According to the Global Triathlon Safety Task Force, between 2015 and 2019, 68% of race fatalities occurred during the swim leg. In 2022, the number was at 83%.  With the prevalence of wetsuits in most races and the terrific swim support by volunteers in kayaks, on paddleboards, and in power craft, they are not drowning. Nearly 100% of those fatalities were due to cardiovascular events—namely, heart attacks and a condition called SIPE.
The task force, chartered to address fatalities in triathlon and how to limit them, has been active in bringing information on how to reduce mortality in triathlon races to race directors, coaches, and athletes. Task force chair Mike Wien has said, “Our initial goal is to save at least one life.” In pursuit of this goal, the task force has published three webinars aimed at race directors, coaches, and athletes on what they can do to make triathlon safer. One of the biggest dangers may be related to low water temperatures at race time.
Imagine this: it’s race morning. The sun is not yet over the horizon. The air is chilly and a slight but nagging breeze is wicking heat from your skin. The water is cold—around 59°F (15°C). The task force recommends getting into the water before the start gun goes off, especially if the water is cold. This might be the last thing you want to do but could be the most important thing.
Years ago, the inaugural HITS Triathlon Series in Palm Springs, California, had a rough start. The air was very chilly (low 40s Fahrenheit) and the water was cold. Not many athletes wanted to get into the water for a warm-up swim, and very, very few did. When the athletes did charge into the frigid waters to sprint for position, many of them experienced pounding chests and could not catch their breath—so much so that several spectators rushed into the water in their jeans and tennis shoes to pull athletes to shore.
These are accomplished triathletes at a championship race, who are competent or even great swimmers, so what on earth happened here? Why could they not catch their breath or shake this terrible pounding in their chest?
The mammalian diving reflex
A physiological phenomenon that can cause such symptoms is called the mammalian diving reflex (DR). It is an evolutionary response to what your brain perceives as drowning. All mammals have this natural ability, as well as certain aquatic birds.
It works like this: When you feel the shock of cold water on your face and hold your breath, the trigeminal nerve in your face sends a signal to your vagus nerve to slow your heartbeat. Your body also shunts blood flow from your extremities to your internal core and brain. According to researcher W. Michael Panneton, “The DR is the most powerful autonomic reflex known.” He goes on to say, in laboratory experiments, “100% of rats get it 100% of the time.” 
There is cold water on your face and you are holding your breath. Well, that could be what drowning is like, so your body transfers blood to your core (let’s keep the heart and vital organs going, shall we?) and away from your arms and legs. Your body also (greatly) reduces your heart rate to conserve oxygen and warmth for the core.
But this is exactly the opposite of what you need in a swim race. You want blood to flow to your arms and legs so that you can swim. And you want to have plenty of ventilation to provide oxygen to those muscles, not have your breathing arrested. And you want your heart rate to elevate, not shut down, to get all that blood moving.
Imagining the shock of cold water, you can maybe even now, in the comfort of your chair, sense the war in your chest—your DR wants you to shut down and wash up on shore before you drown, but your exercise response wants you to ramp up everything and power through the water. Air hunger, tightened chest, and erratic heart rate (like a jackrabbit in your chest) are just awful feelings. Awful enough for spectators in street clothes to have to wade in and drag you to shore.
Many athletes call this a panic attack. It feels like it but is probably not. It is more likely a normal response to warring physiological responses to jumping into cold water and trying to swim fast. According to writer John Rennie, “A disadvantageous consequence, however, is that the muscles in the limbs must then rely more on anaerobic energy metabolism to keep working, so they build up [lactate] and tire more rapidly than they would from comparable excise at the surface.” 
Heart attack or SIPE?
The cardiac health implications are clear and severe—cold-water swimming is not healthy for your heart. Most triathletes who die in races die from heart attacks and most of those fatalities are in older males with existing heart conditions. However, another cause of death might be in play.
Dr. Richard Moon and Timothy Beck describe Swimming-Induced Pulmonary Edema (SIPE) as “a peculiar condition that often occurs in young, healthy swimmers and scuba divers, particularly in cold water. Physiological studies have revealed it to be due to high pulmonary vascular pressures; hence its origin is presumably cardiogenic, yet it occurs in many individuals who are exceptionally physically fit without any cardiac risk factors.” 
Dr. Moon argues that most swim deaths due to SIPE are misdiagnosed as heart attacks.  The catalyst is the same: The mammalian diving reflex moves blood flow to the body’s core, causing shortness of breath. For swimmers who experience SIPE, the buildup of fluid in the lungs also causes coughing and/or bloody sputum, even if they haven’t inhaled water.
In either case, if you’re experiencing chest pain, shortness of breath, and/or coughing up blood during the swim portion, wave down a volunteer immediately. For SIPE, the symptoms resolve when your body is able to warm up. If it’s a heart attack, the chances of survival and recovery are greater the sooner it can be treated.
How to reduce the risk of heart attack or SIPE
There have been small studies conducted that show potential for the drug sildenafil (brand name Viagra) in preventing symptoms of SIPE. [6,7] Though its reputation for treating erectile dysfunction is well-known, sildenafil is also approved for the treatment of cardiac issues such as pulmonary hypertension. The existing studies show promise, but more research is still needed.
