Pulmonary arterial hypertension is a progressive disease of the pulmonary vasculature, ultimately leading to right heart failure and death. Current treatment is aimed at targeting three different pathways: the prostacyclin, endothelin and nitric oxide pathways. These therapies improve functional class, increase exercise capacity and improve haemodynamics. In addition, data from a meta-analysis provide compelling evidence of improved survival. Despite these treatments, the outcome is still grim and the cause of death is inevitable - right ventricular failure. One explanation for this paradox of haemodynamic benefit and still worse outcome is that the right ventricle does not benefit from a modest reduction in pulmonary vascular resistance. This article describes the physiological concepts that might underlie this paradox. Based on these concepts, we argue that not only a significant reduction in pulmonary vascular resistance, but also a significant reduction in pulmonary artery pressure is required to save the right ventricle. Haemodynamic data from clinical trials hold the promise that these haemodynamic requirements might be met if upfront combination therapy is used.