INTRAVENOUS INDUCTION

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

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:

Injection 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

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.

Ketamine

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 you have.

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

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 2–4 minutes.

There are circumstances when suxamethonium should never be given; commonly encountered contraindications are:

Other non-depolarizing relaxants have a longer duration of action and generally require specialist skills to be safe. They are beyond the scope of this book.