During inhalational induction, which action would increase the rate at which alveolar concentration rises (FA/FI)?

During inhalational induction, which action would increase the rate at which alveolar concentration rises (FA/FI)?

• A. Increasing fresh gas flow
• B. Decreasing fresh gas flow
• C. Increasing the fresh gas flow to the vaporizer setting
• D. Decreasing the patient's minute ventilation

Answer: (A)

The main idea is that how fast the alveolar concentration (FA) rises depends on how quickly anesthetic gas is delivered to the lungs. Increasing the fresh gas flow delivers more anesthetic per unit time to the alveoli, which raises the inspired concentration in the circuit and in the alveoli more rapidly. This also reduces dilution with room air and washout of exhaled gas, and minimizes rebreathing of already-adsorbed anesthetic, so FA climbs toward FI faster. Decreasing fresh gas flow would slow the rise by delivering less anesthetic per breath, and decreasing minute ventilation would reduce alveolar ventilation, also slowing uptake. The option describing increasing the flow specifically to the vaporizer setting is not as direct a driver as simply increasing the overall fresh gas flow, which is the most reliable way to speed the rise in FA/FI.

The main idea is that how fast the alveolar concentration (FA) rises depends on how quickly anesthetic gas is delivered to the lungs. Increasing the fresh gas flow delivers more anesthetic per unit time to the alveoli, which raises the inspired concentration in the circuit and in the alveoli more rapidly. This also reduces dilution with room air and washout of exhaled gas, and minimizes rebreathing of already-adsorbed anesthetic, so FA climbs toward FI faster.

Decreasing fresh gas flow would slow the rise by delivering less anesthetic per breath, and decreasing minute ventilation would reduce alveolar ventilation, also slowing uptake. The option describing increasing the flow specifically to the vaporizer setting is not as direct a driver as simply increasing the overall fresh gas flow, which is the most reliable way to speed the rise in FA/FI.