Organophosphate and Carbamate Poisoning

Quick Answer

Organophosphate (OP) poisoning causes a life-threatening cholinergic crisis through irreversible inhibition of acetylcholinesterase (AChE), leading to accumulation of acetylcholine at muscarinic and nicotinic receptors. The primary cause of death is respiratory failure from bronchorrhoea, bronchospasm, and respiratory muscle paralysis.

Key Clinical Features:

• SLUDGE/BBB: Salivation, Lacrimation, Urination, Defecation, GI upset, Emesis + Bradycardia, Bronchospasm, Bronchorrhoea
• Nicotinic effects: Fasciculations, muscle weakness, paralysis, tachycardia
• CNS effects: Seizures, coma, respiratory centre depression

Emergency Management:

1. Decontamination: Remove clothing, copious water irrigation (staff PPE mandatory)
2. Atropine: 1-2 mg IV boluses every 3-5 minutes, titrate to clear secretions (NOT heart rate)
3. Pralidoxime: 30 mg/kg bolus then 8-10 mg/kg/hr infusion within 24-48 hours
4. Airway: Early intubation for bronchorrhoea/respiratory failure (avoid suxamethonium)
5. Benzodiazepines: For seizures (diazepam 10-20 mg IV)

ICU Mortality: 5-20% in developed settings with aggressive atropine therapy

Must-Know Facts:

• Atropine requirements may exceed 1000 mg in severe poisoning
• Aging: OP-enzyme bond becomes irreversible after 24-48 hours (oximes ineffective)
• Intermediate syndrome: Proximal weakness at 24-96 hours despite initial recovery
• Carbamates have reversible binding and shorter duration (pralidoxime less critical)

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CICM Exam Focus

What Examiners Expect

Second Part Written (SAQ):

Common SAQ stems:

• "A 45-year-old farmer is brought to ICU after exposure to an agricultural pesticide. He has profuse secretions, miosis, and respiratory distress. GCS 10. Outline your assessment and management."
• "Discuss the pathophysiology of organophosphate poisoning and the rationale for oxime therapy."
• "A patient with organophosphate poisoning develops respiratory failure on day 3 of ICU admission despite clearing secretions. Discuss the differential diagnosis and management."
• "Compare and contrast organophosphate and carbamate poisoning."

Expected depth:

• Detailed mechanism of acetylcholinesterase inhibition and aging
• Muscarinic vs nicotinic effects with clinical correlation
• Evidence-based atropine and pralidoxime protocols
• Recognition and management of intermediate syndrome
• Complications including cardiac arrhythmias and seizures

Second Part Hot Case:

Typical presentations:

• Day 1: Farmer with cholinergic crisis, profuse secretions, bradycardia, requiring massive atropine
• Day 3: Patient with initial improvement now developing proximal weakness and respiratory failure (intermediate syndrome)
• Agricultural worker with deliberate self-poisoning and mixed ingestion

Examiners assess:

• Systematic A-E examination with focus on secretions and neuromuscular function
• Recognition of cholinergic toxidrome
• Understanding of atropine titration endpoints
• Knowledge of pralidoxime timing and controversy
• Communication with family about prognosis and potential for delayed complications

Second Part Viva:

Expected discussion areas:

• Mechanism of acetylcholinesterase inhibition and aging phenomenon
• Difference between muscarinic, nicotinic, and CNS effects
• Atropine dosing and titration endpoints (clear secretions, not heart rate)
• Pralidoxime controversy: Eddleston trials, WHO recommendations
• Intermediate syndrome pathophysiology and management
• Decontamination priorities and staff safety
• Australian agricultural context and nerve agent terrorism preparedness

Examiner expectations:

• Safe, consultant-level management of life-threatening toxidrome
• Evidence-based practice with knowledge of pralidoxime controversy
• Recognition of delayed syndromes (intermediate syndrome, OPIDN)
• Indigenous health awareness (agricultural workers, remote areas)
• Mass casualty considerations for nerve agent exposure

Common Mistakes

• Titrating atropine to heart rate rather than secretion control
• Failing to recognise intermediate syndrome as cause of delayed respiratory failure
• Using suxamethonium for RSI (prolonged paralysis due to pseudocholinesterase inhibition)
• Not administering pralidoxime early enough (before aging occurs)
• Inadequate decontamination placing staff at risk
• Stopping atropine prematurely leading to recurrent cholinergic crisis
• Forgetting benzodiazepines for seizure prophylaxis/treatment

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Key Points

Must-Know Facts

1. Mechanism of Action: Organophosphates irreversibly inhibit acetylcholinesterase by phosphorylating the enzyme's serine residue, leading to acetylcholine accumulation at muscarinic, nicotinic, and CNS synapses.

