Estimating maximum work rate during cardiopulmonary exercise testing from the six-minute walk distance in patients with heart failure

Abstract

Background: Exercise is recommended for patients with chronic heart failure (CHF) and its intensity is usually set as a percentage of the maximal work rate (MWR) during cardiopulmonary exercise testing (CPX) or a symptom-limited incremental test (SLIT). As these tests are not always available in cardiac rehabilitation due to logistic/cost constraints, we aimed to develop a predictive model to estimate MWR at CPX (estMWR@CPX) in CHF patients using anthropometric and clinical measures and the 6-min walk test (6 MWT), the most widely used exercise field test.

Methods: This is a multicentre cross-sectional retrospective study in a cardiac rehabilitation setting. Six hundred patients with HF in New York Heart Association (NYHA) functional class I-III underwent both CPX and 6 MWT and, through multivariable linear regression analysis, we defined several predictive models to define estMWR@CPX.

Results: The best model included 6 MWT, sex, age, weight, NYHA class, left ventricular ejection fraction (LVEF), smoking status and chronic obstructive pulmonary disease COPD (adjusted R² = 0.55; 95% LoA -39 to 33 W). When LVEF was excluded as a predictor, the resulting model performed only slightly worse (adjusted R² = 0.54; 95% LoA -42 to 34 W). Only in 34% of cases was the percentage difference between estMWR@CPX and real MWR@CPX <10% in absolute value. EstMWR@CPX tended to overestimate low values and underestimate high values of true MWR@CPX.

Conclusions: Our results showed a lack of accuracy in the predictive model evaluated; therefore, for an accurate prescription of cycle-ergometer exercise training, it is necessary to assess MWR by CPX or SLIT.

Keywords: 6-Min walking test; Cardiopulmonary exercise testing; Chronic heart failure.

Fig. 1

Results for model: estMWR@CPX = 17.57 + 0.14*6 MWT +11.50*sex - 0.30*age +0.53*weight - 21.01*NYHA2 - 33.43*NYHA3 - 5.95*smoking - 6.17*COPD. Legend: Top panel: Bland-Altman plot (difference between the mean values of estimated and true MWR@CPX). Bottom left panel: graphical representation of the effect of the considered predictors on estimated MWR@CPX. Diamonds represent the expected average change in estMWR@CPX moving from one extreme value to the other of each variable in the model, adjusting for all other variables. The lines around the circles represent the 95% confidence interval: crossing the zero effect line indicates a lack of significance. Bottom right panel: plot of the residuals as a function of predicted values. Sex: 1 male, 0 female; NYHA2: 1 if NYHA class = 2, 0 otherwise; NYHA3: 1 if NYHA class = 3, 0 otherwise; smoking: 0 non-smoker, 1 ex-smoker, 2 smoker; COPD: 0 no COPD, 1 COPD.

Fig. 2

Histogram showing the distribution of patients according to the percentage difference (E) between estimated and true MWR@CPX.