If both cardiac output and alveolar ventilation are doubled, the net effect on the rate of rise of FA/FI for isoflurane would be

If both cardiac output and alveolar ventilation are doubled, the net effect on the rate of rise of FA/FI for isoflurane would be

• A. Increase
• B. Decrease
• C. No change
• D. Halve

Answer: (A)

The rate at which FA tracks FI depends on two competing processes: delivery of anesthetic to the alveoli via ventilation and removal from the alveoli into the blood via cardiac output. Doubling alveolar ventilation doubles the amount of fresh anesthetic reaching the alveoli each minute, accelerating the rise of FA toward FI. Doubling cardiac output increases the rate at which anesthetic is taken up by the blood, which would slow the rise. For isoflurane, the increase in delivery with higher ventilation dominates the concurrent increase in uptake, so the net effect is a faster rise of FA/FI. In short, faster alveolar delivery outweighs the greater uptake in this scenario, so the rate increases.

The rate at which FA tracks FI depends on two competing processes: delivery of anesthetic to the alveoli via ventilation and removal from the alveoli into the blood via cardiac output. Doubling alveolar ventilation doubles the amount of fresh anesthetic reaching the alveoli each minute, accelerating the rise of FA toward FI. Doubling cardiac output increases the rate at which anesthetic is taken up by the blood, which would slow the rise. For isoflurane, the increase in delivery with higher ventilation dominates the concurrent increase in uptake, so the net effect is a faster rise of FA/FI. In short, faster alveolar delivery outweighs the greater uptake in this scenario, so the rate increases.