Anesthesia Pharmacology: Partial Pressures


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SVP (Saturated Vapor Pressure) and Different Anesthetics

  • 4The 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.  

    • Therefore, anesthetics with a higher SVP would require a smaller ratio of the total gas flow into the vaporizer to pass through the actual vaporizing chamber in order to result in a given final anesthetic concentration delivered to the patient.

  • 4The relationship between SVP and vaporizer flow relative to total flow is illustrated by comparing several volatile anesthetics with differing SVP values.

  • 4In comparing the extremes (halothane (Fluothane) with an SVP of 243 mm Hg with that of methoxyflurane within SVP of 23 mm Hg, the significant differences in the ratio vaporizer flow over total flow is apparent [2.1% versus 32%].




4,,6SVP and Temperature  


  • 4Note the variation in SVP as a function of temperature.  Accordingly, temperature compensation would be required in order to ensure that the vaporizer output would remain constant.  Such constancy requires that other variables are also held constant.  Some of these other variables include:

    1. Changes in the composition of carrier gas which may alter total flow through the vaporizing chamber

    2. Addition of nitrous oxide which may dissolve in the anesthetic volatile liquid component which may change the effective volume passing through the vaporizing chamber

    3. Alteration in gas flow rate (if the flow rate of the gas through the vaporizing chamber is too high, effective equilibration may not be approached therefore the gas leaving the chamber will be relatively less saturated, resulting in a reduced vaporizer output)

    4. Ambient pressure SVP is only temperature dependent; therefore if the ambient pressure were to be reduced, the ratio of the anesthetic vapor pressure to the total pressure would increase resulting in a higher output anesthetic concentration. 

      1. One example might be that if the halothane (Fluothane) vaporizer set (calibrated) for sea level (760 mm Hg) to deliver 2% anesthetic, the percent delivered in Denver (the mile high city) would be about 2.7 % halothane (Fluothane)



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