2. Aging: The OP-enzyme complex undergoes spontaneous "aging" (dealkylation) within 24-48 hours, making the bond irreversible and unresponsive to oxime reactivation. This is why pralidoxime must be given early.

3. Carbamate Difference: Carbamates (e.g., carbaryl, aldicarb) cause reversible carbamylation of AChE with spontaneous hydrolysis within hours. Duration is shorter, and pralidoxime is less critical (though still recommended for severe poisoning).

4. Atropine is Lifesaving: Titrate to control secretions (dry chest auscultation, clear airway) - NOT to heart rate. Doses of 100-1000+ mg may be required in severe poisoning.

5. Respiratory Failure Causes: The primary mechanism of death involves three factors - bronchorrhoea (copious secretions), bronchospasm, and respiratory muscle weakness (nicotinic effect). Early intubation saves lives.

6. Intermediate Syndrome: Occurs 24-96 hours after exposure in 10-40% of patients. Presents with proximal muscle weakness, neck flexion weakness, and respiratory failure despite initial recovery from cholinergic crisis. Due to persistent AChE inhibition at nicotinic junctions.

7. OPIDN (Organophosphate-Induced Delayed Neuropathy): Rare delayed axonal polyneuropathy occurring 2-4 weeks post-exposure. Due to inhibition of neuropathy target esterase (NTE). Presents as ascending paralysis. Pralidoxime does not prevent.

8. Avoid Suxamethonium: Organophosphates inhibit plasma butyrylcholinesterase (pseudocholinesterase), which metabolises suxamethonium. Prolonged paralysis (hours to days) can occur.

9. Pralidoxime Controversy: WHO recommends pralidoxime; however, the Eddleston 2009 trial showed no benefit (possible harm) in Sri Lankan agricultural OP poisoning. May still be beneficial for nerve agents and early presentation before aging.

10. Cholinesterase Levels: Butyrylcholinesterase (plasma/pseudo-ChE) is more sensitive but less specific. RBC acetylcholinesterase correlates better with synaptic AChE and clinical severity.

Memory Aids

Mnemonic SLUDGE/BBB (Muscarinic Effects):

• S: Salivation
• L: Lacrimation
• U: Urination
• D: Defecation
• G: GI upset (cramps, nausea)
• E: Emesis
• B: Bradycardia
• B: Bronchospasm
• B: Bronchorrhoea

Alternative Mnemonic DUMBBELS:

• D: Defecation/Diarrhoea
• U: Urination
• M: Miosis
• B: Bradycardia/Bronchospasm/Bronchorrhoea
• B: (same)
• E: Emesis
• L: Lacrimation
• S: Salivation

Nicotinic Effects "MTW":

• M: Mydriasis (can counteract muscarinic miosis)
• T: Tachycardia (can counteract muscarinic bradycardia)
• W: Weakness (fasciculations, paralysis)

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Definition and Epidemiology

Definition

Organophosphate poisoning is a toxicological emergency resulting from exposure to organophosphorus compounds that irreversibly inhibit acetylcholinesterase (AChE) enzymes, leading to accumulation of acetylcholine at cholinergic synapses throughout the body.

Organophosphate compounds are esters of phosphoric acid and include:

• Agricultural pesticides: Parathion, malathion, chlorpyrifos, dimethoate, fenthion
• Nerve agents: Sarin, soman, tabun, VX (chemical warfare agents)
• Other: TOCP (tri-ortho-cresyl phosphate - industrial), echothiophate (ophthalmology)

Carbamate poisoning involves reversible inhibition of AChE by carbamate esters (e.g., carbaryl, propoxur, aldicarb, methomyl). The clinical presentation is similar but generally shorter duration.

