A-Textbook-of-Clinical-Pharmacology-and-Therapeutics-5th-edition

154 ANAESTHETICS AND MUSCLE RELAXANTS
BENZOCAINE
Benzocaine is 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, lacerations
and carcinoma of the mouth.
TETRACAINE
The use of tetracaine in the UK is restricted to topical application,
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
Chloroprocaine is 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.
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 thiopental and suxamethonium,
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 morphine injection. 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 morphine and pethidine, are thought to be
safe in porphyria. (Ectopic pregnancies should always be
considered in sexually active female patients with abdominal
pain.)
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).
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.