MEDICAL
GASES
Syllabus:
Different gases, color coding of cylinders,
care of cylinders, pressure regulator and accessories.
CARBON DIOXIDE (CO2)
Uses:
1.
Normal concentration of carbon
dioxide in air is 0.04%. If this concentration is raised from normal value then
the depth of respiration is increased. At 3% the dept is doubled and at 5% the
depth becomes thrice the normal depth. It is used in emergency to induce and
improve the respiration rate in newborn babies, drowning persons, and cases of
poisoning by carbon monoxide, morphine, hypnotics and other depressants.
2.
Mixtures of oxygen and carbon
dioxide may be administered to hasten exhalation of anaesthetic gases after
surgical operations and thus reduce the post-operative vomiting and bronchitis.
3.
Liquid carbon dioxide gas (at a
pressure of 50 bars) when escapes suddenly through a pipe the gas freezes and dry ice
(–800C) is formed which is used to remove warts.
CYCLOPROPANE (C3H6)
Uses:
This is a powerful anaesthetic gas. It is
non-irritating and induction and recovery is rapid.
Mixtures of 4%
cyclopropane with oxygen produces analgesia
Mixtures of 8%
cyclopropane with oxygen produces light
anaesthesia.
Mixtures of 20 –
25% cyclopropane with oxygen produces surgical
anaesthesia.
Cyclopropane and oxygen mixture forms an
explosive combination at certain concentration hence this gases are used in
closed circuit.
NITROUS OXIDE (N2O,
Laughing gas)
Uses:
This is the oldest
and safest gaseous anaesthetic. Induction is quick and pleasant, recovery is
rapid, hence it is most popular gas for producing light anesthesia or analgesia
in dentistry and obstetrics.
Disadvantages:
It has low
anaesthetic potency. Large dose of this gas is required to produce anesthesia,
this may cause lack of oxygen.
OXYGEN (O2)
Oxygen is used for
variety of purposes:
(A) During anesthesia, because a person’s need for oxygen continues when
he/she is anaesthetized.
(B)
To relieve anoxia (lack of
oxygen in the tissues) that may be caused by :–
1.
Inadequate oxygenation of the blood by the lungs. This may occur in pneumonia, chronic bronchitis, emphysema,
pulmonary edema, post-operative pulmonary complications, asphyxia in newborns
and barbiturate poisoning.
2.
Reduction of the circulating blood volume. This may result from coronary failure, collapse of the peripheral
circulation and shock.
3.
Reduction of the oxygen-carrying capacity of the blood. This may
be caused by severe anemia, hemorrhage and carbon monoxide poisoning.
(C) To increase
radiation efficiency in tumor therapy
Oxygenation of tumors sensitizes them to radiations. The oxygen must
be used under high pressure with the patient enclosed in a special chamber.
HELIUM (He)
Because of its low
molecular weight helium is easier to breathe than air or oxygen. A mixture of
79% helium and 21% oxygen is used for prolonged asthma or in severe obstruction
or inflammation in the respiratory tract.
NITROGEN (N2)
Nitrogen is used
as an inert gas to replace the air in containers of drug or medicaments
sensitive to oxygen or carbon dioxide.
COLOR CODING OF
CYLINDERS
Medical gases are
stored in heavy steel cylinders made to withstand pressure of about 200bars. No
gas is stored at a pressure of 130 bars.
The cylinders are
similar in appearance. To reduce confusion each type of gas is given a
color-coding that readily identifies the gas. The symbol or name of the gas is
stenciled on the cylinder shoulder.
Gas
|
Color code
|
Liquid or gas
in cylinder
|
Carbon dioxide
|
Grey
|
Liquid
|
Cyclopropane
|
Orange
|
Liquid
|
Helium
|
Brown
|
Gas
|
Nitrogen
|
Grey body
Black top
|
Gas
|
Nitrous oxide
|
Blue
|
Liquid
|
Oxygen
|
Black body
White top
|
Gas
|
Oxygen and
carbon dioxide mixture
|
Black body
Grey & White
top
|
Gas
|
Helium and
Oxygen mixture 79 : 21
|
Black body
Brown and white
top
|
Gas
|
CARE OF CYLINDERS
1.
Cylinders should be stored in a
cool, well-ventilated room free from inflammable materials.
2.
The room should be large enough
for proper grouping according to the cylinder-contents, hence avoiding
confusion.
3.
A special storage rack is
designed to ensure the use of old stock first and is lined to prevent
defacement (damage) of identifying labels and colors.
4.
The color chart is put up on a
visible place in the storage room.
PRESSURE
REGULATORS AND ACCESSORIES
Pressure regulators
§ Before the gas in a cylinder can be used its pressure must be
reduced to much lower level required for administration. This can be achieved
by passing the gas through an automatic pressure regulator that provides a
fixed outlet pressure. For oxygen is usually within the pressure range of 1.5
to 4.5 bars and the flow rate 2 to 4 liters per minute.