Diagnostic Criteria:

• History of exposure to anticholinesterase compound
• Cholinergic toxidrome (muscarinic and/or nicotinic features)
• Reduced cholinesterase activity (plasma or RBC)
• Response to atropine therapy

Severity Classification:

Epidemiology

Global Burden:

• Approximately 3 million cases of pesticide poisoning annually worldwide (PMID: 16461906)
• 300,000 deaths per year, predominantly in developing countries
• 99% of pesticide deaths occur in developing nations where agricultural use is high (PMID: 15797610)
• Deliberate self-poisoning accounts for approximately 2/3 of deaths (PMID: 12543823)
• Case fatality rate: 5-25% overall; up to 70% in resource-limited settings without ICU care

Australian/NZ Data:

• Organophosphate poisoning is relatively rare in Australia compared to Asia
• Agricultural exposure predominates in rural Queensland, NSW, Victoria
• Deliberate self-poisoning occurs but is less common than paracetamol or opioids
• Carbamate insecticides also used in domestic settings (ant baits, pet flea products)
• Nerve agent preparedness is a component of CBRN emergency planning

Risk Factors:

• Occupational: Agricultural workers, pesticide applicators, chemical manufacturing
• Non-modifiable: Male sex (occupational exposure), age >40 years (higher mortality)
• Modifiable: Poor safety practices, inadequate PPE, unsafe pesticide storage
• Psychiatric: Deliberate self-poisoning (most common context globally)
• Geographic: Rural and remote areas with agricultural activity

High-Risk Populations:

• Aboriginal and Torres Strait Islander peoples: May have increased exposure through agricultural work in remote communities, with limited access to tertiary toxicology services
• Seasonal/migrant agricultural workers: Language barriers, inadequate training, limited PPE
• Remote/rural populations: Delayed access to ICU care, limited antidote availability
• Developing countries: High agricultural pesticide use, poor regulation, limited healthcare

Outcomes:

• ICU mortality: 5-20% with aggressive atropine therapy and mechanical ventilation
• Hospital mortality: 10-25% overall
• Delayed mortality from intermediate syndrome: Additional 5-10%
• Long-term sequelae: Neuropsychiatric symptoms (10-20%), OPIDN (<1%)
• Recovery of cholinesterase activity: Days to weeks (RBC ChE takes 90 days for complete regeneration)

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Applied Basic Sciences

Anatomy

Relevant Anatomy for ICU:

Cholinergic Nervous System Distribution:

• Parasympathetic ganglia: Ciliary, pterygopalatine, submandibular, otic ganglia
• Parasympathetic effector organs: Heart (SA/AV nodes), bronchial smooth muscle, GI tract, bladder, exocrine glands
• Sympathetic preganglionic neurons: All sympathetic ganglia, adrenal medulla
• Neuromuscular junction: All skeletal muscles including diaphragm and intercostals
• Central nervous system: Basal forebrain, brainstem, cortex

Airway Anatomy (relevant to bronchorrhoea management):

• Submucosal glands of bronchial tree produce copious secretions
• Smooth muscle of bronchi causes bronchospasm
• Diaphragm and intercostal muscles are affected by nicotinic weakness

Neuromuscular Junction Anatomy:

• Presynaptic motor nerve terminal: Vesicles containing ACh
• Synaptic cleft: 50 nm gap containing AChE bound to basement membrane
• Postsynaptic muscle membrane: Nicotinic ACh receptors clustered at end-plate

Physiology

Normal Cholinergic Neurotransmission:

1. Action potential arrives at nerve terminal
2. Calcium influx triggers vesicle fusion and ACh release
3. ACh diffuses across synaptic cleft and binds receptors
4. Receptor activation: Muscarinic (GPCR) or nicotinic (ligand-gated ion channel)
5. Acetylcholinesterase rapidly hydrolyses ACh to choline and acetate
6. Choline uptake into nerve terminal for ACh resynthesis

Acetylcholinesterase (AChE):

• Serine hydrolase enzyme attached to synaptic basement membrane
• Extremely efficient: Turnover number ~25,000/second
• Essential for terminating cholinergic transmission
• Two main types:
  • "True AChE (RBC AChE): Found at synapses and on RBC membranes"
  • "Butyrylcholinesterase (Plasma ChE/Pseudocholinesterase): Found in plasma, liver, synthesised by hepatocytes"

