For vaporisation to occur, the anaesthetic molecules have to "escape" from the liquid state and become vapor. To escape, the molecules need energy and they take it from the liquid that they escape from. As more and more molecules escape, more and more energy is lost from the liquid. It is the "energy" in a liquid that causes it to have a temperature. Hot liquids have more energy than cold fluids. Therefore, as the escaping molecules take energy, the temperature of the liquid falls (see latent heat of vaporisation). The lower temperature of liquid means that it has less energy and this makes it more difficult for the remaining molecules to escape and become vapor. Thus at lower temperatures, there is less vaporisation.
The less vaporisation then will decrease the concentration of anaesthetic delivered by the vaporiser. I.e. It will deliver an anaesthetic concentration below the setting you dialled.
There are two common solutions to this problem. One is that we can give heat to the liquid to minimise the temperature drop. The other is to increase the flow of fresh gas into the vaporising chamber to compensate for the reduced vaporisation efficiency of the cold fluid.
GIVING HEAT
In most vaporisers, we don't actually give heat "actively". That is, we don't electrically heat it (complicated and needs a power supply) and we don't light a fire under it (absolutely dangerous).
Instead, we make it easy for the vaporiser to use heat from the surrounding air. We do this by making the vaporiser out of metal which is a good conductor of heat and will help transfer heat from the surrounds into the liquid.
In addition, the metal casing is made very thick. Metal has a high specific heat capacity. That is, metal has a high ability to "store" energy and give it up when necessary. When the liquid anaesthetic vaporises, its temperature drops below that of the thick surrounding metal casing. The metal casing then donates the heat it has to the liquid, minimising the temperature drop of the liquid. The thicker the metal, the more energy that it can store. However, the metal casing cannot give up heat indefinitely and after sometime, it also has a significant temperature drop. But because it has a high ability to store heat, the drop is slow. In between your anaesthetic, when you turn the vaporiser off and have coffee before your next case, the metal will continue to "absorb" heat from the surroundings and its temperature will rise, ready to donate heat when you turn the vaporiser on again. To summarise, the metal casing is a good conductor, so allows heat to be easily transferred from the surroundings to the fluid, and is also a good "heat reservoir", so is able to donate heat and help resist the temperature drop.
GIVING MORE FLOW
When the temperature of the liquid drops, we have seen that the output concentration of the vaporiser drops. A way of compensating for that problem is to increase the flow of gas via the vaporising chamber (altering the splitting ratio). One could manually do this by measuring the temperature of the liquid with a thermometer and increasing the dial setting according to some kind of reference chart. This would be quite tedious as you would have to do it all the time. Modern vaporisers have removed the hard work. When the liquid drops its temperature, the flow of gas through the vaporising chamber is automatically increased without you having to turn the dial.
This is accomplished by an automatic temperature compensating valve that influences how much flow occurs into the vaporising chamber. So, the splitting of gas between the by pass pathway and the vaporising chamber is controlled by two valves: the dial you set (splitting valve) and the temperature compensating valve which operates automatically according to the liquid temperature.
The automatic temperature compensating valve uses the physical property that substances (e.g. metals and liquids ) become smaller when the temperature lowers. A metal rod (shown in black below) shortens as the temperature drops. Similarly, a liquid filled in collapsing bellows (shown in green below) becomes smaller in volume when cooled to a lower temperature.
This property is used in the design of temperature compensating valves in vaporisers. In the design that uses a metal rod, the rod offers some resistance to flow into the vaporising chamber. As the vaporiser cools, the rod becomes shorter, making the valve move away from the opening. This reduces the resistance to flow and thus more flow occurs into the vaporising chamber.
Some vaporisers use liquid instead of the metal rod. The liquid is filled inside collapsible bellows. As the temperature falls, the liquid in the bellows contracts into a smaller volume. This makes the bellows shrink, pulling the valve away and thereby increase flow.
Another method uses a "bi metallic" strip. Different metals expand and contract to differing extents when exposed to temperature changes. In the example below, the "green" metal expands and contracts less than the "red" metal.
In a bimetallic strip, two metals with very different degrees of thermal expansion ( "different coefficients of thermal expansion" ) are fixed together. In the example below, when the temperature drops, the "green" metal contracts much more than the "red" metal. Because they are fixed together, they cannot contract independently, like in the diagram above. Instead, the "green" metal "tries" to drag the "red" metal and causes the bimetallic strip to bend.
In the vaporiser, the bimetallic strip is fixed in such a way that it offers a resistance to flow entering the vaporising chamber. When the temperature of the vaporising chamber drops, the bimetallic bends and moves away. This reduces the resistance to flow and thus more flow occurs into the vaporising chamber.
[ Effect of Temperature ] [ Pumping Effect ] [ Desflurane Vaporiser ] [ Inter lock Mechanisms ] [ Vaporiser Fillers ] [ Home ]
