Vapor pressure

In a closed container, molecules from a volatile liquid escape the liquid phase and become vapor. These gaseous molecules strike the wall of the container, exerting what’s known as vapor pressure. Vapor pressure is directly proportional to temperature. Increasing temperature will increase the ratio of gas:liquid molecules, thereby increasing vapor pressure.

Vapor pressure of volatile agents at 20°C (mmHg):

• Sevoflurane: 157
• Desflurane: 669
• Isoflurane: 238
• Enflurane: 172
• Halothane: 243
• N₂O: 38,770

Boiling point

Boiling point is defined as the temperature at which vapor pressure equals atmospheric pressure (760mmHg). Boiling points are listed below (Celsius):

• Boiling point (C)
• Sevoflurane: 58.5
• Desflurane: 22.8
• Isoflurane: 48.5
• Enflurane: 56.5
• Halothane: 50.2
• N₂0: -88

Latent heat of vaporization

Latent heat of vaporization: Energy is required when a molecule changes from liquid to gas. Latent heat of vaporization is the number of calories required to change 1 gram of liquid into vapor without changing temperature. Thus, in the absence of an outside temperature source, volatile liquids will cool significantly and lead to decreasing vaporization. The latent heats of vaporization among the common volatile anesthetic gases are similar. However, desflurane is notable for being less potent than the other agents. Thus, more desflurane molecules are required to achieve a given level of anesthetic depth than with the other agents, leading to greater temperature loss and decreasing vapor pressure.

Desflurane

Desflurane’s unique characteristics (high volatility/moderate potency) require the use of a special vaporizer for proper utilization of this gas. Desflurane vaporizers are heated to 39°C, which increases the vapor pressure in the sump to 1300mmHg, preventing the possibility of boiling in warm OR rooms. Providing an external heat source compensates for the significant heat loss associated with desflurane vaporization. And unlike stand variable bypass vaporizers that pass fresh gas through the vaporizing chamber, desflurane vaporizers add agent directly to the gas stream.