In CTEPH (WHO Group 4), the initiating event is thought to be blockage or narrowing of the pulmonary blood vessels with unresolved blood clots; these clots can lead to increased pressure and shear stress in the rest of the pulmonary circulation, precipitating structural changes in the vessel walls (remodeling) similar to those observed in other types of severe pulmonary hypertension. This combination of vessel occlusion and vascular remodeling once again increases the resistance to blood flow and so the pressure within the system rises.