Atropine Pharmacology

Quick Answer

Atropine is a naturally occurring tropane alkaloid and the prototypical competitive muscarinic acetylcholine receptor antagonist. As a tertiary amine with a pKa of 9.7, it crosses the blood-brain barrier and produces both peripheral and central anticholinergic effects. Atropine non-selectively blocks all five muscarinic receptor subtypes (M1-M5), inhibiting parasympathetic nervous system activity. Key cardiovascular effects include initial paradoxical bradycardia at low doses (<0.5 mg) due to presynaptic M1 receptor blockade enhancing acetylcholine release, followed by tachycardia at therapeutic doses (0.5-1 mg) from postsynaptic M2 receptor blockade at the sinoatrial node. Clinical indications in anaesthesia include treatment of symptomatic bradycardia (0.5-1 mg IV, maximum 3 mg), co-administration with neostigmine for neuromuscular blockade reversal (20 mcg/kg with neostigmine 50 mcg/kg), treatment of the oculocardiac reflex, and as an antidote for organophosphate poisoning (2-4 mg IV boluses until atropinisation). Unlike glycopyrrolate (quaternary amine), atropine's CNS penetration causes central anticholinergic syndrome at toxic doses. Duration of action is 4-6 hours with hepatic metabolism and renal excretion. [1-8]

Pharmacology Overview

Drug Classification and History

Atropine (dl-hyoscyamine) belongs to the belladonna alkaloid class of antimuscarinic drugs, derived from plants of the Solanaceae family including Atropa belladonna (deadly nightshade), Datura stramonium (jimsonweed), and Hyoscyamus niger (henbane). The name "belladonna" (beautiful lady) originated from historical use of the plant extract by Italian women to dilate their pupils for cosmetic purposes. Atropine was first isolated in pure form in 1831 by Mein and has been used clinically for over 150 years, making it one of the oldest drugs still in routine medical practice. [1,2]

Structurally, atropine is a racemic mixture of d- and l-hyoscyamine, though the pharmacological activity resides primarily in the l-isomer. The racemisation occurs during extraction, and pure l-hyoscyamine (available as hyoscine) is approximately twice as potent as the racemic mixture. Atropine remains the reference standard against which all other antimuscarinic agents are compared, and understanding its pharmacology is fundamental to anaesthetic practice. [3,4]

Chemical Structure and Physicochemical Properties

Atropine sulphate (C17H23NO3·H2SO4·H2O) has a molecular weight of 694.8 Da (as the sulphate monohydrate salt commonly used in clinical preparations). The molecule consists of two structural components:

Key Physicochemical Properties:

The tertiary amine structure is critically important: the nitrogen atom is bonded to three carbon groups with no permanent charge, allowing atropine to exist in equilibrium between ionised and unionised forms. At physiological pH 7.4, approximately 99% is ionised (protonated), but the 1% unionised fraction is sufficient for CNS penetration. This contrasts with quaternary ammonium compounds like glycopyrrolate, which carry a permanent positive charge and cannot cross the blood-brain barrier. [5-7]

Mechanism of Action: Muscarinic Receptor Antagonism

Atropine exerts its pharmacological effects through competitive, reversible antagonism at muscarinic acetylcholine receptors (mAChRs). It binds to the orthosteric site of the receptor—the same site where acetylcholine binds—and prevents agonist activation without producing any intrinsic activity (pure antagonist). The binding is surmountable: high concentrations of acetylcholine can displace atropine and restore receptor activation. [8,9]

Muscarinic Receptor Subtypes:

Five muscarinic receptor subtypes (M1-M5) have been identified, all belonging to the G-protein coupled receptor (GPCR) superfamily:

Atropine is a non-selective muscarinic antagonist with similar affinity for all five subtypes. The clinical effects observed depend on which organ systems are most reliant on muscarinic innervation. Importantly, atropine has negligible affinity for nicotinic acetylcholine receptors (at the neuromuscular junction or autonomic ganglia), meaning it does not block neuromuscular transmission or ganglionic transmission at therapeutic doses. [10-12]

