Dependence of peak oxygen uptake on oxygen transport capacity in chronic heart failure: comparison of graded protocol and fixed protocol

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Abstract

Oxygen transport capacity is the most important determinant of maximum oxygen uptake (VO2max) in healthy subjects, however, its role is controversial in patients with chronic heart failure (CHF). The aim of this study was to clarify whether the oxygen transport capacity is an actual determinant of exercise capacity in CHF patients. Thirteen CHF patients underwent two maximum exercise tests, i.e., a graded protocol test and a fixed protocol test, measuring expiratory gases, leg blood flow (LBF), and arterial and venous blood gases. The workload of the fixed protocol was set to exceed the peak workload obtained by the graded protocol. Exercise with the fixed protocol caused significantly larger peak VO2 compared to the graded protocol (813±194 to 971±203 ml/min, P<0.001). Peak LBF increased by 17%, while the peak leg arterio-venous oxygen difference increased by 5% from the graded protocol to the fixed protocol. The linear correlation between leg venous partial oxygen pressure and peak VO2 was more clearly manifested in the fixed protocol (r=0.60, P<0.05) than in the graded protocol (r=0.47, NS). In conclusion, the exercise with graded protocol did not always conduct the upper limit of oxygen demand/supply relationship in CHF patients, whereas, the fixed protocol with a larger workload produced larger peak VO2 and manifested the mechanism to limit VO2 by oxygen transport capacity.

Introduction

Exercise intolerance is a characteristic manifestation of chronic heart failure (CHF), and is related to the severity [1]and prognosis [2]of CHF patients. Oxygen transport capacity, a part of function of cardiac pump reserve, is the most important determinant of maximal oxygen uptake (VO2max) in normal subjects 3, 4, 5. However, it has been noticed that the exercise capacity assessed by a symptom-limited graded exercise protocol does not correlate with left ventricular function 6, 7in CHF. This observation suggests the hypothesis that the exercise tolerance test with a graded protocol and symptom-limited endpoints is not suitable to approach cardiac reserve in patients with CHF. A protocol which can more easily conduct a maximal work may be useful to examine a mechanism of limited VO2max in CHF patients. This hypothesis can be tested by comparing different exercise protocols. Measurement of leg venous partial oxygen pressure (PO2) during exercise is one of the useful methods for ascertaining the limitation of VO2 by oxygen transport capacity [4], the so-called critical capillary PO2 mechanism 8, 9. In the present study, we measured leg venous PO2 and leg blood flow in two exercise tests, i.e., a graded protocol and a fixed protocol, to clarify whether oxygen transport determines exercise capacity in patients with CHF.

Section snippets

Subjects

Thirteen patients with stable CHF were studied. The average age was 58±9 years. The NYHA functional classification was Class I in four cases, Class II in seven cases and Class III in two cases. Clinical diagnoses were as follows: myocardial infarction in seven cases, idiopathic dilated cardiomyopathy in three cases, and valvular heart disease in three cases (mitral regurgitation in two cases and aortic regurgitation in one case). Their left ventricular ejection fraction was 43.6±9.9%. Left

Comparison of VO2 and systemic hemodynamics in two protocols

Exercise time was 9.0±1.5 min in the graded protocol and 2.2±0.2 min in the fixed protocol. Peak VO2 was significantly larger in the fixed protocol than in the graded protocol (813±194 ml/min versus 971±203 ml/min, P<0.001), as shown in Fig. 1. The %increase in peak VO2 obtained in the fixed protocol amounted to 19% of the peak VO2 obtained by the graded protocol.

The values of the other systemic hemodynamic variables are shown in Table 1. Before exercise, the values of variables did not differ

Discussion

The present study showed that the fixed protocol, which featured a larger workload than the graded protocol, caused a significant increase in peak VO2. Linear correlation between leg venous PO2 and peak VO2 was more clearly manifested in the fixed protocol than in the graded protocol.

We designed the present study to measure peak VO2, LBF and leg venous PO2 in the different types of exercise protocol to examine whether the oxygen transport determines aerobic capacity in CHF patients. The

Limitations of the study

The present study includes several limitations. Non-randomized order of exercise protocols is a factor to cause a bias. Nevertheless, we can exclude a training effect on the peak VO2 in the second test. Because, the familiarization to exercise test had been done before the study, and the peak VO2 values with graded protocol were not significantly different. However, the higher resting HR smaller resting SV and lower resting mPCP at the fixed protocol might be an effect of this bias. Another

Clinical implications

Our results indicate that an adequate exercise protocol is important for measurement of VO2max in patients with chronic heart failure, and the decreased peak VO2 observed in patients with CHF is due not only to an impaired circulatory function, but also to the selection of exercise protocol. Measuring leg venous PO2 can contribute to determine whether exercise tolerance is limited by oxygen transport capacity or not in patients.

Conclusions

The exercise with graded protocol did not always conduct the upper limit of oxygen demand/supply relationship during exercise in CHF patients, whereas, the fixed protocol with a larger workload more clearly manifested the mechanism to limit VO2 by oxygen transport capacity.

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