Although peak oxygen consumption is reduced in patients with symptomatic heart failure, the degree of limitation during routine activity often appears greater or lesser than expected from peak capacity. This study was undertaken to determine whether abnormalities could be detected during the initiation of steady-state low-level exercise, approximating routine activity, which were distinct from limitation in peak capacity. We sought to determine whether a delay in the integrated response to the increased metabolic demand caused by exercise, assessed by the oxygen deficit incurred between exercise initiation and the achievement of steady-state oxygen uptake, was present in heart failure. Low-level and symptom-limited maximal exercise tests in 33 ambulatory patients with heart failure were analyzed and compared with 9 tests in control subjects. Oxygen deficit was determined during the 8-minute low-level exercise at 20 W. During maximal exercise, as expected, patients with heart failure had lower peak oxygen uptake (16 +/- 4 vs 31 +/- 6 ml/kg/min) and anaerobic threshold (870 +/- 270 vs 1,180 +/- 370 ml/min) than controls. After 8 minutes of low-level exercise, the oxygen deficit was higher in patients than in controls (550 +/- 230 vs 270 +/- 100 ml, respectively, p < 0.01). Among patients with heart failure, the oxygen deficit did not correlate with peak oxygen uptake or anaerobic threshold. Although the time to steady-state heart rate was later in patients with heart failure, this delay did not correlate with oxygen deficit. The oxygen deficit represents a distinct abnormality of exercise response that may reflect impaired central and peripheral responses to the initiation of exercise.