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ANAESTHETIC MACHINES.

Anaesthetic machines come in many different shapes and sizes, but they all do much the same thing, the controls may be set differently, some may look much more complicated than others with bits and pieces added on all over the place, but underneath it all is a machine that can give a mixture of gases and anaesthetic agents at a safe flow and pressure.

To start off with I will give you an introduction into some of the parts of an anaesthetic machine and the attachments used with one. Some will include, where I can, a little information about maintenance.

This relates to a simple machine made by British Oxygen.

PIN INDEX.
Is a system of making sure that cylinders of a particular gas are only put on the right yoke and thus comes out of the right control.
It is done with a pin or pins built into the yoke, these pins are in different positions for different gases, they have to line up with holes drilled in the pillar valve body of the cylinder.
If they do not line up then the cylinder can not be connected, only when the pins line up with the holes can the cylinder be connected and turned on.

Pin index for different gases.

pinindexYOKE.
Is the clamp which holds the cylinder in place, it may have a cylinder contents gauge attached to it.

BODOK SEAL.
This is a small round seal about 1 cm across made of rubber with a steel outer ring and a hole in the middle, it goes on the yoke around the hole through which the gas passes and makes a seal between the face of the pillar valve and the yoke. Without it there will be a large leak.

PILLAR VALVE.
Is the valve at the top or on the side of a cylinder of gas.
There are two different types of valve you may come across on cylinders depending upon the gas and the size of the cylinder. One on the top and oblong shaped and one on the side which is a square shape, obviously they need different keys to open and close them.

ROTAMETER TUBE.
Many machines use a tube called a rotameter tube to set accurate gas flows. It is called a rotameter because it is designed to rotate in the gas flow.
The true rotameter tube is made of electrically conductive glass and a bore which is tapered, being wider at the top than at the bottom, it has a bobbin inside which has flutes cut out of it at the top, these make it spin in the gas flow showing that it is not stuck, it also stabilises and centres it. There is a spot on the side that helps to see that it is rotating. The bobbin is most often made of aluminium, this makes it light and resistant to corrosion, however I only said that it is RESISTANT to corrosion.
I once saw a machine in the hospital in Pokhara in Nepal where the bobbin in a tube that was never used, in this case cyclopropane, had obviously got a tiny drop of moisture on to it and it had started to corrode, if this is allowed to go on till the corrosion gets out of hand it can have serious consequences, what happens is that the corroding metal expands and this expansion may eventually crack the glass of the tube causing a serious leak.
There are a couple of things you can do to prevent this happening:

a. If you are sure that the tube is never going to be used, consult the senior anaesthetist, and with his written permission take the tube out and replace it with a piece of stainless steel or brass solid rod of the same diameter and length. In an emergency, I have seen a glass drug vial used (see photo below), but that sort of thing should be replaced with the correct tube or a stainless steel rod as soon as possible.

Improvised repair

Drawing of the parts of the flowmeter block on a Boyles anaesthetic machine.

flowmeter block

dcp3_0001

b. If you think that the tube may be used, then during the service take the tube out and remove the bobbin and make sure that every part of it absolutely dry. If, however, you do come across one that has started to corrode, this is what to do:
a. Remove the tube very carefully from the block.

b. Remove the bobbin stop at the top of the tube.

c. If it is free, tip the bobbin out of the tube.

d. If it is not free then you have to, WITH GREAT CARE, put a thin piece of wood that is just smaller in diameter than the inside of the tube, into the tube and tap the bobbin with it to try and force it out. This must be done with the greatest of care, too much force and you WILL crack the tube.

e. Try a little penetrating oil, it should come out in the end.

f. Once you have the bobbin out, clean it up with very fine abrasive paper, you must get rid of all signs of corrosion, and leave the sight of corrosion smooth.

g. Clean out of the tube with soap and water to remove any corrosive material or penetrating oil, then rinse it really well to get rid of any soap solution and dry it properly.

h. Replace the bobbin in the tube, and replace it in the block.

i. Test it really well to ensure that the bobbin does not stick in the tube and that it rotates.

