Recently, an athlete died during the Boney Mountain Trail Half Marathon in my backyard – the Santa Monica Mountains. I have participated in this race and others that are part of the SoCal Trail Series. I know these races to be well-run with a friendly vibe.
While the cause of death has not been officially confirmed, it is well known that a heat advisory was in effect during the race. Temperatures were reportedly climbing by 8 a.m., the official race start time. Multiple sources reported temperatures in excess of 90 degrees. Race results show that 117 finished the half marathon, and the Ventura County Star reported that 21 did not.
This tragedy echoes other recent deaths during endurance events in extreme heat, including that of TikTok creator Caleb Graves. Caleb suffered cardiac arrest and died in September 2024 after crossing the finish line at the Disneyland Half-Marathon in Anaheim, CA. The day before the race, Caleb posted a TikTok video in which he complained about being “marginally worried” after passing out after a 20-minute dog walk in the heat. According to the LA Times, Caleb had no known medical issues.
These stories highlight the urgent need to better understand how heat impacts the heart, especially during endurance events. Below, we explore the science and share precautions that can help protect your health.
What is Heat Stroke?
According to the Cleveland Clinic, Heat stroke is a life-threatening condition in which heat overwhelms your body’s ability to manage its temperature. More specifically, according to the National Institutes of Health (NIH), heat stroke is caused by an imbalance in heat production and heat dissipation. Your body cools itself through four main mechanisms: evaporation of sweat, convection (i.e., air moving around your skin), conduction (i.e., direct contact with something cold), and radiation (i.e., heat emitted from skin to cooler surroundings). When the environmental temperature exceeds your core body temperature, some of these mechanisms reverse, causing heat to transfer into your body instead of out.
Heat stroke symptoms include dizziness, fainting, blurred vision, slurred speech, and confusion. During a heat stroke, your body temperature can climb above 104°F. Heat stroke is life-threatening and requires immediate medical treatment. The risk of complications like organ damage or death increases the longer one’s body temperature remains high.
There are two types of heat stroke:
- Classic / non-exertional heat stroke – usually affects children or older adults during heat waves.
- Exertional heat stroke – occurs when physical activity combines with high temperatures, impacting even healthy, young adults.
Heat stroke can affect one’s thinking, causing confusion, agitation, and aggression. Other symptoms include high heart rate, low blood pressure, fast/shallow breathing, muscle cramps, and vomiting. It’s important to recognize the symptoms in yourself and others and take immediate medical action (i.e., call 911) as necessary.
Complications include disruption of brain activity (coma), muscle breakdown (rhabdomyolysis), reduced blood and oxygen flow to your organs, and severe damage to major organs, including heart failure.
How Heat Affects the Heart
When it’s hot out, the body’s heat production increases and its ability to dissipate heat decreases. As heat production exceeds dissipation, the core body temperature rises, as explained in the NIH’s “The Cardiovascular System in Heat Stroke.”
In response, blood vessels near the surface of the skin dilate (vasodilation), increasing blood flow to help cool the body. As a result, the heart works harder to maintain blood pressure while cooling your skin and pumping blood to working muscles. If this workload becomes too much, the heart can no longer compensate. Blood pressure drops, and the body’s ability to regulate temperature fails, leading to heat stroke.
As the heart is overwhelmed by these demands, several complications can occur:
- Arrhythmias (fast or irregular heartbeat) – One of the first and most common heart responses to heat stroke is a very fast heartbeat (called sinus tachycardia). This happens because you are low on fluids, and the body is under stress, releasing adrenaline. Sometimes the heart can start beating in an abnormal rhythm (e.g., Atrial Fibrillation aka “AFib”), and in extreme cases, this can lead to cardiac arrest.
- Myocardial injury/ischemia (heart damage) – People with heat stroke can show signs of heart injury. According to the NIH, up to 50% show changes on an electrocardiogram (ECG). This is usually accompanied by an increase in troponin levels – proteins that leak into the blood when the heart muscle is damaged. While normal levels are not concerning, elevated levels are a sign of serious cardiac stress or even a heart attack.
- Heart Failure – The intense stress of heat stroke can also trigger stress-induced cardiomyopathy, where the heart temporarily weakens due to a surge in stress hormones like adrenaline. In those with existing heart conditions, this can progress to heart failure. This is most common in people with pre-existing heart conditions.
Sudden Death – Heat waves often lead to sudden death, especially among those with heart disease. These deaths can happen quickly, sometimes before the person reaches the hospital. During the 1995 Chicago heat wave, there were an average of 241 extra deaths per day, and 94% were linked to cardiovascular issues, not the heat alone.
Precautions For Extreme Heat
As you can see, heat stroke can have serious and even fatal consequences, especially for your heart. People with heart conditions need to be especially careful.
Start by understanding your individual heart health: Talk to your doctor, make sure they know about your risk factors (like family history, high blood pressure, or past heart issues), and take steps to manage your cardiovascular health.
It’s important to note that certain medications and substances can increase your chances of heat stroke. These include: beta blockers, antihistamines, laxatives, and diuretics, as well as substances such as alcohol and MDMA (find a complete list here).
Minimizing heat exposure is one of the most important prevention strategies:
- Stay in cool or shaded environments when possible.
- Wear lightweight, breathable, light-colored clothing.
- Hydrate consistently, and include electrolytes—not just plain water.
Avoid overhydration. Drinking too much water without replacing sodium can lead to hyponatremia, a dangerous condition where your blood sodium levels become too diluted. This can cause confusion, seizures, coma, or even death.
Endurance athletes face additional risks due to prolonged exposure, elevated core body temperature, and dehydration during long efforts.
Pre-Race
- Heat Acclimation – Gradually train in hot conditions to help your body adapt. This process improves your ability to sweat, increases plasma volume, and regulates core temperature.
- Pre-Hydration – Begin hydrating several days before the race, using fluids with electrolytes to maintain sodium balance. Pale urine color is a good indicator that you are well-hydrated.
- Adjust Expectations – Recognize that performance may decline in the heat. Use tools like heat index calculators to set realistic goals.
- Cooling Strategies – Use cooling vests or towels before the race to lower core temperature.
During the Race
- Apparel – Wear light-colored, breathable clothing with UV protection.
- Hydrate – Drink fluids regularly, incorporating electrolytes to replace those lost through sweat. Avoid drinking only water to prevent hyponatremia.
- Pacing – Monitor effort levels and be prepared to slow down. Use perceived exertion or heart rate as guides rather than pace alone.
- Cooling Stations – Use water stations to pour water over your head and body to facilitate heat dissipation.
- Monitor your Body – Be vigilant for signs like dizziness, confusion, or excessive fatigue. If symptoms occur, stop immediately and seek medical attention.
Post-Race
- Cool-Down – Move to a shaded or air-conditioned area. Use cold towels or ice packs on the neck, armpits, and groin to lower body temperature.
- Rehydration – Continue drinking fluids with electrolytes to replace losses. Monitor urine color to gauge hydration.
- Rest and Recovery – Allow your body time to recover. Avoid strenuous activity for at least 24 hours post-race.
Conclusion
Heat can be deadly for everyone, not just athletes. It places intense stress on your heart and circulatory system, especially if you’re already at risk.
Knowing your heart health status, speaking to your doctor, and taking simple but serious precautions, like staying cool, hydrating smartly, and recognizing early signs of heat illness, can be life-saving. If you feel the heat is too much, stop. Cool down. Listen to your body and protect your heart.