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. 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).
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. 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. 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).
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