Dose-Dependent Receptor Sensitivity:

Different organ systems show varying sensitivity to muscarinic blockade, reflecting receptor density and physiological importance:

Pharmacokinetic Principles

Absorption

Atropine is well absorbed from multiple routes of administration:

Intravenous Administration:

• Onset of action: 1-2 minutes (heart rate effect)
• Peak effect: 2-4 minutes
• 100% bioavailability
• Preferred route in anaesthesia and emergency settings

Intramuscular Administration:

• Onset of action: 5-15 minutes
• Peak effect: 20-30 minutes
• Bioavailability approximately 100%
• Used when IV access unavailable

Oral Administration:

• Oral bioavailability: 50% (moderate first-pass metabolism)
• Peak plasma concentration: 1-2 hours
• Food does not significantly affect absorption
• Used for premedication (now uncommon)

Intraosseous Administration:

• Equivalent to IV when IV access impossible
• Particularly relevant in paediatric resuscitation

Endotracheal Administration:

• Absorption variable and unpredictable
• Dose 2-3 times IV dose if used
• No longer recommended in current guidelines

Nebulised (Ipratropium Bromide):

• Quaternary derivative used for bronchodilation
• Minimal systemic absorption (<2%) [13-15]

Distribution

Atropine distributes widely throughout the body due to its lipophilic properties:

The tertiary amine structure allows atropine to cross the blood-brain barrier, producing central anticholinergic effects including agitation, confusion, hallucinations, and seizures at toxic doses. This CNS penetration is a key distinguishing feature from quaternary ammonium antimuscarinics like glycopyrrolate. [16,17]

Metabolism

Atropine undergoes hepatic metabolism via multiple pathways:

Primary Metabolic Pathways:

1. Hydrolysis: Ester bond cleavage producing tropine and tropic acid (major pathway)
2. Glucuronide conjugation: Formation of water-soluble glucuronides
3. N-demethylation: Minor pathway producing noratropine

The hepatic metabolism involves cytochrome P450 enzymes, though specific isoforms are less well characterised than for many modern drugs. Approximately 50% of a dose is metabolised, with the remainder excreted unchanged. [18,19]

Elimination

Special Populations:

The pharmacokinetic profile means that a single dose of atropine provides effects lasting 4-6 hours, which is important when considering reversal of neuromuscular blockade or treatment of bradyarrhythmias. [20,21]

Pharmacodynamics: Organ System Effects

Cardiovascular Effects

The cardiovascular effects of atropine are among the most clinically important:

Initial Paradoxical Bradycardia (Low Doses <0.5 mg):

A well-documented phenomenon where low-dose atropine causes transient bradycardia before tachycardia occurs. Two mechanisms are proposed:

1. Peripheral Presynaptic M1 Receptor Blockade:

   • Presynaptic muscarinic autoreceptors (M1/M2 subtypes) on postganglionic parasympathetic nerve terminals normally provide negative feedback, inhibiting further acetylcholine release
   • Low-dose atropine preferentially blocks these presynaptic receptors before achieving therapeutic concentrations at postsynaptic M2 receptors
   • Result: Enhanced acetylcholine release → increased vagal effect → bradycardia

2. Central Vagal Stimulation:

   • Atropine at low concentrations may stimulate medullary vagal nuclei (nucleus ambiguus, dorsal motor nucleus)
   • Increased central parasympathetic outflow produces bradycardia [22-24]

Tachycardia (Therapeutic Doses 0.5-2 mg):

At adequate doses, atropine blocks M2 muscarinic receptors on:

• Sinoatrial node → removes vagal inhibition → increased heart rate
• Atrioventricular node → enhanced conduction velocity → shortened PR interval
• Atrial myocardium → modest positive inotropic effect

The magnitude of tachycardia depends on baseline vagal tone. Young, fit individuals with high vagal tone demonstrate the greatest heart rate increase. In patients with already low vagal tone (elderly, heart failure), tachycardia may be minimal.

