7.3 – Anaesthesia Machine

INTRODUCTION

The function of breathing systems is to deliver oxygen and anaesthetic gases to patients and eliminate carbon dioxide. All breathing systems are composed of similar components but are configured differently. The common components include: fresh gas flow, tubing to direct gas flow, an adjustable pressure limiting valve to control pressure within the system & allow scavenging of waste gas and a reservoir bag to store gas and assist with ventilation.

Each breathing system receives three sources of gas: fresh gas, exhaled dead space gas and exhaled gas from the alveoli. The proportions of each within the system are most greatly influenced by fresh gas delivery. Gas is delivered to spontaneously breathing patients at sub-atmospheric (negative) pressure during inspiration and atmospheric pressure during exhalation. Conversely, ventilated patients receive gas at positive pressure during inspiration and atmospheric pressure during exhalation. In this tutorial, we will explore the different components and types of breathing systems used in common practice.

COMPONENTS OF BREATHING SYSTEMS

A breathing system is made up of components that connect the patient to the anaesthetic machine, and is usually composed of some or all of the following components:

  1. The Adjustable Pressure Limiting (APL) valve allows a variable pressure within the anaesthetic system using a one-way, spring-loaded valve. At a pressure above the opening pressure of the valve, a controlled leak of gas is allowed from the system, which enables control of the patient’s airway pressure. The minimum pressure required to open the valve is 1cm H2O. A safety mechanism exists to prevent pressure from exceeding 60cm H2O, however, be aware that pressures below this can lead to barotrauma.
  2. The reservoir bag allows collection of fresh gas flow during expiration, which in turn minimises the amount of fresh gas required to prevent rebreathing. In addition, it allows the anaesthetist to monitor the breathing pattern of a spontaneously breathing patient. These are usually plastic or rubber, and can come in sizes between 0.5 litres to 6 litres. However, the most common size in the adult system is 2 litres. Laplace’s Law states that pressure is equal to twice the radius divided by the radius of the bag. Therefore, as the bag increases, the pressure within it reduces. This is an important safety measure as the expansion of the bag to accommodate gas limits pressure within the system.
  3. The inspiratory limb allows passage of fresh gas flow to the patient for inspiration. The expiratory limb allows passage of expired gas from the patient. Although tubing length varies depending on the system in use, the diameter is of standard size: 22mm for adult and 18mm for paediatric systems.

The basic design of an anaesthesia machine consists of pressurised gases supplied by cylinders or pipelines to the anaesthetic machine, which controls the flow of gases before passing them through a vapouriser and delivering the resulting mixture to the patient through the breathing circuit. Modern machines have five elements (1) A high pressure supply of gases, (2) pressure gauges on O2 cylinders, with pressure reducing valves, (3) flow meters (4) metal and glass vapouriser bottle for ether and (5) a breathing system.

The anaesthesia machine is a continuous flow machine in which all the components are mounted on a table. Box shaped sections of welded steel or aluminium provide a rigid metal framework mounted on wheels with antistatic tyres (Castors) and brakes. Antistatic measures improve flow meter performance and where flammable vapours are used, reduce the risk of ignition.[]

The basic machine has provision for fixing two O2 cylinders and two N2O cylinders through the yoke assembly with PISS. There is also provision for connecting the pipeline gas source of O2 and N2O (from the wall outlet with quick couplers and yoke blocks at the machine end) instead of one of the cylinders at the yoke assembly. A pressure gauge is mounted on to the yoke assembly to read the pressure in the cylinder. Pressure regulators are located downstream of the yoke assembly, which reduce the high pressure in the cylinders to a low and constant pressure of 45-60 PSIG.[] From the pressure regulators, there are connections through high pressure tubings constructed of heavy duty materials to the flow meter assembly, which is secured to the back bar of the machine by one or more bolts. The back bar supports the flow meter assembly and the vapourisers. At the end of the back bar, there is the common gas outlet to which the breathing circuits are connected to provide the anaesthetic vapour containing O2 enriched gases to the patient.

FUNCTIONS OF ANAESTHESIA MACHINE

The machine performs four essential functions:

  1. Provides O2
  2. Accurately mixes anaesthetic gases and vapours
  3. Enables patient ventilation
  4. Minimizes anaesthesia related risks to patients and staff.

Watch

Video: Introduction to anaesthesia machines

Patient safety is of principal concern in the architecture, installation, commissioning and service of anaesthesia gas supply system. Many inbuilt safety areas are incorporated within the system. Oxygen is one of the hugely utilised medical gases, intended primarily for life support, anaesthesia and respiratory therapy. Primary utility of medical air is as driving source of ventilators and nebulisers. Medical air is commonly combined with air or oxygen for mechanical ventilation of patients in the operation room or critical care unit. Nitrous oxide is frequently mixed with air or oxygen to serve the purpose of analgesia and anaesthesia. A 50% mixture of oxygen and nitrous oxide, popularly known as entonox is used as a labour analgesia in maternity suites. Carbon dioxide is required regularly for insufflations during laparoscopic surgeries. Helium-oxygen mixture is of benefit to treat patients with airway obstruction and also to alleviate respiratory distress. Medical vacuum is catered almost in every clinical area powered by centrally placed vacuum pumps. Handling, transportation and storage of anaesthetic gas should be dealt with extreme caution. Risk assessment should include concerns related to use of oxygen and other gases.

