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.