Vascular and right ventricular remodelling in chronic thromboembolic pulmonary hypertension

In chronic thromboembolic pulmonary hypertension (CTEPH) increased pulmonary vascular resistance is caused by fibrotic organisation of unresolved thromboemboli. CTEPH mainly differs from pulmonary arterial hypertension (PAH) by the proximal location of pulmonary artery obliteration, although distal arteriopathy can be observed as a consequence of non-occluded area over-perfusion. Accordingly, there is proportionally more wave reflection in CTEPH, impacting on pressure and flow wave morphology. However, the time constant, i.e. resistance × compliance, is not different in CTEPH and PAH, indicating only trivial effects of proximal wave reflection on hydraulic right ventricular load. More discriminative is the analysis of the pressure decay after pulmonary arterial occlusion, which is more rapid in the absence of significant distal arteriopathy. Structure and function of the right ventricle show a similar pattern to right ventricular hypertrophy, namely dilatation and wall thickening, as well as loss of function in CTEPH and PAH. This is probably related to similar loading conditions. Hyperventilation with hypocapnia is characteristic of both PAH and CTEPH. Ventilatory equivalents for carbon dioxide, as a function of arterial carbon dioxide tension, conform to the alveolar ventilation equation in both conditions, indicating a predominant role of increased chemosensitivity. However, a slight increase in the arterial to end-tidal carbon dioxide tension gradient in CTEPH shows a contribution of increased dead space ventilation.