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The anaesthetic machine


The anaesthetic machine receives medical gases (oxygen, nitrous oxide, air) under pressure and provides a continuous and accurate flow of each gas individually. A gas mixture of the desired composition at a defined flow rate is created before a known concentration of an inhalational agent vapour is added. Gas and vapour mixtures are continuously delivered to the common gas outlet of the machine, as fresh gas flow (FGF), and to the breathing system and patient ( Figs. 2.1 and 2.2 ). The anaesthetic machine consists of:

  • 1.

    gas supplies (see Chapter 1 )

  • 2.

    pressure gauges

  • 3.

    pressure regulators (reducing valves)

  • 4.

    flowmeters

  • 5.

    vaporizers

  • 6.

    a common gas outlet

  • 7.

    a variety of other features, e.g. high-flow oxygen flush, pressure relief valve and oxygen supply failure alarm and suction apparatus

Fig. 2.1
(A, B) The Penlon Prima 465 with its control panel.

Courtesy Penlon, Abingdon, UK.

Fig. 2.2
Diagrammatic representation of a continuous flow anaesthetic machine. Pressures throughout the system: 1, O 2 : 13,700 kPa, N 2 O: 4400 kPa; 2, pipeline: about 400 kPa; 3, O 2 supply failure alarm activated at <250 kPa; 4, regulated gas supply at about 400 kPa; 5, O 2 flush: 45 L/min at a pressure of about 400 kPa; 6, back-bar pressure 1–10 kPa (depending on flow rate and type of vaporizer).

Most modern anaesthetic machines or stations incorporate a circle breathing system (see Chapter 4 ) and a bag-in-bottle–type ventilator (see Chapter 8 ).

To ensure the delivery of a safe gas mixture, safety features of a modern anaesthetic machine should include the following:

  • Colour-coded pressure gauges

  • Colour-coded flowmeters

  • An oxygen flowmeter controlled by a single touch-coded knob

  • Oxygen is the last gas to be added to the mixture

  • Oxygen concentration monitor or analyser

  • Nitrous oxide is cut off when the oxygen pressure is low

  • Oxygen:nitrous oxide ratio monitor and controller

  • Pin-index safety system for cylinders and non-interchangeable screw thread for pipelines

  • Alarm for failure of oxygen supply

  • Ventilator disconnection alarm

  • At least one reserve oxygen cylinder should be available on machines that use pipeline supply

Pressure gauge

This measures the pressure in the cylinder or pipeline. The pressure gauges for oxygen, nitrous oxide and medical air are mounted in a front-facing panel on the anaesthetic machine ( Fig. 2.3 ).

Fig. 2.3, Pressure gauges for oxygen, air and nitrous oxide.

Some anaesthetic machine designs have a digital display of the gas supply pressures ( Fig. 2.4 ).

Fig. 2.4, Digital display of pressure gauges for oxygen (cylinder and pipeline), nitrous oxide (pipeline) and air (pipeline).

Components

  • 1.

    A robust, flexible and coiled tube, which is oval in cross section ( Fig. 2.5 ). It should be able to withstand the sudden high pressure when the cylinder is switched on.

    Fig. 2.5, The Bourdon pressure gauge.

  • 2.

    The tube is sealed at its inner end and connected to a needle pointer that moves over a dial.

  • 3.

    The other end of the tube is exposed to the gas supply.

Mechanism of action

  • 1.

    The high-pressure gas causes the tube to uncoil (Bourdon gauge).

  • 2.

    The movement of the tube causes the needle pointer to move on the calibrated dial, indicating the pressure.

Problems in practice and safety features

  • 1.

    Each pressure gauge is colour-coded and calibrated for a particular gas or vapour. The pressure measured indicates the contents available in an oxygen cylinder. Oxygen is stored as a gas and obeys Boyle’s law (pressure × volume = constant). This is not the case in a nitrous oxide cylinder since it is stored as a liquid and vapour.

  • 2.

    A pressure gauge designed for pipelines should not be used to measure cylinder pressure and vice versa. This leads to inaccuracies and/or damage to the pressure gauge.

  • 3.

    Should the coiled tube rupture, the gas vents from the back of the pressure gauge casing. The face of the pressure gauge is made of heavy glass as an additional safety feature.

Pressure gauge

  • Measures pressure in cylinder or pipeline.

  • Pressure acts to straighten a coiled tube.

