Basic components of anaesthetic machine and breathing system

• Anaesthetic machine is an assembly of components
• Functions of anaesthetic machine


  1. Receive medical gases (oxygen, nitrous oxide and air) under pressure

  2. Compound them into percentages

  3. Add known concentration of inhalational agent vapour

  4. Deliver the mixture (gases and vapour) to the common gas outlet as fresh gas flow, and from there to the patient through the breathing circuit
• Fundamental to ALL anaesthetic machines is the ability to perform these functions
• Boyles vs Modern anaesthetic machine


  Modern anaesthetic machine are of a continuous flow design, based on the machine introduced by Henry Boyle in 1917
• Basic components of anaesthetic machine


  • Medical gas supply
  • Pressure regulators
  • Pressure gauge
  • Flowmeters
  • Vapourizer
  • Common gas outlet
• Additional components of modern anaesthetic machine


  • Ventilator
  • Patient and machine monitoring
  • Suction apparatus
  • High-flow oxygen flush
  • Pressure-relief valve
  • Oxygen supply failure alarm
  • Scavenging system
• Medical gases


  • Oxygen
  • Nitrous oxide
  • Medical air
• Medical gas supply


  Cylinders or a piped gas system, depending on the requirements of the hospital
• Oxygen cylinders


  • Made of thin-walled molybdenum steel to withstand high pressures (13 700 kPa)
  • Size E cylinders are used on the anaesthetic machine
• Nitrous oxide cylinders


  • Contain a mixture of liquid and vapour
  • In the UK, 75% filled with liquid nitrous oxide (filling ratio)
  • In hotter climates, 67% filled
• At a constant temperature, the pressure in a gas cylinder decreases linearly and proportionally as it empties. This is not true in cylinders containing liquid/vapour
• Oxygen, nitrous oxide, Entonox, compressed air and medical vacuum are commonly supplied through the pipeline system
• Manifolds


  Used to supply nitrous oxide, Entonox and oxygen
• Manifold operation


  1. Large cylinders (e.g. size J each with 6800 L capacity) are usually divided into two equal groups, primary and secondary

  2. All cylinders in each group are connected through non-return valves to a common pipe

  3. This in turn is connected to the pipeline through pressure regulators - which reduce gas/vapour pressure to 400kPa, the standard pipeline pressure
• Changeover between manifold groups

  1. The supply is automatically changed to the secondary group when the primary group is nearly empty

  2. The changeover is achieved through a pressure-sensitive device that detects when the cylinders are nearly empty

  3. The changeover activates an electrical signaling system to alert staff to the need to change the cylinders
• Liquid oxygen storage (VIE)

  • A vacuum-insulated evaporator (VIE) is the most economical way to store and supply oxygen
  • Liquid oxygen is stored (up to 1500 L) at a temperature of −150°C to −170°C and at a pressure of 1000 kPa
• At a temperature of 15°C and atmospheric pressure, liquid oxygen can give 842 times its volume as gas
• Oxygen concentrator


  • Extracts oxygen from air by differential adsorption using zeolite molecular sieve columns
  • Achieves a maximum oxygen concentration of 95% by volume
• Pressure regulators


  • Primary regulators reduce high cylinder pressures (13 700 kPa) to a constant safer operating pressure of around 400 kPa
  • Secondary regulators are located prior to the flowmeters to smooth out fluctuations in the pipeline supply pressure
• Pressure gauge


  • Measure the pressure in the cylinder or pipeline
  • Colour-coded and calibrated for a particular gas or vapour
• Flowmeters


  • Measure the flow rate of a gas passing through them
  • Individually calibrated (at room temperature and atmospheric pressure – sea level) for each gas
  • Have an accuracy of about ± 2.5%
• Flowmeter operation


  1. Has a colour-coded flow control (needle) valve, a tapered (wider at the top) transparent plastic or glass tube, and a lightweight rotating bobbin or ball

  2. When the needle valve is opened, gas is free to enter the tapered tube

  3. The bobbin is held floating within the tube by the gas flow passing around it

  4. The higher the flow rate, the higher the bobbin rises within the tube
• Vapourizer


  • Designed to add a controlled amount of an inhalational agent (in form of a vapour) to the fresh gas flow
  • Variable bypass vapourizers are the most commonly used in modern practice
• Ideal vapourizer characteristics


