1. We have examined the effects of (a) reducing the density of the inspired gas and of (b) increasing inspired oxygen concentration on the exercise performance of eight men with chronic obstructive bronchitis (COB).

2. Each subject performed two types of exercise test breathing three different gas mixtures: air, 35% oxygen in nitrogen and 21% oxygen in helium. The exercise capacity, ventilation and heart rate were recorded. Indices of pulmonary mechanics were measured to determine where and how any change in ventilatory capacity might have been achieved.

3. (a) Effort tolerance at a constant load (70% capacity breathing air) was greater when the inspired gas mixture was 35% oxygen in nitrogen than when air or 21% oxygen in helium was breathed.

(b) Peak expiratory flow (PEF) was greater when the helium mixture was breathed, but the forced expiratory volume in 1 s (FEV1) and maximum expiratory flow at 50% vital capacity (MEF50%) were the same as when breathing air. Effort tolerance was the same breathing the helium mixture and air in both the progressive and constant load tests.

(c) PEF, FEV and MEF50% were greater immediately after than before exercise on all gas mixtures.

4. It is concluded that a reduction in the density of the inspired gas does not improve effort tolerance in patients with COB. Possible reasons for this lack of benefit are discussed.

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