Vaporizers

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Chapter 3 Vaporizers

Many inhalational anaesthetic agents are liquids under normal storage conditions and need to be in a vapour form before they can be administered to a patient. In order that they may be administered safely, an understanding of the phenomenon of vaporization is required.

Laws of vaporization

Molecules of a liquid have a mutual attraction for each other (a phenomenon called cohesion), which is sufficiently great for them to remain in close proximity. But they also possess varying degrees of kinetic energy and are in constant motion, colliding with each other. If the liquid has a surface exposed to air or other gasses, or to a vacuum, some molecules with a high kinetic energy will escape from this surface, resulting in the process of evaporation or vaporization. The molecules from the liquid, which exist in the gaseous phase, are known collectively as a vapour. This vapour exerts a pressure on its surroundings, which is referred to as vapour pressure. If the space above the liquid is enclosed, some of the molecules that have escaped while moving freely in the gaseous state will collide with the surface of the liquid and re-enter it. Eventually, there will occur an equilibrium in which the number of molecules re-entering the liquid equals the number leaving it. At this stage the vapour pressure is at a maximum for the temperature of the liquid and so is called the saturated vapour pressure (SVP).

Factors affecting vaporization of a liquid

Temperature

Vaporization is increased if the temperature of the liquid is raised, since more molecules will have been given sufficient kinetic energy to escape. Fig. 3.1 shows the vapour pressure curves of volatile anaesthetic agents (as well as water) and shows how they vary with temperature. If the liquid is heated, a point is reached at which vaporization now occurs not only at the surface of the liquid, but also in vapour bubbles that develop within its substance. The liquid is now boiling and this temperature is its boiling point. At this temperature, the SVP of the liquid is equal to the ambient atmospheric pressure.

Figure 3.1 Vapour pressure curves for anaesthetic agents.

The boiling point of a liquid may therefore vary with atmospheric pressure. At high altitudes (where the air is thinner, has a lower ambient pressure and therefore exerts less pressure on the surface of a liquid) there is a significant depression of the boiling point. This may render the administration of agents with low boiling points, such as ether, difficult. Fig. 3.2 shows the depression of the boiling point for water with change in atmospheric pressure.

Figure 3.2 Variation of boiling point of water with atmospheric pressure or altitude.

Vaporizing systems

The various organic liquids that possess anaesthetic properties are too potent to be used as pure vapours and so are diluted in a carrier gas such as air and/or oxygen, or, nitrous oxide and oxygen. The device that allows vaporization of the liquid anaesthetic agent and its subsequent admixture with a carrier gas for administration to a patient is called a vaporizer.

A modern vaporizer needs to be constructed so that it provides a suitable, stable and predictable concentration of anaesthetic vapour for admixture with other patient gasses. The delivered amount of vapour must not be affected by changes in temperature, and flow rates of other gasses.

Types of vaporizer

Appropriate vaporization may be achieved by either:

• splitting the patient gas flow so that only a portion passes through the vaporizer. This picks up saturated vapour and then leaves to mix with the remainder of the gas that has gone through a bypass. The final concentration may be altered by varying the splitting ratio between bypass gas flow and vaporizer gas flow, using an adjustable valve. This type is often referred to as a variable bypass vaporizer (Fig. 3.3); or

• alternatively, the vaporizer can be constructed so that it heats the anaesthetic agent to a temperature above its boiling point (in order that it may behave as a gas) and which can then be metered into the fresh gas flow (Fig. 3.4A). Similarly, a vaporizer may contain a fine metal sieve that is submerged in the anaesthetic agent and through which a small independent and metered gas supply (normally oxygen) can be made to pass. The minute bubbles produced have a very large surface area and produce a saturated vapour at ambient pressure, which can then be passed into the fresh gas flow (Fig. 3.4B). These types of vaporizer are often referred to as measured flow vaporizers.

Figure 3.3 A schematic diagram of a variable bypass vaporizer. A flow splitting valve that can be rotated to alter the relative diameters of the vaporizer and bypass channels and so vary the flows through them. L, liquid; C, vaporizer chamber.

Figure 3.4 Schematic diagrams of measured flow vaporizers. A. H, electric heater; L, liquid anaesthetic agent; C, vaporizer chamber; V₁, V₂, flow control valves; E, electronic valve controller to proportion flows. B. L, liquid anaesthetic agent; C, vaporizer chamber; V₁, V₂, flow control valves.

It should also be noted that the various anaesthetic inhalational agents currently available have widely differing potencies and physical properties and hence require devices constructed specifically for each agent. Very potent agents (halothane, enflurane, sevoflurane, isoflurane and desflurane) require vaporizers that can accurately control the concentration of vapour leaving the vaporizer. However, agents such as diethyl ether, with a lower potency, may be used safely with simpler apparatus (if necessary), in which the vapour concentration is not accurately known, since there is less risk of over-dosage (see Chapter 27).

Variable bypass vaporizers

Design features

Surface area of contact between carrier gas and the liquid

Vaporizers, which are required to be very accurate, should always present a saturated vapour to the bypass gas across a wide range of flows. To ensure sufficient vaporization at the highest planned flow, a sufficiently large surface area of liquid should be present. This size is also governed by the volatility of the agent used. A highly volatile liquid will require a smaller surface area. The surface area for vaporization of a liquid can be increased by causing it to spread (by capillarity) over a large sheet of porous material which may be folded in such a way that the carrier gas passes across its entire surface (Fig. 3.5).

Figure 3.5 Vapour pick-up in vaporizers. A. Carrier gas passing over a small surface area of liquid anaesthetic agent. B. The surface area for vaporization is increased by porous wicks dipped into the liquid. The carrier gas is also made to pass close to the surface of the liquid by the baffles, thus increasing vapour pick-up.

Temperature

As vaporization progresses, the vaporizing liquid as well as the vaporizer cools and the quantity of vapour produced decreases. In an attempt to retain the performance of the device, the temperature drop is minimized or prevented by the incorporation of a heat source (heat sink). This normally takes the form of a water bath or substantial metal jacket or even a heating element surrounding the vaporizing liquid. Metal jackets and water baths, however, can only transfer a finite quantity of heat and so only minimize the inevitable fall in temperature.

