Apneic Thresholds

The apneic threshold is defined as the highest arterial carbon dioxide tension at which a subject remains apneic. The apneic threshold is approximately 4 or 5 mm Hg less than resting arterial carbon dioxide tension achieved during spontaneous ventilation. There are no significant agent-specific or depth-related effects of ether, halothane, or isoflurane on the relationship between apneic thresholds and resting arterial carbon dioxide tension in humans ( Fig. 6-21 ).^[303] The difference between resting arterial carbon dioxide tension and apneic threshold is not related to the slope of the carbon dioxide response curves or to the level of resting arterial carbon dioxide tension. This suggests that assisted ventilation to a lower arterial carbon dioxide tension less than the apneic threshold during anesthesia ultimately becomes controlled ventilation. Another clinically important aspect of this observation is related to the return of spontaneous ventilation in the mechanically hyperventilated patient. Carbon dioxide stores in the body must accumulate to return the arterial carbon dioxide tension toward the apneic threshold after cessation of mechanical ventilation. The duration of apnea required before the patient commences spontaneous ventilation is proportional to anesthetic depth. Continuation of mechanical ventilation

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.)