Acute respiratory failure can be caused by anything that results in inadequate gas exchange in the lungs. When a person breathes, carbon dioxide (CO2) is absorbed out of the bloodstream by the lungs, and oxygen is absorbed into the blood; inadequate gas exchange results in high levels of carbon dioxide or low levels of oxygen in the blood. "Acute" refers to an imbalance that develops very quickly, anywhere from a few minutes to an hour. Acute respiratory failure can be cause by an injury, a disease, or problems with blood flow.
The causes of acute respiratory failure are classified as either hypoxemic or hypercapnic. Hypoxemic failure, known as type 1 respiratory failure, refers to causes that lower the amount of oxygen in the blood to pressures lower than 60 millimeters mercury (mmHg). The normal pressure of oxygen in the blood ranges from 85 to 100 mmHg. Main causes of hypoxemic acute respiratory failure are bleeding or a buildup of fluid in the lungs.
Hypercapnic, or type 2 respiratory failure, is due to a buildup of carbon dioxide in the blood. Normal carbon dioxide pressure in the blood ranges from 35 to 45 mmHg; hypercapnic levels are higher, above 50 mmHg. The main causes of hypercapnic acute respiratory failure include a loss of consciousness, lung disease, and hypoventilation or low breathing rate.
Several types of lung disease can cause both type 1 and type 2 acute respiratory failure. Pneumonia and cystic fibrosis cause the lungs to fill up with fluid. Emphysema and severe asthma can result in the accumulation of CO2 as the lungs or lung cells become blocked. In each these conditions, blood oxygen levels are depleted and CO2 cannot be released from the blood.
Conditions that limit blood flow to the lungs, such as pulmonary embolism, also result in inadequate gas exchange. A pulmonary embolism is a blockage of the lung arteries so that blood cannot reach the lung cells to exchange carbon dioxide for new oxygen. High levels of carbon dioxide build up in the blood stream, eventually reaching levels high enough for respiratory failure.
Injuries to specific parts of the body can cause both hypecapnic and hypoxemic failure. A severe blow to the head or alcohol or drug overdose can alter brain functions that control the lungs, lowering the breathing rate. A heavy blow to the chest can damage the ribs or lung tissue, resulting in improper breathing. If a broken rib punctures the lung, then acute respiratory failure may occur due to bleeding.