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INHALATIONAL INDUCTION
Inhalational anaesthesia forms the basis of most general anaesthetic
techniques in common use, although intravenous techniques are
an alternative. There are two different systems available for
delivering anaesthetic gases and vapours to the patient:
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Draw-over
system: uses air as the carrier gas with added volatile
agents or compressed medical gases |
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Continuous-flow
system: compressed medical gases (which must have a minimum
of 30% oxygen) pass through flow meters and vaporizers to
supply anaesthetic to the patient. |
Draw-over
systems can be used with either cylinders or oxygen concentrators
as their oxygen source; Boyle’s machines function only
if cylinders are available. The draw-over system is capable
of producing first-class anaesthetic and surgical conditions.
Modern draw-over apparatus has proved extremely reliable,
easy to understand and maintain and economical in use. However,
some small hospitals, and many larger ones, are equipped
with continuous-flow machines. Detailed descriptions of both
systems are found in Anaesthesia at the District Hospital (WHO, 2001).
Inhalation (gas) induction is the technique of choice for inducing
anaesthesia when the patient’s airway is difficult to
manage. If you use an intravenous induction for such a patient
and “lose” the airway, the patient
may die of hypoxia if you are unable to ventilate the lungs. In contrast, inhalational
induction can proceed only if the patient has a clear airway
down which the anaesthetic can pass. If the airway becomes
obstructed, the patient will stop taking up further anaesthetic
and redistribution of the drug in the body will lighten the
anaesthesia. As this happens, the patient will clear the obstruction.
Inhalational induction is also preferred by some children who may object to
needles. Inhalational induction is an important technique. Practise regularly;
it is simple and requires only patience, care and observation. Either draw-over
or continuous-flow apparatus can be used for inhalational induction (Figure
14.2)
but slightly differing techniques are needed.
Using draw-over apparatus
The best agents to use are ether (for example from the EMO or PAC vaporizer)
and halothane (for example from the PAC or Oxford Miniature Vaporizer (OMV)).
If oxygen is available (1 litre/minute), it should be added with a Tpiece.
For a smooth induction:
| 1 |
Gently
apply a well fitting face mask and begin induction with
halothane. Halothane is preferable because, unlike ether,
it is not an irritant. Gradually increase the concentration
until the patient is asleep (maximum 2–3% halothane). |
| 2 |
Then,
slowly turn on the supply of ether and increase the concentration
by 1% every five breaths. If the patient coughs or holds
his or her breath, reduce the ether concentration immediately
by a third of its setting and try again. |
| 3 |
When
you reach 8% ether, turn off the halothane. You may then
proceed to laryngoscopy and intubation after further
deepening the anaesthesia by increasing the ether concentration
to about 15%.
Watch for the onset of paralysis of the
lower intercostal muscles to show that the anaesthesia
is deep enough. The addition of oxygen is desirable at
least until after intubation. At high altitude, you will
need more oxygen supplement. |
| 4 |
If
your attempt at intubation does not succeed, reapply
the face mask and deepen the anaesthesia again for a second
attempt:
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If
intubation is still impossible, but you can maintain
a clear airway
using a face mask, you may proceed to give anaesthetic with the face
mask, using ether at 7–10% to provide relaxation if required |
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If
relaxation is not required, reduce the ether concentration
to 6%.
With ether at this concentration your patient can, if necessary, manage
without oxygen supplementation, provided that he or she is not very
young, old, ill or anaemic. |
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Take special care if halothane is used as an alternative to ether; it depresses
the heart and respiration. Give additional oxygen if at all possible and
use a tracheal tube and controlled ventilation for all but brief operations.
Using a compressed gas machine
| 1 |
Check
your machine, making sure that you have adequate supplies
of gas for the duration of anaesthesia:
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If
intubation is still impossible, but you can maintain
a clear airway using a face mask, you may proceed
to give anaesthetic with the face mask, using ether
at 7–10% to provide relaxation if required |
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Use
an oxygen analyser at the gas outlet, if possible |
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If
you are using a one-way breathing valve at the
patient end of the circuit, set the total gas flow
(oxygen or oxygen plus nitrous oxide) higher than
the patient’s minute volume. |
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| 2 |
If
you are using halothane as your main anaesthetic agent,
place the face mask on the patient’s face and gradually
increase the halothane concentration up to a maximum
of 3%, reducing it to 1.5% after the patient has settled
or after intubation. |
| 3 |
If
you are using ether (without halothane) as your volatile
agent:
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Turn
on the supply of ether from the Boyle’s bottle
with the face mask
held about 30 cm above the patient’s face |
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Gradually
lower the mask over the next minute to slowly increase
the ether concentration in the inspired gas; this
is usually well tolerated |
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Once
the mask is in contact with the face, slowly increase
the ether concentration over a few minutes |
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The
patient will be ready for intubation when you can
see paradoxical (inward) movement of the lower ribs
during inspiration. |
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