An up-dated method for determining v̇ a/Q̇ inequalities and direct shunt using 0 2, CO 2 and N 2

Abstract

An attempt was made to determine the contributions of uneven v̇ a/Q̇ distribution and direct shunting to the impairment of gas exchange by alveolar-arterial differences for O 2, CO 2 and N 2 in five surgical patients anesthetized with halothane in 38% O 2 and N 2 (A-a)D0 2 varied from 122 to 146 mm Hg, (a-A)DN 2 from −3 to 55 mm Hg and (a-A)DCO 2 from 1.5 to 9.5 mm Hg. A lung model was constructed for each patient with one low v̇ a/Q̇ compartment, one high v̇ a/Q̇ compartment and a direct right to left shunt. When the low v̇ a/Q̇ compartment had a value less than 0.10, uneven distribution of ventilation and perfusion was the prime cause of the (A-a)DO 2. In two patients whose low v̇ a/Q̇ compartments had values greater than O.20 direct right to left shunting was the predominant cause of impaired gas exchange even though ventilation and perfusion were unevenly distributed. In the fifth patient, direct shunting was the sole cause of the (A-a)DO 2. We suggest the “triple gas difference” as a useful method for separating the effects of v̇ a/Q̇ mismatching from direct shunt.