Mixed venous O ₂ tension is a determinant of pulmonary vascular resistance during maximum incremental exercise

Background: An influence of mixed venous oxygen tension on pulmonary vasoconstriction has been suspected since the classic animal experiments of Marshall, et.al. We hypothesize pulmonary vascular resistance (PVR) response to exercise is influenced by mixed venous O 2 tension (PvO 2 ). Methods: We examined maximum invasive cardiopulmonary exercise test (iCPET) results of 257 patients studied for unexplained exertional intolerance over 2 years. We calculated PVR as mean pulmonary artery pressure(mPAP) minus pulmonary artery wedge pressure (PAWP) divided by direct Fick cardiac output (Qt), at rest & peak exercise. We used PVR as dependent variable in univariate linear regression to assess the association with PvO 2 or with alveolar oxygen tension (P A O 2 ), & in multivariate models where we adjusted for potential confounding variables as P A O 2 , hemoglobin (Hb), peak VO 2 , & peak mixed venous pH. Results: A significant association was found between PVR & PvO2 at rest (B=-1.7; p<0.001) & at peak (B:-1.0; p=0.03). We also found an independent significant association between PVR & P A O2 at peak exercise (B=-2.4; p=0.002) but not at rest (B=-0.6; p=0.29). Univariate analysis at peak exercise shows the amount of variance of PVR accounted by SmvO2 was 5.4%. In the multivariate model, SmvO2 improves the model in 2.6% (28.7 to 31.3%). Using the change in mPAP from peak to rest (delta_mpap) as the dependent variable, an independent statistically significant association was found with peak PvO2. Univariate results: B=-5.85 (p=0.017). Multivariate Conclusions: PvO 2 influences pulmonary vascular tone in a dynamic fashion during incremental exercise.