Pathophysiology

Mechanism of Organophosphate Toxicity:

1. Phosphorylation of AChE:

   • OP binds to serine hydroxyl group at active site of AChE
   • Forms stable phosphorylated enzyme (covalent bond)
   • Enzyme is inactivated and cannot hydrolyse ACh
   • ACh accumulates at all cholinergic synapses

2. Aging Phenomenon:

   • Phosphorylated enzyme undergoes dealkylation ("aging")
   • Aging time varies by OP compound: Soman (2 minutes), Sarin (5 hours), VX (>40 hours), most pesticides (24-48 hours)
   • Aged enzyme cannot be reactivated by oximes
   • Recovery requires synthesis of new AChE (weeks)

3. Spontaneous Reactivation:

   • Very slow for most OPs (days to weeks)
   • Carbamates: Rapid spontaneous reactivation (minutes to hours)
   • This is why carbamate poisoning has shorter duration

Acetylcholine Accumulation Effects:

Muscarinic Effects (Parasympathetic end-organs):

• Eye: Miosis, lacrimation, blurred vision
• Respiratory: Bronchospasm, bronchorrhoea (PRIMARY CAUSE OF DEATH)
• Cardiovascular: Bradycardia, hypotension
• GI: Hypersalivation, nausea, vomiting, diarrhoea, abdominal cramps
• Urinary: Urinary frequency, incontinence
• Skin: Sweating

Nicotinic Effects (Autonomic ganglia, neuromuscular junction):

• Muscle: Fasciculations, weakness, paralysis (including diaphragm)
• Ganglia: Tachycardia, hypertension (can counteract muscarinic bradycardia)
• Adrenal medulla: Catecholamine release

Central Nervous System Effects:

• Anxiety, restlessness, tremor
• Confusion, ataxia, slurred speech
• Seizures (often refractory)
• Coma, respiratory depression
• Central respiratory paralysis

Intermediate Syndrome Pathophysiology (PMID: 18565562):

• Occurs 24-96 hours after exposure in 10-40% of severe cases
• Distinct from cholinergic phase and OPIDN
• Affects proximal muscles, neck flexors, respiratory muscles, cranial nerves
• Mechanism: Persistent AChE inhibition at nicotinic receptors causing receptor desensitisation and dysfunction
• Not prevented by oximes (may occur despite pralidoxime therapy)
• Recovery over 5-18 days with supportive care

OPIDN (Organophosphate-Induced Delayed Neuropathy) (PMID: 8093958):

• Rare delayed syndrome occurring 2-4 weeks post-exposure
• Due to inhibition of neuropathy target esterase (NTE) in nervous tissue
• Not related to AChE inhibition
• Predominantly motor distal axonopathy
• Presents as ascending weakness, sensory loss in stocking-glove distribution
• Specific OPs implicated: TOCP, chlorpyrifos, methamidophos
• Not prevented by atropine or oximes

Pharmacology

Atropine (PMID: 16728458):

• Class: Competitive muscarinic receptor antagonist
• Mechanism: Blocks ACh at muscarinic receptors; does NOT affect nicotinic receptors
• ICU Indication: First-line antidote for muscarinic symptoms (secretions, bronchospasm, bradycardia)
• Dosing Protocol:
  • "Initial: 1-2 mg IV every 3-5 minutes"
  • Double dose every 5 minutes if no response
  • "Titrate to: Dry secretions, clear chest auscultation, HR >80 bpm"
  • "Maintenance: Infusion 0.02-0.08 mg/kg/hr OR intermittent boluses Q1-2 hours"
  • Total dose may reach 100-1000+ mg in severe poisoning
• Monitoring: Heart rate, secretion control, pupil size (mydriasis indicates adequate atropinisation)
• Adverse Effects: Hyperthermia, urinary retention, tachyarrhythmias, agitation, delirium
• PBS/TGA: Widely available, no restrictions; stock large quantities for mass casualty

Pralidoxime (2-PAM) (PMID: 19342097, PMID: 16461906):

