Vaporizers

Roy Esaki

Alex Macario

▪ INTRODUCTION

Inhalational agents are drugs with anesthetic properties administered in the form of a gas. For a drug injected intravenously, the dosing relates to the mass the patient receives (e.g., in grams), in the form of a specific volume at a specific concentration. Inhalational agents, however, are delivered as a concentration in a volume of gas. The “volume” is being continuously delivered with each breath the patient receives. As these agents normally exist as liquids at room temperature and atmospheric pressure, a vaporizer is used to turn the liquid into a gas that the patient can inhale.

In the middle of the 19th century, the first available “vaporizers” were merely devices that allowed the patient to breathe evaporated liquid agents. The device used by William Morton in the first public demonstration of ether anesthesia in 1846 was a container that contained a sponge soaked with ether (Fig. 28.1). The patient breathed in the agent as it evaporated off the sponge.

Later, chloroform was administered by dropping the liquid agent using special dropper bottles (Fig. 28.2) over a cloth that was placed either directly over the patient’s mouth or draped over a wire mask. Although such devices allowed the liquid to evaporate into a gaseous form, the concentrations of the agent could not be controlled. Modern vaporizers were thus developed to deliver a precise and constant concentration of the agent.

▪ PHYSICAL CHEMISTRY

To understand the basic principles of how modern vaporizers work, we need to review some principles of physical chemistry: the concepts of vapor, vapor pressure, and gas concentrations.

Vapor and Vapor Pressure

A vaporizer turns the liquid anesthetic agent from a liquid form to a gas, or vapor. All substances can exist in liquid, solid, or gas forms, depending on the pressure and temperature of the substance. As a gas is compressed under increasing pressure, the particles are pushed closer together until the gas turns into a liquid. For example, when nitrogen gas is compressed enough, it turns into liquid nitrogen. For some gases, there is a critical temperature above which a gas cannot exist as a liquid, no matter how much pressure is applied.

A vapor is a substance in the gaseous phase at a temperature below its critical point. That is, it is a gas that has the potential to become a liquid when compressed, or subjected to a higher pressure. When a volatile liquid is placed in a closed container, a certain percentage of the liquid molecules evaporate to become vapor. This vapor creates a pressure, called the vapor pressure. As more heat is applied, more molecules enter the gaseous phase, resulting in a greater pressure. As such, the vapor pressure of any substance increases with temperature. The concentration of an agent delivered by a vaporizer depends on the vapor pressure of the agent. Because different agents have different vapor pressures, each modern vaporizer is calibrated for use with a specific agent. Of note, desflurane has a much higher vapor pressure at room temperature than other agents, and thus requires a vaporizer with unique features (see below).

Gas Concentration: Partial Pressure and Volume Percent

The concentration of a vapor can be expressed as either a partial pressure or a volume percent. In a mixture of gases, each gas independently
contributes part of the total pressure, which is the sum of the partial pressures of all gases present. The portion of the total pressure created by any given vapor is called the partial pressure of that gas. Although the partial pressure of the gas is what actually corresponds to the clinical effect of an anesthetic gas in the body, concentrations delivered by a vaporizer are commonly expressed as a volume percent for practical convenience. Volume percent is the fraction of the total pressure attributable to the gas of interest expressed as a percent (partial pressure of gas/total ambient pressure × 100). This term may be a slight misnomer as the gas molecules are mixed together in a shared volume. Nonetheless, because the volume of a gas is proportional to the number of particles, given a constant pressure and temperature, the volume percent of an agent can be thought of as a percentage of the total number of gas molecules delivered to the patient.

▪ FIGURE 28.1 Early vaporizer. A replica of the inhaler used by Dr. William Morton to demonstrate the use of ether anesthesia in 1846. An ether-soaked sponge was placed inside, and the patient breathed in the evaporated vapor. (Photo courtesy of the Wood Library-Museum of Anesthesiology, Park Ridge, IL [http://www.woodlibrary-museum.org/museum_view.php?id=2].)

