Historically, anaesthetic volatile agents (such as ether or chloroform) were often administered using the open-drop technique, which is that a thick pad of gauze was held over the patient's mouth and nose whilst the anaesthetist dripped liquid volatile agent onto it as the patient breathed.
This technique was hopelessly unreliable. Liquid anaesthetic could drip into the patient's mouth and nose, where it was extremely irritant, and the anaesthetist (and often the onlookers) were exposed to a high concentration of vapour.
Pressure and demand for a more reliable method of administrating volatile agents prompted a variety of improvements, such as the patient breathing from a tube attached to a simple vaporiser, typically a reservoir of volatile liquid. Many different designs evolved, all trying to solve the same problems of cooling of the agent as it evaporated, variation in efficiency with the respiratory efforts of the patient, and trying to ensure the vapour was delivered only to the patient and not to the operating room staff.
The invention of the anaesthetic machine in 1917 allowed the vaporiser to be permanently mounted on the machine, allowing it to be considerably refined, and, while several designs exist, they now have much in common.
There are generally two types of vaporisers: plenum and drawover. Both have distinct advantages and disadvantages. A third type of vaporiser is exclusively used for the agent desflurane (see below).
The plenum vaporiser is driven by positive pressure from the anaesthetic machine, and is usually mounted on the machine. The performance of the vaporiser does not change regardless of whether the patient is breathing spontaneously or is mechanically ventilated. The internal resistance of the vaporiser is usually high, but because the supply pressure is constant the vaporiser can be accurately calibrated to deliver a precise concentration of volatile anaesthetic vapour over a wide range of fresh gas flows. The plenum vaporiser is an elegant device which works reliably, without external power, for many hundreds of hours of continuous use, and requires very little maintenance.
The plenum vaporiser works by accurately splitting the incoming gas into two streams. One of these streams passes straight through the vaporiser in the bypass channel. The other is diverted into the vaporising chamber. Gas in the vaporising chamber becomes fully saturated with volatile anaesthetic vapour. This gas is then mixed with the gas in the bypass channel before leaving the vaporiser.
A typical volatile agent, isoflurane, has a saturated vapour pressure of 32kPa (about 1/3 of an atmosphere). This means that the gas mixture leaving the vapourising chamber has a partial pressure of isoflurane of 32kPa. At sea-level (atmospheric pressure is about 101kPa), this equates conveniently to a concentration of 32%. However, the output of the vaporiser is typically set at 1-2%, which means that only a very small proportion of the fresh gas needs to be diverted through the vaporising chamber (this proportion is known as the splitting ratio). It can also be seen that a plenum vaporiser can only work one way round: if it is connected in reverse, much larger volumes of gas enter the vaporising chamber, and therefore potentially toxic or lethal concentrations of vapour may be delivered. (Technically, although the dial of the vaporiser is calibrated in volume percent (e.g. 2%), what it actually delivers is a partial pressure of anaesthetic agent (e.g. 2kPa)).
The performance of the plenum vaporiser depends extensively on the saturated vapour pressure of the volatile agent. This is unique to each agent, so it follows that each agent must only be used in its own specific vaporiser. Several safety systems, such as the Fraser-Sweatman system, have been devised so that filling a plenum vaporiser with the wrong agent is extremely difficult. A mixture of two agents in a vaporiser could result in unpredictable performance from the vaporiser.
Saturated vapour pressure for any one agent varies with temperature, and plenum vaporisers are designed to operate within a specific temperature range. They have several features designed to compensate for temperature changes (especially cooling by evaporation). They often have a metal jacket weighing about 5kgs, which equilibrates with the temperature in the room and provides a source of heat. In addition, the entrance to the vaporising chamber is controlled by a bimetallic strip, which admits more gas to the chamber as it cools, to compensate for the loss of efficiency of evaporation.
The drawover vaporiser is driven by negative pressure developed by the patient, and must therefore have a low resistance to gas flow. Its performance depends on the minute volume of the patient: its output drops with increasing minute ventilation.
The design of the drawover vaporiser is much more simple: in general it is a simple glass reservoir mounted in the breathing attachment. Drawover vaporisers may be used with any liquid volatile agent (including older agents such as diethyl ether or chloroform, although it would be dangerous to use desflurane). Because the performance of the vaporiser is so variable, accurate calibration is impossible. However, many designs have a lever which adjusts the amount of fresh gas which enters the vaporising chamber.
The drawover vaporiser may be mounted either way round, and may be used in circuits where rebreathing takes place, or inside the circle breathing attachment.
Drawover vaporisers typically have no temperature compensating features. With prolonged use, the liquid agent may cool to the point where condensation and even frost may form on the outside of the reservoir. This cooling impairs the efficiency of the vaporiser. One way of minimising this effect is to place the vaporiser in a bowl of water.
The relative inefficiency of the drawover vaporiser contributes to its safety. A more efficient design would produce too much anaesthetic vapour. The output concentration from a drawover vaporiser may greatly exceed that produced by a plenum vaporiser, especially at low flows. For safest use, the concentration of anaesthetic vapour in the breathing attachment should be continuously monitored.
Despite its drawbacks, the drawover vaporiser is cheap to manufacture and easy to use. In addition, its portable design means that it can be used in the field or in veterinary anaesthesia.
Dual-circuit gas-vapour blender
The third category of vaporiser (a dual-circuit gas-vapour blender) was created specifically for the agent desflurane. Desflurane boils at 23.5C, which is very close to room temperature. This means that at normal operating temperatures, the saturated vapour pressure of desflurane changes greatly with only small fluctuations in temperature. This means that the features of a normal plenum vaporiser are not sufficient to ensure an accurate concentration of desflurane. Additionally, on a very warm day, all the desflurane would boil, and very high (potentially lethal) concentrations of desflurane might reach the patient.
A desflurane vaporiser (e.g. the TEC 6 produced by Datex-Ohmeda) is heated and pressurised to 200kPa (and therefore requires electrical power). It is mounted on the anaesthetic machine in the same way as a plenum vaporiser, but its function is quite different. It evaporates a chamber containing desflurane using heat, and injects small amounts of pure desflurane vapour into the fresh gas flow. A transducer senses the fresh gas flow.
A warmup period is required after switching on. The desflurane vaporiser will fail if mains power is lost. Alarms sound if the vaporiser is nearly empty. An electronic display indicates the level of desflurane in the vaporiser.
The expense and complexity of the desflurane vaporiser have contributed to the relative lack of popularity of desflurane, although in recent years it is gaining in popularity.