In the present series (Part VII) A. V. Hill, C. N. H. Long and H. Lupton* showed an interesting result with normal subjects, on the oxygen intake during exercise while breathing rich oxygen mixtures. They found that there is a considerable rise, reaching 50 per cent., in the maximal oxygen intake when breathing oxygen mixtures, as compared with breathing air; and, further, they calculated under such conditions the work done by the heart. This phenomenon has been re-observed, and some possible errors that might occur in the method are here discussed. The method used was that of the Douglas bag, and the expired air was analysed by the Haldane apparatus. The exercise was “standing-running”. The most serious error that might complicate or affect the results is that due to contamination, by air, of the expired gases. The following simple calculation is sufficient to show the large effect of such contamination by air if it occurred. Suppose that 118 litres of a 50 per cent. oxygen mixture be mixed with 2 litres of air; then calculate how much oxygen has apparently disappeared. The oxygen in 120 litres of the new mixture is 118 × 0·5 + 2 × 0·2 = 59·4 litres, so that the oxygen percentage is now 49·5 per cent. Thus, assuming equality of the inspired and expired nitrogen, apparently about 1 per cent. of oxygen has disappeared,
i.e.
about 1·2 litres of oxygen corresponding to the 120 litres of expired gas. Now assume that a subject records a ventilation of 120 litres per minute and a maximal oxygen intake of 3·8 litres breathing atmospheric air; and that no rise in the oxygen intake actually occurs, when breathing a rich oxygen mixture, but that 120 litres of apparently expired gas now consist of 118 litres of actual expired gas and 2 litres of air leaked in. The result obtained in such an experiment uld give an apparent oxygen intake of 5·0 litres per minute. The following experiments, therefore, were performed with every possible care taken to avoid leakage. The mouthpieces were tested and found to be air-tight, while the Douglas bag and the corrugated rubber tubing were carefully washed with oxygen mixture. Special attention was paid to any possible leakage round the mouth, since the breathing is so heavy that there would appear to be a risk of sucking air inwards.
Muscular exercise, lactic acid, and the supply and utilisation of oxygen. Part X.—The oxygen intake during exercise while breathing mixtures rich in oxygen