Other Cardiovascular Effects:

• Minimal direct effect on vascular smooth muscle (M3 receptors less important in vasomotor control)
• May reveal underlying atrial arrhythmias (atrial fibrillation, flutter) by increasing AV conduction
• Does not cause significant blood pressure changes in normotensive patients [25-27]

Central Nervous System Effects

Due to its tertiary amine structure, atropine crosses the blood-brain barrier and produces dose-dependent CNS effects:

Central Anticholinergic Syndrome (CAS):

Also termed "anticholinergic delirium," this syndrome results from excessive central muscarinic blockade. Classic features include:

• Central features: Agitation, confusion, hallucinations (visual > auditory), memory impairment, dysarthria, ataxia, seizures
• Peripheral features: Mydriasis, dry skin, tachycardia, urinary retention, hyperthermia

Classic description: "Blind as a bat, mad as a hatter, red as a beet, hot as a hare, dry as a bone"

Treatment of CAS involves supportive care and, in severe cases, physostigmine (1-2 mg IV slowly)—a tertiary amine anticholinesterase that crosses the BBB and reverses central effects. [28-30]

Ocular Effects

Atropine produces marked effects on the eye through blockade of M3 receptors:

Systemic atropine administration produces less pronounced ocular effects than topical application. However, even IV atropine can precipitate acute angle-closure glaucoma in susceptible individuals (shallow anterior chamber, hyperopia). [31,32]

Gastrointestinal Effects

Muscarinic receptors (primarily M3) regulate GI motility and secretions:

The antisialagogue effect was historically the main indication for preoperative atropine—reducing secretions to facilitate airway management and prevent reflex laryngospasm. This practice has largely been replaced by modern airway techniques. [33,34]

Genitourinary Effects

Muscarinic blockade affects bladder function:

• Detrusor muscle relaxation (M3 blockade) → reduced bladder contractility
• Internal sphincter stimulation → urinary retention
• This effect limits use in patients with prostatic hypertrophy or bladder outflow obstruction [35]

Respiratory Effects

While atropine has bronchodilator properties, its systemic side effects limit usefulness. Ipratropium bromide (quaternary derivative) is preferred for bronchodilation in COPD/asthma—minimal systemic absorption when nebulised. [36]

Clinical Pharmacology

Clinical Indications in Anaesthesia

1. Symptomatic Bradycardia:

Atropine is first-line pharmacological treatment for symptomatic bradycardia (ANZCOR, AHA guidelines):

Note: Atropine is not recommended in cardiac arrest (asystole/PEA) since 2010 guidelines—no survival benefit demonstrated. [37,38]

2. Reversal of Neuromuscular Blockade:

Co-administered with neostigmine to prevent muscarinic side effects of acetylcholinesterase inhibition:

Atropine vs Glycopyrrolate for Reversal:

Glycopyrrolate is generally preferred because:

• No CNS effects (no central anticholinergic syndrome risk)
• Onset of action better matched to neostigmine
• Less tachycardia
• More stable heart rate profile [39-42]

3. Oculocardiac Reflex:

The oculocardiac reflex (OCR) is a trigeminovagal reflex causing bradycardia, arrhythmias, or asystole with traction on extraocular muscles or pressure on the globe:

Some centres now prefer glycopyrrolate for OCR prophylaxis due to absence of CNS effects, though atropine's faster onset may be advantageous for acute treatment. [43,44]

4. Organophosphate and Carbamate Poisoning:

Atropine is the primary antidote for organophosphate/carbamate poisoning (cholinergic crisis):

Endpoints of Atropinisation:

• Clear lungs (drying of secretions)
• Heart rate >80 bpm
• Systolic BP >80 mmHg
• Pupils NOT an endpoint (may remain miotic)

Pralidoxime (2-PAM) is co-administered to reactivate acetylcholinesterase before irreversible "aging" occurs. [45-47]

5. Other Indications:

• Premedication (historical, now rarely used)
• Treatment of sinus bradycardia during anaesthesia
• Diagnostic mydriasis (though shorter-acting agents preferred)
• Reduction of secretions for bronchoscopy/laryngoscopy

