Factors Affecting the Pulmonary Dead Space as Determined by Single Breath Analysis

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Abstract
The pulmonary dead space was measured in a normal subject by the single breath method of Fowler, making use of continuous simultaneous recordings of expired volume and CO2 concentration. Experimental data included, for each dead space measurement, the end-inspiratory values of lung volume and intraesophageal pressure, tidal volume and time of breath holding. Dead space (Vd) increased with increasing end-inspiratory lung volume (Vl). It did not change significantly with tidal volume unless Vl was allowed to change. The relationship between Vd and Vl was not linear (e.g. Vd = 130 ml at Vl = 3.2 L, 190 ml at 6.0 L and 245 ml at 7.7 L). The relationship between Vd and the esophageal-mouth pressure differences (Pe – Pm) was very nearly linear. (Vd = 120 – 3.46 (Pe – Pm); S.D. 10 ml). The volume distensibility of the total dead space was only a little less, on a percentage basis, than that of the alveoli. Since Vd becomes essentially the same for all lung volumes after 20 seconds of breath holding, it seems likely that most of the distensibility of the dead space resides in the small conducting airways. If this is true, the percentage volume distensibility of these airways must be nearly equal to that of the alveoli. Submitted on March 14, 1957

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