Flooded or fluid-filled lungs, or pulmonary edema, occurs when air sacs in the lungs fill with fluid, according to the Mayo Clinic. Causes of pulmonary edema include a weakened left ventricle in the heart, trauma, drowning, high altitude and reactions to drugs or toxins.
Alveoli, tiny air sacs in the lungs, fill with air with each breath to collect oxygen and release carbon dioxide, explains the Mayo Clinic. Some conditions, however, cause the alveoli to fill with fluid instead of air. The most common cause is a weak left ventricle due to heart valve problems, hypertension, heart muscle damage or coronary artery disease. The weakened left ventricle causes pressure to build up in the left atrium, resulting in pressure increases in the blood vessels of the lungs. This pressure allows fluid to force its way through blood vessel walls into the alveoli.
Pulmonary edema is a major concern for anyone hiking or camping at high altitudes, reports the Mayo Clinic. The most likely cause in this situation is constriction of blood vessels and capillaries, allowing fluid to enter the alveoli. Allowing for an acclimation period at high altitudes before strenuous physical activity helps minimize the risk. Injury to the lungs, resulting in collapse, is another cause of pulmonary edema, as are certain types of blood clots and reactions to certain substances, from illegal drugs to aspirin. Viral diseases like dengue fever and hantavirus may also cause pulmonary edema.