Abstract
OBJECTIVE—To describe the kinetics of metabolic gas exchange at the onset and offset of low level, constant work exercise in patients with chronic heart failure. SETTING—Tertiary referral centre for cardiology. PATIENTS—10 patients with chronic heart failure and 10 age matched controls. METHODS—Each subject undertook maximum incremental exercise testing with metabolic gas exchange measurements, and a fixed load exercise test at 25 watts with metabolic gas exchange measurements before, during, and after the test. A monoexponential curve was fitted to the data to describe the kinetics of gas exchange at onset and offset of fixed load exercise. OUTCOME MEASURES—Peak oxygen consumption; time constants of onset and offset for metabolic gas exchange variables during constant load exercise. RESULTS—Peak oxygen consumption (mean (SD)) was higher in controls (26.1 (4.3) v 15.3 (5.3) ml/kg/min; p < 0.001) than in heart failure patients. Oxygen consumption during steady state was the same in both groups (9.2 (1.8) ml/kg/min in controls v 8.6 (1.6) in patients). The time constant of onset was the same in each group, but the time constant of offset was longer in patients (1.29 (0.14) v 0.82 (0.07); p < 0.005). There was a relation between peak oxygen consumption and time constant of offset (R = 0.56; p < 0.001). CONCLUSIONS—The dynamics of gas exchange at the onset of low level exercise are normal in heart failure, but the recovery is delayed. The delay is related to the reduction in exercise capacity. A patient may spend a greater portion of the day recovering from exercise, and may not begin the next bout from a position of true recovery, perhaps contributing to the sensation of fatigue. Keywords: chronic heart failure; metabolic gas exchange; constant load exercise
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Figure 1 .
The relation between oxygen deficit and debt shown schematically. At the onset of steady state exercise, oxygen consumption increases in an exponential fashion to reach a plateau. The gap between the eventual plateau and the level seen during the incremental period is the oxygen deficit. The oxygen debt is the oxygen consumption in excess of the baseline levels immediately at the end of exercise.
Figure 2 .
The relation between oxygen deficit and oxygen debt.
Figure 3 .
The relation between the time constant of offset at the end of exercise (τ O2 ) and peak oxygen consumption.
Figure 4 .
The relation between peak oxygen consumption (V̇O2 ) and the ventilatory equivalent for CO2 (VEq CO2 ) during steady state exercise.
Selected References
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