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Six-minute walk distance and exercise testing had similar prognostic value for hospitalization and mortality in outpatients with chronic heart failure, according to a head-to-head comparison study.
Using C-index to reflect statistical-model accuracy, the 6-minute walk distance had adjusted values of 0.62 and 0.72 for the composite of all-cause hospitalization/mortality and for all-cause mortality alone, respectively.
With respect to exercise testing, C-index values for peak oxygen consumption (VO2) were 0.61 for all-cause hospitalization/mortality and 0.71 for all-cause mortality. Corresponding values for ventilator equivalents for exhaled carbon dioxide (VE/VCO2) slope were 0.73 and 0.71. Combining information from both parameters did not change the results.
Neither test, alone or in combination, added more than modest discriminatory power to demographic and clinical information, as reported online in the Journal of the American College of Cardiology.
"Although cardiopulmonary exercise testing is often assumed to provide superior function-based prognostic assessment in heart failure patients, we demonstrated that 6-minute walk distance provided prognostic value that was similar to peak VO2, VE/VCO2 slope, and their combination in a relatively stable heart failure population," Daniel Forman, MD, of Brigham and Women's Hospital in Boston, and colleagues wrote.
"These data suggest that a 6-minute walk test may be substituted for cardiopulmonary exercise testing as an inexpensive, practical clinical tool to help gauge prognosis in the large and growing heart failure population," they said.
Cardiopulmonary exercise testing is widely used to evaluate and distinguish the underlying mechanisms of dyspnea and exercise limitation. Additionally, 6-minute walk distance is used as a prognostic parameter for functional status.
Given the lower cost and logistical simplicity of a 6-minute walk test, the latter offers advantages as a prognostic tool for patients with heart failure. Moreover, proponents have argued that the 6-minute walk test provides more relevant information regarding routine activity as compared with bicycle or treadmill exercise, the authors pointed out.
To compare the relative prognostic utility of the two tests, the authors analyzed data for a subgroup of 2,054 participants in HF-ACTION, a randomized trial that evaluated exercise training for patients with systolic heart failure. The trial limited enrollment to ambulatory patients with NYHA class II or III heart function.
All patients included in the analysis had undergone both 6-minute walk test and exercise testing. HF-ACTION had protocol-specified uniform methods for conducting both tests.
Outcomes were the prognostic utility of 6-minute walk distance and the exercise-test outcomes for VO2 and VE/VCO2 slope for all-cause mortality/hospitalization and all-cause mortality. Results were expressed as C- indices, reflecting statistical models' accuracy for discriminating between clinical events and non-events.
The authors found significant correlation between 6-minute walk distance and the exercise-test parameters (P<0.0001). With and without adjustment for covariates, 6-minute walk distance had better correlation with VO2.
Covariates included in adjusted models were height, weight, hospitalizations in the 6-months before baseline, geographic region, NYHA class, age, race, peripheral vascular disease, ECG ventricular abnormality, body mass index, sex, ejection fraction, and diabetes.
After adjustment, correlations between the outcome parameters were substantially weaker, indicating the covariates accounted for a substantial proportion of the unadjusted correlations, the authors noted.
In the unadjusted analysis, 6-minute walk distance was associated with a C-index value of 0.58 for hospitalization/mortality. Corresponding values for peak VO2 and VE/VCO2 were 0.61 and 0.56.
Unadjusted C-index values associated with all-cause mortality were 0.66 for 6-minute walk distance, 0.68 for VO2, and 0.66 for VE/VCO2.
Adjusted models were associated with overlapping confidence intervals and modest integrated discrimination improvement, reflecting similar prognostic discrimination for 6-minute walk distance and the two exercise parameters.
Finally, the authors evaluated models that included only the covariates or covariates combined with the tests results. Adjusted models yielded the following values for all-cause hospitalization/mortality and all-cause mortality, respectively:
- Covariates only: 0.60, 0.69
- With 6-minute walk distance: 0.62, 0.72
- With peak VO2: 0.63, 0.73
- With VE/VCO2 slope: 0.61, 0.71
- With VO2 and VE/VCO2 slope: 0.63, 0.74
- With VO2 and 6-minute walk distance: 0.64, 0.74
- All three: 0.64, 0.74
Likening the results to his wife's request to simplify and get rid of outdated and obsolete clothes in his wardrobe, the author of an accompanying editorial suggested the same might be true for "selecting an exercise test for the 'anticipation of future events' in heart failure patients."
"So, is simple better?" Stanley Rubin, MD, of the University of California Los Angeles, wrote. "If the question is which exercise test ... to include as part of a heart failure research study, then the answer is a qualified 'yes' in favor of the 6-minute walk test. In addition, the 6-minute walk test is useful adjunct for the clinician who is trying to determine the cause of a patient's dyspnea or fatigue, and is also likely useful in charting the progress or the clinical course of some patients who have heart failure or pulmonary arterial hypertension."
"However, predicting adverse events in heart failure ... largely rests on clinical and demographic variables," Ruben cautioned, adding that the authors demonstrated that "prediction of prognosis was little enhanced by adding the results of either the 6-minute walk test or the cardiopulmonary exercise test."
Rubin also suggested that clinical/demographic variables might have performed even better with the inclusion of additional information, such as B-type natriuretic peptide.
From the American Heart Association:
Neither the authors nor Rubin reported any conflicts of interest.
- Reviewed by Robert Jasmer, MD Associate Clinical Professor of Medicine, University of California, San Francisco and Dorothy Caputo, MA, BSN, RN, Nurse Planner
Journal of the American College of CardiologySource Reference: Forman DE, et al "6-minute walk test provides prognostic utility comparable to cardiopulmonary exercise testing in ambulatory outpatients with systolic heart failure" J Am Coll Cardiol 2012; DOI: 10.1016/j.jacc.2012.08.1010.
Journal of the American College of CardiologySource Reference: Rubin SA "Exercise tests in heart failure patients: is simpler better?" J Am Coll Cardiol 2012; DOI: 10.1016/j.jacc.2012.08.1012.