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

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

doi:10.1172/JCI107097

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

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

Published October 1, 1972 - More info

View PDF

Abstract

When physiological dead space (Vd_p) 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 (Vd_m) appears in the measured dead space (Vd_n). Under these conditions the traditional subtraction of Vd_m from Vd_n leads to underestimation of Vd_p and can give a falsely small ratio of Vd_p to tidal volume (Vt) when, in fact, an abnormally large Vd_p/Vt exists. To make the proper correction for Vd_m, two equations have been derived and validated with seven subjects having Vd_p/Vt from 0.29 to 0.87, using Vd_m's from 120 to 322 ml. With only a small modification, these equations are suitable for routine clinical use and give Vd_p/Vt within 0.02 of that by the validated equations (32 of 33 comparisons). The fraction of Vd_m subtracted from Vd_n is the square of the ratio of effective alveolar to total alveolar ventilation and is never > 1. This fraction is (Pa_CO₂/Pa_CO₂)², where Pa_CO₂ and Pa_CO₂ are the mean partial pressures of expired alveolar and of arterial CO₂; in the other equation this fraction is [Pe_CO₂/Pa_CO₂ (Vt — Vd_an — Vd_m)]² where Pe_CO₂ is mixed expired Pco₂ and Vd_an is anatomical dead space. The second equation requires an estimated Vd_an and is applicable when Pa_CO₂ is not measured or does not plateau (as in exercise).