Dosing Summary

Contraindications and Cautions

Absolute Contraindications:

• Hypersensitivity to atropine or belladonna alkaloids
• Narrow-angle glaucoma (untreated)

Relative Contraindications/Cautions:

Drug Interactions

Adverse Effects and Toxicity

Anticholinergic Toxidrome

The anticholinergic toxidrome results from excessive muscarinic blockade:

Mnemonic: "Hot as a hare, blind as a bat, dry as a bone, red as a beet, mad as a hatter, full as a flask"

Management:

1. Supportive care (airway, breathing, circulation)
2. Active cooling if hyperthermic
3. Benzodiazepines for agitation/seizures
4. Urinary catheterisation if retention
5. Physostigmine 1-2 mg IV slowly (for severe central symptoms)
   • Caution: Can cause bradycardia, bronchospasm, seizures
   • Contraindicated with tricyclic antidepressant co-ingestion

Lethal dose in adults is approximately 100 mg, though significant toxicity occurs at much lower doses (>10 mg). [48-50]

Comparison: Atropine vs Glycopyrrolate

Australian/NZ Specific Considerations

TGA-Approved Formulations

Atropine is available in Australia in several formulations:

The standard hospital formulation is 600 mcg/mL in 1 mL ampoules. This concentration is designed for adult dosing where 1 ampoule approximates a single dose. For paediatric use, dilution is necessary.

PBS Listing

Atropine injection is not PBS-listed for general use (hospital pharmacy supply). Atropine eye drops (1%) are PBS-listed under:

• Authority Required: Amblyopia in children under 6 years
• Streamlined Authority: Uveitis, pre/post-operative mydriasis

ANZCOR Guidelines

The Australian and New Zealand Committee on Resuscitation (ANZCOR) provides specific guidance:

ANZCOR Guideline 11.6.1 (Bradycardia):

• Atropine 500-600 mcg IV every 3-5 minutes
• Maximum total dose 3 mg
• Consider transcutaneous pacing if atropine ineffective

ANZCOR Guideline 11.2 (ALS):

• Atropine NOT recommended in cardiac arrest (asystole/PEA)
• Removed from arrest algorithms in 2010

Local Availability

Atropine is universally available in Australian and New Zealand hospitals, emergency departments, and ambulance services. It is included in emergency drug boxes and resuscitation trolleys as a core medication. The AtroPen autoinjector (2 mg) is stocked by Defence Forces and some emergency services for nerve agent/organophosphate exposure scenarios.

Indigenous Health Considerations

When caring for Aboriginal and Torres Strait Islander patients and Māori patients requiring atropine administration, several cultural and clinical considerations apply. There are no documented pharmacogenomic differences in muscarinic receptor function or atropine metabolism specific to Indigenous populations; however, the broader context of healthcare delivery remains important.

Higher rates of cardiovascular disease, including ischaemic heart disease and rheumatic heart disease, in Aboriginal and Torres Strait Islander communities may increase the proportion of patients presenting with bradyarrhythmias or requiring emergency cardiovascular management. When administering atropine for bradycardia in these contexts, clinicians should be aware that tachycardia may be poorly tolerated in patients with underlying cardiac disease, necessitating careful dose titration. Similarly, higher rates of diabetes mellitus may be associated with autonomic neuropathy, potentially affecting the heart rate response to atropine.

Remote and rural healthcare delivery presents particular challenges. Many remote health services are staffed by nurses, Aboriginal Health Workers, or paramedics who may need to initiate atropine therapy before physician consultation. Clear protocols, appropriate training, and access to telehealth support are essential. Culturally safe communication—involving family members where appropriate, using plain language, and respecting cultural protocols around medical treatment—enhances patient care and trust. The involvement of Aboriginal Health Workers and liaison officers in perioperative care ensures culturally appropriate communication and supports informed decision-making.