In order to maintain the expected output of the vaporizer when this occurs, a greater proportion of carrier gas is required to pass through the vaporizing chamber in order to collect sufficient vapour molecules. This is achieved by using devices that are sensitive to changes in temperature (temperature-compensating devices, Fig. 3.6) and which then increase the flow through the vaporizing chamber.

Figure 3.6 Temperature-compensating devices. A. The bi-metallic strip in a vaporizer bypass operating at ambient temperature. B. The same device operating at a cooler temperature; the bi-metallic strip has moved closer to the inflow increasing the resistance in the bypass and increasing the amount of gas passing through the vaporizing chamber and therefore increasing vapour pick-up. C. A bi-metallic arrangement that works on a similar principle. The inner rod is made of Invar, a relatively non-expansile metal. The outer jacket is in contact with the vaporizing liquid and is made of an expansile metal (brass). D. When this contracts (with cooling) it drags the choke on the inner rod into the bypass, increasing the resistance to flow through it.

Two types are commonly used:

1. The first (Figs 3.6A and B) consists of two dissimilar metals or alloys placed back to back (i.e. a bi-metallic strip). As the two metals have different rates of expansion and contraction with temperature, the device has the ability to ‘bend’. It can, therefore, be used to vary the degree of occlusion in the aperture of a gas channel (usually the bypass) and thus alter the flow of carrier gasses through it.

2. In the second arrangement (Figs 3.6C and D), the bi-metallic device consists of a central rod made of Invar, a metal alloy with a low coefficient of expansion, sitting inside a brass jacket, the top part of which is attached to the roof of the vaporizing chamber. The rod is attached only at the base of the brass jacket, which has a higher coefficient of expansion. The outer surface of the jacket is immersed in liquid anaesthetic agent in the vaporizing chamber. As the aforementioned liquid cools, the brass jacket contracts more than the Invar, which is pushed upwards into the bypass, restricting the flow of bypass gas and increasing the flow of carrier gas through the vaporizing chamber.

In heated vaporizers, the heating elements are thermostatically controlled. They are, therefore, automatically temperature compensated and do not require the addition of the devices above.

Potency of anaesthetic agent

As described above, current anaesthetic vapours are too potent to be administered as saturated vapours and require suitable dilution. Therefore, only a proportion of the gas intended for the patient is diverted in the vaporizer to collect vapour. This amount may be varied to produce the desired concentration by using an adjustable flow-splitting valve (see Fig. 3.3). This is usually a rotary valve incorporated within the vaporizer outlet. It proportions the flow of gas between the vaporizing chamber and the vaporizer bypass system, thus controlling the final vapour composition (i.e. the more gas going through the vaporizer chamber, the greater the amount of vapour leaving the vaporizer). The flow-splitting valve is calibrated in percentage of the vapour in the final gas/vapour composition. However, this valve is accurate only if the vaporizer is temperature-compensated (see above). As both the temperature-compensating mechanism and the flow-splitting valve work by altering resistance through the vaporizer, the devices are dependent on each other. Therefore, each vaporizer for a designated anaesthetic agent is individually calibrated at the factory (see below) for that agent and for a specific range of temperatures and flow rates of carrier gas.

As the potency of anaesthetic agents varies widely, the flow-splitting ratios must be individually tailored for each agent and vaporizer design. Agents with high potency will require a wide splitting ratio so that a smaller amount of gas passes through the vaporizer. This produces a lower final concentration when mixed with bypass gas.

Volatility

The flow-splitting ratio is dependant on the volatility of the agent. For example, at any given temperature, a very volatile agent will produce a higher saturated vapour pressure than a less volatile agent, even though they may have similar potencies. The former, however, requires a flow-splitting valve with a wider ratio so as to increase the dilution in order to provide a similar concentration. Table 3.1 shows the relative potency and volatility of some liquid anaesthetic agents that influence vaporizer design.

Table 3.1 Relative potency and volatility of some liquid anaesthetic agents

MAC, minimum alveolar concentration;
SVP, saturated vapour pressure

Types of variable bypass vaporizers

Draw-over vaporizers

The early vaporizers relied on the patient’s respiratory effort to draw gas over the vaporizing surface (hence their name). Unfortunately, draw-over systems are subjected to very variable flow rates, i.e. from 0 to 601 min⁻¹ (the peak inspiratory flow in a hyperventilating adult). At these higher flows the carrier gas may fail to pick up a saturated vapour resulting in a reduced concentration leaving the vaporizer. Furthermore, the gas pathways must offer little resistance to flow so as not to compromise the patient’s inspiratory effort. This restricts the design of the vaporizer components, especially the flow-splitting valve (see above), which must have sufficiently wide a bore. It is very difficult to design a flow-splitting valve that will work accurately over a wide range of flow rates, i.e. 1–60 l min⁻¹. As discussed above, the valve must present a low flow resistance so that at flows of 40 l min⁻¹ (the peak flow in a patient breathing spontaneously), no respiratory embarrassment is caused. However, if the flow across this valve drops to about 4 l min⁻¹, the resistance through the valve will be so low that carrier gas will preferentially pass across the bypass channel rather than through the vaporizing chamber where it has to mix with and then push the ‘heavy’ vapour out into the attached breathing system. At this flow and below, there is thus bound to be a marked fall in vaporizer performance.

Plenum vaporizers

A vaporizer can be made more accurate if the carrier gas is pressurized to make it as dense as the vapour, so that at lower flows it more readily mixes with this rather than tending to pass above it in the vaporizing chamber. Furthermore, if a smaller, continuous flow of gas, i.e. 0–15 l min⁻¹, is used, there is a less rapid removal of vapour, ensuring that a saturated vapour is present at all times. This allows the vaporizer to be calibrated very accurately. This type is usually referred to as a plenum vaporizer (plenum being the term which describes a pressurized chamber). The typical flow resistance (2 kPa (20 cm H₂O) at 5 l min⁻¹) found in plenum vaporizers renders them unsuitable for use as draw-over vaporizers. With plenum vaporizers, the problem of high intermittent flow rates in a breathing system, generated by a spontaneously breathing patient or a mechanical ventilator, is accommodated by siting a reservoir (bag or bellows) downstream, which stores inspiratory gas and vapour during the exhalation phase.