Troubleshooting Anaesthesia Machines

Required Tools for Troubleshooting Anaesthesia Machines

  • Normal service tools
  • Silicone grease
  • Light oil
  • Service manuals
  • Timer
  • Spare parts
  • Mercury sphygmomanometer
  • Anaesthetic gas analyser (if available)
  • Device for measuring flow, pressure and tidal volume

Testing procedure

If there is an official service manual, follow the steps outlined in it; otherwise, follow the procedure below.

Check for leaks in the high-pressure system.

  1. Turn off all flowmeters, and disconnect any ancillary equipment such as ventilators and suckers.
  2. Turn on each cylinder in turn and allow the system to pressurize, then turn the cylinder off. Watch the pressure gauge; if the needle drops, there is a leak.
  3. Remove the covers and brush each joint, or suspect point, with soapy water. Do not forget to check inside the back of the pressure gauge. A leak will be indicated by the formation of bubbles. Do this for each cylinder in turn.

Check the operation of each flowmeter. Make sure the control knob stays where it is set, and is not liable to be turned by mistake.

  1. Close all valves on the machine. Open all cylinder valves one full turn, noting any movement of the flowmeter floats. Float movement indicates a leaky flowmeter valve. If so, adjust the stop so that gas flow ceases 1/8 turn before the knob reaches the stop.
  2. Verify flowmeter accuracy ( ± 2.5 % full scale), with the measuring device connected to the common gas outlet.
  3. Check that the needle-valve stems are tight enough to remain where set unless deliberately turned by the operator.

Check the low-pressure system, which is the part from the control knobs to the
outlet.

  1. Check the top and bottom seals on the flow tubes with a low-pressure test.
    – Connect a mercury sphygmomanometer to the outlet.
    – Turn on the oxygen flowmeter very slowly.
    – Pressurize the back-bar to 30 mrnHg (4 kPa). When this pressure is reachedturn down the flow until the pressure on the gauge remains constant at 30 mmHg (4 kPa).
    – If the flow is less than 100 ml/min, it is acceptable; if it is greater than this, look for a leak
  2. Check all the joints on the back-bar vvith a high-pressure test.
    – Pressurize the system to 150 mmHg (20 kPa).
    – Reduce the flow to maintain that pressure.
    – If the flow is 100 ml/ min or less, it is acceptable; if it is greater than this, look for a leak. Brush each suspect point with soapy water; bubbles will appear at the site of the leak.

Check the correct operation of the oxygen-failure warning whistle (if fitted). Pressurize the intermediate system, turn off the supply, and open the oxygen-flowmeter valve to reduce the pressure slowly. The whistle should sound for a minimum of 10 seconds when the pressure falls to between 180 and 250 kPa. Check that the flow of nitrous oxide is cut off when the oxygen is turned off (if
that system is fitted).

Check the oxygen-flush valve. It should allow a flow greater than 35 litres/min, but not more than 75 litres/min (or as required by local regulations).


If there are hoses for connection to a wall supply, check these. Check the oxygen flow from the oxygen flow tube when the oxygen probe is plugged in and the nitrous oxide disconnected. Similarly, when the nitrous oxide probe is plugged in and the oxygen disconnected, nitrous oxide must flow from the nitrous oxide flow tube, and nothing from the oxygen flow tube.
Check the non-return valve on each of the yokes. Pressurize the system, turn off the cylinder, and remove it. There should be no gas leak. If there is a leak, look for the simple non-return valve in the yoke. Dismantle and clean the valve, then reassemble. If the anaesthetist has been having difficulty in getting patients to sleep, or waking them up, there may be. a problem with the vaporizer. Inspect the mountings and connectors to ensure that they are secure and leak-free. When servicing vaporizers, be sure to keep them in an upright position when they contain liquid anaesthetic. To check the vaporizer, an anaesthetic gas analyser is required. Conned the gas analyser to the common gas outlet. Set the oxygen flow to 3 litres/min, and after zeroing the gas analyser with 100% oxygen, test
each vaporizer at each full percentage setting. Determine that there is no concentration of gas when the vaporizers are in the “off” position. Replace any vaporizer for which the concentration is incorrect by more than 0.3 % of the reading, or 10% of the measured value, whichever is greater. If an anaesthetic gas analyser is not available, you can only check (a) that the vaporizer is off when it is turned off, (b) that it gives an output when it is turned on again, and (c) that the concentration of gas increases as the control is turned up. Check that the control knob turns smoothly. Vaporizers should be returned to the manufacturers, or their agents, for checking every few years. The interval depends upon the model; some models need a check by the company only every 10 years.
Check the smooth operation of the pressure gauges. The pointer should move smoothly and come to rest before the flow in the flow tube stops. If the movement is not smooth, lubricate the linkage in the back with silicone grease.
Check the absorber. Check for smooth operation of the controls and for leaks. Change the filling if required.

Check any other back-bar-mounted equipment.
Check the output pressure of the regulators. This should be around 390 kPa (or as required by local regulations), and in any case should be about 35 kPa lower than the output pressure from the wall outlet.
Check all flexible tubing on the machine.
Check all attached equipment, such as suckers and blood pressure machines.
Check the drawers, wheels, and the general frame of the machine. Lubricate lightly as required.
Clean the machine.
Tick off all tests on the service sheet, and sign it.
Return the machine to the user. The doctor in charge should test it to make sure that it operates satisfactorily.

Watch

Video: Troubleshooting anaesthesia machines

Attributions

Portions of this chapter were adapted from:

 References

License

Icon for the Creative Commons Attribution-NonCommercial 4.0 International License

Biomedical Instrument Troubleshooting Copyright © by Brendan Chapman, Soheil Ghoreyshi, Centennial College is licensed under a Creative Commons Attribution-NonCommercial 4.0 International License, except where otherwise noted.

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