Shunting in the lung: A two-compartment model

Several indicator dilution techniques are described for the measure of anatomic shunting. During breath-holding, three of 20 in vitro lungs demonstrated a xenon-133 dilution curve which could be readily separated into a slow and rapid component. The slow component corresponded to the air volume contained in the lung. The rapid component described the anatomic shunt since its transit time was equivalent to the intravascular reference tracer. Secondly, the area described by this component was similar to, although somewhat lower than, the xenon-133 area measured during breathing. The fractional xenon area, or recovery during breathing, was in accord with oxygen measured Q_S Q. Both measurements tended to overestimate anatomic shunting because of the contribution of low v_a q ratios to gas recoveries. Gas solubility was an important factor in determining this error which occurred even when Q_S Q was measured during 100 percent oxygen breathing. In addition, if the lungs functioned as a multicompartmental system, tracer recovery was increased beyond that predicted by a single-compartment model. A two-compartment system is offered as a practical method of measuring anatomic shunting. The use of two gases, xenon-133 and krypton-85, led to calculated shunts which were significantly lower than those measured during inhalation of 100 percent oxygen. The measurements may be done during breathing, without control of the airway.