Indirect Determination of Respiratory Dead Space and Mean Alveolar Air Composition

¹ From the Department of Human Physiology, University of Milan, Milan, Italy

A method is described of determining respiratory dead space and the composition of alveolar air from pulmonary ventilation and CO₂ production (or O₂ consumption) data obtained at different metabolic levels. This method is based on the assumption a) that alveolar CO₂ is constant when CO₂ production is increased or CO₂ is added to inspired air and b) that dead space is constant; then tidal volume is linearly related to the CO₂ content of a single breath. These assumptions are valid only for a limited increase of ventilation. From data obtained at four metabolic levels the standard error for dead space is <10%, for the composition of alveolar air <3%. Respiratory dead space so determined is ‘functional,’ i.e. the volume of air passages in which there are no gas exchanges with blood. It appears to have a constant value up to moderate levels of pulmonary ventilation. The composition of alveolar air is the mean resulting from the mixing of air coming from all the single alveoli.