systems should take such environmental factors into account when preparing for temperature extremes. This is especially important against the backdrop of climate change, when heat waves are becoming more frequent and prolonged, and medical systems continue to face cold-related risks at the same time.

Thus, the work does not come down to the conclusion that cold or heat alone are the cause of heart failure. It shows something else: for people with a pre-existing disease, the ambient temperature can become an additional stressful factor, and the consequences can manifest themselves at different rates depending on weather conditions.