Limitations of 6-minute walk test-based equations in predicting peak oxygen uptake and risk assessment in patients with pulmonary hypertension and COPD

Background: Peak oxygen uptake (VO₂ peak) is essential for assessing exercise capacity and mortality risk in chronic respiratory diseases. Due to limited accessibility, VO₂ peak is often estimated using prediction equations from the 6-min walk test (6MWT). We evaluate the accuracy of VO₂ peak prediction equations from the 6-min walk distance (6MWD) in patients with pulmonary hypertension (PH) and/or chronic obstructive pulmonary disease (COPD), and risk assessment. Methods: Patients with PAH, COPD with PH (COPD-PH), and COPD without PH were followed at Hospital Clinic, Barcelona, Spain. All underwent the 6MWT and direct VO₂ peak measurement via CPET. Estimated VO₂ peak from 6MWD-based equations was compared with measured values. Reliability and agreement were assessed using intraclass correlation coefficients (ICC) and Bland-Altman analysis. We also evaluated the accuracy of VO₂ peak estimation in identifying high-risk patients. Results: A total of 225 patients participated: 81 with PAH, 64 with COPD-PH, and 80 with COPD without PH. ICC values showed moderate reliability for VO₂ peak prediction in all groups, and despite moderate overall agreement, variability indicated inconsistencies. The equations consistently underestimated mortality risk. Conclusions: Our findings revealed moderate reliability, poor agreement, and a high bias, with 6MWD-based equations overestimating VO₂ peak and underestimating risk classification. While the 6MWT is useful when CPET is unavailable, clinicians should interpret these equations cautiously for risk stratification and decision-making.