• Class: Oxime - acetylcholinesterase reactivator
• Mechanism: Nucleophilic attack on phosphorylated AChE, cleaving OP-enzyme bond, regenerating active enzyme
• ICU Indication: Reactivation of AChE before aging occurs; primarily affects nicotinic symptoms
• Dosing Protocol (WHO/ATSDR):
  • "Loading: 30 mg/kg IV over 15-30 minutes (max 2 g)"
  • "Maintenance: 8-10 mg/kg/hr continuous infusion (or 30 mg/kg Q4-6H intermittent)"
  • "Duration: Continue for 24-48 hours after last atropine dose"
• Timing: Most effective within 24 hours of exposure; ineffective after aging (24-48 hours for most pesticides)
• Monitoring: Muscle strength, cholinesterase levels, clinical response
• Adverse Effects: Hypertension (rapid infusion), headache, nausea, visual disturbance, laryngospasm
• Controversy: Eddleston 2009 trial (PMID: 19342097) showed no benefit and possible harm in Sri Lankan OP poisoning; however, may still benefit nerve agent exposure and early presentation
• PBS/TGA: Available on Section 100 (Special Access Scheme for non-PBS items); ensure adequate stock

Obidoxime and Other Oximes:

• Obidoxime (Toxogonin): Used in Europe; may be superior for some OPs
• HI-6: Primarily for nerve agents; not widely available
• MMB-4: Investigational oxime

Diazepam/Benzodiazepines (PMID: 10475300):

• Class: GABA-A receptor positive allosteric modulator
• Mechanism: Enhances GABAergic inhibition; controls seizures and anxiety
• ICU Indication: Seizure control and prophylaxis (may be neuroprotective)
• Dosing: Diazepam 10-20 mg IV; repeat as needed; consider infusion for status epilepticus
• Alternative: Midazolam 5-10 mg IV/IM
• Evidence: Animal studies show benzodiazepines improve outcomes when added to atropine + oxime (PMID: 10475300)

Lipid Emulsion Therapy:

• Limited evidence for lipophilic organophosphates
• Case reports suggest possible benefit (PMID: 23642305)
• Consider in refractory cases: 20% lipid emulsion 1.5 mL/kg bolus, then 0.25 mL/kg/min infusion

Pathology

Histopathology:

• Pulmonary oedema (cardiogenic and non-cardiogenic)
• Aspiration pneumonia
• Cerebral oedema in severe poisoning
• Skeletal muscle necrosis (rhabdomyolysis in severe cases)
• Delayed: Axonal degeneration in OPIDN

Laboratory Pathology:

• Reduced plasma butyrylcholinesterase (BuChE) - more sensitive
• Reduced RBC acetylcholinesterase - better correlation with clinical severity
• Metabolic acidosis (lactic acidosis from hypoxia, seizures)
• Elevated creatine kinase (rhabdomyolysis, muscle fasciculations)
• Elevated troponin (myocardial ischaemia from hypoxia/arrhythmias)

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Clinical Presentation

ICU Admission Scenarios

Scenario 1: Acute Agricultural Exposure

• History: 48-year-old male farmer found unconscious in shed after spraying crops
• Examination: GCS 8, profuse salivation, miosis 1mm, HR 45, RR 8, copious pulmonary secretions, fasciculations
• Severity: Severe - requires immediate intubation and massive atropine

Scenario 2: Deliberate Self-Poisoning

• History: 32-year-old female brought by ambulance after ingesting "bug spray" in suicide attempt
• Examination: GCS 14, nausea, vomiting, diarrhoea, miosis, HR 110, sweating, fasciculations
• Severity: Moderate - may deteriorate over hours; early atropine and pralidoxime

Scenario 3: Delayed Presentation with Intermediate Syndrome

• History: 45-year-old male initially recovered from cholinergic crisis, Day 3 develops progressive weakness
• Examination: Alert, proximal muscle weakness 3/5, neck flexion weak, cannot lift head off pillow, decreasing FVC
• Severity: Intermediate syndrome - requires ventilatory support

Symptoms and Signs

History:

• Chief complaint: Vomiting, diarrhoea, excessive sweating, weakness, confusion
• Exposure history: Pesticide application, ingestion (deliberate or accidental), inhalation, dermal contact
• Product identification: Crucial for determining specific OP, expected duration, aging time
• Time since exposure: Affects prognosis and pralidoxime efficacy
• Quantity: Large ingestions (>100 mL) predict severe toxicity
• Symptoms onset: Usually within 30 minutes to 6 hours of exposure (longer for lipophilic compounds)

