A Textbook of Clinical Pharmacology and Therapeutics

154 ANAESTHETICS AND MUSCLE RELAXANTS

BENZOCAINE

Benzocaineis a topical anaesthetic which is comparatively
non-irritant and has low toxicity. Compound benzocaine
lozenges (containing 10 mg benzocaine) are used to alleviate
the pain of local oral lesions, such as aphthous ulcers, lacer-
ations and carcinoma of the mouth.

TETRACAINE

The use of tetracainein the UK is restricted to topical applica-
tion, especially in ophthalmic surgery. However, it is popular
in the USA for use in spinal anaesthesia because of its potency
and long duration of action.

CHLOROPROCAINE

Chloroprocaineis claimed to have the most rapid onset of all
and has low toxicity. Although not available in the UK, it is
widely used in North America.

FURTHER READING

Allman KG, Wilson IH. Oxford handbook of anaesthesia, 2nd edn.

Oxford: Oxford University Press, 2006.

Saseda M, Smith S. Drugs in anaesthesia and intensive care. Oxford:

Oxford Publications, 2003.

Key points

Toxicity of local anaesthetics

• Inadvertent intravenous injection may lead to
  convulsions and cardiovascular collapse.


• Initial symptoms of overdose (excess local dose resulting
  in high plasma concentrations and systemic toxicity)
  may include light-headedness, sedation, circumoral
  paraesthesia and twitching.


• The total dose of lidocaine should not exceed 200 mg
  (or 500 mg if given in solutions containing adrenaline).

Case history

An 18-year-old white South African girl who had recently

commenced the oral contraceptive was admitted with

abdominal pain and proceeded to have a laparotomy.

Anaesthesia was induced using thiopentalandsuxametho-

nium, and was maintained with isoflurane. A normal

appendix was removed. Post-operatively, the patient’s

abdominal pain worsened and was not significantly

improved with a morphineinjection. A nurse reported that

the patient’s urine appeared dark in colour and her blood

pressure was high.

Question

What is the likely post-operative diagnosis and what may

have precipitated this?

Answer

Acute intermittent porphyria in association with:

• oral contraceptive pill;


• thiopental.

Opiates, such as morphineandpethidine, are thought to be

safe in porphyria. (Ectopic pregnancies should always be

considered in sexually active female patients with abdominal

pain.)