The European heatwave of 2003 resulted in between 22,000 and 45,000 deaths in excess of those expected for that time of year. Similarly, in Victoria in 2009, there were 374 “extra” deaths beyond what would have been expected over the summer.
Many of the excess deaths occur two or three days after the heatwave starts, as the physiological stress of the heat accumulates and takes its toll on those vulnerable individuals in our community.
The elderly and the very young, in particular, are unable to react adequately to high ambient temperatures. Individuals who are very obese, very malnourished or have poor physical fitness are also more physically vulnerable to extreme heat.
A range of medications and illicit drugs may also reduce the body’s capacity to react to heat or increase the stress effects of heat, placing a person at increased risk of heat-related illness and death.
How does heat affect the body?
As warm-blooded mammals we have built-in mechanisms that allow us to maintain our body temperature within safe limits, whether the environment is cold or hot.
Some of these mechanisms involve physiological responses and feedback loops that drive involuntary reactions such as sweating which cools us down and shivering which warms us up.
We are able to change our skin blood flow ten-fold in response to thermal stress, shunting warm blood from our body core to increase our heat loss when we are too warm, and lowering skin blood flow when we are cold.
But these mechanisms can fail in extreme heat, leading to heat-related illness.
Death from heatstroke
Deaths from pure “heat stroke” in otherwise young, fit people are uncommon. Local data isn’t available, but in the United States some 200 deaths each year are attributed to this cause.
Heat stroke deaths usually involve high temperatures together with high-risk situations, such as leaving children in cars and participating in intense sports or military training. Indeed, many of these situations may involve individuals or organisations being found to have some legal liability for the fatal outcome.
The exact mechanism of death from heat-related illnesses is still subject to some conjecture and it is likely to be a multifactorial process.
One important process seems to be the triggering of a systemic inflammatory response, as a result of damaged cells from the the lining of the bowel leaking endotoxins. These find their way into the systemic circulation and trigger a range of inflammatory processes in body organs that can lead to tissue damage and eventually organ failure.
Fluid balance and changes in electrolyte levels can also have significant effects on heart function and muscle blood flow. This may be part of the mechanism that causes muscle cramps and muscle injury in heat-related illnesses.
In contrast to pure “heat stroke” deaths, the numbers of deaths that have been in part “caused” by the heat is probably far greater. These cases are difficult to prove and in many cases the doctor’s death certificate will not record heat as a contributing cause.
The problem lies in the fact that the people who are most at risk of dying in the heat are people with existing diseases.
During moderate environmental conditions, they are able to manage the various minor physiological stresses on their body because their organ systems, though diseased, still have sufficient capacity to deal with the changes around them.
But when these environmental factors become more extreme, they can exceed the limited physiological reserves their diseased organ systems have to cope with factors such as extreme heat. This leads to a variety of organ failures that are defined as much by the degree of pre-existing disease as the severity of the heat.
Determining the cause of death
An autopsy takes place when the functions of the body’s organs have ceased. A pathologist therefore cannot “see” organ failure and instead has to infer the previous state of organ failure from the changes they can see.
The problem is that the changes of heat-related organ failure are often exactly the same as those cause by natural disease-related organ failure. Diabetes, heart disease, and infectious diseases are good examples of disease types that can impair organ function to varying degrees and increase the risk of dying in a very hot environment.
My clinical and paramedical colleagues have been coping with the increased health care demands of this heatwave with increased ambulance call-outs and hospital presentations. Unfortunately, on day four of the southern Australian heatwave, forensic pathologists are now facing these issues.
David Ranson is a Clinical Associate Professor in the Department of Forensic Medicine at Monash University.
This article was first published in The Conversation.