5. Supply of medical gases Flashcards
The supply of medical gases (oxygen, nitrous oxide, Entonox and medical air)
to a typical hospital.
Central gas supply
Piped gas (oxygen, nitrous oxide, Entonox and medical air)
is supplied through high-quality copper pipelines.
The outlets have a non-interchangeable coupling in
the form of a Schrader-type valve.
The hoses from the gas outlet to the
anaesthetic machine are colour-coded.
Gas is supplied at a pressure of 4 bar,
apart from the medical gas that is used to drive
surgical instruments which is supplied at 7 bar.
The gases may come from a manifold of large cylinders. They may be arranged in
banks of cylinders, each of which should contain enough gas to supply a hospital
for at least 2 days.
Oxygen
Oxygen is usually supplied from a liquid oxygen source.
Liquid oxygen is stored below its critical temperature (-118 C)
at around -160 C and at a pressure of 7 bar,
which is the vapour pressure of oxygen at that temperature.
The low temperature is maintained both by a vacuum insulated
shell and by the fact that as the oxygen evaporates,
its temperature will fall.
The contents of the storage device can be determined
either by weight or by pressure gauges,
which measure the pressure difference between the top and bottom of the liquid oxygen.
Oxygen
Oxygen is usually supplied from a liquid oxygen source.
Liquid oxygen is stored below its critical temperature (-118 C)
at around -160 C and at a pressure of 7 bar,
which is the vapour pressure of oxygen at that temperature.
The low temperature is maintained both by a vacuum insulated
shell and by the fact that as the oxygen evaporates,
its temperature will fall.
The contents of the storage device can be determined
either by weight or by pressure gauges,
which measure the pressure difference between the top and bottom of the liquid oxygen.
Gas cylinders
— The cylinders on an anaesthetic machine are usually size ‘E’,
which contain 680 litres of oxygen,
1,800 litres of nitrous oxide and 680 litres of medical air.
Designed to withstand very high pressures (they are tested to 250 bar),
they are made of molybdenum, chromium steel, manganese and high-carbon manganese
steel. (Cylinders for domiciliary oxygen can be made of lightweight aluminium
alloy.)
— Their features include colour coding
(which is not international; oxygen cylinders
in the UK are black with white shoulders, whereas in the USA they are green),
a pin index system to ensure attachment only to the correct yoke and information
about the contents of the cylinder.
The coloured plastic collar indicates the date of the last cylinder test (the interval is between 5 and 10 years).
The bodok seal is a neoprene washer enclosed within an aluminium surround which provides a
gas-tight seal between the cylinder head and the yoke.
Cylinder contents
— Oxygen is stored as a gas at a pressure of 13,700 kPa (137 bar).
— Entonox is a 50:50 gas mixture of oxygen and nitrous oxide at a pressure of
13,700 kPa.
— Medical air is stored in cylinders of high tensile steel at a pressure of 13,700 kPa.
Larger cylinders (which contain 4,600 litres) are designed for a higher maximum
working pressure of 23,000 kPa.
— Nitrous oxide is in a mixed liquid and vapour phase whose pressure is 4,400 kPa.
Oxygen concentrators
— These provide an alternative method of providing oxygen,
although their low flow rates (4 l min/1) and
pressures (70 kPa) mean that they are more commonly used
to provide domiciliary supplies for individual patients.
— They comprise zeolite-containing columns. Zeolites are hydrated aluminium
silicates which are ion-exchangers and molecular sieves. The flow of air into the
cylinders is directed so that nitrogen and water vapour are absorbed from one
cylinder, and absorbed gas from the other is extracted by a vacuum pump. Every
30 seconds a solenoid valve switches the flow to ensure a constant flow of 95%
oxygen to the reservoir. The remaining 5% is argon, which appears to have no
adverse effects. (Although in higher concentrations it has the same effect as xenon
in stimulating erythropoietin release.)
Medical air
Medical air: this can be supplied from a central compressor or from cylinders.
It must be dry, free from particulate matter, including the mineral oils used to
lubricate the compressor, and free from bacteria. The air is, therefore, desiccated
and filtered.
the critical temperature
the critical temperature of a gas is the temperature
at which it cannot be liquefied no matter how great the pressure that is applied
(oxygen: 118.6 C, nitrous oxide: 36.4 C, xenon: 16.6 C).
The critical pressure
The critical pressure of a substance is the pressure required to liquefy a gas at its critical temperature.
Filling ratio
Filling ratio:
this is the mass of gas used to fill a cylinder divided by the mass of
water needed to fill the cylinder completely.
It applies to gases that are stored in the liquid phase,
and for nitrous oxide, it is 0.75. If the cylinder is to be used in hotter
climates, this is reduced to 0.67.0
An overfilled cylinder that is exposed to high
ambient temperatures will generate dangerously high pressures.
Safe storage
This is largely common sense.
Cylinders should be kept in a secure and
dry environment, free from extremes of temperature.
Full and empty cylinders should ideally be kept in separate areas to avoid the risk of substitution.
Large cylinders are usually stored upright; smaller ones may be laid horizontally.
Entonox:
Entonox: this is a 50:50 N2O/O2 mixture.
Cylinders should be stored flat to prevent
the risk of delivering 100% nitrous oxide when the cylinder is first used.
Nitrous oxide
Nitrous oxide:
you may be asked what happens when a nitrous oxide cylinder empties.
In theory, the pressure, which is the vapour pressure,
should remain constant until the liquid phase is exhausted,
after which the pressure would fall to zero as the cylinder emptied.
In practice, because the temperature of the liquid nitrous oxide falls
as it vaporizes, the cylinder pressure also drops. The pressure returns to 4,400 kPa only
if the gas flow ceases and the cylinder is allowed to return to room temperature
Gas supply or oxygen failure
failure of the liquid oxygen source triggers supply
from a reserve manifold of large oxygen cylinders, which are also remote from the
site of delivery to the patient. There should also be reserve cylinders available in
theatre. Should there be a complete failure of oxygen delivery, the anaesthetic
machine should discontinue the flow of nitrous oxide and entrain air.
