For the simplest anaesthetics, as in Figure 1, a patient can provide the energy to move anaesthetic gases.

Clover apparatus with chloroform
Anaesthetics: J.T. Clover. Credit: Welcome Collection.

This approach is very limitted and fails as:

• The requirements for supplemetal gases increase (a source of compressed gas quickly become the only practical way in which to deliver what is required)
• The circuit resistance increases (the patient can no longer sustain the work of breathing
• There is a need for muscle relaxation.

As a result, we are used to having gasses supplied in cylinders or via pipelines. Since the volume of a cylinder is limitted by practical considerations, it is unsurprising that the pressures within cylinders are as high as possible (e.g. oxygen 137 bar / 2000 psi) to maximise the quantity of gas which they contain. By contrast, a pipeline need only run at the lowest pressure (e.g. 4 bar) needed to reliably deliver the required flow.

In order to control the supply of a compressed gas we require devices which will limit pressures (regulators) and flows (valves). Because we use gases from sources with very different pressures, these may be cascaded i.e. a high pressure regulator is used in series with a low-pressure regulator.

These systems will be discussed in the followng sections

Physics of gases

See physics - gases

Cylinders

A fatal cylinder rupture

Cylinders are a convenient means of providing access to small amounts of gas, but they must be regarded with caution. Some cylinders contain gases at high pressure, whilst others contain liquids and vapors. A rupture may allow the contents to expand violently and some may also support combustion (oxygen or nitrous oxide). As a result, a great deal of effort is expended to make cylinders safe.

• Material: molybdenum or chromium steel. These are strong and corrosion resistant.
• At the time of manufacture 1% of cyclinders are cut into strips for testing.
• At intervals, each cylinder is subject to high-pressure, hydraulic testing (steel cylinders every ten years and composite cylinders every five years). It is filled with water pressurised to 200-250 bar to confirm its ability to withstand these pressures and measure the elastic recoil of the metal. For safety, this occurs in a water tank. The cylinders are also inspected endoscopically. The date when the next test is due appears on the plastic disc attached to the neck of every cylinder (see below).

Next test date appears on the cyclinder neck. In this case, May (Green label), 2018 (Grey label).

Fuseable cylinder plug. The low melting-point material in the centre of the plug melts as the temperature rises, allowing the cylinder to vent.

Fuseable seal. The low melting-point material in in the seal melts as the temperature rises, allowing the cylinder to vent.

No matter how strong the cylinder, if its temperature is raised sufficiently, then rupture becomes inevitable. In order to prevent this, a low-metling point material is used to allow controled venting. In the US, this generally takes the form of a fuseable plug. In the UK, the sealant between the valve and cyclinder body fulfills the same purpose. Clearly, the venting gas may exacerbate any fire, but this remains a much better options than an explosion.

In recent times, composite cylinders have appeared (e.g. BOC medical, industrial). These consist of a steel or aluminium inner liner, externally reinforced by a fibre-glass. Kevlar or carbon-fibre winding in epoxy resin. They have the advantage of being lighter and stronger than steel cylinders, can be filled to higher pressures and include integral regulators and valves. Unfortunately, even quite minor damage to the fibre-reinforcement can result in failure, although literature reports indicate the regulator as the initial seat of several fires ([1], [2]). Although in widespread use, the cylinders should be treated with even greater care than standard cylinders and they should be turned on away from the patient and only transported in cylinder holders (i.e. NOT on flammable bedding where a devastating fire can take hold within seconds).

Identification

Although gas cylinders are colour coded, the correct way to identify the contents is via the cylinder label. This records the followinf information, as shown in the images below.

