Antimuscarinics are as competitive, reversible antagonists of ACh at the muscarinic receptor. They are divided into:
- Naturally occurring tertiary amines
These can cross the blood-brain barrier, and have central effects.
- Synthetic quaternary amines
Do not cross the blood-brain barrier.
|Class||Quaternary amine. Muscarinic antagonist||Tertiary amine|
|Uses||Bradycardia, antisialagogue||Antisialagogue, motion sickness|
|Presentation||Clear, colourless solution at 200μg.ml-1. Incompatible with diazepam and thiopentone.||Racemic, only L-isomer active|
|Route of Administration||IV/IM||PO, SC, IV/IM|
|Dosing||200-400μg||20-40mg IV slow push or IM|
|Absorption||Minimal PO absorption - not used via this route.||< 50% PO bioavailability|
|Distribution||Crosses placenta but not BBB, VD 0.5L.kg-1||VD 2L.kg-1|
|Metabolism||Minimal hepatic hydrolysis||Extensive metabolism by hepatic esterases|
|Elimination||Renal of 85% unchanged drug||Renal of metabolites|
|Resp||Bronchodilation, antisialagogue||Bronchodilation, greatest antisialagogue effect|
|CVS||Initial bradycardia due to partial agonist effect. Reverses vagal causes of bradycardia, may cause tachycardia in doses >200μg. HR peaks at 3-9 minutes following administration.||Least likely anticholinergic to cause tachycardia|
|CNS||Most likely anticholinergic to cause central anticholinergic syndrome|
|GIT||Reduced oral and gastric secretions, and gastric motility||Reduced oral and gastric secretions, and gastric motility. Increases biliary peristalsis|
- Smith S, Scarth E, Sasada M. Drugs in Anaesthesia and Intensive Care. 4th Ed. Oxford University Press. 2011.
- Petkov V. Essential Pharmacology For The ANZCA Primary Examination. Vesselin Petkov. 2012.