Reduced oxygen uptake increase to work rate increment (ΔVO2/ΔWR) is predictable by VO2 response to constant work rate exercise in patients with chronic heart failure

Patients with reduced peak oxygen uptake ((V) over dot O-2) due to chronic heart failure (CHF) exhibit abnormal (V) over dot O-2 kinetics even during mild to moderate exercise. This is characterized by a reduced ratio of the (V) over dot O-2 increase to the work rate increment (Delta(V) over dot O-2/DeltaWR) during ramp exercise, and by a slow increase in (V) over dot O-2 during constant work rate exercise. Because the slow (V) over dot O-2 kinetics alone is unlikely to explain the reduced Delta(V) over dot/DeltaWR on theoretical grounds, we can postulate that the linearity between work rate and (V) over dot O-2 may be impaired when exercise is imposed in a ramp fashion. The present study was designed to address this issue. In 21 CHF patients and 17 normal controls, we performed both symptom-limited exercise testing (15 W . min(-1) ramp) and a constant work rate exercise test (0 W followed by 50-W step). The (V) over dot O-2 step response was used to mathematically derive the hypothetical ramp response by time integration. Although peak (V) over dot O-2 and work rate were both significantly lower in patients, the attenuation in peak (V) over dot O-2 was more prominent (p<0.05), which could be explained by a lower Delta(V) over dot O-2/DeltaWR in patients compared with controls [8.1 (SD 1.0) and 9.8 (0.5) ml . min(-1) . W-1, p<0.01]. The hypothetical Delta(V) over dot O-2/DeltaWR derived from the (V) over dot O-2 step response was also significantly lower in patients [8.7 (1.0) and 10.0 (0.7) ml . min(-1) . W-1, p<0.01]. The hypothetical and measured Delta(V) over dot O-2/DeltaWR were highly correlated (r=0.78, p<0.001). Thus, we can reasonably predict the (V) over dot O-2 ramp response from the (V) over dot O-2 response to a step increase in work rate, indicating that linearity between (V) over dot O-2 and work rate is held constant irrespective of loading patterns. Additional studies, such as those including evaluation of anaerobic bioenergetics, are needed to further elucidate the precise mechanism(s) of this phenomenon.