However, flowmeters on an anaesthetic machine are calibrated for use at or around atmospheric pressure. Therefore, the final design of a plenum vaporizer develops from a compromise between the high carrier gas pressures required for accurate vapour delivery and the low pressures required to maintain the accuracy of the flowmeters.

Factors affecting vaporizer performance

Extremes of temperature

It is obvious that a temperature-compensating mechanism can operate only within a reasonable temperature range. At too low a temperature, vaporization will be low, and it may be uncontrollably high when it is too hot.

Barometric pressure

Ideally, a vaporizer should also be calibrated at a specific barometric pressure. Strictly speaking, as a saturated vapour is only altered by temperature, one might expect the calibration of a vaporizer to be independent of barometric pressure. However, changes in barometric pressure will affect the carrier gas composition passing through the vaporizer, which in turn will affect the concentration of vapour in the mixture leaving it. For example, when the barometric pressure is reduced (at altitude), the number of molecules of carrier gas flowing through the vaporizer is reduced. However, the number of vapour molecules collected by the gas in the vaporizing chamber remains unchanged, although these now represent a higher percentage of the total number of molecules leaving the vaporizer. The final output concentration of vapour is thus now higher than at sea level, although its partial pressure remains unchanged. As anaesthetic effect is governed by the partial pressure of the agent in the body, there is no effective change under the normal conditions. However, extremes of pressure may have a significant effect. For example, at very high altitude (low barometric pressure), a very volatile liquid such as ether may boil at ambient temperature. This may render the use of such agents difficult. Fig. 3.2 shows the variation of boiling point with atmospheric pressure.

Pumping effect

When a resistance is applied to the outlet of the anaesthetic machine, such as that which occurs when manually assisted inspiration or controlled ventilation is used, there is an intermittent increase in the anaesthetic gas pressure, which is transmitted back to the vaporizer. When this happens, it causes carrier gas within the vaporizer to be compressed. Gas in the outlet is already saturated and, therefore, cannot pick up any more vapour. When the back pressure is released, the expanding carrier gas, which is also saturated with vapour, surges out through both the inlet and outlet of the vaporizer chamber. The gas that leaves the inlet enters the bypass and adds to the vaporizer output to increase in the final vapour output. (Fig. 3.7 demonstrates the sequence of events.)

Figure 3.7 A. Flow-through vaporizer with no back pressure. B. Back-up pressure and/or reverse flow causing build-up of carrier gas in the vaporizer chamber. C. Release of back pressure causing gas and saturated vapour to escape through the vaporizer inlet and outlet and into the bypass. D. Anaesthetic vapour in the bypass gas added to that from the vapour outlet so increasing the final vapour concentration.

This effect can be minimized by the fitting of internal compensating mechanisms. It may be achieved by either:

• increasing the resistance to flow through the vaporizer and bypass so that the carrier gas develops a higher pressure within the vaporizer, so as to reduce the pumping effect. However, the pressure increase due to vaporizer design should be as small as possible as these pressures are transmitted back to the flowmeters, which are calibrated for use at near atmospheric pressure; or

• building an elongated flow passage into either the inlet or outlet of the vaporizer to minimize the effect of surges in pressure (Fig. 3.8).

Figure 3.8 Elongation of the inflow channel in a vaporizer preventing saturated vapour reaching the bypass.

Some vaporizer designs employ both mechanisms. The former cannot be fitted to draw-over vaporizers as they would produce too great a resistance to flow (see below, Temperature-compensated vaporizers).

Furthermore, where plenum vaporizers are fitted, some anaesthetic machines now incorporate a non-return valve on the end of the back bar, so that the back pressure surges on the vaporizer are reduced. However, pressure still builds up to some extent in the back bar when the non-return valve closes due to higher downstream pressure.

Liquid levels

The liquid level within the vaporizing chamber may affect performance. If the vaporizer is overfilled, insufficient exposed surface area of wick may cause a drop in vapour output. Additionally, over-filling may result in dangerously high concentrations, due to spillage of liquid agent into the bypass.

Anaesthetic agents

The anaesthetic agent halothane contains a stabilizing agent, thymol. This is a waxy substance which, if left in the vaporizer, would clog the felt or cotton wick found in older models, reducing the potential surface area for vaporization. This would then reduce the vaporizer performance. Thymol may also ‘gum up’ the vaporizer, making the control knob difficult to adjust, as well as compromising the internal mechanism. The manufacturers, therefore, used to advise that the liquid agent be drained off and replenished at intervals. This advice should be tempered by consideration of economy and the frequency with which the vaporizer is employed. Recent models from most manufacturers for use with halothane have wicks made from synthetic materials that do not absorb thymol into the fabric, so many can now recommend a service interval (often with a change of wick) of 5 years. Vaporizers used with other agents often have a 10-year service interval.

Anaesthetic agents are susceptible to various types of degradation including decomposition in sunlight. Hence, all these agents are stored in dark brown glass or treated metal containers. Sevoflurane is also degradable by Lewis acids (such as metal oxides and metal halides) to hydrofluoric acid and other toxic compounds. Water inhibits such degradation. Hence, different manufacturers add varying amounts of water to the compound for stabilization. For example, Abbott incorporates 352 ppm water whereas other generic versions contain 19–57 ppm. Some low water formulation agents have been shown to undergo substantial degradation by reacting with the internal metal components of some vaporizers. This damage and the accumulation of toxic by-products present a potential patient safety issue. Care must taken to identify those vaporizers at risk and use only high water content sevoflurane in them. Some manufacturers coat the insides of their vaporizers with Teflon in order to be able to use the generic products.

