“Group 5” pulmonary hypertension with unclear and/or multifactorial mechanisms

World Health Organization (WHO) group 5 disorders with pulmonary hypertension is an interesting group because it combines several diseases with multiple pathologies (see Classification of Pulmonary Hypertension [1]). A common feature of these diseases is that the mechanisms of pulmonary hypertension are poorly understood and may include many or some of the following as causes: pulmonary vasoconstriction, proliferative vasculopathy, extrinsic compression, intrinsic occlusion, high-output cardiac dysfunction, vascular destruction, and left heart failure.