Examination:

General:

• Appearance: Diaphoretic, "wet" patient with copious secretions
• Odour: Garlicky or petroleum-like smell (some OPs)
• Distress: May be anxious, agitated, or obtunded

A - Airway:

• Copious oral secretions (salivation)
• Pooling of secretions in hypopharynx
• Risk of aspiration
• May require repeated suctioning

B - Breathing:

• Respiratory rate: Decreased (CNS depression) or increased (compensation for secretions)
• Work of breathing: Increased (bronchospasm, secretions)
• Auscultation: Widespread coarse crackles, rhonchi (bronchorrhoea), wheeze (bronchospasm)
• Percussion: Normal or dull (pulmonary oedema, aspiration)
• SpO2: Often <90% on room air

C - Circulation:

• Heart rate: Bradycardia (muscarinic predominance) OR tachycardia (nicotinic/catecholamine predominance)
• Blood pressure: Hypotension (muscarinic) OR hypertension (nicotinic)
• Perfusion: Variable; may be poor with bradycardia
• JVP: May be elevated with pulmonary oedema
• Heart sounds: Regular; may have gallop with fluid overload

D - Disability/Neurology:

• GCS: 3-15; commonly reduced
• Pupils: Miosis (classic) but may be normal or dilated if nicotinic effects predominate
• Fasciculations: Visible muscle twitching, especially deltoid, tongue
• Weakness: Generalised; more prominent in severe poisoning
• Seizures: May be present or develop; often refractory

E - Exposure/Everything Else:

• Temperature: Variable; may be elevated (atropine effect) or normal
• Skin: Diaphoretic (sweating), may have dermal contamination
• Abdomen: Hyperactive bowel sounds, incontinence
• Urine output: May have urinary incontinence

Severity Scoring

Peradeniya Organophosphorus Poisoning (POP) Scale (PMID: 11239329):

• Score 0-3: Mild
• Score 4-7: Moderate
• Score 8-11: Severe

Poisoning Severity Score (PSS) - International classification

General ICU Scores:

• APACHE II: Variable; higher with respiratory failure, seizures, shock
• SOFA: Elevated due to respiratory and neurological dysfunction

Differential Diagnosis

Key Differentials:

1. Carbamate poisoning: Similar cholinergic syndrome but reversible; shorter duration; product identification differentiates
2. Nerve agent exposure: G-agents and VX cause identical syndrome; context (terrorism, military) key; extremely rapid aging with soman
3. Myasthenic crisis: Weakness without cholinergic autonomic features; fatigable weakness; respond to edrophonium (worsens OP)
4. Overdose of cholinergic drugs: Donepezil, rivastigmine, pyridostigmine overdose; clinical similar
5. Nicotine poisoning: Early cholinergic then paralytic phase; vaping, nicotine patches
6. Muscarinic mushroom poisoning: Clitocybe, Inocybe species; similar muscarinic syndrome; no nicotinic features
7. Carbon monoxide poisoning: Altered consciousness; check COHb
8. Septic shock: Multi-organ dysfunction; may coexist with aspiration pneumonia

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Investigations

Laboratory Investigations

Bedside Tests:

Arterial Blood Gas:

• pH: Low (respiratory and metabolic acidosis)
• PaCO2: Elevated (respiratory depression, increased dead space from secretions)
• PaO2: Low (VQ mismatch from bronchorrhoea, shunt from aspiration)
• HCO3: Low if lactic acidosis (hypoxia, seizures, muscle hyperactivity)
• Lactate: Elevated in severe poisoning (tissue hypoxia, seizures)
• Interpretation: Mixed respiratory and metabolic acidosis with hypoxaemia

Blood Tests:

Cholinesterase Levels (PMID: 8093958):

• Butyrylcholinesterase (BuChE/Plasma ChE/Pseudocholinesterase):

  • "Normal: 5,000-12,000 U/L (laboratory-specific)"
  • More sensitive marker of exposure
  • Less specific for clinical severity
  • Affected by liver disease, pregnancy, genetic variants

• RBC Acetylcholinesterase (RBC AChE):

  • "Normal: 26-44 U/g Hb (laboratory-specific)"
  • Better correlation with synaptic AChE
  • Better predictor of clinical severity
  • Half-life 120 days (regeneration requires new RBC synthesis)

