Health risks to open water swimmers: risk factors for swimming-induced pulmonary oedema

Background Open water swimming is increasingly popular, with an estimated 4.4 million people in England participating in 2022/23. However, there is growing evidence that it is associated with a dangerous condition known as swimming-induced pulmonary oedema (SIPE). SIPE occurs when fluids from the blood leak into the lungs during swimming, causing acute shortness of breath and a cough, which is productive of blood-tinged sputum. Symptoms usually improve once on dry land, however, SIPE can be life-threatening if a swimmer is unaccompanied or cannot leave the water quickly. We set out to identify what might increase the risk of developing SIPE to guide policy and practice on safe open water swimming. Methods We carried out an unmatched case-control study of outdoor swimmers. Cases and controls were identified using a questionnaire survey of the online open water swimming community and at competitive open water swimming events. We used whole population sampling of event competitors and a combination of convenience and snowball sampling to recruit members of the online open water swimming community. Cases were those who had experienced shortness of breath during a swim that did not occur on entering the water and was not associated with water aspiration. Controls were those who described no such experience. As well as experience of shortness of breath, the questionnaire asked about demographics, height, weight, medical history, swimming habits and health behaviours. We calculated adjusted odds ratios for hypothesized risk factors using logistic regression, controlling for age, sex and potential confounders. Results Our study included 107 cases and 871 controls. Risk factors for having ever had SIPE included wearing a tight wetsuit or non-tight wetsuit compared to no wetsuit (tight wetsuit: adjusted OR 5.83, 95% CI 3.15 to 10.80; non-tight wetsuit: adjusted OR 1.87, 95% CI 1.14 to 3.04). The associations were robust to attempts to control for potential confounding factors. Age, sex, ethnicity, smoking, medication use, history of cardiovascular disease, drinking before swimming, swimming distance, maximum effort levels, month of swimming, height and BMI were not associated with SIPE. Sensitivity analyses using different case definitions for SIPE showed consistent findings. Conclusion Wearing a wetsuit during open water swimming increases the risk of SIPE. It is important to further explore whether this is causal and what the mechanisms might be so that we can advise swimmers, event organisers and health professionals on prevention and management.