Gas exchange dynamics and responses to exercise in chronic thromboembolic pulmonary hypertension

Chronic thromboembolic pulmonary hypertension (CTEPH) is mainly caused by mechanical obstruction by thrombi associated with pulmonary vascular remodeling. Bronchial collateral circulation has been studied as a protective mechanism or implicated in the pathogenesis of microvascular disease, also contributing to changes in gas exchange at rest and during exercise. In this study, we sought to characterize the gas exchange abnormalities in patients with CTEPH with particular attention to bronchopulmonary shunt.

Prospective, cross-sectional study with diagnostic intervention. After undergoing right heart catheterization, subjects performed a 50 % maximal workload steady-state exercise at room air and receiving 100 % oxygen with a face mask from a Douglas bag. Arterial and mixed venous blood samples were obtained for gas analysis and calculation of shunt.

Data from 14 subjects (7 women, 49 ± 15 years) with CTEPH were analyzed. All participants showed an increased shunt volume/fraction detected by 100 % oxygen breathing at rest that decreased during exercise (17.0 ± 3.6 % versus 9.8 ± 3.0 %; P < 0.001). Shunt fraction was negatively correlated with hemodynamic severity at rest, degree of pulmonary vascular obstruction and dilation of the pulmonary artery trunk. A drop in PaO₂ was observed in 71 % of patients with RVP > 750 dynes.s.cm⁵, compared to 14 % among those with less severe hemodynamic profile. Multiple linear regression analysis revealed that both PVR and the Qanadli score were independently associated with bronchopulmonary shunt fraction during exercise.

Bronchopulmonary shunt seems to be an adaptive mechanism in CTEPH at rest and during exercise.