An old enflurane vaporizer with a vapor pressure of 170 mmHg is used. Which volatile would deliver an output when the dial setting equals the vaporizer output?

An old enflurane vaporizer with a vapor pressure of 170 mmHg is used. Which volatile would deliver an output when the dial setting equals the vaporizer output?

• A. A Halothane
• B. B Sevoflurane
• C. C Isoflurane
• D. D Desflurane

Answer: (B)

The key idea is how a variable bypass vaporizer sets output. The dial controls how much saturated vapor is returned to the carrier gas, and the actual amount delivered depends on the agent’s saturated vapor pressure at the vaporizer’s temperature. If you use a vaporizer designed around a certain vapor pressure (like the old enflurane unit set to about 170 mmHg), the agent that will give you an output that matches the dial setting is the one whose saturated vapor pressure is close to that value. Among the common agents listed, sevoflurane has a saturated vapor pressure around 157–160 mmHg, which is closest to 170 mmHg. Halothane and isoflurane have higher pressures (roughly 238–244 mmHg), and desflurane has a much higher pressure (about 669 mmHg). Because their vapor pressures are quite different from the 170 mmHg design point, they wouldn’t produce an output that lines up with the dial setting as neatly as sevoflurane would. So, sevoflurane is the best match for delivering an output when the dial is set to the vaporizer’s output.

The key idea is how a variable bypass vaporizer sets output. The dial controls how much saturated vapor is returned to the carrier gas, and the actual amount delivered depends on the agent’s saturated vapor pressure at the vaporizer’s temperature. If you use a vaporizer designed around a certain vapor pressure (like the old enflurane unit set to about 170 mmHg), the agent that will give you an output that matches the dial setting is the one whose saturated vapor pressure is close to that value.

Among the common agents listed, sevoflurane has a saturated vapor pressure around 157–160 mmHg, which is closest to 170 mmHg. Halothane and isoflurane have higher pressures (roughly 238–244 mmHg), and desflurane has a much higher pressure (about 669 mmHg). Because their vapor pressures are quite different from the 170 mmHg design point, they wouldn’t produce an output that lines up with the dial setting as neatly as sevoflurane would.

So, sevoflurane is the best match for delivering an output when the dial is set to the vaporizer’s output.