Correction for mechanical dead space in the calculation of physiological dead space.

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 CO(2); in the other equation this fraction is [Pe(CO2)/Pa(CO2) (Vt - Vd(an) - Vd(m))](2) where Pe(CO2) is mixed expired Pco(2) 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).