An overall estimate of VA/Q can be obtained by simple arterial blood gas analysis. A three-compartment analysis can be performed by recording the O 2 and CO2 content in arterial and mixed venous blood and mixed expired gas. This enables the calculation of one compartment that is perfused but not ventilated (venous admixture), one compartment that is ventilated but not perfused (dead-space), and an ideal compartment that is both ventilated and perfused ( N..u..n,n l.9.93). However, venous admixture in this three-compartment model includes perfusion not only of non-ventilated lung tissue (true pulmonary shunt), but also of regions which are poorly ventilated or perfused in excess of their ventilation (low VA/Q regions). Similarly, the dead-space compartment includes not only true dead-space (airways plus non-perfused alveoli) but also regions which are ventilated well in excess of their perfusion (high VA/Q regions).
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