The alveolar rate of rise toward the inspired concentration (Fa/F1) is accelerated by an increase in alveolar ventilation from 2 to 4 and from 4 to 8 liters per minute (constant cardiac output). The increase is greatest with the more soluble agent, halothane, and smaller with the least soluble anesthetic, nitrous oxide. (Reprinted with permission from Eger EI II [ed.]. Anesthetic Uptake and Action. Baltimore: Williams & Wilkins, 1974.)
Figure 25.5. An explanation is that when high inspired tensions of anesthetics are used, particularly if they are highly soluble, a large uptake from the alveoli will occur. Consequently, the lung volume may tend to shrink, causing negative pressure. However, the shrinkage is opposed by the pulling in of fresh gases from nonrespi-ratory conducting airway passages between inspirations, thus effectively increasing the total ventilation. Since greater uptake will occur with 75% N2O than with 40%, the effect will be greater at higher inspired anesthetic tensions.
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