A multicompartment analysis of the lung.

Abstract

Intravascular inert-gas tracers have been employed in a multicompartment lung model to measure the anatomic shunt. This includes blood flow to atelectatic areas, to regions with closed nonventilated airspaces and to regions where there are diffusion blocks to tracer passage. The volume of perfused airspaces without diffusion-block distal to bronchial obstruction has been determined. The gases xenon 133 and krypton 85 were used to measure shunting. Data indicate that this shunt is 30–50% less than that calculated with 100% oxygen breathing. This is related to the effects of low\({{\dot V_A } \mathord{\left/ {\vphantom {{\dot V_A } {\dot Q}}} \right. \kern-\nulldelimiterspace} {\dot Q}}\) ratios in the shunt calculations using 100% oxygen. A system employing two compartments is analysed, and a graphical technique is presented which allows calculation of the ventilation, perfusion and alveolar volume of each compartment. Comparisons are made between single- and 2-compartment analyses, and it is shown that the single-compartment model will underestimate the true\({{\dot V_A } \mathord{\left/ {\vphantom {{\dot V_A } {\dot Q}}} \right. \kern-\nulldelimiterspace} {\dot Q}}\) ratio except in very special cases. Where possible, the results for 2-compartment systems are extended to N compartments. Clinical application of the methods described is facilitated by establishing a steady-state tracer distribution with a constant tracer infusion. Such an equilibrium condition is not required, but reduces the number of tracer sample measurements to four.