CO2 levels and pulmonary shunting in anesthetized man.

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Abstract
Eighteen anesthetized patients undergoing nonthoracic surgical procedures had controlled hyperventilation with gas mixtures containing about 40% or 99% O2. When arterial CO2 tension was increased toward normal levels by adding CO2 to inspired gas, and again reduced with CO2-free mixtures, arterial O2 tension also usually increased and decreased concomitantly. Similar changes of CO2 levels were induced in 11 other patients in whom the effect on cardiac output was measured and the extent of right-left pulmonary shunting was estimated. With initial hypocapnia, shunting exceeded 8%. When cardiac output was unchanged with increased CO2 levels, increased arterial O2 tension reflected decrease of shunting. Change of cardiac output was accompanied by similarly directed change of shunting, interacting with and at times overriding the apparent "direct" effect of CO2. The mechanisms by which CO2 levels affect shunting are obscure.