Anaesthetic vaporiser

An anaesthetic vaporizer is a device generally attached to an anaesthetic machine which delivers a given concentration of a volatile anaesthetic agent. It works by controlling the vaporisation of anaesthetic agents from liquid, and then accurately controlling the concentration in which these are added to the fresh gas flow.

The design of these devices takes account of varying: ambient temperature, fresh gas flow, and agent vapour pressure.

There are generally two types of vaporizers: plenum and drawover. Both have distinct advantages and disadvantages. The dual-circuit gas-vapour blender is a third type of vaporizer used exclusively for the agent desflurane.

The plenum vaporizer is driven by positive pressure from the anaesthetic machine, and is usually mounted on the machine. The performance of the vaporizer does not change regardless of whether the patient is breathing spontaneously or is mechanically ventilated. The internal resistance of the vaporizer is usually high, but because the supply pressure is constant the vaporizer can be accurately calibrated to deliver a precise concentration of volatile anaesthetic vapour over a wide range of fresh gas flows. The plenum vaporizer 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 vaporizer works by accurately splitting the incoming gas into two streams. One of these streams passes straight through the vaporizer 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 vaporizer.

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 vaporising 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 vaporizer 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 vaporizer 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 vaporizer is calibrated in volume percent (e.g. 2%), what it actually delivers is a partial pressure of anaesthetic agent (e.g. 2kPa)).

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