Hypoxic respiratory failure is the general term used to describe organs of breathing not being capable of maintaining effective gas exchange in the lungs. Blood gas tests are used to ascertain whether respiratory failure is present.
In infants with hypoxic respiratory failure the blood vessels in the lungs are constricted. This limits the blood flow from the heart to the lungs and means that when the blood low in oxygen returns to the heart, it is conveyed past the lungs through ducts that remain from the foetal period and flows back out into the body.
Many infants with hypoxic respiratory failure have a sporadic lung disease which means that the air flow (and therefore oxygenation) is good in some areas of the lungs and less good or non-existent in other areas.
When nitric oxide is inhaled, the constricted blood vessels in the lungs relax, so that the blood flow from the heart to the lungs increases, and the quantity of blood that goes outside the lungs decreases. The blood flow increases in those areas of the lungs where the air flow is greatest and where the best gas exchange can take place.