• Product name, chemical symbol and form
• Product specifications
• Product license
• Cylinder size code and contents in l
• Instructions for handling and use
• Batch label showing the batch number, expiry date, size and gas
• Hazard warning diamonds
• Emergency contact details

Cylinder labels

Cylinder sizes

Cylinder sizes are described by alphabetic codes, the commonest of which are shown below

Code	Construction	Water capacity (l)	Valve type	Outlet	Dimensions (mm)	Oxygen	Nitrous oxide	Air	Entonox	CO2	Heliox
Neck
Content						Gas	Liquid	Gas	Gas	Liquid	Gas
CD	Composite	2.0	Handwheel	Integral Schrader/firtree	520*100	230 bar 460 l	N/A	N/A	N/A	N/A	N/A
D	Standard	2.3	Key	Pin-index	535*102	137 bar 340 l	44 bar 900 l	N/A	137 bar 500 l	50 bar 900 l	N/A
E	Standard	4.7	Key	Pin-index	865*102	137 bar 680 l	44 bar 1800 l	137 bar 640 l	N/A	50 bar 1800 l	N/A
F	Standard	9.4	Key	Bullnose	930*140	137 bar 1360 l	44 bar 3600 l	137 bar 1280 l	137 bar 2000 l	N/A	N/A
HX	Composite	10.0	Handwheel	Integral Schrader/Firtree	930*140	230 bar 2300 l	N/A	N/A	N/A	N/A	200 bar 1780 l

Common cylinder properties.

Valves

In order to contain the gas in the cylinder, there must be some form a valve. For medical gases, you are likely to encounter:

• Pin-index (below)
• Bull-nose (below)
• Integral with regulator. Standard with composite cyclinders
• Handwheel - unusual in anaesthetic practice.

Opening a valve without a regulator in situ used to be a standard way to clear any dust or debris before connecting the cylinder. This was referred to as 'cracking' the cylinder and is dangerous. It is easy to rupture an eardrum by standing in-line with the escaping gas, so great is the pressure. Modern cylinders are supplied with dust seals which are removed immediately before use.

>Pin-index

Cylinder yoke with pins

Cylinders are connected to the machine via a yoke (Fig. 7). It is essential that the correct gas is connected in each position. In order to prevent the wrong cylinder from being inserted, the yokes employ a system of pins which will only engage with cylinders with corresponding drillings (Fig.7).

Cylinders have matching drillings in any of 7 positions.

The pins are numbered in a clockwise direction with reference to the yoke (anti-clockwise when considering the cylinder drillings) and run in the order 1237456 (Fig. 8).

A Bodok seal

Simply tightening the screw tensioner (Fig. 5) is not enough to provide a gas-tight seal between the metal surface of the cylinder valve and that of the yoke. A washer is required. Unfotunately, given the high pressure involved, a simple washer would be likely to fail. The solution is to use a washer with a peripheral metal reinforcement. This is know as a 'Bodok seal' (Fig.9). The neoprene from which the seal is made, is resistant to combustion even when exposed to the adiabatic heating which occurs as the cylinder valve is opened.

Bull-nose valves

These are found on larger clyinders (F and above) and require the attachement of a regulator.

Bull-nose connections.

Cylinder contents

A cylinder may contain a compressed gas or a vapour in equilibrium with liquid. Although the 'packaging' appears identical (a cylinder), they can have very different properties.

Differing behaviours of liquid and gas-filled cylinders.

Cryogenic liquid systems

A cryogenic liquid oxygen system

As demand for gas (almost exclusively, oxygen) increases, bulk cryogenic tanks become the only practical solution. This allows the delivery of liquified gas by tanker and the system can meet very high peak demands. The layout of a typical system is as shown below.

A VIE and main evaporator

The main component is the 'vacuum insulated evaporator' (VIE) which is a double-walled pressure vessel. The inner vessel contains the cooled, pressurised, liquified gas, while the space between the two vessels is maintained at high vacuum. Since no insulation can be perfect, some heat will enter the vessel and vaporize some of the liquid. This can be vented by pressure relief valves (which will open at a pre-determined pressure and then close as the pressure falls), but often the gas can be used immediately and is simply passed into the pipepline. In case of failure, there is also a 'rupture' valve in the system. This will break if the pressure in the system becomes excessive and discharge the entire contents of the VIE.