▪ FIGURE 28.2 Chloroform dropper. From left to right: a chloroform drop flask, a drop bottle with a control valve, and an alembic flask. Such devices allowed careful titration of the liquid agent. (Photo courtesy of the Wood Library-Museum of Anesthesiology, Park Ridge, IL [http://www.woodlibrarymuseum.org/museum_view.php?id=25].)

▪ PRINCIPLES OF MODERN VAPORIZERS

Vaporizers vary greatly in their design and construction. Figure 28.3 shows one common type of modern vaporizer. All modern (concentrationcalibrated vaporizers) are placed out of circuit— that is, between the flowmeter and the common gas outlet, rather than within the breathing system or between the common gas outlet and the breathing system. The intent of this chapter is not to go over the specifics of the operation of any specific vaporizer model but to provide an overview of the principles underlying the operation of modern vaporizers.

▪ VARIABLE BYPASS VAPORIZERS

As mentioned previously, the basic purpose of a vaporizer is to deliver a set concentration of anesthetic gas in a volume of inert gas, such as oxygen. Figure 28.4 shows the general schematic
of a vaporizer. In this schematic, fresh gas flow enters from the top left, corresponding to the vaporizer inlet. The inert gas can then flow across the top bypass chamber, without being exposed to any volatile agent. Alternatively, some of the fresh gas flow can be diverted down into the vaporizing chamber, where it becomes saturated with a certain concentration of the volatile agent, as determined by the partial pressure of the agent. The concentration dial can vary the percentage of gas, called the splitting ratio that bypasses the vaporizing chamber; this construction is thus called the variable bypass vaporizer.

▪ FIGURE 28.3 The Drager Vapor 2000 series; from left to right, a desflurane, a sevoflurane (turned on), and an isoflurane vaporizer.

▪ FIGURE 28.4 Schematic of a variable bypass vaporizer. Simple schematic of modern vaporizer: Fresh gas flow enters the vaporizer inlet (top left) and is directed either down into the vaporizing chamber to become saturated with vapor or into a bypass chamber across the top. By varying the ratio of the split, the output vaporizer concentration (top right) can be changed. (Figure courtesy Dr. Guy Watney [http://www.asevet.com/resources/vaporizer/index.htm].)

The end result is that the partial pressure of the volatile agent in the vaporizing chamber is diluted by the fresh gas flow through the bypass to obtain the desired concentration of anesthetic. The gas with the desired vapor concentration exits through the outlet of the vaporizer, shown at the top right of the schematic. In an older type of vaporizer, called the copper kettle, the gas flows for both the flow directed to the vaporization chamber and for the flow that would skip the chamber had to be manually adjusted to achieve a desired output gas concentration. Modern variable bypass vaporizers do this automatically when the concentration dial is set to a desired concentration.

Although the input fresh gas flow rate theoretically can increase the output gas concentration, this effect is minimal with most modern vaporizers. The input gas composition (i.e., if nitrous oxide is used in addition to oxygen) can also affect the concentration. To account for these effects, many electronic vaporizers have a feedback system that adjusts its internal settings based on the actual gas output. In other types of vaporizers, such as the Tec 6 used for desflurane, there are two separate input circuits, rather than a single fresh gas flow that is split.

▪ VAPORIZATION METHOD

Older vaporizers such as the copper kettle bubbled the carrier gas up through the liquid anesthetic to saturate the carrier gas with the anesthetic. Most modern vaporizers have the carrier gas flow over the liquid agent where it takes up the anesthetic. Increasing the surface with internal wicks and baffles makes the vaporization and uptake of the anesthetic more efficient. To accommodate the higher vapor pressure of desflurane, the Tec 6 vaporizer uniquely uses a gas/vapor blender in which the desflurane is heated to a constant temperature to produce a vapor that is then injected into the gas flow in a regulated fashion (Fig. 28.5).

▪ FIGURE 28.5 Schematic of Tec 6 desflurane vaporizer. The liquid desflurane (liquid) is heated (H) to a vapor form (gas), which is then mixed with the carrier gas to produce a vaporizer output of the desired concentration. (Figure courtesy Dr. Guy Watney [http://www.asevet.com/resources/vaporizer/index.htm].)

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