Positive pressure ventilation result in intermittent pressure changes. During the positive pressure, there is a pressure rise and during expiration, there is a sharp drop in pressure. These pressure changes can be transmitted back into the vaporiser and can affect the concentration of anaesthetic agent delivered. The effect of changing pressure affecting the output of the vaporiser is called the "pumping effect". 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. This "back pressure" is transmitted to both, the "by pass" channel and also to the vaporising chamber. This "back pressure" opposes the flow of the fresh gas in both the "by pass" channel and the vaporising chamber. The fresh gas tries to move forward and gets compressed both in the 'by pass' channel and the vaporising chamber. However, the vaporising chamber volume is much larger than the 'by pass' channel volume, and thus, more fresh gas gets compressed into it than into the 'by pass' channel.
This extra fresh gas that enters the vaporising chamber collects anaesthetic vapor 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.
Some of the rapidly expanding gas (containing vapor) enter the inlet of the vaporiser and cross over into the 'by pass' channel as shown below.
Normally, a vaporiser 'by pass' channel does not have vapor. So this vapor due the 'pumping effect' is additional. When this 'by pass' vapor flows across to the exit of the vaporiser, it meets the vapor from the vaporising chamber. The addition of the 'by pass' vapor to the vapor from the vaporising chamber raises the final concentration of anaesthetic delivered. i.e. The 'pumping effect' increases the delivered concentration of anaesthetic agent.
Vaporiser designers have various tricks to reduce the 'pumping effect' and some of these are discussed below:
LARGE 'BY PASS CHANNEL
The 'by pass' channel can be made larger, ideally equal to the volume of the empty space of the vaporising chamber. Therefore, when there is 'back pressure', the effects will be equal in both; in the vaporising chamber and the 'by pass' channel. Therefore, when there is 'back pressure', no longer will extra fresh gas go into the vaporising chamber.
LONG INLET TUBE
The vaporiser inlet tube can be made longer. When the 'back pressure' is suddenly released during expiration, as discussed before, the extra gas in the vaporising chamber will suddenly expand. However, thanks to the long inlet tubing, the extra gas containing vapor expands into the long inlet tube and doesn't reach the 'by pass' channel.
INCREASED RESISTANCE
The vaporiser can be designed to have a high internal resistance to flow. This high resistance "resists" changes to flow caused by the intermittent 'back pressure' of positive pressure ventilation.
ONE WAY VALVE
A 'one way' valve (also called unidirectional valve) can be put between the vaporiser outlet and the ventilator / breathing system. 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. This prevents the transmission of 'back pressure' to the vaporiser.
[ Pumping Effect ] [ Desflurane Vaporiser ] [ Inter lock Mechanisms ] [ Vaporiser Fillers ] [ Home ]