In order to ensure that the pressure within the VIE does not fall below its operating level, a valve will release liquified gas into a 'pressure maintaining vaporizer' when the VIE pressure decreases. This uses environmental heat to vaporize the gas and maintain the required pressure.

A similar principle is used when a demand is placed on the VIE. Liquified gas is allowed to enter the main vaporizer where it warms and feeds the main gas piplelines. As this process occurs, the vaporizer cools and you will often see them with a thcik coating of ice.

A differential pressure guage provides an indication of the quanity of liquid in the tank (since it is measuring the differential pressure between the top and bottom of the VIE, it provides a measurment of the height of the fluid column above the bottom of the tank NOT the 10.5bar working pressure of the system). However, this is not regarded as a reliable enough for filling. Instead, a pipe built into the VIE and placed at a fixed position opens as part of the 'tri-cock filling valve'. As the fluid level in the VIE rises, eventually the pipe opening is submerged and it will vent liquid rather than gas. This is the indicator that the VIE is filled to its maximum capacity.

For more details, this is a link to a US cryogenic gas supplier which describes the process very well.

Regulators

A high-pressure cyclinder regulator in an anaesthetic machine. The narrow-bore copper pipe coming from the back of the regulator supplies the pipeline pressure gauge.

Pipelines delivers medical gases at a pressure of 400kPa (4bar). Cylinder pressures are much higher (O₂ and air 137bar and N₂O 44bar) and yet both must supply the same systems despite wide variations in demand. In order to achieve this, reulators are required. These mechanical devices will accept variable input pressure and provide a constant output pressure across a range of flows. A simple design is show below.

Schematic of a simple pressure regulator

When the cylinder is opened, the force pushing the poppet valve upwards is P_cyl*A_pop. If we tighten the set screw such that the downward force provided by the spring just opens the poppet when P_out is at the desired pressure, the system will tend to maintain this pressure. If P_out falls, then the upward force on the diaphragm (P_out*A_d) will fall. This allows the spring to push the diaphragm downwards and opens the poppet valve wider. Conversley, if P_out rises, then the upward force on the diaphragn (opposing the spring) increases and tends to close the valve. In use, tthe forces exisit in balance, so the valve is help partically open, maintaining P_out at the desired pressure.

The area of the diaphragm (A_d) is shown as being much larger than that of the poppet valve seat (A_pop). This is important because we want the position of the diaphragm to be controled solely by the balance between the force of the spring balanced against P_out*A_d. In reality, P_cyl*A_pop also plays a role; this is just an unfortunate side-effect of the design but means that as the cylinder pressure falls, the valves tends to open more and P_out will rise. By making A_d much larger than A_pop, this effect is minimised since changes in P_out produce much larger movements of the diaphragm than do changes in P_cyl.

Anaesthetic machines generally use piped gases and provide a cyclinder as an emergency backup. Cylinders should be checked during the machine check and their valves should then be closed to prevent them from being emptied inadvertently. An additonal safety feature is provided by careful setting of the high-pressure (cylinder) regulator; this is usually set to deliver an output pressure of a little below pipeline pressure. In this way, when the pipline is connected, the output of the cylinder regulator is exposed to a higher pressure than it is set to deliver and it should close preventing accidental use of the cylinder.

In applications where very stable pressure control is required, regulators may be cascaded so that a 'first-stage' regulator, receiving widely varying input pressures (e.g. 0-137 bar from a cylinder) supplies a 'second-stage' regulator with a much more closely controlled (e.g. 9-11 bar) gas supply. The second stage regulator can now be expected to operate within much closer tolerances. In the anaesthetic machine, however, we have a slightly different requirement. Here we genrally want the machine to be able to deliver constant flows rather than constant pressures and as a result, the gas from the cylinder regulator or pipeline is fed to a flow-meter.

See Physics - pressure measurement

N₂O flow-meter tube

Flow-meters

In the anaesthetic machine we generally want to be able to control gas flow and so the term 'flow-meter' generally refers to two, linked components; a needle valve and a variable area, constant differenial pressure flow-meter.

