Physiological Considerations in Open Water Swimming (Part 3)

The two previous posts outlined the stimulating effects of cold water shock and the fight or flight response, which, when countered with a suppressing effects of the diving reflex, can lead to what is known as an autonomic conflict, causing a higher than normal risk of cardiac arrhythmia, a rare but potentially lethal event in open water swimming. This post reviews some other important physiological responses that open water swimmers may encounter.

Swim Induced Pulmonary Edema(SIPE).
When fluid builds up your lungs to a point where it interferes with the ability to absorb oxygen, you will feel short of breath and your breathing rate increases. The diagnosis of SIPE has become more common in recent years, likely due to the popularity of endurance swimming and longer-distance triathlons and is thought to occur in about 1-2% of athletes doing swims longer than 2Km. These swimmers are more often female, in wetsuits, and competing in colder water. Other factors, such as sustained heavy exertion, excessive swim pre-hydration, a lack of a swim warm-up, a history of hypertension, the use of anti-inflammatory medications (such as ASA), and the use of fish oil supplements have also been associated with SIPE. Interestingly, SIPE does not seem to be associated with age, or with lung or heart disease.

SIPE is characterized very simply as a combination of fairly sudden onset of shortness of breath and crackling sounds in the chest, associated with a cough productive of pink, frothy or blood-tinged secretions. It is thought that fluids from the pulmonary capillaries leak into the lungs’ air spaces (also known as “alveolar flooding”), in turn causing the shortness of breath, the crackling sounds in the chest, and the cough productive of a frothy pink (or blood-tinged) discharge. The good news is that it is usually self-limited after getting out of the water. Deaths from SIPE are rare.

So what is going on here? Most likely, significant cold water immersion causes blood vessels in the arms and legs to close down to preserve heat (“peripheral vasoconstriction”), redirecting and increasing blood flow (“shunting”) blood to the chest and lungs. The peripheral vasoconstriction forces the left ventricle to pump harder which reduces the left ventricular stroke volume, and tends to back blood up into the lungs, increasing the blood back-pressure into the pulmonary circulation. This effect can be enhanced in thinner individuals who are more likely to feel the cold, or in swimmers who have not adequately warmed up, or with the wearing of a snug wetsuit, which causes more compression of the limbs and therefore further increases the shunting of blood flow to the lungs.

On the other side of the heart, the right ventricle has to deal with the increased volume coming into the heart due to this shunting effect, which increases the blood pressure in the small arteries in the lungs. When there is increased blood pressure on either side of the delicate oxygen-absorbing tissues (the alveoli) in the lungs, the alveoli are swamped with blood, creating edema, and progressive shortness of breath, and blood-tinged sputum. SIPE is different than the cold shock response: SIPE only occurs after some significant exertional time in the water, although it seems to come out of nowhere very quickly; the cold shock response occurs within seconds of cold exposure, loading the heart so dramatically as to risk straining the heart with an ischemic event.

SIPE is more likely to occur if there had been significant hydration prior to the event—more fluid on board, more fluid pressure in the lungs. Medicines like ASA-related drugs, which by their platelet inhibition increase bleeding tendencies, can also enhance SIPE by contributing to impairing the “bleeding tendency” in the alveoli. Fish oil supplements also have anti-platelet and vasodilatory effects, which would also enhance the development of SIPE.

Since SIPE can also kill you, there are strategies here that are important to consider. Do not over-hydrate, especially before a cold water event. Warm up well. Do not take fish oil supplements or anti-inflammatory medications prior to the swim. Consider how tight your wetsuit is. Confirm that you do not have untreated hypertension – talk to your doctor about this.

If you do develop a frothy pink productive cough, with shortness of breath, and the feeling of crackling sounds in your chest, get out of the water, and get prompt medical attention. Although many SIPE victims are admitted to hospital for assessment, treatment and observation, often just getting out of your wetsuit, rewarming, and time is all that is necessary.

Near-drowning Reflexes.
Drowning events have, of course, occurred in warm swimming pools as well, and in good swimmers; as a result, research has attempted to replicate what happens to the heart with the aspiration of water. In 1985, Japanese researchers were able to record what happens to heart rate, heart rhythm, blood pressure, respiratory efforts, and consciousness immediately after water enters the trachea. Here is what they found:

It appears that the entry of water into the trachea (such as choking on water while swimming) may be another possible potent trigger for dramatic slowing down of the heart rate, a response that can occur within just seconds of aspiration. In rats, the heart rate dropped to under 10 beats per minute, or even stopped, causing a dramatic drop in blood pressure, loss of consciousness and a terminal arrhythmia called ventricular fibrillation. This sensitivity of the trachea is thought to be increased with fatigue. Cardiac arrest would follow quickly.

In open water conditions, irregular waves, collisions with other swimmers, and lack of experience in open water swimming may increase the risk of aspiration and the triggering of this reflex, which may trigger the above cascade of cardiac events–very quickly.

Experience in open water swimming remains a key element of risk reduction to avoid this pattern of aspiration, as is improved breathing pattern flexibility, the ability to cough while the face is in water, and keeping clear of any unnecessary contact with other swimmers.

Photo credit: smith_cl9

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