On the basis of previous work, our own experience and findings, and the considerations discussed above, we propose a set of four diagnostic criteria for the hepatopulmonary syndrome: 1. presence of chronic hepatic disease (alcoholic, postnecrotic, or primary biliary cirrhosis or active chronic hepatitis)--severe liver dysfunction may not be mandatory; 2. absence of intrinsic cardiopulmonary disease, with normal chest radiograph or with nodular basal shadowing; 3. pulmonary gas exchange abnormalities--an increased alveolar-arterial oxygen gradient (> or = 2.0 kPa) with or without hypoxaemia; 4. the extrapulmonary appearance of intravenous radiolabelled microspheres or a positive contrast enhanced echocardiogram, suggesting intrapulmonary vascular abnormalities. Although these four criteria appear straightforward, there may be other features that are not always present--namely: 1. low transfer factor (diffusing capacity); 2. shortness of breath, with or without platypnoea and orthodeoxia; 3. increased cardiac output and reduced pulmonary vascular pressures; 4. small (or no) increase in pulmonary vascular resistance when the patient is breathing low oxygen mixtures. From the physiological viewpoint, the hepatopulmonary syndrome provides an excellent model for clinical research in the pathophysiology of pulmonary gas exchange. So far it has been possible to show that arterial hypoxaemia in this condition is (1) partitioned into components resulting from VA/Q mismatching, intrapulmonary shunt, and limitations of oxygen diffusion; (2) modulated by the interplay between the intrapulmonary and the extrapulmonary determinants of PaO2, such as cardiac output and minute ventilation; (3) vulnerable to the influence of inadequate pulmonary vascular tone; and (4) resolved when the injured liver is replaced and hepatic function is restored to within normal limits.
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