Prediction of maximal oxygen uptake from 6-min walk test in pulmonary hypertension

Maximal oxygen uptake (V'_{O max}), assessed by cardiopulmonary exercise testing (CPET), is an important parameter for risk assessment in patients with pulmonary hypertension (PH). However, CPET may not be available for all PH patients. Thus, we aimed to test previously published predictive models of V'_{O max} from the 6-min walk distance (6MWD) for their accuracy and to create a new model. We tested four models (two by Ross et al. (2010), one by Miyamoto et al. (2000) and one by Zapico et al. (2019)). To derive a new model, data were split into a training and testing dataset (70:30) and step-wise linear regression was performed. To compare the different models, the standard error of the estimate (SEE) was calculated and the models graphically compared by Bland-Altman plots. Sensitivity and specificity for correct prediction into low-risk classification (V'_{O max} >15 mL/min/kg) was calculated for all models. A total of 276 observations were included in the analysis (194/82 training/testing dataset); 6MWD and V'_{O max} were significantly correlated (r=0.65, p<0.001). Linear regression showed significant correlation of 6MWD, weight and heart rate response (HRR) with V'_{O max} and the best fitting prediction equation was: V'_{O max} = 1.83 + 0.031 × 6MWD (m) - 0.023 × weight (kg) - 0.015 × HRR (bpm). SEEs for the different models were 3.03, 3.22, 4.36 and 3.08 mL/min/kg for the Ross et al., Miyamoto et al., Zapico et al. models and the new model, respectively. Predicted mean V'_{O max} was 16.5 mL/min/kg (versus observed 16.1 mL/min/kg). 6MWD and V'_{O max} reveal good correlation in all models. However, the accuracy of all models is inadequate for clinical use. Thus, CPET and 6MWD both remain valuable risk assessment tools in the management of PH.