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. I.e. desflurane is said to have a very steep "Vapor Pressure versus Temperature curve".
These physical properties of desflurane created a big headache for vaporiser designers.
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.g. by having thick metal construction). However, these mechanisms are not perfect and in practice small changes in vaporiser temperature still occur. This is not a big problem with anaesthetic agents such as Isoflurane or Sevoflurane which have a relatively less steep "Vapor Pressure versus Temperature curves". 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. With Desflurane, with its steep "Vapor Pressure versus Temperature curve", even these small temperature changes can cause large changes in vapor pressure which cannot be compensated for with simple devices such a 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 heated vapor is then "injected" into the fresh gas flow.
Before discussing the desflurane vaporiser in detail, let us first understand the concept of vaporisers "injecting anaesthetic" ( No, I am not referring to you "injecting" propofol into a patient! ).
You will recall that "standard" vaporisers work by splitting the fresh gas flow into two pathways, one going through the vaporising chamber and picking up anaesthetic agent and the other "by passes" the chamber and thus has no anaesthetic. The two streams then mix at the end of the vaporiser to give the final concentration of anaesthetic.
Another option is to "inject" the anesthetic agent directly into the fresh gas flow. In this method, the fresh gas flow coming from the flow meters does not split into two streams. There is only one stream for the fresh gas flow, and into this stream, the anaesthetic agent is directly injected.
The desflurane vaporiser works in a similar way. There is a tank (sump) which contains desflurane which is electrically heated to a highly controlled constant temperature (approximately 40 degrees C). Because of the heat, above the liquid Desflurane is gaseous Desflurane at a pressure of about two atmospheres (about 1500 mmHg or 200 kPa). This Desflurane gas is injected into the fresh gas flow.
The amount of Desflurane concentration in the fresh gas is controlled by the dial setting set by you. The dial moves a valve which varies the resistance to Desflurane flow from the tank to the fresh gas.
If you want a higher concentration of desflurane, the valve attached to the dial reduces the resistance to flow of desflurane and more of it gets injected into the fresh gas.
Conversely, if you want a lower concentration of desflurane, the valve attached to the dial increases the resistance to flow of desflurane and less of it gets injected into the fresh gas.
THE PROBLEM of FLOW
You may recall that the "standard" vaporiser is flow dependant. That is, at higher flows, the vaporisation is inadequate and the concentration of anaesthetic delivered will fall unless special modifications are made to the basic design.
The Desflurane vaporiser discussed so far will also be affected by the flow rate of fresh gas going through it. However, it is very important to note that the mechanism of how flow can affect a "normal" vaporiser and a Desflurane vaporiser are very different.
In an "ordinary vaporiser", the high flow rates cause the vaporiser output concentration to drop because of inadequate vaporisation. In the Desflurane vaporiser, this is not the reason for drop in concentration at high flows, as there is no problem with vaporisation. In fact, because of its low boiling point, Desflurane very easily becomes vapor.
In the desflurane vaporiser, the reason for a drop in concentration of anaesthetic at high flows is that the anaesthetic becomes more diluted by that high flow.
If you keep the rate of injection of Desflurane constant, and increase the fresh gas flow, the injected Desflurane will be diluted more and the delivered concentration will drop.
Conversely, if you keep the rate of injection of Desflurane constant, and decrease the fresh gas flow, the injected Desflurane will be less diluted and the delivered concentration will increase.
One solution would be for you to manually adjust the dial setting to match the fresh gas flow. For low flows, you will have to reduce the dial setting to reduce the rate of Desflurane injection, and for high fresh gas flows, you will need to do the opposite. This would be really tedious in our modern times. Fortunately, the Desflurane vaporiser automatically adjusts the rate of injection of desflurane to match the flow rate, and thus keeps the delivered concentration constant.