The graduations on the tube are calibrated with the bobbin inside, on a true rotameter tube you will see that the bobbin has a number on it, there should be the same number on the glass tube, they cannot be changed around from one tube to another except in a real emergency, as this would make the graduations slightly inaccurate, and they should never be changed from one gas tube to another.
Tubes cannot be changed between gases as each tube is calibrated only for that gas marked on it. In any case very often the tubes are of different sizes and physically could not fit into another place.
They should have an accuracy of about 2%.

Flowmeters used on anaesthetic machines will have a bobbin, these are read from the top as below.

flowbob

Other flowmeters such as used for oxygen delivery to a patient in a bed will have a ball, these are read in the middle as below.

flowball


There are other types of flow indicators that you may come across here is a description of a few of them:

Coxeters dry bobbin flowmeter.
A bobbin floats in a vertical glass tube of uniform bore. Gas enters from below.
As the flow increases the bobbin rises in the tube and allows the gas to pass out through a series of holes in the back of the tube. At some point the gas leaking from the holes is such that the bobbin stays in one position. The different gas  tubes sit in a sealed outer tube which delivers the gas to the top manifold
Now replaced by the more accurate rotameter type.

Bourdon flowmeters
Are found on flow-limited regulators (for delivery of oxygen directly to patients) but are not usually used on anaesthetic machines. They consist of an adjustable pressure supply, a pressure gauge (calibrated in units of flow) and a special orifice. As the pressure is increased, flow through the orifice increases, which is reflected by the gauge.

One problem with this type of flowmeter is that if the orifice becomes partially blocked, the flowmeter will over-read (since, for any given pressure, the flow will be less because of the increased resistance of the orifice), and vice versa.

The Heidbrink meter.
A black inverted float is free to rise within a metal tube with a varying taper. The upper-end projects into a glass tube.
It is accurate at low flows and will also accommodate high flows.

flowheid

The Connel meter.
A pair of stainless steel balls move within a tapered glass tube on an inclined plane.
I have seen some of these meters on anaesthetic machines on but as a rule they will be the older ones, newer machines will have the rotameter tube and a bobbin.

RESERVOIR BAGS.
These are usually made of anti-static rubber and should be large enough to supply the patients inspiratory volume.
They also act as an excellent visual monitor of spontaneous breathing.
They come in various sizes, 0.5 litre, 1 litre, 2 litre, 3 litre, 4 litre and 6 litre.
The 2 litre bag being the normal adult size that you will come across.
These tend to perish and develop holes, they should be thrown away when they have got like this.
Quite often people will stick tape over the hole, this may be all right to last you through an operation if you are unable to get a new one quickly, but it should not be left in this condition.
The problem is that once one hole has developed others will appear soon, and it is most probable that it will appear at a moment of crisis.
As an outside repair, use a patch from a bicycle inner tube repair kit.

HEIDBRINK VALVE.
This is a pressure relief valve used in the patient circuit for controlling the pressure of gas that the patient gets when being inflated by hand with a rubber bag, they should have a minimal resistance to expiration, and during spontaneous breathing should always be fully open.
Their opening pressure must always be greater than the collapsing pressure of the reservoir bag or the bag will not act as a reservoir.
It also allows the expired gas to escape.
It consists of a spring-loaded disc, and by turning the adjustment screw you are increasing or decreasing the pressure of the spring on the disc.
This will regulate how much pressure the patient gets, and the excess will blow off to the atmosphere

REDUCING VALVE.
These valves are used to reduce the high pressure in a cylinder of gas down to a more safe level.
In the case of oxygen the pressure is reduced from 2000 psi to about 55 or 60 psi.

OXYGEN FLUSH VALVE.
This is a valve which will, when pressed, give a high flow of oxygen to the patient, it is taken straight off the reducing valve, thus bypassing the flow tubes.
Many machines have a valve which when pressed and turned will lock in the open position, but it is now considered to be a bad thing, and the newer machines have a valve which cannot be locked.
The sort of flow that you should expect out of one of these will be about 35 lpm. and up to about 75 lpm.

Drawing of an oxygen flush valve and blow off assembly on a Boyles anaesthetic machine.

Emergency flush and blow off

Drawing of a different type of emergency flush valve on a Boyles anaesthetic machine.

emergency o2 (1)

Drawing of a Boyles ‘M’ anaesthetic machine.

Boyles machine

dcp3_0003

Drawing of the pipework in a Boyles anaesthetic machine.

Boyles anaesthetic machine pipe diagram

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