Carrier gas composition

Vaporizer output may be affected when the carrier gas composition is changed. This is due to changes in viscosity and density, which alter the performance of the flow-splitting valve. Increasing the concentration of nitrous oxide reduces the vapour concentration. This is of little importance in clinical practice at present. However, the interest in the gas xenon (which is five times as heavy as air), as a potential anaesthetic, may change this. There is also a further mechanism causing a change in vaporizer output when nitrous oxide concentrations are increased. Nitrous oxide dissolves in volatile agents, so that the effective total gas flow through the vaporizer is temporarily reduced.

Stability

Some vaporizers, if tilted or inverted, may allow the liquid agent to contaminate the bypass. This has caused a fatality in the past when a vaporizer was accidentally overturned prior to attachment to the machine. Modern designs of vaporizer aim to eliminate this risk by sealing off the vaporizing chamber from the bypass and outflow gas channels when the vaporizer is set to off or transport mode as they must be to allow disconnection from the back bar of the anaesthetic machine.

Calibration of vaporizers

Vaporizers designed to give an accurate output are individually calibrated prior to leaving the factory. Typically, they are filled with the designated anaesthetic agent and left in a room at a standard temperature (23°C) for 4 hours. A blank control dial (linked to a computer) is attached and rotated at various carrier gas flow rates. The output concentration is measured using a sample that is analyzed by a refractometer (see Fig. 15.3). The dial (which has a unique serial number) is then removed and a calibration scale etched onto it from the information stored on the computer. It is then re-attached to that same vaporizer and the calibration confirmed prior to leaving the factory. Vaporizers may also have the calibration confirmed in a similar manner, following servicing.

Filling of vaporizers

In the original plenum vaporizers, a screw-threaded stopper in the filling port was simply unscrewed, liquid agent was added and the stopper reconnected. These systems were criticized and have largely disappeared from use as the vaporizer could easily be filled with the wrong agent. Despite this, they are still supplied by all the major manufacturers for certain countries (Fig. 3.9) and are referred to as ‘screw fill’ systems.

Figure 3.9 Screw fill port on a vaporizer.

Agent-specific filling devices (in the UK) (Fraser Sweatman pin safety system) were introduced by Cyprane (which became Ohmeda and is now part of GE Healthcare) in the early 1980s. In these the distal end was keyed to fit a vaporizer calibrated, labelled and colour coded for a specific agent and the proximal end keyed to fit only the neck of the bottle for that agent (Fig. 3.10A).

Figure 3.10 A. Fraser Sweatman pin safety system for isoflurane. Note that the top has unique grooves (shape, colour and position) that fit only the specific filling ports on matching vaporizers. B. The Dräger-Fil system for isoflurane. C. GE Healthcare Easy-Fil for generic sevoflurane.

This is now referred to commonly as the ‘key fill’ system. Dräger have a similar system (Dräger-fil) for their Vapor range (Fig. 3.10B) as do GE Healthcare (Easy-Fil) (Fig. 3.10C).

Although these devices go some way to reducing the potential for filling vaporizers with the wrong agent, it is by no means foolproof. For example it would still be possible to decant one agent from an open bottle to another agent-specific one. Some manufacturers of sevoflurane (Abbott) and desflurane (Baxter) produce sealed bottles to which the agent-specific filling device is already fitted and made tamper-proof with a crimped metal seal. These filling devices also have valves, which are only opened when inserted fully into their respective filler ports, so as to prevent spillage (Figs 3.11A and B). The filling system for sevoflurane and the older agents can be designated at ordering of the vaporizer but sevoflurane from Abott Laboratories only uses their own, Quik-Fil system (Fig. 3.11A). The desflurane system (Fig. 3.11B) is called Saf-T-Fil (Baxter Healthcare) and operates across all vaporizer manufacturers.

Figure 3.11 A. A sealed bottle of Abbott sevoflurane with its integral uniquely shaped filler (Quick-Fil). Note that the filler on the bottle is secured by a crimped metal seal. B. Similarly, a sealed bottle of desflurane; with Saf-T-Fil system. The filler caps are shown alongside the bottles. As the contents are pressurized at ambient temperature, the glass bottle is encased in a plastic coat to prevent it exploding if damaged.

Although it is increasingly difficult to fill a vaporizer with the wrong drug, it is far easier to fail to fill a vaporizer because of the multitude of filling systems between drug and vaporizer manufacturers.

Examples of variable bypass vaporizers

Temperature-compensated vaporizers

TEC 5 (GE Healthcare)

Fig. 3.12 shows a TEC 5 (with a keyed filler), its mode of operation and performance charts. Although now owned by GE Healthcare, these vaporizers and some TEC 7s (see below) are still badged as Datex–Ohmeda.

Figure 3.12 A. The TEC 5 vaporizer. B. Filling the vaporizer. (1) insertion of keyed filler B into vaporizer filling port C, (2) applying the lock D to make the filler/vaporizer connection secure, (3) inverting and raising the bottle of agent to create a filling pressure, (4) opening the chamber lock E to fill the chamber, (5) lowering the bottle below the vaporizer to empty the chamber if required. C. Working principles of Tec 5 and Tec 7 vaporizers. (1) inlet, (2) elongated passage that prevents ‘pumping effect’, (3) helical wick, (4) base of vaporizing chamber, (5) rotary valve for metering vapour saturated carrier gas, (6) mixing chamber, (7) bypass, (8) temperature-compensating device, (9) outlet.

When the vaporizer is attached to the back bar of an anaesthetic workstation, gasses destined for the patient pass through it via three channels. In the OFF position, with the rotary valve (5) closed, the gasses pass through the inlet (1) and into one channel (6). From here the gasses pass across the top of the vaporizer, without coming into contact with the vaporizing chamber and leave through the outflow (9).

In the ON position, this pathway closes and the other two channels (2 and 7) are open. The vaporizer channel has an elongated passage (2) that funnels carrier gas into the centre of the wick assembly. This comprises a hollow tube of Teflon cloth held open by a steel wire spiral and wound into a helix (3) within the vaporizer. The lowest helix is in contact with the liquid agent. This arrangement greatly increases the surface area for vaporization over previous Tec models. The gas passing through the wicks becomes saturated with vapour and enters a rotary valve assembly (calibrated control knob).

