Acute respiratory distress syndrome: Respiratory failure of sudden onset due to fluid in the lungs (pulmonary edema), following an abrupt increase in the permeability of the normal barrier between the capillaries in the lungs and the air sacs. The muscles used in breathing are forced to work harder, causing labored and inefficient breathing. An abnormally low level of oxygen in the blood (hypoxemia) occurs. The types of acute lung injury that may lead to ARDS include, but are not limited to, aspiration of food or other items into the lungs, inhalation of a toxic substance, widespread infection of the lungs, blood infection (sepsis), and near-drowning. Treatment frequently involves temporary use of a mechanical ventilator to help the patient breathe.