Hypoxic pulmonary vasoconstriction is a natural process that redistributes blood flow to areas of the lung with higher ventilation. Pathological processes or high-altitude environments can cause it, while medical conditions or vasodilators can interfere. Treatment involves CPAP and proper physical positioning.
Hypoxic pulmonary vasoconstriction is a natural protective mechanism that redistributes blood flow to areas of the lung with a higher degree of ventilation. Starting before birth, the homeostatic process of hypoxic pulmonary vasoconstriction occurs. Pathological processes or high-altitude environments can cause the event. Medical conditions or situations, including hypothermia and the presence of vasodilators, can interfere, inhibit or reverse the process prematurely.
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 pushes the blood into the alveoli where the oxygen concentration 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 suffers from altitude sickness.
At high altitudes, full pulmonary hypoxia occurs due to a decrease in atmospheric pressure. The hypoxic condition leads to vasoconstriction, which leads to high-altitude pulmonary edema. When edema occurs, emergency specialists can give the steroid dexamethasone, which reduces inflammation and helps fluid reabsorption. Mountain climbers generally carry oxygen supplements with them which reduce the possibility of hypoxia occurring.
Once oxygen levels normalize in all lung fields, hypoxic pulmonary vasoconstriction reverses on its own. 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. The sensors and mediators inhibit the oxygen-sensitive potassium channels, which depolarizes the arteries. This action activates voltage-gated calcium channels, causing an influx of calcium and narrowing of the arteries. The entire series of events occurs within seconds after the body detects moderate hypoxia.
The body may be unable to respond normally to hypoxic events if an individual is simultaneously suffering from the effects of lower than normal carbon dioxide levels or metabolic derangements including acidosis or alkalosis, which accompany hyperventilation and other medical conditions. Situations that cause increased pulmonary vascular resistance and traumatic chest injury also inhibit hypoxic pulmonary vasoconstriction. These circumstances cause imbalanced areas of perfusion and ventilation which can prevent deoxygenated blood from receiving oxygen.
Treatment for interference from hypoxic pulmonary vasoconstriction typically requires re-inflating the collapsed arterioles using continuous positive airway pressure, otherwise known as CPAP. Proper physical positioning also aids in lung expansion. When the condition affects one lung, patients who lie on their side must be on the healthy lung.
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