Figure 17-31 In conscious controls (heavy solid line), increasing end-expiratory PCO₂ increases pulmonary minute volume. The dashed line is an extrapolation of the CO₂ response curve to zero ventilation and represents the apneic threshold. An increase in anesthetic (halothane) concentration (end-expiratory concentration) progressively diminishes the slope of the CO₂ response curve and shifts the apneic threshold to a higher PCO₂ . The heavy line interrupted by dots shows the decrease in minute ventilation and the increase in PCO₂ that occur with increasing depth of anesthesia. (Redrawn with modification from Munson ES, Larson CP Jr, Babad AA, et al: The effects of halothane, fluroxene and cyclopropane on ventilation: A comparative study in man. Anesthesiology 27:716, 1966.)