Background: Nocturnal desaturation sometimes observed in patients with chronic thromboembolic pulmonary hypertension (CTEPH) despite the absence of parenchymal lung disease, however the mechanism remains unclear. We aimed to clarify the clinical features of nocturnal desaturation in CTEPH.

Methods: Data from 173 patients with CTEPH, who underwent BPA from January 2013 to August 2022, were retrospectively analyzed. We evaluated hemodynamics with right heart catheterization, arterial blood gas examination, respiratory function tests, nocturnal oximetry, and exercise capacity (cardiopulmonary exercise testing [CPET]) which were routinely performed at baseline and after BPA.

Results: Nocturnal desaturation at baseline was -13.3±5.8%. Higher physiologic dead-space fraction (VD/VT) (p<0.001) and higher alveolar-arterial oxygen difference (A-aDO2) (p=0.039) at baseline were associated with larger nocturnal desaturation in the multivariable linear analysis. After BPA, nearly normal hemodynamics was achieved (mean pulmonary artery pressure (PAP): 37.5±10.0 to 20.2±4.9 mmHg, p<0.01). Nocturnal desaturation also improved from -13.3±5.8% to -10.3±5.4% (p<0.01). The improvement of VD/VT was well correlated with the improvement of nocturnal desaturation after BPA (p<0.001, r=0.425).

Conclusions: Nocturnal desaturation often coexisted with CTEPH patients. VD/VT; the marker of physiologic dead-space fraction, and A-aDO2; the marker of ventilation-perfusion mismatch and lung diffusing capacity were strongly associated. Nocturnal desaturation slightly improved after BPA, and the improvement was associated to the decrease of physiologic dead-space fraction.