CLASS
Anti-cholinergic
PRESENTATION
Clear, colourless solution.
Formulations
- 1ml vial, 600mcg, 600mcg/ml
INDICATIONS & DOSING
Bradycardia
- Adult; 300-600mcg IV/IO/IM/subcut, repeat 3-5 minutes
- Paediatric; 20mcg/kg IV/IO/IM/subcut, repeat 3-5 minutes
PRACTICALITIES
Administration
- Drawn and administered neat
Incompatibilities
- Adrenaline, thiopentone, heparin, metaraminol, sodium bicarbonate
Practice tips
- Follow with a rapid saline flush and consider chest compressions to aid circulation of drug if circulation compromised
- Particularly efficacious when bradycardia is parasymapthetically driven (e.g. oculocardiac reflex)
- Maximum dose due to limit in efficacy with complete parasympathetic blockade, not toxicity (adult ~3mg)
- Avoid low dose which can cause paradoxical bradycardia via the Bezold-Jarisch reflex (adult <500mcg)
PHARMACOKINETICS
Onset
- IV; 2-4 minutes
- IM; 15-50 minutes
Duration of action
- IV/IM; maximum effect 20-60 minutes, duration up to 5 hours
Metabolism
Metabolised in the liver and tissues by hydrolysis to tropine and tropic acid which are renally excreted
Elimination
~30-50% of drug excreted unchanged renally
MECHANISM
Competitive antagonism of acetylcholine at central and peripheral muscarinic receptors. Inhibits effects of parasympathetic outflow.
DESIRED CLINICAL EFFECTS
Airway
- Positive chronotropy, dromotropy, increase in cardiac output
Respiratory
- Bronchodilation
- Decrease in bronchial secretions
Gastrointestinal
- Antisialagogue (less effective cf. hyoscine)
- Reduces gastrointestinal tone, aiding procedural instrumentation
- Anti-emetic
OTHER CLINICAL EFFECTS, ADVERSE EFFECTS & TOXICITIES
Respiratory
- Increase in physiological dead space
Cardiovascular
- Paradoxical bradycardia (low dose, cardiac denervation post-transplant)
- Tachycardia, precipitation of myocardial ischaemia
- Dysrhythmias due to dromotropic effect
Neurological
- ‘Central anticholinergic syndrome’; excitatory (agitation, delirium, seizures) or depressive (sedation, coma)
- Mydriasis, blurred vision
Genitourinary
- Urinary retention
Other
- Pyrexia in paediatric population due to inhibition of diaphoresis
Antidote
- Physostigmine
CONSIDERATIONS
Precautions
- Ischaemic heart disease, myocardial ischaemia
- Cardiac transplant (paradoxical bradycardia)
- Elderly and other patients at high risk of delirium/cognitive impairment
- Obstructive urinary tract disease
- GI disease; obstructive, paralytic ileus/atony, infective
- Myasthenia gravis; exacerbation
Obstetric
ADEC category A
Drug interactions
- Metoclopramide, antagonism
- MAO-inhibitors; potential for hypertensive crisis
- Oral anticholinergics; precipitation of central anticholinergic syndrome
REFERENCES
Drug information has been compiled from multiple sources including
- Drugs in Anaesthesia and Intensive Care (Scarth & Smith)
- Micromedex (IBM)
- BJA Education (Oxford Academic)
- Pharmacology for Anaesthesia and Intensive Care (Cambridge)
- Australian Prescriber (NPS MedicineWise)