Anesthetic Vaporizers

The purpose of an anesthetic vaporizer is to produce a controlled and predictable concentration of anesthetic vapor in the carrier gas passing through the vaporizer.

Most vaporizers are of the plenum type, which consists of a vaporizing chamber containing the liquid anesthetic, and a bypass. Gas passing through the vaporizing chamber volatilizes the anesthetic and is then mixed with the anesthetic-free gas bypassing the chamber, the proportion of vapor-containing gas and bypass gas being controlled by a tap.

Saturated vapor pressure
The most important factor governing vaporizer design is the saturated vapor pressure (SVP) of the anesthetic. SVP is a measure of the volatility of the liquid anesthetic in the carrier gas: after equilibration between the carrier gas and the liquid anesthetic, the concentration of highly volatile anesthetics (e.g. isoflurane) in the gas will be higher than that of poorly volatile anesthetics (e.g. methoxyflurane).

Anesthetics with a high SVP will require a smaller proportion of the total gas flowing through the vaporizer to pass through the vaporizing chamber to produce a given concentration than will anesthetics with a low SVP. The following table shows the SVP of some anesthetics at 20^oC and the proportion of the total gas flow required to pass through the vaporizing chamber to produce an ouput concentration of 1% at a barometric pressure of 760 mmHg:

It follows that it can be extremely dangerous to deliver anesthetics from vaporizers for which they were not designed. A vaporizer intended for use with methoxyflurane filled with isoflurane and with the dial set to 1% would in fact be producing about 15 % isoflurane.

SVP and temperature

Saturated vapor pressure varies as a function of temperature:

Unless some means of compensation is employed, the vaporizer output will increase as temperature rises, and vice versa.

Factors affecting vaporizer output

• Flow through the vaporizing chamber
  Varying the proportion of gas passing through the vaporizing chamber and bypass is the method by which vaporizer output is controlled.
• Efficiency of vaporization
  Vaporizers may incorporate a system of wicks and channels in the vaporizing chamber to improve efficiency of vaporization and increase the output concentration of anesthetic.
  
• Temperature
  As temperature increases, the output of the vaporizer will increase, unless some compensatory mechanism is used.
• Time
  Vaporization causes the liquid anesthetic to cool since heat is lost because of the latent heat of vaporization of the anesthetic. Therefore, the output concentration will tend to fall over time.
• Gas flow rate
  Changes in carrier gas flow rate may affect vaporizer output by:
  • Altering the proportion of the total gas flow that passes through the vaporizing chamber.
  • Altering the efficiency of vaporization. For example, at high flow rates, the gas leaving the vaporizing chamber will tend to be less saturated (since the gas spends less time in the chamber), so the output of the vaporizer will tend to fall.
• Carrier gas composition
  The composition of the carrier gas may affect vaporizer output by:
  • Changes in the viscosity and density of the gas mixture affecting the proportion of the total flow that passes through the vaporizing chamber
  • Nitrous oxide dissolving in the anesthetic, thus altering the effective volume that passes through the vaporizing chamber.
• Ambient pressure
  Saturated vapor pressure is solely a function of temperature. Therefore, if ambient pressure is reduced, the (constant) SVP becomes a greater proportion of the total (reduced) pressure, and the ouput concentration (in volumes %) rises.
       For example, a halothane vaporizer calibrated at sea level and set to deliver 2% will produce about 2.7% halothane if used in Denver, Colorado.

Types of vaporizer

Simple vaporizers do not compensate for these effects. Their output varies widely under different conditions.
Precision vaporizers incorporate mechanisms to compensate for these effects. Their output is reasonably constant over a wide range of conditions.
Low-resistance vaporizers have a low internal resistance to gas flow and are designed to be used within the breathing circuit, either in drawover apparatus or as in-circuit vaporizers in the circle absorber system.

Simple vaporizers