Background: Peak oxygen consumption ((V) over dot o(2)peak) is a strong independent predictor of prognosis in patients with severe chronic heart failure (CHF) and is used to guide optimal timing of transplantation. However, its assessment is relatively expensive and time-consuming and requires sophisticated equipment and highly trained personnel. The purpose of this study was to determine whether changes in 6-minute walk test (6WT) distance, a simple, inexpensive potential alternative measure of functional capacity, can predict changes in (V) over dot o(2)peak in patients with severe CHF.Methods: Sixteen subjects (ejection fraction 23 +/- 2%, (V) over dot o(2)peak 16.2 +/- 1.1 ml kg(-1) min(-1)) underwent repeated 6WT and (V) over dot o(2)peak assessments that included familiarization and 4 serial measures, 6 weeks apart (baseline and at Weeks 6, 12 and 18). Analysis compared baseline performance with each subsequent testing occasion.Results: At baseline, mean (+/- SE) (V) over dot O(2)peak was 16.3 +/- 1.1 ml kg(-1) min(-1) and 6WT distance was 458 +/- 21 m. 6WT and (V) over doto(2)peak were strongly correlated at all timepoints (average r = 0.82; all p < 0.05). However, mixed model analysis, assessing the capacity of the changes seen in 6WT to predict changes in (V) over dot(2)peak, showed no statistical significance (F = 0.11; p = 0.74).Conclusions: The 6WT is commonly used to assess functional capacity in patients with heart failure. This study demonstrates that, despite a strong cross-sectional correlation with (V) over doto(2)peak, changes in the 6WT are not a reliable predictor of changes in (V) over dot o(2)peak within patients. Therefore, the 6WT has limited utility as a serial measure to assess changes in the clinical status of patients with severe heart failure.