Anaesthetic machines can be classified as being either intermittent flow or continuous flow machines. The Entonox® valve is widely used in the delivery suite to supply nitrous oxide/oxygen analgesia to mothers in labour, but otherwise intermittent flow anaesthetic machines are virtually obsolete and do not merit further consideration. Continuous flow machines provide a mixture of gases and vapours at a constant flow rate into a reservoir, usually a bag, from which the spontaneously breathing patient can inhale. The same flow can be delivered to a ventilator if the lungs are to be inflated mechanically. In either case, the flow rate must be adequate for the patient's requirements otherwise carbon dioxide will accumulate. Continuous flow machines can vary in size from a small wall mounted system used in the anaesthetic room to the larger machines used in the operating theatre. These are mounted on a frame together with monitoring devices, ventilators, spare gas cylinders, have one or more storage drawers, and usually have a metal cover over the pipework to serve as a work surface. The basic functions of the machine are: to deliver compressed gases to patient at a safe pressure, to allow the flow and composition of the gases to be easily adjusted, to permit the addition of a precise concentration of a volatile anaesthetic such as isoflurane, and to deliver this mixture to a common gas outlet and, hence, to a breathing circuit or a ventilator. All hospital machines arrange for the expired gas to be ducted away to a scavenging system. The complexity of a modern operating theatre system is increased by various inbuilt monitors and safety features but the principles of an anaesthetic machine are shown in Figure EQ. 1.
With the exception of portable anaesthetic machines (such as the Triservice® apparatus) modern machines are designed to use compressed medical gases supplied from the hospital pipelines. Oxygen, nitrous oxide and medical air are piped to the operating theatres throughout the hospital at a pressure of 400 kPa. Colour coded hosepipes connect the machine to the gas outlet and incorrect connections are prevented by a non interchangeable screw thread (NIST) at the machine end and a sleeve indexed Schrader probe at the supply terminal. While this method of supply is very reliable, all machines are fitted with a back-up supply of gas cylinders for oxygen and nitrous oxide. Cylinders are also used as the primary source of carbon dioxide though this is being phased out as this gas is not used in contemporary practice. The cylinders are mounted onto a yoke on the back or side of the machine and secured with a wing nut. A system of projecting pins on the yokes engage with matching holes on the cylinder to ensure that the correct one is fitted. Gas leaks are prevented by the interposition of a small rubber and metal seal between the cylinder and the yoke. This "Bodok' seal can easily become lost or damaged and should be routinely inspected whenever the cylinders are exchanged.
When the cylinder valve is opened with a spanner, gas flows through a unidirectional check valve and pressure regulator before entering the internal pipework. The regulator reduces the pressure to 400 kPa and maintains this almost constant despite the cylinder pressure falling with use. The cylinder pressure is displayed on a gauge. Gas supplied from the hospital pipeline is already at 400 kPa pressure and can pass directly into the machine. The flow of each gas (or vapour) through the machine is controlled by a bank of needle valves whose control knobs are again colour coded to aid identification. By convention, the oxygen valve is placed at the left of the bank and is often fitted with a distinctive octagonal knob.
Unscrewing the needle valve allows gas to enter the flowmeter and continue to the manifold known as the "back bar', on which are mounted one or more vaporisers, and thence to the common gas outlet. The vaporisers can be permanently fixed to the back bar but are usually docked onto spring loaded ports (e.g. Selectatec System, Ohmeda, Hatfield, UK) so they can be easily removed. The common gas outlet is a port with a standard 15 mm internal diameter and 22 mm external diameter that can either be fixed or allowed to swivel through 90 degrees (Cardiff swivel) and is strong enough to take the weight of the breathing attachment or ventilator tubing. A pipe runs directly from the oxygen source to the common gas outlet, which bypasses the flowmeter block and back bar. By this method a high flow of oxygen to the patient circuit can be delivered even if there are leaks in the system.
The modern machine has several safety features to ensure that a hypoxic mixture of nitrous oxide cannot be delivered to the patient. Failure of the oxygen supply is indicated by an audible alarm. Typically, this device also cuts off the nitrous oxide supply but leaves the supply of medical air unaffected. The anaesthetist is prevented from setting too low a flow of oxygen either by a system that links the oxygen and nitrous oxide flowmeters (Link System) or by a pressure sensitive valve which prevents the flow of oxygen falling below 25% (minimum ratio system). A combined master switch for electrical power and gas flow is now commonplace on anaesthetic machines. This ensures that the integral oxygen analyser is always active when the machine is in use. Safety devices such as ventilator pressure, minute volume or low oxygen alarms are most useful if they are activated without further conscious effort. Such alarms are rendered ineffective if the warning tone can be silenced for significant periods—most can only be muted for a few seconds.
It is important to note that although anaesthetic machines in the UK are fitted with pressure relief valves, these are set at 30 - 40 kPa to protect the apparatus and not the patient. In some countries the valves are set at 5 kPa to prevent pulmonary barotrauma to the patient.
To the student of design, the contemporary anaesthetic machine might initially appear to be dangerously unergonomic since it has so obviously just evolved from early designs. However, this evolution has resulted in it becoming a reliable device with several important safety features. The challenge for designers is to produce a machine that is easier to use, conveys information about its performance to the anaesthetist in an instantly accessible format, but that also can still be seen to be operating correctly so that malfunctions or disconnections can be quickly rectified. The major manufacturers have responded to this challenge by replacing manual controls with electronically actuated devices. For example, flowmeter blocks can be replaced with solenoid operated valves and conventional vaporisers with "fuel injection' techniques. This permits feedback from sensors to automatically adjust gas flows and mixtures or ventilator settings thus "closing the loop'. These advances are unlikely to be accepted until the software can recognise artefacts in the signals from the sensors and also be made immune to interference from other electrical apparatus such as surgical diathermy or from voltage spikes in the mains power supply.