The second channel guides bypass gas through the base of the vaporizer, through the temperature compensating device and towards the outflow (9).

The proportion of gasses passing the two channels is determined by (a) the resistance to flow in the temperature-compensating device (8), and (b) the calibrated control knob (5) which varies the resistance from the vapour chamber exit (6). The percentage of vapour at the outlet depends on the amount of vapour-laden gasses that is mixed with the fresh gasses passing through the bypass. As the temperature within the vaporization chamber falls (reducing the vapour concentration produced), the thermostatic valve closes. This causes a greater proportion of the total gas flow to pass through the chamber; by this means the vapour concentration in the output is kept constant.

The vaporizer has some significant design features. If it is accidentally inverted, the liquid agent will not spill into the bypass. It also incorporates an interlock facility. If two vaporizers with this latter feature are placed on a back bar (see below), the first vaporizer to be switched on extends lateral rods that impinge on the adjacent vaporizer preventing its operation. Also, the vaporizer dial cannot be turned if the vaporizer is improperly mounted on the anaesthetic machine, i.e. not seated correctly and locked on to the back bar.

TEC 7 series

These vaporizers (Fig. 3.13B) are identical in function to the TEC 5. However, they also offer a wider selection of filler assemblies and a change in the cosmetic appearance over the TEC 5 (Fig. 3.12) to match the manufacturer’s workstation.

Figure 3.13 A. TEC 7 isoflurane and sevoflurane vaporizers. B. Performance of TEC 5 and TEC 7, as influenced by changes in flow rate and temperature.

Blease Datum

Blease is now part of Spacelabs Healthcare and the vaporizer range is now labelled as Spacelabs Healthcare. However, the design is still identical to the latest Blease Datum and there are many still labelled as such in circulation.

This range fulfils all the criteria for a temperature compensated vaporizer. It has:

• a heat sink

• a thermal compensating device

• a large surface area for vaporization

• an elongated inlet to minimize the damping effect.

The earlier version had a very large heat sink made of brass, in order to reduce the cooling effect of vaporization: hence the vaporizer weighed 11 kg. The latest version (Fig. 3.14A) has an improved thermal conductivity and now weighs 7.5 kg. The earlier version had a stainless steel outer covering but this was separated from the brass body of the vaporizer by a layer of plastic. The latter has been removed to improve the absorption of radiant heat from the surroundings and has enabled less brass to be used. The heavy weight of the vaporizer helps to seat the vaporizer firmly on the back bar when it is connected to the anaesthetic workstation and so reduces the potential for leaks. The working principles of the vaporizer are shown in Figure 3.14B.

Figure 3.14 A. The Blease Datum vaporizer with agent-specific filler for sevoflurane. B. Working principles: (1) fresh gas input, (2) control dial, (3) zero lock, (4) thermal compensator rod, (5) filler block for agent-specific device, (6) agent reservoir, (7 and 9) wick extension, (8) elongated Teflon wick, (10) vapour control valve, (11) combined gas and vapour output, (12) variable resistance bypass valve. C. Performance characteristics for variations in temperature and flow.

The wick is made from a long tube of PTFE (Teflon) wadding that is supported internally by a wire coil (8). This is wound into a spiral to create a large surface area for contact with carrier gas that passes through the tube. At the base of the spiral, the Teflon is made to drape into the liquid reservoir (7). The material has a high capillarity to ensure that the wick is saturated at all times, even when the reservoir is low (liquid level compensation).

In the OFF position, there is a ‘zero lock’ (3) on the dial that isolates the vaporization chamber so that all the patient designated gas travels through the bypass. In the ON position this gas is split into two flows. One passes through the elongated wick (8), collecting vapour; the elongated passage behaves as a damping device to counteract the pumping effect. From here, the gas, which is now saturated with anaesthetic vapour, travels through to the vapour control valve (10), operated by the control dial (2). It then joins the remainder of the gas in the bypass. A thermal compensator (4) alters the flow through the bypass (12), to correct for changes in vapour production at lower temperatures. The bi-metallic device consists of a central rod made of Invar, a metal alloy with a low coefficient of expansion, part of which sits inside a brass jacket, the top part of which is attached to the roof of the vaporizing chamber. (see above: Temperature compensating devices: p. 44, and Fig. 3.6C and D).

Unlike the TEC vaporizers, the control dial may be turned on when it is not connected to a machine and so if this is left on and the vaporizer is inadvertently tipped, there is the potential for liquid anaesthetic agent to enter the bypass. However, the relevant channels are placed towards the front of the vaporization chamber near the filler block. If the vaporizer falls on its side, the filler block prevents these channels from being submerged in liquid agent. Figure 3.14C shows the performance curves for the vaporizer.

Dräger ‘Vapor’ 2000 series of vaporizers

Figures 3.15A, B and C show the vaporizer, working principles and performance curves. Models in the range are all compensated for temperature and pumping effects. In the OFF position, gasses destined for the patient are directed through the bypass (12) in the vaporizer without coming into contact with anaesthetic vapour.

Figure 3.15 A. Dräger Vapor 2000 vaporizer. B. Working principles: (1) gas input, (2) damping the chamber, (3) tubular wick, (4) wick extension, (5) reservoir chamber, (6) concentration control valve, (7) calibration dial, (8) temperature compensation device, (9) mixing a chamber, (10) output, (11) bypass tap, (12) bypass tap. C. Performance characteristics.

When the vaporizer control dial is switched ON, these gasses split into two flows with one part initially flowing through a series of baffles (2) that counteract pressure surges (the pumping effect). From here, it is directed through the vaporizing system, where it becomes saturated with vapour. This takes the form of a tubular wick (3) coiled in a spiral, through which the gas passes. The outer surface of the spiral is attached to a sleeve of similar material (4) that dips into the liquid anaesthetic agent in the reservoir (5), so as to keep the spiral wick soaked. The wick is made of a synthetic material with a high capillarity, but which does not absorb agent. Therefore, a stabilizing agent, such as thymol that is added to halothane, will not clog the wick and reduce its efficiency. This allows the vaporizer to be used for prolonged periods between services.