• Interpretation:

  • 50% of normal: Mild

  • 20-50% of normal: Moderate
  • <20% of normal: Severe
  • Serial measurements show recovery over days to weeks

Full Blood Count:

• WCC: May be elevated (stress response, aspiration)
• Hb: Usually normal
• Platelets: Usually normal

Urea, Electrolytes, Creatinine:

• Potassium: May be low (GI losses, diaphoresis) or high (AKI, rhabdomyolysis)
• Sodium: Variable
• Creatinine: Elevated if AKI from hypotension, rhabdomyolysis

Creatine Kinase:

• Often elevated from fasciculations, seizures, rhabdomyolysis
• Serial monitoring important

Liver Function Tests:

• Usually normal unless pre-existing liver disease
• May affect BuChE interpretation

Troponin:

• May be elevated with hypoxic myocardial injury

Blood Glucose:

• Hyperglycaemia common (catecholamine release)

Toxicology Screen:

• Qualitative urine drug screen for co-ingestants
• Specific OP metabolites not routinely available

Coagulation:

• Usually normal
• Check for co-ingested anticoagulants

Imaging

Chest X-Ray:

• Typical findings: Diffuse bilateral infiltrates (pulmonary oedema, aspiration)
• Sensitivity/specificity: Variable
• Indications: All patients with respiratory symptoms

CT Head:

• Indicated if prolonged coma, seizures, focal neurology
• May show cerebral oedema in severe poisoning

Electrocardiogram (PMID: 12556948)

ECG Changes in OP Poisoning:

• QT prolongation: Significant risk; monitor continuously
• Sinus bradycardia: Muscarinic effect; responds to atropine
• Sinus tachycardia: Nicotinic effect, hypoxia, or atropine effect
• ST-T changes: Hypoxia, myocardial ischaemia
• Ventricular arrhythmias: Torsades de Pointes (QT prolongation), VT, VF
• Conduction abnormalities: AV block, bundle branch block

Physiological Monitoring

Non-Invasive Monitoring:

• Continuous ECG: For arrhythmias and QT prolongation
• SpO2: Target ≥92%; may be unreliable with poor perfusion
• NIBP: Frequent monitoring; hypotension common
• Temperature: Fever may indicate atropine toxicity or aspiration pneumonia
• Respiratory rate: Early indicator of deterioration
• Capnography: If intubated

Invasive Monitoring:

• Arterial line: For continuous BP and frequent ABG sampling
• Central venous pressure: If requiring significant fluid resuscitation
• ScvO2: If available; guides resuscitation

Neuromuscular Monitoring:

• Serial muscle strength assessment (MRC scale)
• Forced vital capacity (FVC): <15 mL/kg predicts need for intubation
• Neck flexion strength: Early indicator of intermediate syndrome
• Ability to count to 20 in single breath

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ICU Management

Initial Resuscitation (First Hour)

PRIORITY: Airway Protection and Secretion Control

A - Airway:

• Assessment: Look for pooling secretions, stridor, gurgling
• Positioning: Head-up 30°, lateral if unconscious
• Suctioning: Aggressive, frequent suctioning of oropharynx
• Airway adjuncts: OPA if no gag reflex; NPA if gag present

Intubation Indications:

• GCS <8 or inability to protect airway
• Respiratory failure (RR <10 or >30, SpO2 <90% despite O2)
• Copious bronchorrhoea requiring continuous suctioning
• Refractory bronchospasm
• Seizures (for airway protection)
• Anticipated deterioration

RSI Considerations (CRITICAL):

• AVOID SUXAMETHONIUM: Prolonged paralysis (hours to days) due to inhibited pseudocholinesterase
• Use rocuronium 1.2 mg/kg (onset 60 seconds)
• Induction: Ketamine 1-2 mg/kg or propofol 1-2 mg/kg (reduced dose if hypotensive)
• Prepare for difficult airway (secretions, bronchospasm)
• Have atropine ready at induction
• Video laryngoscopy preferred

Post-Intubation:

• Generous suctioning via ETT
• Initial ventilator settings: Vt 6-8 mL/kg PBW, RR 12-16, PEEP 5-8, FiO2 as needed
• Expect high ventilation requirements due to secretions