  • Colour-coded and calibrated for a particular gas or vapour.

Pressure regulator (reducing valve)

Pressure regulators are used because:

  • Gas and vapour are stored under high pressure in cylinders. A regulator reduces the variable cylinder pressure to a constant safer operating pressure of about 400 kPa (just below the pipeline pressure) ( Fig. 2.6 ).

    Fig. 2.6, The principles of a pressure regulator (reducing valve).

  • The temperature and pressure of the cylinder contents decrease with use. In order to maintain flow, constant adjustment is required in the absence of regulators.

  • Regulators protect the components of the anaesthetic machine against pressure surges.

  • The use of pressure regulators allows low-pressure piping and connectors to be used in the machine. This makes the consequences of any gas leak much less serious.

They are positioned between the cylinders and the rest of the anaesthetic machine ( Figs. 2.7 and 2.8 ).

Fig. 2.7, Cylinder pressure regulators (black domes) positioned above the cylinder yokes in the Datex-Ohmeda Flexima anaesthetic machine.

Fig. 2.8, Cylinder pressure regulator (the machine’s tray has been removed).

Components

  • 1.

    An inlet, with a filter, leading to a high-pressure chamber with a valve.

  • 2.

    This valve leads to a low-pressure chamber and outlet.

  • 3.

    A diaphragm attached to a spring is situated in the low-pressure chamber.

Mechanism of action

  • 1.

    Gas enters the high-pressure chamber and passes into the low-pressure chamber via the valve.

  • 2.

    The force exerted by the high-pressure gas tries to close the valve. The opposing force of the diaphragm and spring tries to open the valve. A balance is reached between the two opposing forces. This maintains a gas flow under a constant pressure of about 400 kPa.

Problems in practice and safety features

  • 1.

    Formation of ice inside the regulator can occur. If the cylinder contains water vapour, this may condense and freeze as a result of the heat lost when gas expands on entry into the low-pressure chamber.

  • 2.

    The diaphragm can rupture.

  • 3.

    Relief valves (usually set at 700 kPa) are fitted downstream of the regulators and allow the escape of gas should the regulators fail.

  • 4.

    A one-way valve is positioned within the cylinder supply line. This prevents backflow and loss of gas from the pipeline supplies should a cylinder not be connected. This one-way valve may be incorporated into the design of the pressure regulator.

Pressure regulator

  • Reduces pressure of gases from cylinders to about 400 kPa (similar to pipeline pressure).

  • Allows fine control of gas flow and protects the anaesthetic machine from high pressures.

  • A balance between two opposing forces maintains a constant operating pressure.

Second-stage regulators and flow restrictors

The control of pipeline pressure surges can be achieved either by using a second-stage pressure regulator or a flow restrictor ( Fig. 2.9 )—a constriction—between the pipeline supply and the rest of the anaesthetic machine. A lower pressure (100–200 kPa) is achieved. If there are only flow restrictors and no regulators in the pipeline supply, adjustment of the flowmeter controls is usually necessary whenever there is change in pipeline pressure.

Fig. 2.9, A flow restrictor. The constriction causes a significant pressure drop when there is a high gas flow rate.

Flow restrictors may also be used downstream of vaporizers to prevent a back pressure effect (see later).

One-way valve or backflow check valves

These valves are usually placed next to the inlet yoke. Their function is to prevent loss or leakage of gas from an empty yoke. They also prevent accidental cross-filling between paired cylinders.

Flow control (needle) valves

These valves control the flow through the flowmeters by manual adjustment. As the valve is opened, the orifice around the needle becomes larger and flow increases. They are positioned at the base of the associated flowmeter tube ( Fig. 2.10 ). Increasing the flow of a gas is achieved by turning the valve in an anticlockwise direction. These valves reduce the gas pressure from around 44 kPa to just above atmospheric pressure before entry to the flowmeter block.

Fig. 2.10, A flow control (needle) valve and flowmeter.

Components

  • 1.

    The body, made of brass, screws into the base of the flowmeter.

  • 2.

    The stem screws into the body and ends in a needle. It has screw threads allowing fine adjustment.

  • 3.

    The flow control knobs are labelled and colour-coded.

  • 4.

    A touch-coded knob controls the oxygen flowmeter.

  • 5.

    A flow control knob guard is fitted to protect against accidental adjustment in the flowmeters.

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