Hypoxic pulmonary vasoconstriction is a natural protective mechanism that redistributes blood flow to the more ventilated areas of the lung. Beginning before birth, the homeostatic process of hypoxic pulmonary vasoconstriction occurs. Disease processes or high altitude environments may cause the event. Medical conditions or situations, including hypothermia and the presence of vasodilators, can interfere with, inhibit, or prematurely reverse the process.
Mechanisms that occur within the lungs cause the pulmonary arteries to constrict when an area of the lung has low oxygen levels. The medical term for this condition is hypoxia. This action forces blood into the alveoli, where the concentration of oxygen is higher. The process usually occurs in specific areas of one or both lungs when an individual has pneumonia or tumors. Global hypoxia involves all fields of both lungs and occurs when an individual has sleep apnea or experiences altitude sickness.
At high altitudes, complete pulmonary hypoxia occurs due to decreased atmospheric pressure. The hypoxic condition leads to vasoconstriction, leading to high-altitude pulmonary edema. When edema occurs, emergency specialists may administer the steroid dexamethasone, which decreases swelling and stimulates fluid reabsorption. Mountaineers generally carry oxygen supplements that reduce the chance of hypoxia occurring.
Once oxygen levels normalize in all lung fields, the hypoxic pulmonary vasoconstriction reverses itself. The researchers believe that the physiological factors that initiate the process include chemical sensors that generate chemical mediators, which regulate protein, potassium, and calcium channel responses. Sensors and mediators inhibit oxygen-sensitive potassium channels, which depolarizes the arteries. This action triggers voltage-gated calcium channels, causing an influx of calcium and constriction of the arteries. The entire series of events occurs a few seconds after the body detects moderate hypoxia.
The body may not be able to respond normally to hypoxic events if an individual is simultaneously suffering from the effects of lower than normal carbon dioxide levels or metabolic imbalances including acidosis or alkalosis, accompanying hyperventilation and other medical conditions. Conditions that cause increased pulmonary vascular resistance and traumatic chest injuries also inhibit hypoxic pulmonary vasoconstriction. These circumstances cause imbalanced areas of perfusion and ventilation that can prevent deoxygenated blood from receiving oxygen.
Treatment for hypoxic pulmonary vasoconstriction interference typically requires reinflating collapsed arterioles through the use of continuous positive airway pressure, also known as CPAP. Proper physical positioning also helps lung expansion. When the condition affects one lung, patients who lie on their side should lie on the unaffected lung.