If cardiac output and alveolar ventilation are doubled, the effect on the rate of rise of FA/FI for isoflurane compared with that which existed immediately before these interventions would be

If cardiac output and alveolar ventilation are doubled, the effect on the rate of rise of FA/FI for isoflurane compared with that which existed immediately before these interventions would be

• A. Doubled
• B. Somewhat increased
• C. Unchanged
• D. Somewhat decreased

Answer: (B)

The rate at which FA/FI for an inhaled anesthetic rises depends on two opposing forces: how quickly fresh anesthetic reaches the alveoli (driven by alveolar ventilation) and how quickly it is removed from the alveolar gas into the blood (driven by tissue uptake and cardiac output). When alveolar ventilation is doubled, delivery to the alveoli increases, so FA climbs toward FI more rapidly. When cardiac output is doubled, more anesthetic is taken up by the blood and tissues, which tends to slow the rise of FA. Isoflurane has moderate blood solubility, so increasing alveolar ventilation still accelerates the rise somewhat even though uptake into blood also increases. The result is a net effect that is greater than before, but not doubled or unchanged. Hence the rate of rise is somewhat increased compared with the baseline.

The rate at which FA/FI for an inhaled anesthetic rises depends on two opposing forces: how quickly fresh anesthetic reaches the alveoli (driven by alveolar ventilation) and how quickly it is removed from the alveolar gas into the blood (driven by tissue uptake and cardiac output). When alveolar ventilation is doubled, delivery to the alveoli increases, so FA climbs toward FI more rapidly. When cardiac output is doubled, more anesthetic is taken up by the blood and tissues, which tends to slow the rise of FA.

Isoflurane has moderate blood solubility, so increasing alveolar ventilation still accelerates the rise somewhat even though uptake into blood also increases. The result is a net effect that is greater than before, but not doubled or unchanged. Hence the rate of rise is somewhat increased compared with the baseline.