Clinical meaning
Hepatopulmonary syndrome (HPS) occurs in patients with liver disease when pulmonary capillaries dilate abnormally (from 8-15 micrometres to 15-500 micrometres). The dilation is driven by increased hepatic production and impaired clearance of vasodilators, primarily nitric oxide and endothelin-1. In dilated capillaries, red blood cells passing through the center of the vessel are too far from the alveolar wall for effective oxygen diffusion, creating a functional right-to-left shunt. Hypoxemia worsens in the upright position (orthodeoxia) because gravity increases blood flow to the dilated basilar capillaries. The triad of liver disease, intrapulmonary vascular dilation, and impaired oxygenation defines HPS.
Exam relevance
Risk factors: - Portal hypertension (most common association) - Cirrhosis from any cause - Chronic hepatitis B or C - Alcoholic liver disease - Non-cirrhotic portal hypertension (portal vein thrombosis) - Budd-Chiari syndrome
Diagnostics: - Contrast-enhanced echocardiography (agitated saline bubble study) showing late opacification of left heart - ABG showing PaO2 < 80 mmHg or A-a gradient >= 15 mmHg on room air - Pulse oximetry with positional changes (supine vs upright) - Tc-99m MAA lung perfusion scan (shunt fraction > 6%) - Liver function tests and assessment of liver disease severity (MELD score) - CT pulmonary angiography to rule out other causes of hypoxemia