The major function of the lung is to exchange respiratory gases (O2 and CO2). Arterial PO2 and PCO2 are the directly measurable endpoint variables used in practice to interpret respiratory failure properly. The calculated variables P(A-a)O2 (difference between alveolar and arterial PO2), venous admixture ratio, and physiological dead-space, which are viewed as simple functional markers of the overall status of pulmonary gas exchange, are also used. However, these indices consider not only

the state of the lung, i.e. ventilation-perfusion ( VAIQ) mismatching, intrapulmonary shunt, and limitation on 02 diffusion, but also the external conditions under which

the lung is operating. The components that distinctively determine P02 and PC02 in any single gas exchange unit of the lung are the VktQ ratio, the composition of the inspired gas, and the composition of the mixed venous blood.

The multiple inert gas elimination technique gives both quantitative and qualitative estimates of the distributions of VpJQ ratios, without producing changes in either

the pulmonary vascular caliber or the airway tone. In addition, it aids in the interpretation of the complex interplay between the intrapulmonary components ( VpJQ imbalance, intrapulmonary shunt, and O2 diffusion limitation) and extrapulmonary components (inspired O 2 concentration, overall ventilation, cardiac output, and O 2 consumption) of gas exchange.

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