Pulmonary vascular resistance occurs when the pulmonary artery creates resistance against the blood flowing into it from the right ventricle. The resistance is naturally created by the arrangement of blood vessels in the lungs and is healthy in low levels. Problematic vascular resistance is created when there is an increased amount or viscosity of blood flow to the pulmonary artery and therefore an increase in resistance.
The heart, when healthy, functions as a pumping and filtration system for blood. Deoxygenated and used blood enters the right atrium from the inferior and superior vena cava and flows into the right ventricle. The right ventricle contracts and pumps blood into the pulmonary artery, which carries the blood into the heart for filtration and oxygenation. This new blood flows into the left atrium and subsequently into the left ventricle, which also contracts, forcing blood into the aorta and out to the rest of the body.
There is some natural resistance to the blood flow created by gravity, particularly when the veins and arteries flow up at a vertical angle. The contractions in the ventricles of the heart normally provide enough force to allow enough blood to flow despite the resistance. Pulmonary vascular resistance is a particular type of resistance created by the vasculature, or arrangement of blood vessels, of the lungs. The heart comes up against this resistance at the pulmonary artery, where the blood enters the lungs for filtration.
The most common cause of pulmonary vascular resistance are circulatory problems. Changes in the viscosity of blood, which may be caused by a change in hematocrit, will affect the level of resistance at the pulmonary vasculature. Another factor that affects resistance are the arterioles. Arterioles can expand and contract to a limited degree in order to increase or reduce blood flow. When the arterioles are too small, vascular resistance can result.
When communication between the left and right side of the heart is disrupted, usually due to circulatory problems, the blood will flow to the area of least resistance. This will often result in an increase in blood flow to the pulmonary artery. The increase in blood flow creates an increase in pulmonary vascular resistance. If left untreated, the increased resistance can cause permanent damage to the blood vessels of the lungs.
Pulmonary vascular resistance is quite difficult to detect because it involves the inner workings of the cavities of the heart and lungs. Scientists are working on non-invasive methods to detect this disorder. One of these methods that has undergone limited testing is echocardiographic assessment. This method is effective in detecting low levels of vascular resistance but is not as effective when dealing with higher levels.