Vaporizers are closed containers where the conversion of a volatile anesthetic from liquid to vapor takes place. Modern vaporizers are specific to the particular anesthetic agent and account for temperature and flow to deliver a consistent concentration of agent. The operator controls precise delivery of volatile agent concentration with a calibrated dial.
At operating room temperatures, volatile anesthetics exist in both liquid phase and gas phase in vaporizers. The latent heat of vaporization is the number of calories required at a specific temperature to convert 1 g of a liquid into a vapor. As the temperature of the liquid decreases, the heat of vaporization necessary for molecules to leave the liquid phase increases. When equilibrium between the liquid phase and vapor phase is reached, vaporization ceases as an equal number of molecules enter and leave the liquid phase.
Specific heat is the calories required for 1 g of a substance to increase by 1°C. Knowledge of the specific heat of an anesthetic agent allows for vaporizers to be designed such that the correct amount of heat can be added to maintain the temperature of the liquid as vaporization occurs. In addition, vaporizer components are designed with a high specific heat to minimize temperature change.
As anesthetic agent molecules collide with each other in the walls of the vaporizer, a pressure is created, known as the saturated vapor pressure, which is unique for each volatile anesthetic (Table 8-1). Vapor pressure is independent of atmospheric pressure, but dependent on the physical characteristics of the liquid. Vapor pressure also depends on temperature such that a decrease in temperature corresponds to lower vapor pressure (fewer molecules in vapor phase). The boiling point of a liquid is the temperature at which the vapor pressure equals atmospheric pressure. Cooling the liquid anesthetic is undesirable because it lowers the vapor pressure and, therefore, limits the attainable vapor concentration. Modern vaporizers are temperature compensated.
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TABLE 8-1 Vapor Pressure
|Volatile Anesthetic Agent ||Vapor Pressure (mm Hg) at 20°C |
|Halothane ||243 |
|Isoflurane ||240 |
|Desflurane ||681 |
|Sevoflurane ||160 |
VARIABLE BYPASS VAPORIZERS
Most vaporizers (Tec 4, Tec 5, SevoTec, Vapor 19.n, Vapor 2000, and Aladin) are considered to have a variable bypass carrier gas flow and a flow-over vaporization method. Not all of the entering gas is exposed to the anesthetic liquid; some gas is exposed whereas the rest bypasses the agent. These vaporizers are agent specific, temperature compensated, and are located outside of the circuit, between the flowmeters and the common gas outlet.
Basic Principles and Components
Generic schematic of agent-specific variable bypass vaporizer. (Reproduced with permission from Barash PG, Clinical ...
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