Heat and the Heart: How temperatures affect our health

One heatwave is barely over before the next one begins. Whilst people, particularly in the east and in the cities, are breathing a sigh of relief that the long June heatwave has subsided, the next heatwave is already approaching and threatens to have Europe firmly in its grip once again for some time. It is now widely known that heat has negative effects on our health, but what happens in our bodies when we are exposed to heat for prolonged periods, and how can we best protect ourselves from the negative consequences of temperatures exceeding 30 degrees? Markus Wallner from the Division of Cardiology at the Medical University of Graz discusses this. Recent figures show just how serious the consequences of heat can be: during the record-breaking summer of 2022, there were around 61,600 heat-related deaths in Europe. The cardiovascular system, in particular, comes under strain.

Strain on the circulatory system

To ensure that all the body’s functions can run smoothly, stable conditions must prevail: body temperature, electrolyte balance, blood pressure and much more must be kept in constant equilibrium. The process of keeping these aspects in harmony is called homeostasis and is one of the fundamental and most important functions of many systems in the human body. In the context of a heatwave, thermoregulation – that is, the regulation of body temperature – is a particular focus.

When we talk about body temperature in everyday language, we are specifically referring to the temperature at the body’s core – that is, the area where organs such as the heart, liver and brain are located. The body strives to maintain a constant temperature of around 37 degrees. The hypothalamus in the brain is primarily responsible for controlling these values; it responds to information from various parts of the body and, depending on the perceived temperature, takes measures to cool the body down or protect it from the cold.

When temperatures are high, a series of measures is triggered to reduce the elevated body temperature. As a first step, the blood vessels dilate to allow more heat to be transported from the body’s interior to the surface and released via the skin. To support this, the body begins to sweat. We secrete fluid via sweat glands, which cools us down as it evaporates. However, at particularly high temperatures or high humidity, even sweating reaches its limits. Furthermore, the body’s responses carry their own risks. Increased sweating causes the body to lose fluid and important electrolytes (such as sodium and potassium), which can subsequently lead to a variety of problems. These range from tiredness and difficulty concentrating, through dizziness and headaches, to light-headedness or loss of consciousness. Regular fluid intake is therefore essential.

Why the heat puts particular strain on the heart

Heat means hard work for the cardiovascular system. When the blood vessels in the skin dilate to release heat, blood pressure drops. To ensure that the circulation and organs are still adequately supplied, the heart must beat faster and more forcefully to maintain a higher cardiac output. This increased workload leads to higher oxygen consumption by the heart. At the same time, the body loses fluid through sweat, causing blood volume to drop and the blood to become more viscous. This increases the risk of blood clots and, consequently, the risk of heart attack and stroke.

The loss of electrolytes such as potassium and magnesium can also contribute to cardiac arrhythmias. For people with existing heart failure, coronary heart disease or cardiac arrhythmias, this places an additional strain on the body. Analyses of heatwaves show that cardiovascular diseases account for a large proportion of the additional deaths. An important, often overlooked factor is medication such as diuretics and certain blood pressure medicines, which can affect the body’s fluid and heat balance and may need to be adjusted during the hot summer months. 

Protection from the heat

Dealing with the heat properly can save lives. Older or unwell people, infants and young children in particular should be given special protection during a heatwave. In general, exposure to direct sunlight should be avoided and care should be taken to drink enough fluids.

When it comes to drinking, the most important thing is to drink regularly throughout the day, ideally water or diluted fruit juices. Whether a drink is ice-cold or lukewarm plays a smaller role in the body’s heat balance than is often assumed. As ice-cold drinks in large quantities can slightly reduce sweat production, pleasantly cool to room-temperature drinks are therefore a good choice. Alcohol and drinks high in caffeine should be avoided, as they cause the body to lose additional fluid. Anyone who needs to monitor their fluid intake due to a heart or kidney condition should follow their doctor’s advice.

Particular caution is also advised when jumping into cool water. Anyone who is very hot should not jump abruptly into cold water, but should allow the body to cool down gradually, as the sudden cold stimulus can, in rare cases, trigger cardiac arrhythmia. The same applies to showering. A lukewarm to cool shower is one of the simplest and most effective ways to cool down. An ice-cold shower, on the other hand, is generally not recommended: very cold water causes the blood vessels in the skin to constrict briefly, temporarily reducing the body’s ability to release heat. Damp, cool towels placed on the nape of the neck or the forearms are simple and effective alternatives.

Heatstroke and sunstroke

In everyday language, the two terms are often used interchangeably, yet they describe two different conditions, both of which, however, constitute a medical emergency.

Heatstroke: Heatstroke is defined as the body’s temperature rising above 40 degrees, accompanied by a disturbance of the central nervous system (such as confusion or loss of consciousness). This can lead to circulatory collapse or swelling of the brain. Heatstroke occurs when the body is no longer able to lower its temperature, for example due to high humidity, unsuitable clothing or being in an enclosed space with poor ventilation. Depending on the severity of the heatstroke, symptoms such as nausea, headaches, loss of consciousness or seizures may occur. Heatstroke is a life-threatening emergency; move the affected person into the shade, actively cool their body – for example, with cool water and damp cloths – and call the emergency services. 

Sunstroke: Sunstroke is a form of heat-related illness that can result from prolonged, direct exposure of the head to the sun. If the body is unable to dissipate the heat effectively, this leads to irritation of the meninges and a local inflammatory reaction, which can result in cerebral oedema. Older people, those with bald heads and children are particularly susceptible to sunstroke. Initial symptoms such as headaches, dizziness or vomiting may not appear until hours after exposure to the sun. Head coverings provide effective protection, and direct sunlight should be avoided, particularly at midday. Those affected should be taken to a cool, shaded place and positioned with their head and upper body slightly raised. The head and neck should ideally be cooled with damp cloths. If there are signs of impaired consciousness, persistent vomiting or seizures, seek medical assistance immediately.