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.

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.

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.

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

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.

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.

Types of variable bypass vaporizers

Factors affecting vaporizer performance

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.)

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

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

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