For Māori patients in New Zealand, similar principles apply: involvement of whānau (family) in care decisions, recognition of tikanga (customs), and awareness of health disparities affecting cardiovascular and metabolic health. The Treaty of Waitangi principles of partnership, participation, and protection guide culturally safe healthcare delivery.

ANZCA Primary Exam Focus

Common MCQ Patterns

ANZCA Primary MCQs frequently test the following atropine concepts:

1. Chemical structure: Tertiary amine vs quaternary ammonium (atropine vs glycopyrrolate)
2. Mechanism of action: Competitive muscarinic antagonist, non-selective (M1-M5)
3. Paradoxical bradycardia: Low-dose (<0.5 mg) mechanism via presynaptic M1 blockade
4. Receptor subtypes: M2 at SA node, M3 at smooth muscle/glands
5. CNS penetration: Tertiary amine crosses BBB; central anticholinergic syndrome
6. Comparison with glycopyrrolate: Onset, duration, CNS effects, preferred uses
7. Dose for bradycardia: 0.5-1 mg IV, max 3 mg (NOT recommended in arrest)
8. NMB reversal: Neostigmine:atropine ratio (2.5:1), alternative glycopyrrolate (5:1)
9. Oculocardiac reflex: Trigeminovagal, prevention/treatment dose
10. Organophosphate poisoning: Titrate to atropinisation, not pupil size

Primary Viva Themes

Typical viva scenarios include:

• Intraoperative bradycardia: atropine vs glycopyrrolate selection, dosing
• Reversal of neuromuscular blockade: choice of anticholinergic, rationale
• Strabismus surgery: oculocardiac reflex prophylaxis and treatment
• Organophosphate exposure: resuscitation approach, atropine dosing
• Elderly patient with confusion postoperatively: differential including central anticholinergic syndrome
• Comparison of antimuscarinic agents in tabular format

Key Equations and Calculations

Paediatric Dosing:

• Bradycardia: 20 mcg/kg IV (minimum 100 mcg, maximum 600 mcg per dose)
• NMB reversal: 20 mcg/kg with neostigmine 50 mcg/kg

Neostigmine:Anticholinergic Ratios:

• Neostigmine 2.5 mg : Atropine 1 mg (2.5:1)
• Neostigmine 2.5 mg : Glycopyrrolate 0.5 mg (5:1)

Assessment Content

SAQ Practice Question (20 marks)

Question:

A 68-year-old man (75 kg) undergoes laparoscopic cholecystectomy under general anaesthesia with rocuronium for neuromuscular blockade. At the end of surgery, train-of-four monitoring shows a ratio of 0.6. The anaesthetist plans to administer neostigmine for reversal.

(a) Describe the mechanism of action of neostigmine and explain why an antimuscarinic agent must be co-administered. (5 marks)

(b) Compare and contrast atropine and glycopyrrolate as antimuscarinic agents for use with neostigmine reversal. Include physicochemical properties, pharmacokinetics, and clinical effects in your answer. (8 marks)

(c) State the doses of neostigmine and atropine you would administer for this patient. Justify the dose ratios used. (3 marks)

(d) The patient develops confusion and agitation in the recovery room with tachycardia, dilated pupils, and dry mouth. Discuss the likely diagnosis and outline your management. (4 marks)

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Model Answer:

(a) Mechanism of Neostigmine and Rationale for Antimuscarinic (5 marks)

Neostigmine Mechanism (3 marks):

• Neostigmine is a quaternary ammonium acetylcholinesterase inhibitor
• It reversibly inhibits acetylcholinesterase at the neuromuscular junction
• This prevents breakdown of acetylcholine (ACh), increasing ACh concentration
• Elevated ACh competitively displaces non-depolarising muscle relaxants (rocuronium) from nicotinic receptors
• Result: Restoration of neuromuscular transmission and muscle strength

Rationale for Antimuscarinic (2 marks):