  • Its performance is not affected by changes in fresh gas flow, volume of the liquid agent, ambient temperature and pressure, decrease in temperature due to vaporization, or pressure fluctuation due to the mode of respiration
  • Low resistance to flow, light weight with small liquid requirement, economy and safety in use with minimal servicing requirements, corrosion- and solvent-resistant construction
• Common gas outlet


  • Where the anaesthetic machine 'ends'
  • The gas mixture made at the flowmeters, plus any inhaled anaesthetic agent added by the vaporizer, exits the machine and enters the fresh gas tubing that conducts it to the breathing system
• Safety features of modern anaesthetic machines

  • Colour-coded pressure gauges
  • Colour-coded flowmeters
  • 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
  • NIST - 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
• Anaesthetic Breathing System (ABS)


  An assembly of components which connects the patients airway to the anaesthetic machine and serves as a conduit for gases from the anaesthetic machine to the patient
• Essential functions of ABS


  • To reliably deliver the intended gas mixture from the machine to the patient
  • To effectively eliminate carbondiaoxide
• Desirable functions of ABS

  • Low resistance to flow
  • Have minimal apparatus dead space
  • Economy of fresh gas flow
  • Adequate humidification of inspired gas
  • Conserve heat and moisture
  • Light weight
  • Convenient during use
  • Efficient during both spontaneous and controlled ventilation
  • Safe to use in all age groups
  • Permits scavenging of waste products
• Components of ABS


  • APL valve
  • Reservoir bag
  • Fresh gas flow tubing
  • Corrugated tubing
• APL valve


  • A spring-loaded pressure release valve in which the release pressure can be varied to suit the situation
  • Allows control of the pressure within the breathing system and therefore the airway
  • Permits control of the airway pressure during positive pressure ventilation, permits application of PEEP, and facilitates scavenging
  • Barotrauma may result from inadvertent closure of the APL valve and it adds bulk to the breathing system
• Reservoir bag


  An essential component of most breathing systems that improves efficiency and permits manual ventilation
• Desirable properties of anaesthetic breathing systems


  • Effective elimination of carbon dioxide
  • Low resistance to flow
  • Minimal apparatus dead space
  • Economy of fresh gas
  • Adequate humidification of inspired gas
  • Conserve heat and moisture
  • Light weight
  • Convenient during use
  • Efficient during both spontaneous and controlled ventilation
  • Safe to use in all age groups
  • Permits scavenging of waste products
• Components of anaesthetic breathing systems


  • APL valve
  • Reservoir bag
  • Fresh gas flow
  • Corrugated tubing
• One or two components may be missing in some breathing systems
• Anatomical and Mapleson's classifications are based on the arrangement of the components
• APL valve


  • A spring-loaded pressure release valve in which the release pressure can be varied to suit the situation
  • Essential component of most breathing systems except the Mapleson E or F
  • Allows control of the pressure within the breathing system and therefore the airway
  • Permits control of the airway pressure during positive pressure ventilation, permits application of positive end expiratory pressure and facilitates scavenging
  • Barotrauma may result from inadvertent closure of the APL valve and it adds bulk to the breathing system
• Reservoir bag


  • An essential component of most breathing systems
  • Improves efficiency and permits manual ventilation
  • Made of anti-static rubber or plastic, latex-free versions also exist
  • The standard adult size is 2 L, with paediatric sizes down to 0.5 L, larger sizes are occasionally used for inhalational induction in adults
  • Acts as a reservoir for oxygen and anaesthetic gases
  • Manual ventilation is achieved by squeezing the bag
  • Acts as a visual indicator of spontaneous ventilation
  • In the event of a valve becoming stuck or being left unintentionally closed, pressure will build up in the breathing system and the reservoir bag will then distend, limiting the pressure to around 60 cmH2O and thus reducing barotrauma
  • It is relatively easy to tear the bag
  • It is not self-inflating and must therefore be used with a pressurized gas supply
• Breathing system tubing


  • Connects one part of a breathing system to another
  • Also acts as a reservoir for gases in certain systems
  • Mostly made of plastic, but also of silicone rubber and silver-impregnated bactericidal plastics
  • They are of variable length depending on the configuration of the breathing system used
  • They must promote laminar flow wherever possible, reason they have uniform and large diameter
  • Size for adults is 22-mm wide, paediatric tubing is 15-mm wide to reduce bulk
  • The corrugations resist kinking and increase flexibility, but produce greater turbulence than smooth-bore tubes
• Classifications of anaesthetic breathing systems


  • Anatomical classification
  • Functional classification
  • Mapleson's classification