Open water swimmers are being warned about a potentially dangerous condition that causes fluid in the lungs.
Medics writing in the journal BMJ Case Reports said swimmers should be told more about the risks of swimming-induced pulmonary oedema (SIPE), which is linked to cold water swimming.
They said the condition leads to fluid accumulation in the lungs resulting in difficulty breathing, low levels of oxygen in the body and a cough.
Open water swimming is a popular sport and saw a growth during the pandemic.
Yet cases of SIPE are likely to be under-reported and often occur in those who are otherwise fit and healthy.
Among the factors that increase the risks are being older, being female, having high blood pressure, long distance swimming, cooler temperatures and pre-existing heart disease.
The medics, from the Royal United Hospitals Bath NHS Foundation Trust, describe treating a woman in her 50s who is a keen competitive long distance swimmer and triathlete.
Otherwise fit and well, she was struggling to breathe and coughing up blood after taking part in an open water swimming event at night in water temperatures of around 17C while wearing a wetsuit.
Her symptoms started after swimming 300 metres.
The woman had experienced breathing difficulties during an open water swim a fortnight earlier, which had forced her to abandon the event and left her feeling breathless for some days afterwards.
On arrival at hospital, her heartbeat was rapid, and a chest x-ray revealed pulmonary oedema, the medics said.
Further scans showed fluid had infiltrated her heart muscle and caused strain, but she had no structural heart disease.
The woman's symptoms settled within two hours of arrival at hospital but she was monitored overnight and discharged the following morning.
The experts said it is not clear exactly what causes SIPE but is likely to be linked to how the blood responds to a cold environment, combined with an exaggerated constriction of blood vessels in response to cold and increased blood flow during physical exertion.
They suggested recurrence is common and has been reported in 13%-22% of scuba divers and swimmers.
In advice to swimmers, the doctors said people should consider swimming at a slower pace, not wear a tight-fitting wetsuit in warmer temperatures and avoiding tablets such as ibuprofen.
For those experiencing symptoms for the first time, the authors recommend stopping swimming and getting out of the water straight away, then sitting upright, and calling for medical assistance if required.
Writing in the journal, the woman said: "While swimming in a quarry at a night swim I started to hyperventilate and realised I couldn't swim any further.
"Luckily, I was able to call for help and got guided back to the quay by a paddleboard. When I got out, I undid my wetsuit and immediately felt the sensation of my lungs filling with fluid.
"I started to cough and had a metallic taste in my mouth. When I got into the light, I could see my sputum was pink and frothy."
She said her husband drove her to hospital for treatment.
The woman said: "Two weeks prior to this incident I had experienced a much milder event while open water swimming in the sea that I hadn't attributed to SIPE at the time, but had also experienced difficulty in my normal running and pool swimming training.
"I had just assumed I was a bit under the weather.
"Other than this I have had no other symptoms and am now fully recovered and back to full training."
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'Extremely rare'
However, a consultant cardiologist has said swimming-induced pulmonary oedema is "extremely rare".
Speaking on RTÉ's News At One Professor Bill McEvoy, also a cardiology professor at University of Galway, said the condition was not seen commonly in the context of cold-water swimming.
"This is a potential rare complication, we need more research to understand how real this even is, and it probably applies to competitive, high-intensity sea swimming moreso than the recreational sea swimming we commonly see," he said.
"I think the report reflects a rare condition. It is a case report published in a medical journal and case reports are quite uncommon, rare presentations," he added.
"In cardiology, we see a lot of pulmonary oedema, which is fluid in the lungs, which is usually caused by a back-up of fluid behind the heart, from a weak heart, and it is a common presentation for patients to have pulmonary oedema, but we don’t see it commonly in the context of cold-water swimming," Prof McEvoy said.
"So I believe that this condition is uncommon, probably actually on the rare side, and shouldn’t be a major concern to current seawater swimmers that are doing so for leisure or recreation."
Prof McEvoy said he had never witnessed a case of swimming-induced pulmonary oedema.
"It is not commonly seen. It may be slightly complicated by the fact that when someone is near drowning and inhales water, that reaction can induce oedema in the lungs," he said.
He added: "So whether or not the oedema is due to swimming in cold water, or if it’s because swimmers inhaled water and had a near-drowning episode, it is hard to distinguish.
"The idea that cold water immersion could cause pulmonary oedema is not common".
Prof McEvoy said sea-swimming, like any other physical activity, is more healthy than harmful, especially in light of the scale of obesity in Ireland.
"I don’t want the message to get out there that its unsafe to conduct physical activity like sea swimming. People should be careful, should swim with a friend and shouldn’t get out of their comfort zone … but if you’re just doing it recreationally, I think this condition is extremely rare," he said.
