Sildenafil for Swimming-Induced Pulmonary Edema (SIPE) Prevention

Immersion pulmonary edema (also known as swimming-induced pulmonary edema, SIPE) is a
condition in which the lungs fill with fluid (pulmonary edema) during a dive or vigorous
swim, causing cough with bloody sputum, shortness of breath and reduced blood oxygen levels.
In the Navy this usually occurs in young, healthy individuals such as SEAL and Special
Warfare Combatant-Craft trainees. SIPE generally resolves spontaneously within 24 hours but
it can be fatal. It is believed that SIPE is responsible for some deaths in civilians during
triathlons. This proposal aims to test a drug to prevent SIPE.

Depending upon severity, the prevalence of SIPE is reported in up to 60% during 2.4-3.6 km
open sea swimming trials in the Israel Defense Force. In SEAL training in the US
approximately 40 cases per year (around 3%) have been reported, more commonly during winter,
when it is observed in up to 5% of trainees. Return to duty time can be up to 7 days. SIPE
also affects other groups of highly fit individuals such as triathletes. In susceptible
individuals it tends to recur, thus a preventive medicine would be useful for both Navy SEALs
and civilians.

The aim of this study will be to provide the Navy an FDA-approved drug that can be used to
prevent SIPE. The investigators hypothesize that sildenafil administration to
SIPE-susceptible individuals one hour before a swim in cold water will reduce or eliminate
the risk of SIPE. The method to be used to test this hypothesis will be a 40-minute period of
exercise immersed to the neck in 20°C water, a test that results in SIPE symptoms in the
majority of susceptible individuals. The investigators plan to study 20 individuals who have
previously experienced SIPE. Each volunteer will be tested twice. Either sildenafil or an
inactive drug (placebo) administered in random order will be given prior to each exercise.
For each participant exercise periods will be performed at least 7 days apart. The identity
of the drug and placebo will be concealed from the investigators and the volunteers until the
end of the study. The number of instances in which SIPE manifestations after sildenafil and
placebo will then be compared.

Availability of a drug that can prevent SIPE would provide the Navy with a useful tool that
could be administered to SEALs who have experienced SIPE prior to critical missions. It would
also be useful for civilians who have experienced SIPE but wish to continue with the
precipitating exercise such as swimming or competing in triathlons, and also for patients
with heart failure for whom swimming induces shortness of breath or pulmonary edema.

Inclusion Criteria:

- Healthy volunteers between 18 and 45 years

- History of swimming-induced pulmonary edema

Exclusion Criteria:

- Pregnant women

- Significant heart valve disease

- Cardiomyopathy

- Uncontrolled hypertension

- Coronary artery disease

- Obstructive lung disease

- VO2max <25 mL/kg as estimated by the University of Houston Non-Exercise Test

- Previous adverse reaction to sildenafil

- Use of antihypertensives or other drugs that are known to interact adversely with
sildenafil (e.g. nitrates, alpha adrenergic blockers)