Overview
Why Oxygen Falls
Hepatopulmonary syndrome (HPS) is a pulmonary complication of chronic liver disease or portal hypertension.
Hepatopulmonary syndrome (HPS) is a pulmonary complication of chronic liver disease or portal hypertension. Excess nitric oxide production causes diffuse dilatation of the pulmonary vessels. Blood then travels through vessels that are abnormally wide, increasing the distance between red blood cells and the alveolar oxygen interface and producing ventilation-perfusion mismatch with an intrapulmonary shunt. The result is arterial hypoxemia that may be worse than the lung examination suggests. Upright posture directs more blood flow toward dependent lung regions, where the dilated vessels magnify the mismatch. This explains orthodeoxia: oxygen saturation or PaO₂ falls when the patient sits or stands. Platypnea is the accompanying increase in dyspnea while upright. HPS is not the same as portopulmonary hypertension. HPS primarily causes pulmonary vascular dilatation and shunting; portopulmonary hypertension causes increased pulmonary vascular resistance and may lead to right-ventricular strain. The distinction matters because their diagnostic tests and treatment pathways differ.