• Neostigmine increases ACh at ALL cholinergic synapses, not just the NMJ
• At muscarinic receptors (parasympathetic effectors), excess ACh causes:
  • Bradycardia (M2 at SA node)
  • Excessive salivation (M3 at salivary glands)
  • Bronchospasm and bronchorrhoea (M3 at airways)
  • Increased GI motility (M3 at gut smooth muscle)
• Antimuscarinic agents (atropine, glycopyrrolate) block these muscarinic effects
• They do NOT block the desired nicotinic (NMJ) effects of neostigmine

(b) Comparison of Atropine and Glycopyrrolate (8 marks)

Clinical Implications (2 marks):

• Glycopyrrolate is generally PREFERRED for neostigmine reversal because:
  • Better onset matching prevents early tachycardia followed by relative bradycardia
  • No risk of central anticholinergic syndrome
  • More stable heart rate profile
• Atropine preferred when rapid onset essential (acute bradycardia treatment)

(c) Dosing Calculation (3 marks)

Neostigmine dose (1 mark):

• Standard dose: 50 mcg/kg
• For 75 kg patient: 50 × 75 = 3,750 mcg = 3.75 mg
• Maximum dose: 5 mg (this is within range)
• Administer: Neostigmine 3.75 mg IV (or round to 4 mg)

Atropine dose (1 mark):

• Standard dose: 20 mcg/kg (when used with neostigmine)
• For 75 kg patient: 20 × 75 = 1,500 mcg = 1.5 mg
• Maximum dose: 1.2 mg (use maximum)
• Administer: Atropine 1.2 mg IV

Dose ratio justification (1 mark):

• Neostigmine:Atropine ratio = 2.5:1 (approximately)
• At 50 mcg/kg neostigmine with 20 mcg/kg atropine: ratio = 50:20 = 2.5:1
• This ratio provides adequate antimuscarinic cover for expected muscarinic effects
• Alternative: Glycopyrrolate 10 mcg/kg with neostigmine 50 mcg/kg (5:1 ratio)

(d) Diagnosis and Management (4 marks)

Likely Diagnosis (1 mark):

• Central Anticholinergic Syndrome (CAS) / Anticholinergic toxicity
• Triad: Central effects (confusion, agitation) + Peripheral effects (tachycardia, mydriasis, dry mouth)
• Risk factor: Atropine administered (tertiary amine crosses BBB)

Differential Considerations (1 mark):

• Emergence delirium
• Hypoxia/hypercarbia
• Pain
• Drug effects (residual anaesthesia, opioids)
• Metabolic derangement (hypoglycaemia)

Management (2 marks):

1. Supportive care:

   • Ensure adequate oxygenation and ventilation
   • Check glucose, temperature, pain assessment
   • Calm environment, reassurance

2. Symptomatic treatment:

   • Benzodiazepines (midazolam 1-2 mg IV) for agitation

3. Specific treatment (if severe/confirmed CAS):

   • Physostigmine 1-2 mg IV slowly (over 5 minutes)
   • Physostigmine is a tertiary amine anticholinesterase that crosses BBB
   • Reverses both central and peripheral anticholinergic effects
   • Caution: Can cause bradycardia, bronchospasm, seizures
   • Have atropine available to reverse excessive cholinergic effects

4. Monitoring:

   • Continuous ECG, pulse oximetry
   • Repeat physostigmine if symptoms recur (short half-life 1-2 hours)

Total: 20 marks

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Primary Viva Scenario (15 marks)

Examiner: A 4-year-old child (18 kg) is undergoing strabismus surgery. During manipulation of the medial rectus muscle, the surgeon notes sudden bradycardia to 40 bpm on the monitor. Describe your assessment and management.

Candidate:

Immediate Assessment (2 marks):

"This is an emergency—I would immediately:

1. Call for help and ask the surgeon to stop all surgical manipulation
2. Confirm the bradycardia is real (check pulse, arterial trace if available)
3. Assess for associated hypotension, arrhythmia (heart block, junctional rhythm), or cardiovascular collapse
4. Consider the differential diagnosis—most likely the oculocardiac reflex given the context"

Examiner: The surgeon releases the muscle and the heart rate increases to 60 bpm but remains below baseline (previously 100 bpm). What is your diagnosis and what would you do?

