Fig. 1. Curve A represents the normal carbon dioxide response of an awake individual; the “hockey stick” appearance at low values of Paco2corresponds to the observation that following hyperventilation, awake individuals do not become apneic but rather show a modest decrease in VEuntil Paco2returns to its resting value. Curve B represents the carbon dioxide response curve following administration of a sedative or anesthetic medication, which decreases its slope by 50%. Note that the curve no longer has a hockey stick shape but rather falls linearly to a VEof 0 (the apneic threshold). Once apnea develops, the Pco2must increase to approximately the resting value before ventilation restarts, accounting for the hysteresis loop (line B ). Curve C represents the carbon dioxide excretion hyperbola, which depends on the principle of conservation of mass: Assuming constant carbon dioxide production, increasing VEwill decrease Paco2, whereas decreasing VEtends to increase Paco2. In the awake state, point X (the intersection of carbon dioxide response curve A with carbon dioxide excretion hyperbola C) defines the resting Paco2and VE, whereas point Y represents the values of Paco2and VEduring sedation or anesthesia.