On the fluctuation in the composition of the alveolar air during the respiratory cycle in muscular exercise

THE measurement of the circulation rate in man has been attempted by various methods, none of which has come into general use. The problem is important in physiology, and possibly more so in medicine, where physiological explanations are sought for the phenomena of disease. For that reason, among others, the recently devised ethyl iodide method of measuring the circulation rate published by Yandell Henderson(5) in 1925, has attracted considerable attention. In this method the subject breathes a certain concentration of ethyl iodide vapour for a given time, during which the amount of ethyl iodide taken up by his blood and the concentration of the vapour present in his alveolar air are determined; from these results, given a knowledge of the partition coefficient of ethyl iodide between blood and air, the circulation rate can be calculated. In such an experiment, the reliability of the method of sampling alveolar air is obviously of fundamental importance. Yandell Henderson uses an automatic modification of Krogh's Copenhagen method, whereby in successive expirations small samples are taken from the last portions of air expired, and it is assumed that the composition of the mixed sample obtained in this way is the same as the composition of alveolar air. Alveolar air, however, varies in composition during the respiratory cycle; in expiration its CO2 increases and its oxygen decreases, while the opposite changes take place in inspiration. For calculation of the circulation rate it is necessary to know the average alveolar composition throughout the experimental period, which comprises many respiratory cycles. How far the composition of Yan d ell H end e r s o n's sample approximates to that average is not clear. The fractions of his sample are withdrawn from the alveoli at the same phase of successive respiratory cycles; that phase may coincide with the moment at which