If the thought of popping a little blue pill before a race makes you uncomfortable, fortunately, there is one simple and effective way to reduce the shock of mammalian diving reflex and therefore the chance of cardio-pulmonary problems in the swim: a proper warm-up. 
- Get in the water. Let the water trickle into your wetsuit. Float around for a few moments and feel the temperature of the water on your hands, feet, and especially face and head.
- Bob. Take a breath. Put your face in the water and slowly exhale bubbles for 10 to 15 seconds. Lift your face into the air, take a deep breath, then bob again. Repeat this for a minute or two.
- Swim a little. Do some easy swimming for 25 to 50 meters back and forth along the shoreline to get your muscles and your heart on the same page.
- Go swim. By gently bobbing before swimming, you get your mammalian diving reflex to realize that you indeed are not drowning. Then you can settle into your swim warm-up and carry on.
When you cannot get in the water in advance, or the warm-up area is too far from the swim start, consider bringing a few bottles of water to splash on your face and pour down the front and back of your wetsuit. The Escape from Alcatraz Triathlon in San Francisco, California, features a swim start of 3,000 athletes jumping off a boat into 59°F water in less than eight minutes. No swim warm-up is possible, so doing something like the water bottle trick can help.
What happens when the symptoms really are psychological (like a panic attack) and not physiological? Athletes can, even after only one or two intensely unpleasant swim episodes, anchor those past events and project them toward their next open-water swim. That’s how Post-Traumatic Stress Disorder (PTSD) works. 
If your heart races and you experience rapid, shallow breathing to a detectable degree when you just think about jumping into cold water surrounded by fellow athletes, you might benefit from any technique that reduces the physiological effects of your thoughts.  Cognitive behavioral therapy with a trauma focus holds the greatest promise, for example Reconsolidation of Traumatic Memories Protocol (disclosure: I am a certified RTM administrator). 
Managing swimming PTSD is important not just for your mental health, but physical as well. A 2017 report found that people with PTSD are 17-20% more likely to develop cardiovascular disease compared to the general population.  Reducing your symptoms of PTSD will in turn reduce your risk of heart attack in the swim.
Don’t forget to enjoy yourself
This is a serious topic but not one meant to cause fear, so let’s be clear: Fatalities in triathlon are exceedingly rare, so go swim and enjoy every minute. Consider the precautions that you can take to enhance your safety, well-being, and enjoyment by a proper water warm-up, and address any unpleasant memories of swims that bring on uncomfortable symptoms.
You likely got into triathlon because you love it, and now you are more armed than ever to stay safe in the swim. Just don’t forget to have fun.
About the author
Will Murray is a USA Triathlon Level 1 coach and co-author with Craig Howie of The Four Pillars of Triathlon: Vital Mental Conditioning for Endurance Athletes. Will also is a certified administrator of the Reconsolidation of Traumatic Memories Protocol (RTM) for Post-Traumatic Stress and is the mental skills coach at D3 Multisport.
- Education Hub: Global Triathlon Safety Task Force [Internet]. education.triathlon.org. [cited 2023 Aug 17]. Available from: https://education.triathlon.org/mod/page/view.php?id=10899
- Michael Panneton W. The Mammalian Diving Response: An Enigmatic Reflex to Preserve Life? Physiology [Internet]. 2013 Sep;28(5):284–97. Available from: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3768097/
- Rennie J. How the Dive Reflex Extends Breath-Holding [Internet]. Scientific American. 2012. Available from: https://www.scientificamerican.com/article/breath-holding-dive-reflex-extends/
- Moon RE, Beck TP. Cold Water, Hard Swim, Shortness of Breath. Chest. 2020 Oct;158(4):1329–30.
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- Moon RE, Martina SD, Peacher DF, Potter JF, Wester TE, Cherry AD, et al. Swimming-Induced Pulmonary Edema: Pathophysiology and Risk Reduction With Sildenafil. Circulation [Internet]. 2016 Mar 8 [cited 2021 May 13];133(10):988–96. Available from: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5127690/
- Martina SD, Freiberger JJ, Peacher DF, Natoli MJ, EA Schinazi, Kernagis D, et al. Sildenafil: Possible Prophylaxis against Swimming-induced Pulmonary Edema. 2017 Sep 1;49(9):1755–7.
- Avoiding Open Water Panic Feelings [Internet]. www.usatriathlon.org. 2023. Available from: https://www.usatriathlon.org/articles/training-tips/avoiding-open-water-panic-feelings
- Gray RM, Bourke F. Remediation of intrusive symptoms of PTSD in fewer than five sessions: a 30-person pre-pilot study of the RTM Protocol. Journal of Military, Veteran and Family Health. 2015 Nov;1(2):13–20.
- Kitchiner NJ, Lewis C, Roberts NP, Bisson JI. Active duty and ex-serving military personnel with post-traumatic stress disorder treated with psychological therapies: systematic review and meta-analysis. European Journal of Psychotraumatology. 2019 Nov 8;10(1):1684226.
- Gray R, Davison A, Bourke F. Reconsolidation of Traumatic Memories, The RTM Protocol: Albuquerque trainee results. 2021 [cited 2023 Oct 10]; Available from: https://randrproject.org/pdf/2022/5.%20Albuquerque%20trainees%20PsyArxiv.pdf
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