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Vaporizers: Altitude effects

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

However, variable-bypass vaporizers are not suitable for desflurane as its vapor pressure at 20°C is almost 1 atmosphere (664 mm Hg).1 This required the development of a special desflurane vaporizer (Tec 6, Datex-Ohmeda), which electrically heats desflurane to 23°C to 25°C and pressurizes the vaporizer with a backpressure regulator to 1500 mmHg to create an environment which the anesthetic has a lower, but predictable volatility.1 This creates a constant output of desflurane across a range of barometric pressures.1 Therefore, if the dial is set at 6% it will deliver a 6% desflurane regardless of altitude. However, when the 6% desflurane leaves the vaporizer at high altitude it is at a decreased ambient (local) pressure so the partial pressure of desflurane will be less than it would be at sea level. Therefore, at high altitudes the partial pressure of desflurane will be lower at a given vaporizer setting (volume percent) than at sea level, leading to underdosing of the anesthetic to the patient.1

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