Correction for Mechanical Dead Space in the Calculation of Physiological Dead Space

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Abstract

When physiological dead space (Vdp) is calculated for a patient who has alveolar dead space, e.g., after pulmonary vascular occlusion, less than the full volume of attached mechanical dead space (Vdm) appears in the measured dead space (Vdn). Under these conditions the traditional subtraction of Vdm from Vdn leads to underestimation of Vdp and can give a falsely small ratio of Vdp to tidal volume (Vt) when, in fact, an abnormally large Vdp/Vt exists. To make the proper correction for Vdm, two equations have been derived and validated with seven subjects having Vdp/Vt from 0.29 to 0.87, using Vdm's from 120 to 322 ml. With only a small modification, these equations are suitable for routine clinical use and give Vdp/Vt within 0.02 of that by the validated equations (32 of 33 comparisons). The fraction of Vdm subtracted from Vdn is the square of the ratio of effective alveolar to total alveolar ventilation and is never > 1. This fraction is (PaCO2/PaCO2)2, where PaCO2 and PaCO2 are the mean partial pressures of expired alveolar and of arterial CO2; in the other equation this fraction is [PeCO2/PaCO2 (Vt — Vdan — Vdm)]2 where PeCO2 is mixed expired Pco2 and Vdan is anatomical dead space. The second equation requires an estimated Vdan and is applicable when PaCO2 is not measured or does not plateau (as in exercise).

Authors

Gloria J. Singleton, C. Robert Olsen, Richard L. Smith