Candidate:

Diagnosis: Oculocardiac Reflex (2 marks):

"This is the oculocardiac reflex (OCR)—a trigeminovagal reflex characterised by:

• Afferent limb: Trigeminal nerve (V1 ophthalmic division via ciliary ganglion)
• Central integration: Brainstem (main sensory nucleus → reticular formation → vagal motor nucleus)
• Efferent limb: Vagus nerve to SA node

Surgical stimuli that trigger OCR include traction on extraocular muscles (especially medial rectus), pressure on globe, orbital manipulation, and intraorbital injection."

Immediate Management (3 marks):

"The heart rate has partially recovered to 60 bpm but remains below baseline. My management:

1. Ensure cessation of surgical stimulus—confirmed the surgeon has released the muscle
2. Continue monitoring—may recover fully without pharmacological intervention
3. Ensure adequate oxygenation and ventilation—exclude hypoxia/hypercarbia as contributors
4. Optimise depth of anaesthesia—light anaesthesia increases OCR incidence; deepen if appropriate
5. If bradycardia persists or recurs with surgery: Administer atropine 10-20 mcg/kg IV

For this 18 kg child: Atropine 20 mcg/kg = 360 mcg (round to 300-400 mcg)"

Examiner: Why might you choose atropine over glycopyrrolate in this situation?

Candidate:

Rationale for Atropine (3 marks):

"For acute treatment of the oculocardiac reflex, atropine has advantages:

In an acute bradycardic event, the faster onset of atropine is beneficial. The brief duration of CNS exposure during a single dose is unlikely to cause significant central effects in a child.

However, for prophylaxis (before surgery begins), glycopyrrolate may be preferred because:

• No CNS effects
• Longer duration covers the procedure
• Better antisialagogue effect

Some anaesthetists give prophylactic IV atropine or glycopyrrolate before strabismus surgery in patients with previous OCR or those at high risk."

Examiner: The surgery continues. 10 minutes later, with surgical manipulation, the child develops asystole for 3 seconds followed by sinus bradycardia at 50 bpm despite the atropine given earlier. What now?

Candidate:

Management of Refractory OCR (3 marks):

"This represents a severe oculocardiac reflex refractory to initial atropine. My approach:

1. Immediate: Ask surgeon to stop all manipulation

2. Repeat atropine: Administer another dose of atropine 20 mcg/kg IV (360 mcg)

3. Consider adrenaline: If bradycardia/asystole persists, adrenaline 10 mcg/kg IV as per paediatric bradycardia protocol

4. If refractory:

   • Request retrobulbar block with local anaesthetic to block afferent limb
   • Request surgical pause to allow recovery
   • Prepare for CPR if cardiovascular collapse

5. Communication: Inform the surgeon that repeated severe OCR may necessitate abandoning the procedure if haemodynamics cannot be maintained safely"

Examiner: How can the oculocardiac reflex be prevented?

Candidate:

Prevention Strategies (2 marks):

"Prevention of OCR includes:

1. Pharmacological prophylaxis:

   • IV atropine 10-20 mcg/kg OR glycopyrrolate 5-10 mcg/kg before surgical manipulation
   • Evidence for efficacy is mixed; does not abolish OCR in all patients but reduces incidence and severity

2. Gentle surgical technique:

   • Avoid sudden, forceful traction on extraocular muscles
   • Gradual, careful manipulation

3. Regional anaesthesia:

   • Retrobulbar or peribulbar block blocks the afferent trigeminal pathway
   • Useful for repeat surgery or known severe OCR

4. Adequate anaesthetic depth:

   • Light anaesthesia increases OCR incidence
   • Appropriate depth of volatile or IV anaesthesia

5. Avoid hypoxia and hypercarbia:

   • These augment vagal reflexes"

Examiner: Thank you. Good understanding of the oculocardiac reflex and atropine pharmacology.

Total: 15 marks (2 + 2 + 3 + 3 + 3 + 2)

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