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 has been 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, with more than three million people thought to take part every year in England, they said.
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 A&E for treatment.
“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.”