§ Once the control has been set the regulator automatically maintains
a constant rate until the cylinder is exhausted.
§ If the distal end of the tube is blocked the regulator prevents
excessive pressure build-up in the apparatus and in the tubing.
§ 
A
cylinder contents gauge is fitted to the high-pressure side of the instrument.
A
cylinder contents gauge is fitted to the high-pressure side of the instrument.
Flowmeters
The rate of flow
of gas to the patient can be measured by different ways:
1.
By observing the movement of
rebreathing bag connected to the patient’s mask.
2.
By gauge flow-meters the
gas flows through a fixed orifice and the pressure is measured on a dial
reading pressure gauge behind the orifice.
3.
Float flow-meters consists of a graduated vertical tube housed in a clear plastic
container. The gas is led into the lower end and its pressure causes a small
lightweight colored float to rise in the tube to a point where equilibrium is
established between the pressure of the gas and the weight of the float. The
flow rate, usually in liter / min is indicated by the height of the float
relative to the graduated scale.
4.
In domiciliary oxygen therapy a
separate flow meter is not required. The cap of the regulator consists of a
flow selector where three positions are marked:
(a) OFF (b) MED (flow rate 2lit/min) and (c) HIGH (flow rate 4lit/min)
Nebulizers
During oxygen therapy a stream of dry oxygen on a small area of
mucous membrane may cause considerable pain. To reduce this pain the oxygen is
humidified by passing the oxygen through water kept inside the nebulizer at
room temperature.
MASKS
An oxygen mask is necessary to provide appropriate oxygen therapy to
the patient.
Low concentration masks
Use: These are used for long standing respiratory conditions like
chronic bronchitis, emphysema, and bronchial asthma.
§ With these masks a 23 to 30% oxygen concentration can be at a flow
rate of 2 lit/min.
Examples:
1.
Mask with venturi device: Its upper part is transparent, flexible, loose fitting plastic
network with a foam-lined edge for comfort. It is sealed to a clear, rigid,
plastic funnel incorporating a venturi device that ensures a 28% oxygen at a
rate of 2 lit/min.
2.
Mask with many holed
centered (MC Mask): This is a transparent, rigid,
foam-edged, plastic mask with 14cventillation holes around a central, angled,
inflow pipe that opens near to the mouth.
3.
Mask with open center (Edinburg Mask): It has a translucent, loose-fitting, plastic face
piece with a foam-covered, flexible strip at the edge and a thick-rimmed,
cylindrical opening, 5cm in diameter, at the front. The inflow tube passes
through the rim, close to the mouth and at right angles to the movement of the
respiratory air.
High concentration mask
These are used for
acute conditions in which there is little risk of carbon-dioxide narcosis, e.g.
acute lobar pneumonia, severe anaemia, coronary thrombosis and severe shock.
1.
Kidde Mask: This plastic mask provides 40 to 60% oxygen concentration at a
rate of 4lit/min. It is avilable in two versions:
(a)
Re-breathing type: This consists of a
semi-transparent, moulded, plastic face piece containing a ventilating orifice
and attached to a plastic re-breathing bag. The bag deflates and expands as the
patient inhales and exhales respectively. At the beginning of the exhalation
the breath is richest in oxygen and is returned to the bag along with fresh
oxygen from the cylinder. It restricts
escape of exhaled air until the bag is full. The rest of the air full of carbon
dioxide is expelled.
(b)
Non-rebreatnig type: In this case a
non-return valve prevents the exhaled air from entering the bag. Here the bag
acts as oxygen reservoir. This mask is used when it is necessary to prevent
rebreathing of exhaled carbon dioxide.
2.
MC Mask: This mask is used for high concentration oxygen therapy. It delivers
60% oxygen concentration at a flow rate of 6 lit/min.
OXYGEN
EQUIPMENTS
1.
Flat bottomed cylinder of
oxygen of 48ft3 (1300 litters)
capacity and fitted with bull-nose valves.
2.
Ancillary equipment comprising–
(a) a control head containing (i) a pressure-reducing valve, (ii) a
contents gauge, calibrated in units of pressure and with ¼ , ½ and FULL markings, (iii) a cap comprising a
two-flow (MED and HIGH) selector, (iv) an outlet carrying the male side of the
bayonet connection, (v) a bull-nosed cylinder adapter for finger tightening.
3.
1.37 m of polythene connecting
tube with a bayonet fitting at one need to link with (iv) above. Light weight
plastic tube is preferred.
4.
A key spanner, for opening the
cylinder.
5.
A disposable mask.
6.
A cylinder stand.