From the vaporizing chamber, the saturated gasses pass to a conical control valve (6), whose aperture is adjusted by the calibration dial (7). From here they pass to a mixing chamber (9), where they blend with bypass gasses prior to leaving the vaporizer. If the operating temperature of the latter drops, a compensating device (8) (see also Figs 3.6C and D) proportionately decreases the flow of gasses through the bypass so as to maintain the output of the vaporizer.

In the OFF position, the vaporizing chamber has a small connection to atmosphere (11) that allows some gas to escape when liquid agent is added. This makes the filling process easier.

Penlon Sigma Delta vaporizer

The Penlon Sigma Delta, shown in Figs 3.16A and B, includes all the features of a modern Plenum vaporizer.

Figure 3.16 A. Penlon Sigma Delta vaporizer. B. Working principles: (1) inlet, (2) bypass, (3) outlet, (4) control knob, (5) helical damping coil, (6) vaporizing chamber, (7) wick, (8) needle valve, (9) temperature compensating element. C. Performance characteristics.

In the OFF position, gasses destined for the patient are directed via the inlet (1) through the bypass (2) to the outlet (3). A closing mechanism (not shown) prevents any gasses coming into contact with anaesthetic vapour in the vaporizer.

When the control knob (4) is turned on, the closing mechanism is released and a second channel is opened that ducts a portion of these gasses through the vaporizing system. They pass initially through a helical damping coil (5) that prevents saturated vapour tracking back through the vaporizer and contaminating other gasses in the back bar (the ‘pumping effect’).

From here they pass into the vaporizing chamber (6) and around the wick (7). The latter is novel in that it is made of sintered polyethylene (1 m long), which is held in close proximity to a copper backing plate. The two are then coiled into a spiral, the top and bottom of which are made gas-tight. The carrier gasses, therefore, have to pass around the spiral, coming into contact with the whole surface area of the wick. The wick assembly is designed as a cartridge for ease of removal and cleaning, and has a long service life. The recommended service interval is 5 years for halothane vaporizers and 10 years for all others.

Gasses saturated with anaesthetic vapour leave the chamber and pass through an orifice whose aperture is varied by a needle valve (8) attached to the control knob. The latter, therefore, controls the amount of vapour-laden gasses flowing through the device to mix with the bypass gasses.

A bi-metallic temperature-compensating element (TCE) (9) is placed in the vaporizing chamber, so that its base is immersed in the vaporizing liquid. When the liquid agent cools, the aperture of the TCE closes and diverts more gas through the vapour chamber pathway, in order to maintain the accuracy of the vaporizer output.

The vaporizer is interesting in that it is relatively light (5 kg), being made of aluminium. Aluminium has a better thermal conductivity than brass and so, despite its weight, it conducts a similar amount of radiant heat as does an equivalent sized brass device. The rotary control can be turned on when the vaporizer is disconnected from an anaesthetic workstation. If this occurs and the vaporizer is tipped on its side or inverted, then liquid vapour could enter the outflow from the vaporizing chamber.

Plenum vaporizers with electronic control

Conventionally, vaporizers have two main parts, a chamber for producing a saturated vapour, and a mechanical system for regulating gas flow through the chamber. The control of accurate vapour output requires precision engineering of expensive materials with each component contributing a degree of variability in performance. Hence, there is the need to calibrate each vaporizer individually and to be able to identify each component by serial number to ensure correct reassembly when serviced. With the introduction of electronics into the anaesthesia machine, it is now possible to regulate vapour concentration with electropneumatic proportional flow valves controlled by microprocessors. A single-control system may be installed on a workstation and can be used for all volatile anaesthetic agents.

GE Healthcare – Aladin and Aladin 2 vaporizers

Both vaporizers function in a similar manner. The Aladin 2 has a few added refinements which will be described below. The Aladin range has a conventional vaporizing chamber in the form of a detachable cassette and an electronic vapour control unit built into the anaesthetic workstation (see also Chapter 4, Aisys and ADU).

The cassette

The cassette (Fig. 3.17A) is a leakproof metal box that is divided into two sections. The larger rear section is the vaporizing chamber which is filled with a synthetic material that behaves as a wick. It is formed into lamellae (1) interspersed with metal plates (2) to create a convoluted pathway, so as to maximize vapour pick-up. The back panel has inflow (3) and outflow (4) spring-loaded mechanical ball valves, to prevent a leak of agent when transported. There is also a mechanical contact for the temperature sensor (6) that is placed within the vaporizer to measure the temperature of the liquid. The front section of the unit has a handle (7) and a locking mechanism (8) for securing it to the workstation. There is a conventional vapour-specific filling system (9) with a clear glass window (larger in the Aladin 2) displaying the liquid level (10). The Aladin 2 has an additional liquid level sensor that feeds information back to the anaesthetic workstation via an electronic bus (5) which also conveys vaporizer temperature data. Capacitor plates that sense the level of agent, are fitted inside this device and the four copper contacts of the electronic bus, on the top of the unit, power the capacitor and transmit information. Agent level is then displayed graphically.

Figure 3.17 (A) Aladin 2 cassett. (B) Diagram of the back of the casette and cutaway showing carrier gas flow – (1) wicks, (2) lamellae, (3) inlet valve, (4) outlet valve, (5) contact for electronic temperature sensor and liquid anaesthetic agent level, (6) temperature sensor strip (7) handle, (8) locking mechanism, (9) fill system (10) liquid level window, (11) agent identification magnets (not visible externally). (C) Diagram of control unit- (12) bypass flow measurement, (13) one way valve, (14) inflow close valve, (15) inlet check valve, (16) outlet check valve, (17) outflow close valve, (18) liquid flow prevention, (19) proportional flow valve, (20) agent flow measurement, (21) cassette pressure sensor, (22) cassette pressure relief valve, (23) flow to scavenging, (24) Aladin cassette. (D) Filling the casette: (25) Colour coded bottle, (26) Colour coded adapter tab, (27) Colour coded adapter cap.

