Intermittent positive pressure ventilation transmitted through a variable bypass vaporizer can raise its output concentration above the dial setting, a phenomenon known as the pumping effect. Which design feature counteracts this?

• A A check valve or long spiral baffle between the vaporizing chamber outlet and the vaporizer outlet port ✓
• B A bimetallic temperature compensating strip in the bypass channel
• C A pressure relief valve venting to the scavenging system
• D Wicking material that increases the surface area of liquid agent

Explanation

Back pressure during inspiration compresses the gas in the vaporizing chamber, and on release this enriched gas surges into the outlet, raising delivered concentration. B one way check valve placed at the outlet, or a long tortuous baffle, blocks retrograde flow and pressure transmission into the chamber. Wicks maintain saturation against temperature change, and the bimetallic strip corrects output drift with temperature, neither addresses pressure driven surges.

Reference: Understanding Anesthesia Equipment (Dorsch and Dorsch), 6th ed.

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Written and medically reviewed by the StethoPrep medical team.