pulmonary hypertension is a pathophysiologic condition with many possible causes. indeed, this condition frequently accompanies severe heart or lung conditions. a 1973 world health organization meeting was the first attempt to classify pulmonary hypertension by its cause, and a distinction was made between primary ph (resulting from a disease of the pulmonary arteries) and secondary ph (resulting secondary to other, non-vascular causes). further, primary ph was divided in the "arterial plexiform", "veno-occlusive" and "thromboembolic" forms. in 1998, a second conference at évian-les-bains addressed the causes of secondary ph. subsequent third, fourth, and fifth (2013) world symposia on pah have further defined the classification of ph. the classification continues to evolve based on improved understanding of the disease mechanisms.