The basic vaporiser discussed previously has a very simple design. Unfortunately, this simple design has some problems:
The flow rate of fresh gas going through the vaporiser can affect its output.
The temperature of the vaporiser drops with use and this can affect its output.
Some ventilators transmit a "positive pressure" back into vaporiser which can affect its output.
The problem of flow is discussed below. The others will be discussed elsewhere.
PROBLEM of HIGH FLOW
As discussed before, part of the fresh gas flow enters the vaporisation chamber and picks up vapor.
However, this vaporisation is not very efficient. If one uses a high fresh gas flow, the vaporisation process can't keep up with so much gas arriving into the vaporisation chamber. 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 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. I.e. 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.g. "Copper Kettle") use bubbles to increase the surface area for vaporisation. 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.
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