A previously validated computer model of the normal pulmonary circulation is adapted to simulate pulmonary arterial hypertension (PAH) in humans. Model predictions are used to explore the suitability of currently accepted criteria for diagnosing PAH by correlating hemodynamic data with the degree of vascular compromise (disease severity).Model predictions demonstrate a hyperbolic relationship between vascular compromise, mean pulmonary artery pressure (PAPm) and pulmonary vascular resistance (PVR). PAPm and PVR change very little from disease initiation until a vascular compromise of 65% to 70% (surface area of 0.35 to 0.3 of baseline, respectively) is reached. Following that, further compromise is associated with a steep rise in PAPm and PVR.The relationship between vascular compromise and hemodynamics may explain the relative stability of cardiac output early in this disease process and, therefore, the lack of symptoms. It also explains the rapid deterioration following diagnosis if the disease remains untreated.Model predictions demonstrate the inadequacy of the current hemodynamic criteria for diagnosing PAH over a wide range of left atrial pressure and cardiac output combinations and for early detection of disease. The model provides an alternative approach to diagnosing PAH by translating hemodynamic data to degree of vascular compromise.