Minimal Change in Measured Functional Pulmonary Capillary Surface Area After First Session of Balloon Pulmonary Angioplasty For Chronic Thromboembolic Pulmonary Hypertension

Introduction:
Chronic thromboembolic pulmonary hypertension (CTEPH) is caused by partial or complete occlusion of pulmonary arteries by organized thrombus, affecting perfusion of downstream pulmonary capillaries in regions fed by the occluded arteries. We have already demonstrated that functional capillary surface area (FCSA) is decreased in resting humans with CTEPH. Balloon pulmonary angioplasty (BPA) to re-establish flow through the affected arteries has become an effective treatment option in appropriate patients. The amount of capillary perfusion that is regained during a session of BPA is unknown. We hypothesized that we could measure an increase in FCSA in patients undergoing BPA.
Methods:
10 Subjects underwent their first session of elective BPA (one lung, mean 6.8 ± 1.7 segments). FCSA assessment involves measuring the first-pass transpulmonary hydrolysis of the substrate ³H-benzoyl-Phe-Ala-Pro (BPAP) as it interacts with the endothelial ectoenzyme, angiotensin converting enzyme. At cardiac catheterization, trace quantities of BPAP are injected into the right atrium and transit the lung circulation, and then effluent systemic arterial blood is collected and processed. Radioactivity levels (parent and metabolite) are measured. The instantaneous % metabolism and hydrolysis are calculated, as well as FCSA. After completion of the BPA session, the extent of which is based on clinical needs, FCSA is re-measured with another injection of BPAP.
Results:
There were no consistent or major hemodynamic improvements after the first session of BPA. Despite radiographic evidence of newly reperfused pulmonary segments, FCSA increased in only 2 patients, and decreased slightly in others. (NCT05719415).
Conclusions:
The lack of significant improvement in measurable FCSA may be due to a decrease in perfusion of the contralateral lung by a “steal’ phenomenon into the newly reperfused lung, thus appearing as no net change in total FCSA.