This record can be used to calculate dead space, as discussed in the text.Īnatomic Versus Physiologic Dead Space. Record of the changes in nitrogen concentration in the expired air after a single previous inspiration of pure oxygen. In making this measurement, the subject suddenly takes a deep breath of oxygen.This Air expired (ml) A simple method for measuring dead space volume is demonstrated by the graph in Figure 37-7. Therefore, the dead space is very disadvantageous for removing the expiratory gases from the lungs. On expiration, the air in the dead space is expired first, before any of the air from the alveoli reaches the atmosphere. This air is called dead space air because it is not useful for gas exchange. The second equation requires an estimated Vd an and is applicable when Pa CO 2 is not measured or does not plateau (as in exercise).Some of the air a person breathes never reaches the gas exchange areas but simply fills respiratory passages where gas exchange does not occur, such as the nose, pharynx, and trachea. This fraction is ( Pa CO 2/Pa CO 2) 2, where Pa CO 2 and Pa CO 2 are the mean partial pressures of expired alveolar and of arterial CO 2 in the other equation this fraction is 2 where Pe CO 2 is mixed expired Pco 2 and Vd an is anatomical dead space. 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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