Antimuscarinics (Cardiac)
Antimuscarinics used for bronchodilation are covered under Antimuscarinics (Respiratory), whilst atropine is covered separately.
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.- Atropine
- Hyoscine
- Synthetic quaternary amines
Do not cross the blood-brain barrier.- Glycopyrrolate
| Property | Glycopyrrolate | Hyoscine |
|---|---|---|
| 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 | |
| MSK | Anhydrosis | |
| GIT | Reduced oral and gastric secretions, and gastric motility | Reduced oral and gastric secretions, and gastric motility. Increases biliary peristalsis |
| GU | Difficult micturition |
References
- 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.