A volatile anesthetic has a saturated vapor pressure of 360 mm Hg at room temperature. At what carrier-gas flow would this agent be delivered from a bubble-through vaporizer if the carrier-gas flow through the vaporizing chamber is 100 mL/min?

A volatile anesthetic has a saturated vapor pressure of 360 mm Hg at room temperature. At what carrier-gas flow would this agent be delivered from a bubble-through vaporizer if the carrier-gas flow through the vaporizing chamber is 100 mL/min?

• A. 30 mL/min
• B. 60 mL/min
• C. 90 mL/min
• D. 120 mL/min

Answer: (C)

In a bubble-through vaporizer, the vapor leaving with the carrier gas reaches an equilibrium where the partial pressure of the anesthetic in the gas equals its saturated vapor pressure. That means the fraction of the total gas that is anesthetic in the output is the saturated vapor pressure divided by atmospheric pressure. Here, P_sat = 360 mm Hg and P_atm ≈ 760 mm Hg, so the fraction f of the total output that is anesthetic is 360/760 ≈ 0.474. The remaining fraction (1 − f) ≈ 0.526 is carrier gas. If the carrier-gas flow through the chamber is 100 mL/min, this is the portion of the total output that is not anesthetic, so 100 = (1 − f) × Q_out. Solve for the total output Q_out ≈ 100 / 0.526 ≈ 190 mL/min. The flow of anesthetic vapor delivered is f × Q_out ≈ 0.474 × 190 ≈ 90 mL/min. So the agent would be delivered at about 90 mL/min.

In a bubble-through vaporizer, the vapor leaving with the carrier gas reaches an equilibrium where the partial pressure of the anesthetic in the gas equals its saturated vapor pressure. That means the fraction of the total gas that is anesthetic in the output is the saturated vapor pressure divided by atmospheric pressure.

Here, P_sat = 360 mm Hg and P_atm ≈ 760 mm Hg, so the fraction f of the total output that is anesthetic is 360/760 ≈ 0.474. The remaining fraction (1 − f) ≈ 0.526 is carrier gas.

If the carrier-gas flow through the chamber is 100 mL/min, this is the portion of the total output that is not anesthetic, so 100 = (1 − f) × Q_out. Solve for the total output Q_out ≈ 100 / 0.526 ≈ 190 mL/min. The flow of anesthetic vapor delivered is f × Q_out ≈ 0.474 × 190 ≈ 90 mL/min.

So the agent would be delivered at about 90 mL/min.