When the vaporizer is filled, the liquid passes into the rear section through a one-way valve. If the cassette is more than 7° off the horizontal, this closes and the filler will not accept any further liquid into the unit. There is also an air vent between the two halves. When the liquid level in the rear half reaches this, it closes and prevents overfilling. The top of the front section houses a row of five magnets (11) arranged in a sequence that provides unique vapour identification for that cassette. The vaporizer is spill proof when transported and is maintenance free for all agents. When the cassette is plugged into the workstation, matching probes engage and open the mechanical ball valves on the back of the unit. If the workstation is in use, the temperature sensor is activated and the reed switches positioned over the magnets on the control unit identify the agent in use.

In the ADU workstation there is a fan that is activated when the desflurane cassette is fitted and that directs heat through vents from the workstation electronics onto the vaporizing chamber when this has cooled below a critical level. In the Aisys using the Aladin 2 desflurane cassette none of this is required (see Chapter 4).

The control unit

Housed in the anaesthetic workstation (Fig. 3.17B), this behaves as an electronic variable bypass. When the user sets an anaesthetic concentration on the front panel of the workstation, the fresh gas flow is split into two. The bulk of the gasses pass across the bypass where the flow is measured (12). A smaller portion of the gasses pass through a mechanical one way valve (13), through an electronic ‘inflow close valve’ (14), through the open ball valve (15) in the back of the cassette (that is opened when the cassette in plugged into the workstation) and into the vaporizing chamber. It picks up saturated vapour and leaves the cassette via the other open ball valve (16), an electronic ‘outflow close valve’ (17) and a liquid flow prevention valve (18) to the proportional flow valve (19) that controls vapour output. From here it passes to the agent flow measurement device (20) and into the outlet of the control unit, where it joins the bypass gas in a mixing chamber. The electronic inflow and outflow valves are open when the vaporizer is in use. Otherwise they remain closed to prevent a leak of gas destined for the patient when a cassette is not attached.

A microprocessor gathers information regarding the agent used, its temperature in the cassette and the flow of gas in the bypass. It makes a calculation for the amount of agent to add to the bypass gas to provide the desired concentration (set electronically by the user on the workstation display) and instructs the proportional flow valve to open sufficiently to provide this. The presence of a cassette, the specific agent, its level in the cassette and the calculated vapour concentration delivered are displayed electronically on the workstation screen (Aisys range). The outflow pathway from the cassette is fitted with an electronic pressure sensor (21). If the outflow pressure exceeds 2.5 bar, a relief valve opens (22) and vents the flow to scavenging (23).

Draw-over vaporizers

All the plenum vaporizers described above offer resistance to the gas flow. For this reason carrier gasses have to be pressurized to some degree to be driven through them. However, pressurized gas sources are not always available in some countries or in certain situations. Draw-over vaporizers, with their low-resistance gas pathways, can be installed within a breathing system and are therefore, a useful alternative to plenum systems despite not being as accurate. Fig. 3.18 illustrates various breathing systems in which a draw-over vaporizer has been installed. In systems A–D, exhaled gasses are vented to the atmosphere and suitably scavenged where appropriate. However, in system E, the patient’s exhaled gasses are recirculated through the vaporizer. This is of importance, since not only will the concentration of volatile agents be increased by the repeated passage of the gasses through the vaporizer, but the latter must be of a type without cloth wicks, since these could become saturated with water condensed from the expired air and so cease to function.

Figure 3.18 Draw-over anaesthetic systems. Note that they all contain non-return valves to prevent reverse flow through the vaporizer. System A. is used for spontaneous respiration. System B. incorporates a bellows and therefore requires a second non-return valve (V₂). If this is used for controlled ventilation (system C.), an additional non-return valve (NRV) is often substituted for the APL valve. If the former is of a design which has a tendency to jam, the second valve V₂ is either removed, or in the case of the Oxford Inflating Bellows, held open by a magnet. In system D. an oxygen flowmeter has been added. During the expiratory phase, the continuing supply of oxygen flows into the reservoir and is stored for use in subsequent breaths. In system E. the vaporizer has been placed in a circle breathing system. A vaporizer in this position is often referred to as a vaporizer in circle (VIC).

Typical examples of draw-over vaporizers are described below. Further examples may be found in Chapter 27.

The Oxford Miniature Vaporizer (OMV)

This vaporizer (Figs 3.19A and B) is primarily used with portable anaesthetic equipment with the armed forces and has the advantage that it may be drained of one anaesthetic agent and charged with another. Detachable scales are available for several agents. It is very simple to use and needs little in the way of servicing. It is still manufactured but is not marketed in the EU as it does not carry a kitemark.

Figure 3.19 A. OMV vaporizer. B. Working principles: (1) flow splitting valve, (2) bypass gas, (3) gas to vaporizing chamber, (4) saturated vapour and gas in vaporizing chamber, (5) stainless steel wick, (6) liquid agent, (7) water jacket, (8) output from vaporizer. Permission granted by Penion UK Ltd. C. Performance characteristics.

It has a flow-splitting valve (1) that separates carrier gas (2) into bypass gas (3) and gas that passes through the vaporizer to pick up vapour (4). It has stainless steel wicks (5). It is not temperature compensated, but there is a sealed compartment (6), filled with water plus antifreeze, which acts as a heat sink to minimize changes of temperature. The ‘Triservice’ version is described in Chapter 27.

Epstein, Macintosh, Oxford (EMO) ether inhaler

This has been deservedly the most popular draw-over vaporizer (Fig. 3.20) for the administration of ether, and is still widely used throughout the world. For spontaneous respiration, it is often used in conjunction with the OMV (Oxford Miniature Vaporizer). The latter is usually filled with halothane to provide smooth and rapid induction of anaesthesia, which is then continued by ether from the EMO. Both vaporizers may be used in conjunction with self-inflating bellows for techniques employing controlled ventilation.

