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Inhalational anaesthetic agents need to be delivered to the lungs for them to work. Of course one cannot simply pour them into the lungs! A much more acceptable way is to convert the liquid inhalational agent into a form that can be delivered by the inhalational route to the patient.
A device which converts liquid to vapour is called a vaporiser. Before going onto discussing vaporisers, we need to first understand what a vapour is. And to understand what a vapour is, we need to know about something called critical temperature. If you take a gas, and compress it really hard, the particles that compose it are brought ever so close to each other. As you keep compressing , the particles will at some point coalesce and convert the gas into liquid.
If compressed with enough pressure, it will condense into a liquid. A gas that is currently above its critical temperature remains a gas. However hard you compress it, it will not condense into a liquid. Let us take isoflurane as a example. The critical temperature of isoflurane is about degrees centigrade. Therefore, at room temperature e. Now for a moment, let us imagine that you worked on the planet Venus. The surface temperature on Venus is about degrees centigrade.
Let us come back to Earth. Fresh gas enters the inlet of the vaporiser and is divided into two flow pathways. On the other hand, the fresh gas that is sent to the vaporising chamber becomes fully saturated with vapor. At the exit end of the vaporiser, the by pass gas vaporless meets the chamber gas fully saturated with vapor and the two mix. The resultant output depends on how much of fresh gas went though each of the pathways.
When you dial a high anaesthetic concentration requirement, the splitting valve sends more fresh gas via the vaporising chamber. Similarly, when you dial a low anaesthetic concentration requirement, the splitting valve sends less fresh gas via the vaporising chamber. Finally, when you set the dial to zero to make vaporiser deliver no anaesthetic vapor, the splitting valve sends all the fresh gas via the by pass pathway and nothing through the vaporising chamber.
You have seen that the anaesthetic concentration that is output by the vaporiser is determined by the ratio of fresh gas flow that goes through the vaporising chamber and the fresh gas flow that goes through the bypass pathway. The basic vaporiser discussed above has a very simple design. Unfortunately, this simple design has the following problems:. As discussed before, part of the fresh gas flow enters the vaporisation chamber and picks up vapor. However, in the basic design, this vaporisation is not very efficient.
The result is that, relative to the high flow of fresh gas flow, the amount of anaesthetic vaporised is inadequate.
So this means that at high flows, the basic vaporiser delivers less anaesthetic concentration than is set on the dial. The solution employed by modern vaporisers to solve this problem is to make the vaporisation much more efficient by increasing the surface area of contact between the fresh gas and anesthetic agent.
So even when there are high flows, the efficient vaporisation means that all gas going through the vaporisation chamber is fully saturated. Because of this ability to saturate fresh gas at all flow rates, the output concentration remains accurate to the setting on the dial over a wide range of flows. The output concentration is independent of flow.
One method that vaporisers use to increase the efficiency of vaporisation is to dip wicks into the anaesthetic agent. Due to capillary action, the anaesthetic agent rises into the wicks. This dramatically increases the surface area of anaesthetic agent exposed to the fresh gas entering the vaporisation chamber and thereby improves the efficiency of vaporisation.
Certain vaporisers e. In these, some of the fresh gas flow is bubbled through a disk made out of a special material sintered disk that is very porous. The disk is submerged into the anaesthetic agent and when fresh gas is sent through it, a large number of tiny bubbles form. The tiny bubbles of fresh gas have a very large total surface and thus become fully saturated with vapor efficiently.
As more and more molecules escape, more and more energy is lost from the liquid. Therefore, as the escaping molecules reduce the energy left in the liquid, the temperature of the liquid falls. The falling temperature lowering energy of the liquid means that less molecules are able to escape.
The less vaporisation then will decrease the concentration of anaesthetic delivered by the vaporiser. It will deliver an anaesthetic concentration below the setting you dialed.
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. Instead, we make it easy for the vaporiser to use heat from the surrounding air. The metal helps to minimise the temperature drop by two ways. Firstly metal is a very good conductor of heat and therefore is able to efficiently transfer heat from the surrounding air into the anesthetic agent. So in summary, the metal provides heat to minimise the temperature drop by two ways.
When the temperature of the liquid agent 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. 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 goes via the vaporising chamber. The automatic temperature compensating valve uses the physical property that substances e. 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 automatic 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 the expansion or contraction property of a special liquid inside bellows shown in green to control the valve. 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. Different metals expand and contract to differing extents when exposed to temperature changes.
