Reduced oxygen uptake increase to work rate increment (Δ${\dot{V}O_{2} }$/ΔWR) is predictable by${\dot{V}O_{2} }$ response to constant work rate exercise in patients with chronic heart failure

Abstract

Patients with reduced peak oxygen uptake (${\dot{V}O_{2} }$) due to chronic heart failure (CHF) exhibit abnormal${\dot{V}O_{2} }$ kinetics even during mild to moderate exercise. This is characterized by a reduced ratio of the${\dot{V}O_{2} }$ increase to the work rate increment (Δ${\dot{V}O_{2} }$/ΔWR) during ramp exercise, and by a slow increase in${\dot{V}O_{2} }$during constant work rate exercise. Because the slow${\dot{V}O_{2} }$ kinetics alone is unlikely to explain the reduced Δ${\dot{V}O_{2} }$/ΔWR on theoretical grounds, we can postulate that the linearity between work rate and${\dot{V}O_{2} }$ 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${\dot{V}O_{2} }$ step response was used to mathematically derive the hypothetical${\dot{V}O_{2} }$ ramp response by time integration. Although peak${\dot{V}O_{2} }$ and work rate were both significantly lower in patients, the attenuation in peak${\dot{V}O_{2} }$ was more prominent (p<0.05), which could be explained by a lower Δ${\dot{V}O_{2} }$/ΔWR 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 Δ${\dot{V}O_{2} }$/ΔWR derived from the${\dot{V}O_{2} }$ 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 Δ${\dot{V}O_{2} }$/ΔWR were highly correlated (r=0.78, p<0.001). Thus, we can reasonably predict the${\dot{V}O_{2} }$ ramp response from the${\dot{V}O_{2} }$ response to a step increase in work rate, indicating that linearity between${\dot{V}O_{2} }$ 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.