1. Breath-to-breath oxygen uptake at the alveolar capillary membrane has been measured by an improved method during exercise in thirty-two patients with valvular heart disease and twenty-four control subjects. Exercise was of up to 3 min duration on a treadmill usually at 3–4 m.p.h.; the gradient was increased on successive walks up to the breaking point.

2. In the normal subjects the oxygen uptake over the last 0·3 min up to the breaking point (V̇o2 bp) was on average higher than the maximum predicted by the method of Åstrand. In two subjects V̇o2 bp was also higher than that obtained by the conventional method. Thus it is believed to be an approximation of maximal oxygen uptake. In two thirds of the patients the V̇o2 bp was reduced compared with the control subjects.

3. The curves relating oxygen uptake to time for exercise of increasing severity showed poor separation in thirteen of the patients compared with the other subjects studied. This finding is evidence for a defect in oxygen uptake during submaximal exercise in some cardiac patients which is identifiable by physiological means without recourse to clinical data.

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