Because they are fixed together, they cannot contract independently, like in the diagram above. 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. Positive pressure ventilation result in intermittent pressure changes. These pressure changes can be transmitted back into the vaporiser and can affect the concentration of anaesthetic agent delivered.
In this section, this effect and the methods used by vaporiser designers to prevent it from happening are explained. Below is shown a basic vaporiser and beyond it a bag to represent positive pressure ventilation. When the bag is squeezed positive pressure ventilation , pressure is transmitted back into the vaporiser as shown below. Now see what happens when the positive pressure is suddenly released expiration.
The previously compressed gases now suddenly expands in all directions. The vaporiser inlet tube can be made longer. The vaporiser can be designed to have a high internal resistance to flow. On way valves allow flow in one direction, but not in the other. In the diagram below, the one way valve is allowing gases to flow forwards.
However, this valve prevents flow from occurring in the reverse direction. Before we proceed to talk about the desflurane vaporizer, we need to understand what vapor pressure is. The process of evaporation in a closed container will proceed until there are as many molecules returning to the liquid as there are escaping equilibrium. At this point the vapor is said to be saturated, and the pressure exerted by the vapor usually expressed in mmHg is called the saturated vapor pressure.
The temperature at which the vapor pressure is equal to the atmospheric pressure is called the boiling point. Desflurane has a very low boiling point about 23 degrees Centigrade and even at room temperature, has an high vapor pressure.
Also, for small changes in temperature, the vapor pressure of desflurane changes quite dramatically. An operating room temperature is not perfectly constant. It keeps changing slightly depending on various factors including the number of medical students young body heat watching the surgery.
These changes in operating room temperature then change the temperature of vaporisers present in that room. As discussed elsewhere, the standard vaporisers try to resist changes in temperature e.
However, these mechanisms are not perfect and in practice small changes in vaporiser temperature still occur. In them, small temperature changes will lead to only small changes in vapor pressure and this can be compensated by mechanisms such a the bimetallic strip. So a whole new vaporiser design had to be made. The solution chosen for the problem is to have a vaporiser that heats the Desflurane to a very precisely controlled temperature that is not affected by changes in room temperature.
The two streams then mix at the end of the vaporiser to give the final concentration of anaesthetic.
Inhalational anaesthetic agents are usually liquids at room temperature and barometric pressure and need to be converted to vapour before being used and this conversion is effected using a vapouriser. Vapourisers have evolved from very basic devices to more complicated ones. Anaesthetists should understand the basic principles of anaesthetic vapouriser, including the principles that affect vapouriser output and how they influence vapouriser design. Most of the modern vapourisers in use are designed to be used between the flow meter and the common gas outlet on the anaesthesia machine. Modern vapourisers are flow and temperature compensated, concentration calibrated, direct reading, dial controlled and are unaffected by positive-pressure ventilation. Safety features include an anti-spill and a select-a-tec mechanism and a specific vapouriser filling device.
Michael P. Vapor pressure Molecules escape from a volatile liquid to the vapor phase, creating a "saturated vapor pressure" at equilibrium. Vapor pressure VP increases with temperature. VP is independent of atmospheric pressure, it depends only on the physical characteristics of the liquid, and its temperature. So, even although evaporation proceeds at a rate governed by liquid temperature and is independent of altitude barometric pressure , individual vaporizer types may or may not function the same at altitude. Latent heat of vaporization is the number of calories needed to convert 1 g of liquid to vapor, without temperature change in the remaining liquid.
How anaesthesia vaporisers work explained simply.
Inhalational anaesthetic agents need to be delivered to the lungs for them to work. Of course one cannot simply pour them into the lungs! A much more acceptable way is to convert the liquid inhalational agent into a form that can be delivered by the inhalational route to the patient. A device which converts liquid to vapour is called a vaporiser. Before going onto discussing vaporisers, we need to first understand what a vapour is. And to understand what a vapour is, we need to know about something called critical temperature. If you take a gas, and compress it really hard, the particles that compose it are brought ever so close to each other.
Modern Anaesthesia Vapourisers
Vapour - gaseous phase of a substance below its critical temperature. Saturated Vapour Pressure SVP - partial pressure of the vapour phase of a substance when at equilibrium with its liquid phase e. Increases rapidly as boiling point approaches. Boiling Point - temperature at which SVP equals ambient pressure.