As pressure from a cylinder varies with content and temperature, this would result in constant adjustment of the needle valve being required. This problem is overcome by using a pressure reducing or regulating valve. The principles of action of a pressure reducing valve are shown in Figure EQ.2.
Figure EQ.3 Principles of a vaporiser
Flow through a flowmeter tube lifts a bobbin in proportion to the flow. The glass flowmeter tubes are calibrated in litres per minute and are accurate to better 10% over most of their range. Early designs of bobbin were inaccurate because of a tendency to adhere to the inside of the tube as static electricity accumulated. Later designs used grooved bobbins that rotated in the gas stream. This system was patented as the "Rotameter' tube. Modern flowmeter tubes are rendered anti-static with conductive materials.
The range of flow-rates varies with each gas and so may the linearity of the scale. Calibration marks may be at intervals of 200 ml/min initially, but they rise to 2 l/min at the top. This is advantageous when setting low flow rates when using the circle system. Even greater accuracy is achieved by connecting a small diameter tube in series with a standard tube in the "cascade' flowmeter block. The first tube typically has a range from 0 to 1 l/min.
Apart from nitrous oxide, almost all other inhalational anaesthetics are liquids at room temperature and pressure but are administered as vapours. Ether and chloroform were initially given by the open drop method onto a gauze mask. Vaporisers became necessary when nitrous oxide and oxygen were delivered through a machine. Early vaporisers were glass bottles containing the liquid agent over which flowed the carrier gas. Output was regulated by a flow splitting valve that allowed some or all of the gas to pass into the bottle instead of flowing past. Low resistance vaporisers could be placed in the breathing circuit whereas higher flow resistance required the vaporiser to be sited on the back bar. These flow controls were not calibrated and the inhaled concentration varied considerably with the flow rate of the carrier gas and as the liquid level fell and cooled with use. Output concentrations of ether from a Boyle's bottle could vary between 5 and 40%, and although this was many multiples of the MAC value (approximately 2%), overdoses were infrequent because the depth of anaesthesia was slow to change with such a soluble agent. Rather more precision was needed to use halothane safely and this stimulated a change in vaporiser design. The principle employed was to split the flow of anaesthetic gas into two streams, one passing into a chamber to emerge fully saturated with vapour which was then diluted by the remaining flow which had bypassed the vaporiser this is shown in (Figure EQ.3). Providing the chamber always produces a fully saturated output, a predictable concentration can be delivered. This is the principle of the plenum vaporiser which is still the basis for most modern designs. The gas flow is either split by having a separate rotameter supplying the vaporiser, as with the "copper kettle', or alternatively the flow is split inside the vaporiser by the action of the control knob, e.g. TEC (BOC/Ohmeda) series and others. A predictable output is ensured by having a large enough surface area for the liquid to evaporate and thus fully saturate the gas in the chamber. The use of a wick increases the surface area or the carrier gas can be forced through a submerged disc to produce a stream of fine bubbles. Since the saturated vapour pressure depends on the temperature, a predictable output may be achieved by keeping this constant by constructing the vaporiser from a large mass of conducting metal or making some compensation as the vaporiser cooled.
The "copper kettle' and Drager vaporisers were fitted with a thermometer and the anaesthetist read off the output from a table or scale. The "TEC 2' design included a temperature sensitive valve that enabled the flow splitting control to be accurately calibrated. The output concentration from the TEC 2 was somewhat dependent on the flow rate of the carrier gas and this was displayed graphically on a small card, which was attached to the vaporiser by a chain. This variability was effectively eliminated with later designs.
Since each agent has its own physical properties, calibrated vaporisers are specific to a single agent and are clearly marked with the formulary name. Later advances have included colour coding the external surfaces and ensuring that the vaporiser could only be charged with the desired agent. The filling port is shaped to adapt to an indexed tube, which in turn can only fit onto the correct bottle. Examples of these vaporisers include the later TEC series (Ohmeda) or Vapor (North American Drager).
The most recent versions in these series have included additional safety features. The control knob includes a release control so that it cannot be accidentally turned on. The control knob may be linked to the lever that locks the vaporiser to the back bar. Turning on a vaporiser causes a control rod to extend which prevents adjacent vaporisers from operating so that only one agent can be used at a time. The reservoir chambers are protected from overfilling and liquid anaesthetic agent is prevented from spilling into the output pathway if the vaporiser is tilted.
Whereas the recently introduced agent sevoflurane can be administered from a conventional vaporiser, the lower boiling point of desflurane (23°C) requires a heated vaporiser. Heating the reservoir to 39°C increases the pressure to approximately 200 kPa. Opening the control knob allows desflurane vapour to leave the chamber and mix with the carrier gases. Since the vaporiser is electrically powered, additional electric sensors and alarms have been incorporated. A modified filler port is also required.
Some recent anaesthetic machines meter gas flows by electrically controlled solenoid valves. The volatile agent can be similarly metered, with the vaporiser part of an integrated system that is totally controlled by a computer program. Some use a form of "fuel injection' where the liquid agent is injected into a vaporising chamber.
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