Plenum Vaporisers
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 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 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 5 kg, 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 first temperature-compensated plenum vaporiser was the Cyprane 'FluoTEC' Halothane vaporiser, released onto the market shortly after Halothane was introduced into clinical practice in 1956.
Read more about this topic: Anaesthetic Vaporiser