The respiratory dead space measured by single breath analysis of oxygen, carbon dioxide, nitrogen or helium.

Fowler ( 1 ) estimated the respiratory dead space in man by recording simultaneously and continuously the nitrogen concentration and the volume flow at the mouth during a single expiration, following a breath of oxygen. Such an expiration may be divided into three phases. In the first part of expiration there is virtually no nitrogen. This is followed by an S-shaped rise in nitrogen concentration as alveolar gas begins to appear. Finally, when the dead space is washed out, the third phase, the 'alveolar plateau' is recorded. By substituting for the S-shaped curve a vertical line which represents a theoretical square front between dead space air and alveolar air, and computing the expired volume up to this front, Fowler obtained an estimate of the dead space for nitrogen, defined as "the volume of the conducting airway down to the location at which a large change in gas composition occurs." The respiratory dead space for oxygen and carbon dioxide is of interest for several reasons. Firstly, the dead space presumably includes upper airways, bronchi and bronchioles. It is possible that there may be an appreciable transfer of oxygen and carbon dioxide between gas and blood in the smaller bronchioles, though not for the relatively insoluble gas nitrogen; in this case, these fine tubes would no longer be "dead space" for oxygen and carbon dioxide, so that the dead space volume for these gases would be smaller than for nitrogen. DuBois, Fowler, Soffer, and Fenn (2) measured the respiratory dead space for carbon dioxide but did not make comparisons of it with that for other gases in their subjects. Pappenheimer, Fishman, and Borrero (3), using an en-