<p>The influence of partial pressure of oxygen (pO₂) is often overlooked during vasoreactivity testing by inhaling oxygen in congenital heart diseases (CHD). This study aims to evaluate the influence of pO₂ on hemodynamic data. Patients with simple CHDs and suspected pulmonary arterial hypertension undergoing right heart catheterization including vasoreactivity testing by inhaling O₂ were consecutively enrolled from June 2023 to March 2025. Patients with an arterial O₂ saturation below 98% after inhaling O₂ were excluded. Cardiac output was calculated using indirect Fick method, where blood O₂ content (1.39 × hemoglobin × O₂ saturation + 0.032 × pO₂) was excluded (unadjusted) or included (adjusted) with the pO₂ term. Hemodynamic data after inhaling O₂ calculated by both methods were compared. An over 20% decrease in pulmonary vascular resistance with stable blood pressure was considered a positive vasoreactivity testing. During the study period, a total of 136 patients with simple CHDs were enrolled and 129 of them were diagnosed with pulmonary arterial hypertension. In vasoreactivity testing, after adjustment, pulmonary / systemic blood flow ratio decreased from 4.32 ± 5.82 to 2.51 ± 1.67, while pulmonary vascular resistance was increased from 5.62 ± 4.18 WU to 7.04 ± 4.44 WU. The response to O₂ was adjusted to negative in 19 patients. The pO₂-adjusted indirect Fick method incorporates the effect of dissolved O₂ in hemodynamical data calculation, reducing overestimation of pulmonary / systemic blood flow ratio and underestimation of pulmonary vascular resistance in vasoreactivity testing by inhaling O₂ for patients with simple CHDs.</p><p></p>