We are now ready to discuss the workings of the Desflurane vaporiser. You will need to refer to the numbers on the diagram below:
Your flow meters deliver the fresh gas flow [1]. The fresh gas travels through pipe [2]. Note that, unlike other vaporisers, none of the fresh gas goes to the vaporising chamber [4]. The vaporising chamber is electrically heated [3]. Using sensors for feedback, the temperature is kept very constant. The heating causes the Desflurane to become a gas under pressure [4] and this travels down pipe [5]. The dial you control is fixed to a valve [6] that changes the resistance to Desflurane flow. When you increase the concentration setting, the valve opens a bit and lowers the resistance, allowing more Desflurane to flow through. The Desflurane then goes via pipe [7] and meets the fresh gas at [8]. The Desflurane mixes with the fresh gas and a final concentration emerges from the exit of the vaporiser [9].
Now we can discuss how the vaporiser, to keep the output concentration accurate, adjusts the Desflurane flow when the fresh gas flow changes. Here is the same diagram again:
The fresh gas flows in pipe [2]. This pipe has a fixed resistance [10] in its path. For the fresh gas flow to overcome this resistance [10], the pressure in pipe [2] rises. Higher the fresh gas flow in pipe [2], higher will be the pressure rise in pipe [2] since more flow has to occur through the same fixed resistance [10]. Similarly, when the fresh gas flow is decreased, the lesser flow will find it easier to go through the fixed resistance and the pressure in pipe [2] drops. It is important to remember that the pressure in pipe [2] is proportional to the fresh gas flow going through it. Higher the flow, higher is the pressure.
Pipe [5] carries desflurane under pressure from the vaporising chamber [4] to the fresh gas flow at [8]. The flow of Desflurane is resisted by two valves [6,13]. Valve [6] is the valve that you control when you set the dial to a particular concentration. Valve [13] is an electronically controlled valve. Computer [12], the vaporiser's "brain", is able to alter the flow of Desflurane by controlling valve [13]. Device [11] is called a "differential pressure transducer". It has a diaphragm that on one side is exposed to the pressure in pipe [2] carrying fresh gas and the other side of the diaphragm is exposed to the pressure in pipe [5] carrying Desflurane. When the pressure is equal on both sides of the diaphragm, it lies in a neutral position. If one side of the diaphragm is at a higher pressure than the other side, the pressure difference makes the diaphragm move. In this way, the differential pressure transducer [11] is able to measure the pressure difference between the fresh gas pipe [2] and the Desflurane pipe [5]. It continuously keeps computer [12] informed about pressure difference information.
Now let us see how the vaporiser copes when the fresh gas flow is increased.
The fresh gas flow has been increased by you [1]. Increased flow fresh gas flows through pipe [2] and meets fixed resistance[10]. The increased flow through the fixed resistance [10] makes the pressure in pipe [2] to rise and this pressure is experienced by differential pressure transducer [11]. Since the desflurane pressure in pipe [5] is now lower than the fresh gas pressure in pipe [2], the diaphragm in the differential pressure transducer [11] moves and a signal about the pressure difference is sent to the computer [12].
The computer [12], acts on the information provided by the differential pressure transducer. It proceeds to increase the flow of desflurane to inject into the increased fresh gas flow. It commands the electronically controlled valve [13] to reduce the resistance to flow. As the valve [13] opens up and lowers the resistance, the Desflurane flow increases.
The increased flow of Desflurane causes the pressure in pipe [5] to rise and this rise pushes the diaphragm of the differential transducer back to its neutral position and it sends a signal to the computer [12] that the desflurane pressure has increased sufficiently and the computer stops telling the valve to open further.
The increased pressure [5] increases the Desflurane flow which mixes with the increased fresh gas flow and maintains the output concentration [9]. If the fresh gas flow changes the system will again alter the rate of injection of Desflurane.
Click on button for more details about the Desflurane vaporiser
Here is a simple animation to show how the vaporiser increases the desflurane flow to match a rise in fresh gas flow:
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