In 1917, ‘Cocky’ Boyle developed a machine designed to deliver anesthesia. Current anesthetic machines in use are all developed from his original design. The basic principle involves supplying pressurized gas to the anesthetic machine, which regulates the pressure prior to passing through a vaporizer for the addition of a volatile anesthetic. The gas mixture is then delivered to the patient via a breathing system.
Power supply
Modern anesthetic machines are connected to an electrical supply to power the ventilator and integrated monitors. Before use, the machine must be connected directly to mains electricity and switched on. In the event of power failure, modern machines often contain a back-up power source, and the charge status of the battery is checked as part of the machine ‘self-test’.
Gas supply
Gases are required to provide continuous-flow anesthesia. These are delivered to the machine either from a piped supply or from cylinders.
Piped gas supply
Hospitals’ piped medical gases and vacuum (PMGV) systems supply gases. a large device for storing liquid oxygen. Banks of cylinders are used to supply nitrous oxide and air although some hospitals use a compressor for the latter. Some hospitals now use oxygen concentrators to provide their piped supply. These gases are delivered to a terminal outlet that has a collar specific and color-coded for each gas.
Delivery of piped gas to the machine
Flexible hosing links the piped supply to the machine, where it is connected with a non-interchangeable screw thread (NIST) connection.
Cylinder gas supply
All machines must have at least two oxygen sources in case of failure of one. This second source is an oxygen cylinder housed on the back of the machine. The standard size E cylinder is supplied at 134 bar. Other cylinder gases, such as air and nitrous oxide, are also often present. A pin-index system is used as a safety system to ensure the incorrect cylinder cannot be attached to the machine. A Bodok seal separates the cylinder from the yoke and provides a gas tight connection.
Pressure gauges
Color-coded Bourdon gauges indicate the pressures of the piped and cylinder supplies. Many newer machines display numerical pressure values on their screens.
Pressure-regulating valves
Cylinders contain gases at higher pressures than the PMGV supply so pressure-regulating valves bring the cylinder supply to an operating pressure of 400 kPa (4 bar) e consistent with the piped supply.
Oxygen failure alarm
If oxygen pressure falls an alarm must sound. Originally, this alarm was a mechanical device called a Ritchie whistle powered by the nitrous oxide supply but modern anesthetic machines employ electronic alarms. This is checked by disconnecting the oxygen pipeline.
Flowmeters
Gases pass through an adjustable needle valve into flowmeters to accurately measure gas flow through the anesthetic machine. The conventional form of a flowmeter is known as a Rotameter. These are constant pressure, variable orifice devices. Rotameters have a ball or bobbin which moves proportionally to the gas flow within a tapered glass tube (wider at the top than bottom). The pressure across the bobbin is fixed, as the upward flow of gas being measured is opposed by gravity acting on the bobbin. At low flows, the area around the bobbin is small due to the conical shape of the flowmeter, and it behaves like a tube. Here, flow is laminar and therefore dependent on viscosity. At higher flow rates, the clearance between the bobbin and the wall is large in comparison to the length of the bobbin. Flow at this point is as though through an orifice, and therefore dependent on density.
The rotameter has several safety features:
 color-coding display units and needle valve control knobs for each gas.
 the oxygen knob is larger and octagonal in shape and projects out further so that it is identifiable in darkness, and, in the UK, is always located to the left of the other rotameter knobs for nitrous oxide and air.
 oxygen is added last to the gas mixture to prevent any being lost if there is damage to the other rotameters.
 the nitrous oxide is cut off in the event of oxygen failure. This is done either by a ratio-mixer valve, a chain linkage between the oxygen and nitrous oxide needle valves or electronically using oxygen analyzers.
 the rotameters have an internal anti-static coating of precious metal to prevent sticking of the bobbin.
 the bobbins rotate to provide visual confirmation of flow.


By: B.M.T. Montadher Talib