Figure 3.20 A. The EMO ether inhaler. This is a low-resistance vaporizer which is both temperature and level compensated. B. Working principles. Note that there is a mass of water, which provides a heat sink. When the control lever is put to the ‘close for transit’ position, the ether chamber is sealed off to prevent spillage.

Measured flow vaporizers

TEC 6 (Plus) (Desflurane)

This vaporizer (Figs 3.21A and B) was designed specifically for the volatile agent desflurane. This agent is unusual in that its boiling point is around room temperature and so it would not remain as a liquid in the reservoir of a conventional vaporizer. It therefore requires an unusual design which dispenses with most of the conventional compensating devices mentioned above.

Figure 3.21 A. The TEC 6 vaporizer. B. Working principles: (1) heater in the vapour chamber, (2) shut-off valve; (3) reservoir, (4) electronic pressure regulator, (5) concentration dial, (6) vaporizer outflow, (7) fresh gas inflow, (8) restrictor, (9) differential pressure sensors. C. Performance characteristics of TEC 6 vaporizer.

The reservoir of the TEC 6 has two thermostatically controlled electric heating elements (1), which raise the temperature of the desflurane to 39°C. At this temperature, the SVP is 194 kPa (1500 mmHg). When vapour is required, a shut-off valve (2) opens and pure vapour under pressure is allowed to escape from the reservoir (3). It passes to an electronic pressure regulator (4) that reduces the pressure to that normally found in a plenum vaporizer (1–2.5 kPa) and then to a variable flow restrictor linked to the calibrated concentration selection dial (5), from where it is fed into the carrier gas flow leaving the vaporizer (6).

Fresh gas flow into the vaporizer (7) has to pass through a narrow restriction (8) so that its pressure (which increases with flow) matches that normally found in a plenum vaporizer. With increasing flows, two independent sensors (9) in the pathway detect the pressure rise and instruct the desflurane pressure regulator to increase proportionately the desflurane pressure (and flow) to the selection dial, so as to maintain the set vapour concentration. If the readings from the two sensors are not similar, the shut-off valve closes and isolates the vaporizing chamber.

The vaporizer has a number of other features:

• The vaporizer heaters are switched on automatically when the unit is connected to the electricity supply. However, a 5–10 min warm-up time is required to reach operating temperature. During this time the concentration dial cannot be turned on.

• There are two more electric heaters in the upper part of the vaporizer to prevent vapour condensation.

• The concentration dial has graduations of 1%, but from 10–18% these are 2% increments. There is an interim stop at 12%, which can be manually overridden to access the higher concentrations.

• The front panel has five LED lights. From top to bottom they are labelled OPERATIONAL to indicate that the unit is ready to be used; NO OUTPUT for when the agent drops below minimum operating level; LOW AGENT to indicate that refilling is required; WARM UP (see below) and ALARM BATTERY LOW for when the back-up alarm power is either low or disconnected. The latter consists of a 9-volt alkaline battery which requires changing annually. The front panel also houses an LCD (liquid crystal display) of 20 vertically mounted bars that receives electronically processed signals from a sensor in the reservoir. The bars gradually disappear as the vaporizer empties, at which point the heaters are switched off and the low agent LED flashes. There are three symbols displayed on the side of the LCD. The uppermost (equivalent to all 20 bars showing), indicates that the reservoir is full (390 ml). In the middle, a mark indicates that a 240 ml refill is possible (a whole bottle) and the lowest indicates that the reservoir has only 60 ml left.

• At the beginning of the warm-up time, the vaporizer begins a self-testing sequence. The warning alarm sounds for 1 s and all the LEDs flash. When operating temperature is reached, the warm-up light (amber) extinguishes, the operational light glows (green) and the concentration dial unlocks.

• There is a detector which shuts off the vaporizer if it senses more than a 15° tilt off the vertical axis.

• The filler port accepts only the specific filler nozzle (SAF-T-FIl), which is crimped onto the supply bottle for desflurane. To fill the vaporizer, the filler nozzle is pushed into a spring-loaded aperture in the filler port, which is then rotated upwards by inverting the bottle. The contents of the bottle will then decant into the vaporizer reservoir. If the latter is filled only when the LCD bars fall below the 240 ml refill mark, then it will accept the whole bottle. When empty, the bottle may be returned to the starting position at which point the spring in the filler port will eject the filler nozzle. The filling process may be carried out even when the vaporizer is in use. As the bottle is pressurized, it is coated in plastic to prevent the glass splintering in the event of damage. Overfilling is prevented in normal circumstances by placing the outlet from the reservoir above the level attained by the bottle in its filling position. However, should the vaporizer be tilted (and this can only happen if the vaporizer is not in use and not attached to the back bar), overfilling can occur, although liquid will be prevented from leaving the reservoir by the shut-off valve. When the vaporizer is next commissioned, a small amount of liquid might leave the reservoir but would rapidly vaporize.

Dräger D Vapor

This vaporizer (Fig. 3.22) is another example of a measured flow vaporizer for desflurane and works on similar principles to the Tec 6 above. The agent is heated in a chamber so that it behaves as a gas. Its pressure is then regulated electronically to match the patient gas pressure as this alters with changes in flow rate. It is then fed into the gas stream via the calibrated control knob according to the desired concentration. It is fitted with the ubiquitous Saf-T-Fil connector for desflurane.

Figure 3.22 Dräger D Vapor A. Vaporizer. B. Performance characteristics (In area 2 with high gas flows and high dial settings delivered concentrations of desflurane are significantly below those indicated on the control dial.).

The Dräger DIVA

The DIVA (direct injection of volatile anaesthetic) anaesthetic vaporizer is yet another example of a measured flow vaporizer (Fig. 3.23). But unlike the examples above, it has two sections, a plug in vaporizing (and metering) module specific for a particular agent, and a built-in gas supply module that is built into the Dräger Zeus anaesthetic workstation and can accommodate two vaporizing modules.