Hepatopulmonary syndrome: pathophysiology of impaired gas exchange

The hepatopulmonary syndrome (HPS) consists of a triad of liver dysfunction , increased alveolar-arterial oxygen gradient and intrapulmonary vascular d ilations. The mechanisms of impaired arterial oxygenation are still debated but the multiple inert gases elimination technique and more recently contr ast echocardiography, greatly facilitated rite investigation of such mechan isms. Subsequently the cause of hypoxemia can be attributed to several mech anisms such as ventilation-perfusion mismatch, right-to-left intrapulmonary shunts and alveolar-to-capillary diffusion defect, variously implicated in the severity of the disease. SHP may result from intrapulmonary vascular d ilations and angiogenesis but the pathogenesis of such abnormalities is not completely explained. The hypothesis of an imbalance in vasoactive mediato rs and angiogenic factors has been put forward. Increasing data support the theory that rite increase irt synthesis and release of nitric oxide (NO) i s the key factor modulating vascular tone. If this hypothesis is true, the use of competive inhibitors of NO synthesis should restore pulmonary vascul ar tone, reversing the hemodynamic changes and gas exchange impairment of H PS.