The hepatopulmonary syndrome is characterized by a defect in arterial oxygenation induced by pulmonary vascular dilatation in the setting of liver disease; patients of all ages can be affected. This clinical syndrome has three components: liver disease, pulmonary vascular dilatation, and a defect in oxygenation. A classification of the severity of the hepatopulmonary syndrome based on abnormalities in oxygenation is vital because severity influences survival and is useful in determining the timing and risks of liver transplantation. The vascular component includes diffuse or localized dilated pulmonary capillaries and, less commonly, pleural and pulmonary arteriovenous communications. Arterial hypoxemia is common in the context of hepatic disease; its cause is often multifactorial (e.g., ascites, hepatic hydrothorax, and chronic obstructive pulmonary disease in patients with alcoholism), and in the particular case of the hepatopulmonary syndrome, the pathophysiological features are unique. The definition of arterial hypoxemia associated with the hepatopulmonary syndrome is based on measurements of the partial pressure of oxygen that are performed with the patient in a standardized position, preferably sitting and at rest. The use of the more sensitive alveolar–arterial oxygen gradient is important because it can increase abnormally before the partial pressure of oxygen itself becomes abnormally low as the gradient measure compensates for the reduced levels of arterial carbon dioxide and hyperventilation, along with respiratory alkalosis, that are common in cirrhosis.
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