Responses to constant work exercise in patients with chronic heart failure

Objective-To describe the kinetics of metabolic gas exchange at the onset a nd offset of low level, constant work exercise in patients with chronic hea rt 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 me tabolic gas exchange measurements, and a fixed load exercise test at 25 wat ts with metabolic gas exchange measurements before, during, and after the t est. A monoexponential curve was fitted to the data to describe the kinetic s of gas exchange at onset and offset of fixed load exercise. Outcome measures-Peak oxygen consumption; time constants of onset and offse t for metabolic gas exchange variables during constant load exercise. Results-Peak oxygen consumption (mean (SD)) was higher in controls (26.1 (4 .3) upsilon 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 upsilon 8.6 (1.6) in patients). The time constan t of onset was the same in each group, but the time constant of offset was longer in patients (1.29 (0.14) upsilon 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 rel ated to the reduction in exercise capacity. A patient may spend a greater p ortion 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.