Figure 6-21 Ventilatory responses to increased carbon dioxide and apneic thresholds during ether, isoflurane, and halothane anesthesia in patients. The apneic threshold had a relatively fixed relationship to the resting PaCO₂ . With an increase in ventilatory depression at an increasing depth of anesthesia, resting PaCO₂ and apneic threshold increased by approximately the same amount. (From Hickey RF, Fourcade HE, Eger EI, et al: The effects of ether, halothane and Forane on apneic threshold in man. Anesthesiology 35:32, 1971.)