Fig. 7. Compartment models for analysis of dead-space effect in tracer washout. (A ) Two-compartment model: A represents the tracer-containing perfused region and B the tracer-free embolized region at the beginning of the washout. (B ) Four-compartment model: A1 represents the perfused compartment that receives tracer and A2, B1, and B2 the tracer-free embolized regions. V̇= minute ventilation; V^D= volume of common dead space; V^DA= volume of regional dead space for compartments A1 and A2; V^DB= volume of regional dead space for compartments B1 and B2.