The dial of an isoflurane-specific, variable bypass, temperature-compensated, flowover, out-of-circuit vaporizer set on 2% yields 2% output at 700 mL/min. When the flowmeter is set to 100 mL/min and 15 L/min (dial still 2%), how will the outputs compare?

The dial of an isoflurane-specific, variable bypass, temperature-compensated, flowover, out-of-circuit vaporizer set on 2% yields 2% output at 700 mL/min. When the flowmeter is set to 100 mL/min and 15 L/min (dial still 2%), how will the outputs compare?

• A. Output will be less than 2% in both cases
• B. Output will be greater than 2% in both cases
• C. Output will be 2% at 100 mL/min and less than 2% at 15 L/min
• D. Output will be 2% in both cases

Answer: (A)

The main idea is that a variable bypass, flow-over vaporizer delivers a set concentration by mixing vaporized anesthetic with bypass gas, and this delivery is calibrated for a certain range of fresh gas flow. Even though the dial indicates a target percentage, actual output stays close to that target only within the vaporizer’s designed flow window. When you push the flow far below or far above that window, the device can’t maintain the same concentration. Here, setting the dial to 2% yields about 2% output at 700 mL/min, which sits in the intended range. Dropping the flow to 100 mL/min is far below the design range, so there isn’t enough carrier gas to carry and deliver the anesthetic at the dial’s rate—output falls below 2%. Pushing the flow to 15 L/min is well above the design range; the gas moves through too quickly for the vaporizer to saturate it fully, so the delivered concentration again drops below 2%. Thus, the outputs will be less than 2% in both the low-flow and high-flow cases.

The main idea is that a variable bypass, flow-over vaporizer delivers a set concentration by mixing vaporized anesthetic with bypass gas, and this delivery is calibrated for a certain range of fresh gas flow. Even though the dial indicates a target percentage, actual output stays close to that target only within the vaporizer’s designed flow window. When you push the flow far below or far above that window, the device can’t maintain the same concentration.

Here, setting the dial to 2% yields about 2% output at 700 mL/min, which sits in the intended range. Dropping the flow to 100 mL/min is far below the design range, so there isn’t enough carrier gas to carry and deliver the anesthetic at the dial’s rate—output falls below 2%. Pushing the flow to 15 L/min is well above the design range; the gas moves through too quickly for the vaporizer to saturate it fully, so the delivered concentration again drops below 2%.

Thus, the outputs will be less than 2% in both the low-flow and high-flow cases.