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PHYSICS OF VAPORIZATION
|Volatile Anesthetic Agent||Vapor Pressure (mm Hg) at 20°C|
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The vapor pressures of anesthetic agents above 0°C are much greater than those used in the clinical setting; therefore, they are mixed with medical gases to reduce the concentration before the delivery to the patient.
Anesthetic agent vapor is the gaseous phase of an anesthetic agent that is normally a liquid at room temperature and atmospheric pressure. Anesthetic gas is any gaseous substance, e.g., nitrous oxide, used in producing a state of anesthesia. Anesthetic vaporizer is a device designed to facilitate the change of an anesthetic from a liquid to a vapor.
Vapor Pressure of Inhaled Anesthetic Agents (at 20 C) Desflurane: 669 mm Hg. Halothane: 243 mm Hg. Isoflurane: 238 mm Hg. Sevoflurane: 157 mm Hg. Blood:Gas Partition Coefficient of Inhaled Anesthetic Agents (at 20 C) Halothane: 2.50 (MAC 0.75%) Isoflurane: 1.46 (MAC 1.2%) Sevoflurane: 0.65 (MAC 1.8-2.4%)
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 degrees C, which increases the vapor pressure in the sump to 1300mmHg, preventing the possibility of boiling in warm OR rooms.
Alveolar concentration (FA) to inspired concentration (FI): used to determine the speed of induction, works on a 0-1 scale in which the FA/FI will approach 1 as the gas gets administered; 1 being equilibrium. The rate at which the above ratio approaches equilibrium represents the speed of induction.
4 The saturated vapor pressure (SVP) describes the number of anesthetic molecules in the gas phase following equilibration between the carrier gas and liquid anesthetic. Those gases which are more highly volatile would be expected then to have a higher SVP.
The vapor pressure is a measure of the volatility of the drug and is defined as the maximum pressure that can be produced by the anesthetic at a given temperature and pressure. Vapor pressure divided by the atmospheric pressure gives the maximal fraction of anesthetic vapor that can be produced.
concentration: Anesthetic vapor pressure may be represented as absolute pressure (mmHg) or in volumes percent of one atmosphere which is volume of anesthetic vapor per 100 volumes of total gas. The relationship between volume percent in the anesthetic
VAPOR PRESSURE (also called SATURATION VAPOR PRESSURE) The process of evaporation in a closed container will proceed until there are as many molecules returning to the liquid as there are escaping (equilibrium). At this point the vapor is said to be saturated, and the pressure exerted by the vapor (usually expressed in mmHg) is called the saturated vapor pressure.