Concentration-calibrated vaporizers

All vaporizers in common use today in developed countries are calibrated by the output concentration, expressed in volume percent. This is known as a concentration-calibrated (variable bypass, direct-reading) vaporizer. The American Society for Testing and Materials anesthesia workstation standard requires that all vaporizers on the anesthesia workstation be concentration calibrated. In addition, all vaporizer control dials must turn counterclockwise to increase the output concentration. These vaporizers must be placed between the flowmeters and the outlet on the anesthesia machine.

Concentration calibration may be accomplished by splitting the flow of gas that passes through the vaporizer. Some gas passes through the vaporizing chamber (the part of the vaporizer containing the liquid anesthetic agent), and the remainder goes through a bypass to the vaporizer outlet (Figure 6-1). The ratio of bypass gas to the gas flowing to the vaporizing chamber is called the splitting ratio and depends on the resistances in the two pathways. It also depends on the setting of the concentration dial that allows more gas to pass through the vaporizing chamber as higher output concentrations are set. The splitting ratio may also depend on the total gas flow through the vaporizer. Another method of controlling the outlet concentration is to direct enough carrier gas to flow through the vaporizing chamber to achieve the concentration set on the vaporizer. This is determined by a computer.

In many concentration-calibrated vaporizers, the composition of the carrier gas affects vaporizer output (vaporizer aberrance). Most vaporizers are calibrated using O₂ as the carrier gas. Usually, little change in output occurs if air is substituted for O₂. Addition of N₂O to the carrier gas typically results in both temporary and long-lasting effects on vaporizer output. The temporary effect is usually reduced vapor concentration. The duration depends on the gas flow rate and on the volume of liquid in the vaporizer. The longer-term effect may be increased or decreased output concentration, depending on the construction of the vaporizer.

At low barometric pressure (higher altitudes), variable-bypass concentration-calibrated vaporizers will deliver approximately the same anesthetic partial pressure but increased concentrations measured in volume percent. At high barometric pressure (as in a hyperbaric chamber), these vaporizers deliver decreased output measured in volume percent because the vapor pressure of the agent is affected only by temperature and not by ambient pressure. The partial pressure and clinical effects remain relatively unchanged.

In a flow-over vaporizer, carrier gas passes over the surface of the liquid. Increasing the area of the carrier gas-liquid interface can enhance the efficiency of vaporization. This can be done using baffles or spiral tracks to lengthen the pathway of the gas exposed to the liquid. Another method is to use wicks with their bases in the liquid. The liquid moves up the wicks by capillary action.

Temperature compensation

When a liquid is vaporized, energy, in the form of heat, is lost. The vapor pressure decreases as the temperature of the liquid drops. Three methods have been employed to maintain a constant vapor output with fluctuations in liquid anesthetic temperature.