Needle valves

A needle valve

Needle valves allow very precise control over the flow of high pressure gas into the flow-meters. The valve block is usually a separate component so that it can be replaced if damaged. Because of the delicacy of the valve, it shouls never be over-tightened and will often have 'stops' to prevent this from hapening.

Where nitrous oxide is used, the needle valves may be modified to prevent the creation of an hypoxic mixture. This system uses a pair pf cog-wheels on the oxygen and nitrous valves, linked by a chain, such that a minimun oxygen:nitrous ratio is always maintained. Unfortunately, the introduction of low-flow systems renders this safety device ineffective.

Variable area, constant differential pressure flow-meters

Given the complexity of the name, it is hardly surprising that the trade name 'Rotameter' has been adopted as a generic term to describe this type of device (Fig. 22). A cylindrical bobbin is suspended in a tapering tube (usually glass). As can be seen in the highly stylised figure below, at the bottom of the tube, the gap between the bobbin and wall is reminiscent of a long, thin pipe where laminar flow may be expected. As a result, flow is described by the Hagen-Poiseuille equation (equation 1) and the pressure drop is related to the viscosity of the gas (equation 2). Further up the flow-meter tube, the appearance much more resembles an orifice where turbulent flow is to be expected. As a result, ΔP is now related to the density of the gas (equation 4) and the square of the flow ! It is only with very careful design that the gradations on the flow-meter tube can be set at useful intervals (often a log scale).

The design of the Rotameter

The rotameter includes a number of design features intended to enhance safety

• The bobbin has slots cut into it so that the flowing gas tends to make it rotate. This centres the bobbin in the gas stream (improving accuracy) but also makes it easy to see that it has not become stuck
• The top of the rotameter includes a 'stop' so that the bobbin remains visible at maximum flow (or even above) and there can be no doubt that a very high flow of gas is running. This is important when using gases which migh produce an hypoxic mix.
• The rotameter tube is often coated with a gold film. This allows static charge to leak away easily and help to prevent the bobbin from becoming stuck.
• By careful design, the bobbin moves much further at low flows than high. This makes it possible in a single rotatmeter to make delicate adjustments at low flows while still being able to deliver high flows. An alternative approach is to use cascaded rotameters, the first covering a low-flow range. When the bobbin moves off-scale on this tube, the next rotameter will be at the bottom is its range (e.g. two rotameter one with a range of 0-1l and one with a range of 0-15l).

The fact that a rotameter tube must be designed with reference to both the viscosity and density of a gas means that it can only be used with that gas. Rotameters are NOT interchangeable.

Backbar

The first anaesthetic machines assembled the basic components which had come to be standard for anaesthetic use; they tended to be monolithic. In order to allow flexibility, the backbar developed to allow vaporisers to be connected and disconnected at will. Initially, these connections were similar to those in the breathing system (push fit), but gradually a more sophisticated system developed.

The TEC backbar

The TEC backbar allows vaporisers to be added at will and can support multiple vaporisers or none. In order to achieve this, it has mounts are self-sealing valves. In the 'up' position (no vaporizer), gas simply passes along the backbar. In the 'down' position (vaporizer present), gas is diverted via the vaporiser.

A schematic of the TEC backbar

Manufacture and storage of gases and vapours / safety

Individual gases

Oxygen

Oxygen is obtained through fractional distillation of liquified air. This produces large quantities at very low prices, such that we use oxygen with little thought. In hospital, the supply is cheaper and more plentiful than that of air.

Oxygens support combustion and at high pressures or conentrations, some material which would not normally burn do so vigorously. Often very little energy is required to start a fire. On opening a cyclinder, the adiabatic heating which occurs as the residual gas in a regulator is compressed can easily ignite any grease which might have been applied inappropriately. A spark is enough to cause bedding, clothes or hair to burn furiously.

Handling liquid oxygen brings additional risks. Skin will stick to and freeze to any of the cold surfaces exposed to the liquified gas. The drastic expansion which occurs as the liquid vaporizes demands that it be handled in areas which are well ventilated and clear of any combustible materials. Just walking on asphalt which has been drenched in liquid oxygen can be enough to cause ignition - as a resul, you will always see a VIE mounted on a concrete base. Nonetheless, the volumes of gas which can be released in event of failure are so large that the repercussions can be widespread, as shown in the photgraph below.

