Pharmacology for Anaesthesia and Intensive Care

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Section IVOther important drugs

Table 18.3.Effects of some anticholinergics.

Hyoscine Atropine Glycopyrrolate

Antiemetic potency ++ + 0

Sedation/amnesia +++ + 0

Anti-sialagogue +++ + ++

Mydriasis +++ + 0

Placental transfer ++ ++ 0

Bronchodilation +++++

Heart rate + +++ ++

Effects

While hyoscine’s main uses are derived from its central antimuscarinic effects, it

also has peripheral antimuscarinic effects some of which can be useful and are

summarized in Table18.3.

Other central effects – it may precipitate a central anticholinergic syndrome, which

is characterized by excitement, ataxia, hallucinations, behavioural abnormalities and

drowsiness.

Kinetics

Its absorption is variable and its oral bioavailability lies between 10% and 50%. Trans-

dermal administration is effective in reducing PONV and motion sickness despite

very low plasma levels. It is extensively metabolized by liver esterases and only a

small fraction is excreted unchanged in the urine. Its duration of action is shorter

than that of atropine.

Atropine

Atropine is a racemic mixture, but onlyl-atropine is active.

Uses

Atropine is used to treat bradycardia and as an anti-sialagogue. It is also used to

antagonize the muscarinic side effects of anticholinesterases. It is not used to treat

PONV because of its cardiovascular effects.

Effects


Central nervous system – it is less likely to cause a central cholinergic crisis than

hyoscine and is less sedative.


Cardiovascular – it may cause an initial bradycardia following a small intravenous

dose. This may be due to its effects centrally on the vagal nucleus or reflect a partial

agonist effect at cardiac muscarinic receptors.


Respiratory system – bronchodilation is more marked than with hyoscine, leading

to an increase in dead space. Bronchial secretions are reduced.