Acute respiratory distress syndrome (ARDS) is a medical emergency that develops in the lungs in response to damage. Overall, think of it as a disorder of epithelial inflammation.
(Since "I" in IRDS refers to "Infant", don't be tempted to read the "A" as "Adult". ARDS can occur at any age.)
A diverse set of factors can trigger ARDS. The cause may develop in the alveoli themselves. A serious infection such as pneumonia is one possibility. Also, the most dangerous forms of influenza A can lead to ARDS. Or the alveoli may be severely damaged by inhalation of stomach contents or a toxic gas. On the other hand, the cause may be from outside the lungs. Severe trauma involving shock and transfusions can lead to ARDS after several hours to a day. Sepsis is the presence of pathenogenic microorganisms or their toxins in the blood accompanied by a subsequent systemic inflammatory response. This often leads to ARDS. Sepsis might develop, for example, in someone in a hospital with a catheter in a vein, providing a pathway for bacteria to get into the blood.
Whatever the cause, dyspnea signals the onset of serious problems. The blood is not being oxygenated adequately, which is termed hypoxemia, and carbon dioxide is not being blown off fast enough, which is hypercapnia. For someone breathing room air, the normal partial pressure of oxygen in systemic arterial blood (PaO2)is 100 mm Hg. In ARDS, this would be below about 40 mm Hg breathing room air, although, of course, the patient would quickly be put on supplemental oxygen.
A second characteristic is protein-rich infiltrates accumulating in the alveoli, which would be observed in a chest x-ray. But this would appear similar to pulmonary edema developing from left ventricular failure. This is ruled out by estimating the pressure in the pulmonary veins. The infiltrates in ARDS are not due to high pulmonary venous pressure, but to changes occuring in the epithelia in the alveoli.
As ARDS develops, neutrophils begin leaving the pulmonary capillaries and enter the alveoli, joining the macrophages normally present. On the one hand, both of these cells are important for dealing with and resolving the problem. On the other hand, they simultaneously create the dangerous conditions of ARDS. Inflammatory paracrines and cytokines are likely released in large, and even excessive, amounts. The hyper-inflammation is the basis of the disorder.
The epithelia become leaky, allowing proteins and thus fluid to move into the alveoli. Normally, the type I cells are less permeable than the capillary epithelia. The integrity and transport properties of the type I cells is important for keeping just the right amount of fluid on the inside of the alveoli. But this changes dramatically. Also, type I cells likely are dying, with the type II cells unable to replace them fast enough. Damage to the type II cells can also cause surfactant problems. The fluid-filled alveoli poorly oxygenate the blood, and in this sort of situation supplemental oxygen may not be very effective.
In severe cases there may be substantial fibrosis in the extracellular matrix.
Approximately, two thirds of the patients eventually resolve the difficulties. The neutrophils undergo apoptosis and the macrophages, as usual, remove the debris, including the dead cells and extra protein. Some fibrosis may be repaired, although is more severe cases permanent damage to alveoli may result.
In fatal cases, which are about one-third of the total, the lungs become heavy and virtually without air.