Edema, in general, means swelling. This typically occurs when fluid from inside blood vessels seeps outside the blood vessel into the surrounding tissues, causing swelling. This can happen either because of too much pressure in the blood vessels or not enough proteins in the bloodstream to hold on to the fluid in the plasma (the part of the blood that does not contain any blood cells.) Pulmonary edema is the term used when edema happens in the lungs.
The immediate area outside of the small blood vessels in the lungs is occupied by very tiny air sacs called the alveoli. This is where oxygen from the air is picked up by the blood passing by, and carbon dioxide in the blood is passed into the alveoli to be exhaled out. Alveoli normally have a thin wall that allows for this air exchange, and fluids are usually kept out of the alveoli unless these walls lose their integrity. Pulmonary edema occurs when the alveoli fill up with excess fluid seeped out of the blood vessels in the lung instead of air. This can cause problems with the exchange of gas (oxygen and carbon dioxide), resulting in breathing difficulty and poor oxygenation of blood. Sometimes, this can be referred to as “water in the lungs” when describing the condition to patients. Pulmonary edema can be caused by many different factors. It can be related to heart failure, called cardiogenic pulmonary edema, or related to other causes, referred to as non-cardiogenic pulmonary edema.
What causes pulmonary edema?
As mentioned earlier, pulmonary edema can be broadly divided into cardiogenic and non-cardiogenic causes. Some of the common causes are listed below.
Cardiogenic causes of pulmonary edema
Cardiogenic causes of pulmonary edema results from high pressure in the blood vessels of the lung due to poor heart function. Congestive heart failure due to poor heart pumping function (arising from various causes such as arrhythmias and diseases or weakness of the heart muscle), heart attacks, or abnormal heart valves can lead to accumulation of more than the usual amount of blood in the blood vessels of the lungs.This can, in turn, cause the fluid from the blood vessels to be pushed out to the alveoli as the pressure builds up.
Non-cardiogenic pulmonary edema
Non-cardiogenic pulmonary edema can be commonly caused by the following:
• Acute respiratory distress syndrome (ARDS), a potentially serious condition caused by severe infections, trauma, lung injury, inhalation of toxins, lung infections, cocaine smoking, or radiation to the lungs. In ARDS, the integrity of the alveoli become compromised as a result of underlying inflammatory response, and this leads to leaky alveoli that can fill up with fluid from the blood vessels.
• Kidney failure and inability to excrete fluid from the body can cause fluid build-up in the blood vessels, resulting in pulmonary edema. In people with advanced kidney disease, dialysis may be necessary to remove the excess body fluid.
• High altitude pulmonary edema, which can happen due to rapid ascent to high altitudes of more than 10,000 feet.
• Brain trauma, bleeding in the brain (intracranial hemorrhage), severe seizures, or brain surgery can sometimes result in fluid accumulation in the lungs, causing neurogenic pulmonary edema.
• A rapidly expanding lung can sometimes cause re-expansion pulmonary edema. This may happen in cases when the lung collapses (pneumothorax) or a large amount of fluid around the lung (pleural effusion) is removed, resulting in rapid expansion of the lung. This can result in pulmonary edema on the affected side only (unilateral pulmonary edema).
• Rarely, an overdose on heroin or methadone can lead to pulmonary edema.
• Aspirin overdose or chronic high dose use of aspirin can lead to aspirin intoxication, especially in the elderly, which may cause pulmonary edema.
• Other more rare causes of non-cardiogenic pulmonary edema may include pulmonary embolism (blood clot which has traveled to the lungs), transfusion-related acute lung injury (TRALI), some viral infections, or eclampsia in pregnant women.