A well-managed VIE failure

Falures in oxygen delivery systems are so rare, that we give them little thought. Nontheless, they do happen and we are so dependent on having immediate access to a reliable supply, that is is important to consider what to do in an emergency.

Nitrous oxide

Nitrous oxide is supplied either as a piped gas from a cylinder manifold or from individual cylinders. It is present as a compressed liquid at a pressure of 44bar. As vapour is drawn from the cylinder, more of the liquid nitrous oxide evaporated to maintain the equilibrium. Since evaporation requires energy, the cylinder temperature will fall and it is common to see a 'frost line' on small (D or E) cylinders used at high flows. This temperature drop will be associated with a fall in the cylinder pressure, but this cannot be used to gauge content. If the cylinder is allowed to re-warm, the pressure will return to 44bar, even if it almost empty. The only way to determine content is by weight and this is not particularly practical as the weight of gas is small by comparison with that of the cylinder. In the days before pipeline supplies, the solution was to have two cylinders on all machines - one in use (the 'duty' cyslinder) and one as a reserve.

Manufacture

Nitrous oxide is produced by the controlled heating of ammonium nitrate to 240–250°C in a decomposition reactor. It is crucial to control the temperature to prevent explosive decomposition !

The reaction is

NH4NO3 -> N2​O + 2H2​O

The nitrous oxide and water vapor mixture is collected and the the hot gases cooled. This condenses and removes most of the water vapor. The remaining gas is then purified by 'scrubbing', which involves passing it through chemicals which remove impurities such as other nitrogen oxides. Common scrubbing materials include solutions of alkalis like sodium hydroxide (NaOH) to neutralize acidic byproducts. Finally, the gas is dried further using desiccants (e.g., silica gel or molecular sieves) to remove residual moisture.

The purified nitrous oxide is compressed and cooled to liquefy it for storage. It is also tested to ensure it meets medical standards.

Air

Air is supplied in cylinders or via a pipeline. The cylinders have the same properties as their equivalents for oxygen.

The piped gas is at either 7 bar (used to power surgical tools) or 4 bar (all other uses) and comes from local compressors. Care must be taken when deciding where the air intakes are placed in order to avoid drawing in fumes. Contamination can also arise from the oil used to lubricate the compressors' cylinders. The difficulty of maintining a bank of compressors means that air is a much more expensive product than oxygen !

Entonox

Entonox is a 50% mixture of oxygen and nitrous oxide which is used for analgesia. It is provided in cylinders or via pipeline and is usually delivered to a patient via a 'demand valve'.

When mixed with oxygen and compressed, the nitrous oxide does not liquify as you might expect (the oxygen reduces the critical pressure of nitrous oxide). As a result, the 137 bar cylinder contains a gas mixture, ready for use. Unfortunately, this is only true under certain conditions. If entonox is exposed to 'low' temperatures, that is, its 'pseudo-critical' temperature', then the nitrous oxide will liquefy. The pseudo-critical temperature is pressure dependent (see table below). At pipeline pressures, there is no risk. However, at cylinder pressures, the pseudo-critical pressure is within a range to which cylinders may be exposed during transport or while in a hospital gas store. If such a cylinder is used, then the first gas drawn is almost pure oxygen, but the nitrous concentration gradually increases and will be close to 100% as the cylinder empties. This risk can be avoided by storing cylinders horizontally (increases the surface area over which the oxygen can dissolve into the liquid nitrous oxide) at above 5°C for 24hrs. Prior to use, cylinders should be inverted three times (ensures physical mixing of any liquid nitrous with the gas component). All of this was clear to our predecessors, but sadly in the rush to show ourselves 'environmentally friendly', there is a move away from pipeline entonox supply and old tragedies will reoccur.

Psuedo-critical temperatures for entonox