Vaporizers: Altitude effects

Advanced, Clinical Sciences: Anesthesia Procedures, Methods, and Techniques

Volatile anesthetics are liquids at room temperature and atmospheric pressure; therefore, they must be vaporized in order to be safely delivered to patients with accuracy. Variable-bypass vaporizers (used for halothane, isoflurane, and sevoflurane) work by splitting the total fresh gas flow into two fractions. One fraction of the fresh gas flow passes through the bypass chamber of the vaporizer without contacting anesthetic. The second fraction is diverted through a vaporizing chamber, where it becomes saturated with the vapor of the liquid anesthetic. These two fractions mix at the outflow of the anesthesia machine and are delivered to the patient. The amount of the total fresh gas flow diverted to the vaporizing chamber and, therefore, the concentration of volatile anesthetic delivered to the patient is determined by the concentration dial.1 Variable-bypass vaporizers compensate for changes in atmospheric pressure because they deliver a constant partial pressure (i.e., altitude changes maintaining relative anesthetic gas partial pressure).2

The following equation can be used to account for higher altitude:1

Required vaporizer setting = (Desired vaporizer setting at sea level x 760 mmHg)/ (barometric pressure in mmHg)

See Also:

Vaporizer output at altitude

Vapor Pressure


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Cameron Sumner, MD