Physiological Considerations in Open Water Swimming (Part 1)

A simple definition of Physiological: “the organic processes and phenomena of an organism or any of its parts or of a particular bodily process”.

With the growth and immense popularity of open water swimming and triathlon in the last 15 years, a disturbing trend has come to light which is the disproportionate number of deaths in the swim portion of triathlons. The numbers are chilling. In the period 2007-2012 there have been 52 deaths recorded by Triathlon USA, and of those, 44 were in the swim portion of the race, and all of them were in open water, and not one in a pool.

Why at the beginning of a race? No one is exhausted or dehydrated or overheated. Are there any trends to suggest an obvious explanation? Not really. These athletes were a mixture of young and old, male and female, experienced racers and rookies. These people have been in the beginning, the middle, and the end of a swim when they collapsed. The length of race did not seem to matter, whether sprint, Olympic or Ironman distance. They were in fresh and salt water. Some were even able to grasp onto a paddle board before going under, while some were never seen going under. And there are often few witnesses in the water nearby who could accurately describe what happened. The autopsies, when done, do not reliably show lungs full of water, damaged hearts, or significant trauma—since autopsies don’t show everything, they are frequently signed off as death by cardiac arrest. So can we learn anything from what is going on here?

To maximize your ability to survive open water swimming, check out the following basic physiological reflexes that may be contributing to risk of sudden death in open water swimming. They are all easy to understand.

The Cold Shock Response. No mystery here: jumping into cold water is a significant strain on your body (even when the lake is a comfortable 22°C, that is still 15°C less than your body temperature). Your heart rate and breathing rate spirals upward dramatically, as does your blood pressure (due to massive vasoconstriction in your limbs and skin), and your demand for oxygen (as your body now very dramatically needs to generate heat with shivering). In susceptible individuals, this cardiac strain can lead to angina, myocardial infarction, and potentially cardiac arrhythmias.

The solution? Don’t jump or dive into cold water. Ease into it—walk into cool water if you can. Acclimatizing slowly each time you go. And swim in cool water regularly to get comfortable with cool water swimming. Your body can adjust without a shocking response if you give it a chance to. And of course, do not combine cold water swimming with alcohol or a variety of other drugs that might distort your feeling of the cold, which in turn would distort your perceptions of heat loss, and your risk of hypothermia.

The Mammalian Diving Reflex. All mammals use a fundamental survival reflex when diving to conserve oxygen use—this allows aquatic mammals (e.g., seals, dolphins, whales, walruses) and diving birds like penguins to stay underwater for extended periods of time. This reflex is also evident in humans (especially in infants), and has clear implications when swimming in open water.

How does this work? When the face specifically is submerged and exposed to cold water, nerve receptors in the face relay this information to the brain and the vagus nerve to dramatically slow the heart rate (called bradycardia) and the urge to breathe. Blood supply to the periphery is also constricted, to further reduce oxygen demand. The degree of the effect is proportional to the coldness of the water, and it is increased further with breath-holding. Unlike the cold shock response described above, it is not triggered with the limbs or body contacting cold water.

How cold does the water have to be? Although one source suggests 16C or less, the reflex likely varies with how well you are acclimatized to cold water (one the day, and over time), as well as your overall health and fitness. Chances are if it feels cold to you, it is cold enough to elicit a diving reflex.

To lessen the effect of the diving reflex, two things are important: First, get used to swimming in cool open water, by going out into it regularly. And second, initially splashing water on the face before fully entering the water to swim is important. If the water is particularly cold (16C/61F or less), you might consider a series of short swims of only 15-30 seconds, standing with head out of the water in between, to acclimatize the face to the cold. Check your pulse in your neck before you enter the water, and after you have acclimatized to get a sense of how you are doing.

Photo Credit: Dawn – Pink Chick

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