Also shown are P aCO 2, the partial pressure of venous carbon dioxide (P vCO 2), the relationship between P aCO 2 and minute CO 2 production (V̇ CO 2) and alveolar gas volume, the partial pressure of mixed alveolar carbon dioxide (P ACO 2), the partial pressure of end-tidal carbon dioxide (P ETCO 2), and the P ĒCO 2 in relation to the model and the volumetric capnogram. The dead space fraction is equal to V Dphys divided by tidal volume (V T). The dead space in an average adult has been reported to be 150 cc or 2cc/kg ideal body weight. The sum of the regions of alveolar dead space (V Dalv) and anatomic dead space (V Danat) equal the physiologic dead space (V Dphys). Anatomical dead space is thus defined as the volume of the conducting zone (Figure 1). The pure dead-space compartment represents areas of ventilation with no perfusion. The pure shunt compartment represents areas of perfusion without ventilation. The ideal compartment represents areas of perfect alveolar gas volume to Q̇ T matching. ![]() The 3-compartment lung model described by Riley and Cournand 151, 152 represents gas exchange in the lung in regard to the matching of alveolar gas volume (V̇ A) and perfusion (Q̇ T), shunt (Q̇ S), and dead space (V D).
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