13 September 2004

The equipment - the gas delivery system

Should you not be too drowsy when you are transferred into the theatre, you will see the anaesthetic machine at one end of the operating table.


The Equipment
Should you not be too drowsy when you are transferred into the theatre, you will see the anaesthetic machine at one end of the operating table. This is sometimes referred to as the “Boyles” machine, named after the anaesthetist who produced the first such machine. Naturally, the anaesthetic machine is a far more sophisticated machine than the originals. The machine has several components which are described below.

The Gas Delivery System
The basic purpose of the machine is to produce a controllable gas mixture that will be delivered to the patient through a set of tubes called the anaesthetic circuit.

The gases that are required are oxygen, nitrous oxide and air. These gases are supplied to the machine from pipelines connected to containers of gas outside the hospital, and in the case of oxygen and nitrous oxide there are also cylinders of gas mounted on the anaesthetic machine for backup purposes.

The gases enter the machine at high pressures which are reduced and enter the controlling devices that enable the anaesthesiologist to vary the amount of each gas being delivered to the patient.

These controllers are called rotameters and in older machines still in use you will be able to see these graduated glass cylinders containing a little bobbin on the left hand side of the machine. Turning the controller knob on each rotameter causes gas to flow and lift the bobbin, so that the anaesthesiologist can set a precise flow rate by reading the level of the bobbin against the graduations on the cylinder.

The machines are carefully designed to prevent a dangerous gas mixture containing too little oxygen from being delivered to the patient. The mixture of anaesthetic gases (usually nitrous oxide and at least 30% oxygen) is then directed to a device called a vaporiser.

The purpose of the vaporiser is to insert volatile anaesthetic agent into the gas stream. The function of the volatile agent is to produce anaesthesia, and has been described earlier. The commonly used agents are halothane, enflurane, isoflurane, sevoflurane or desflurane.

The vaporiser is fitted with a dial so that the anaesthesiologist can vary the concentration of the volatile agent in the circuit according to the needs of the patient, the stage of the anaesthetic and the operation.

The older agents such as chloroform and ether are no longer used because of their dangerous side-effects, even though they are very effective anaesthetic agents. The gas mixture is then delivered to the anaesthetic circuit at a connection point on the right-hand side of the machine.

There are a number of dials and gauges on the machine which give the anaesthesiologist information about the pressure of the gases in the pipelines and cylinders.

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