Intravenous induction is pleasant for the patient and easy
for the anaesthetist. It will be the technique of choice in
many cases, but care is always needed as it is relatively easy
to give an overdose or to stop the patient from breathing.
If breathing stops, the patient may die unless you can easily
ventilate the lungs with a face mask or tracheal tube.
The first rule of intravenous induction is that it must never
be used in a patient whose airway is likely to be difficult
to manage. For such a patient,
inhalational induction is inherently much safer. Alternatively,
the patient should be intubated while still awake. Intravenous
induction will also suddenly reveal any pre-existing dehydration,
hypovolaemia or hypotension. These conditions must be corrected
preoperatively or there will be a dangerous fall in blood pressure
on injection of the drug.
Thiopental is presented as ampoules of yellow powder that must
be dissolved before use in sterile distilled water or saline
to make a solution of 2.5% (25 mg/ml). Higher concentrations
are dangerous, especially if accidentally injected outside a
vein, and should not be used.
The normal practice is to give a “sleep” dose,
by injecting the drug slowly, until the patient becomes unconscious
and loses the eyelash reflex. The average sleep dose in a healthy
adult is 5 mg/kg of body weight, but much less (2 mg/kg) is
needed in sick patients. An overdose of thiopental will cause:
of thiopental is almost always painless. If the patient reports
pain, stop injecting immediately because the needle is probably
outside the vein and may even have entered an artery. Avoid
injection into the elbow, if possible,
because it is easy to enter the brachial artery by mistake.
Thiopental is cumulative in the body and slowly metabolized. It is therefore not
suitable for the maintenance of anaesthesia.
Propofol is a recently introduced, intravenous anaesthetic that can be used
for induction of anaesthesia. It is a white emulsion which, like thiopental,
produces unconsciousness in one arm to brain circulation. Its depressant effects
on respiration and blood pressure are greater than those of thiopental, especially
if it is injected quickly and, after injection, there is often a respiratory
arrest requiring manual inflation of the patient’s lungs. Injection is
often painful unless a small amount of lidocaine (20 mg of lidocaine in 200
mg of propofol) is added just before injection.
The chief advantage of propofol is in the quality of recovery. Patients are
much less drowsy postoperatively; this is an advantage if they have to leave hospital
the same day.
The normal induction dose of propofol is 2–2.5 mg/kg of body weight.
At present, it is much more expensive than thiopental. To avoid bacterial contamination,
ampoules must be used immediately after being opened.
Induction with ketamine is similar in principle to induction with thiopental
and the same precautions apply. The average induction dose is 1–2 mg/kg
of body weight. The standard formulations are:
Be sure to check which formulation of ketamine
The patient’s appearance as he or she loses consciousness is different
from when barbiturates are used and the patient may not appear to be “asleep”.
The eyes may remain open, but the patient will no longer respond to your
voice or command or to painful stimuli. If you try to insert an oropharyngeal
airway at this stage, the patient will probably spit it out. Muscle tone
in the jaw is usually well maintained after ketamine has been given, as is
the cough reflex. A safe airway is not guaranteed since, if regurgitation
or vomiting of gastric contents occurs, there is still severe danger of aspiration
into the lungs.
After induction with ketamine, you may choose to proceed to a conventional
inhalational anaesthetic, with or without relaxants and intubation. For short
procedures, increments of ketamine may be given intravenously or intramuscularly
every few minutes to prevent the patient responding to painful stimulation.
This method of anaesthesia is simple, but produces no muscular relaxation.
Ketamine is also not a cheap drug. If your supplies are limited, try to reserve
ketamine for cases where there are few suitable alternatives; for example,
for short procedures in children when access to the airway may be difficult.
Suxamethonium is a depolarizing, short-acting muscle relaxant which is widely
used to facilitate intubation, especially for emergencies. The dose is 1–2
mg/ kg or 100 mg for a full size adult or a woman having a caesarean section
under general anaesthesia. It gives a rapid onset of total paralysis when
given intravenously. Intramuscular suxamethonium is also effective within
There are circumstances when suxamethonium should never be given; commonly
encountered contraindications are:
or established airway obstruction: for example, after
potassium level that is already high, such as in renal
that might rise, such as in severe burns
to the drug or a family history of malignant hyperthermia.
Other non-depolarizing relaxants have a longer duration of action and generally
require specialist skills to be safe. They are beyond the scope of