B - Breathing:

• Oxygen therapy: High-flow initially; target SpO2 92-96%
• Bronchospasm management: Salbutamol nebulisers; ipratropium (anticholinergic)
• Atropine: Early administration reduces bronchorrhoea and bronchospasm

Ventilatory Support:

• NIV generally contraindicated (aspiration risk, secretions)
• Early intubation preferred for severe poisoning
• Lung-protective ventilation if developing ARDS

C - Circulation:

Fluid Resuscitation:

• IV crystalloid (0.9% saline or Hartmann's)
• Careful fluid balance (pulmonary oedema risk)
• Target MAP ≥65 mmHg

Bradycardia Management:

• Atropine is the primary treatment
• Usually resolves with adequate atropinisation

Hypotension:

• Fluid bolus 10-20 mL/kg
• Noradrenaline if refractory after adequate atropinisation

Cardiac Monitoring:

• Continuous ECG; monitor QTc
• Arterial line for continuous BP

D - Disability:

• GCS monitoring every 15-30 minutes initially
• Pupil assessment (miosis expected; may indicate atropine adequacy)
• Seizure management: Benzodiazepines first-line
• Glucose control: Target 6-10 mmol/L

E - Everything Else:

DECONTAMINATION (CRITICAL FOR STAFF SAFETY) (PMID: 24314871):

• PPE for staff: Nitrile gloves (double), impermeable gown, N95 mask, eye protection
• Remove clothing: All clothing removed, placed in sealed bags
• Skin decontamination: Copious water irrigation; soap and water for oily substances
• Hair decontamination: Wash thoroughly if dermal exposure
• Do NOT induce vomiting: Risk of aspiration
• Gastric lavage: Consider within 1 hour if large ingestion and airway protected; controversial
• Activated charcoal: 50-100 g if within 1-2 hours of ingestion, airway protected, not aspirating

Specific Antidotal Therapy

ATROPINE (PMID: 16728458, PMID: 23642305)

Mechanism: Competitive muscarinic receptor antagonist; reverses muscarinic effects (secretions, bronchospasm, bradycardia); does NOT affect nicotinic effects (weakness, paralysis)

Dosing Protocol (WHO Guidelines):

Initial Bolus:

• Adults: 1-2 mg IV every 3-5 minutes
• Increase dose (double) if no response after first 2-3 doses
• Continue until adequate atropinisation achieved

Endpoints of Atropinisation (TARGET):

• Clear chest on auscultation (no crackles/rhonchi from secretions)
• Heart rate >80 bpm
• Dry axillae
• Systolic BP >80 mmHg

NOT endpoints:

• Tachycardia >120 bpm (indicates over-atropinisation)
• Pupil dilation (delayed indicator; not reliable)

Maintenance:

• Continuous infusion: 0.02-0.08 mg/kg/hr (10-20% of initial loading dose per hour)
• OR intermittent boluses every 1-4 hours
• Titrate to maintain clear secretions
• Duration: Continue until no recurrence of secretions for 12-24 hours

Expected Doses:

• Mild poisoning: 5-20 mg total
• Moderate poisoning: 20-100 mg total
• Severe poisoning: 100-1000+ mg total (cases up to 3000 mg reported)

Adverse Effects of Over-Atropinisation:

• Tachycardia, arrhythmias
• Hyperthermia (thermoregulation impaired)
• Urinary retention
• Paralytic ileus
• Agitation, confusion, delirium
• Seizures (rare)

PRALIDOXIME (2-PAM/Protopam) (PMID: 19342097, PMID: 16461906, PMID: 21131908)

Mechanism: Nucleophilic attack on phosphorylated serine, reactivating AChE before aging occurs

Dosing Protocol (WHO/ATSDR):

Loading Dose:

• 30 mg/kg IV over 15-30 minutes (maximum 2 g)
• Administer as soon as possible after exposure

Maintenance:

• Continuous infusion: 8-10 mg/kg/hr (approximately 500 mg/hr for 60 kg adult)
• OR intermittent boluses: 30 mg/kg every 4-6 hours

Duration:

• Continue for at least 24-48 hours after atropine discontinued
• Longer for highly lipophilic compounds (fat storage, delayed release)

Timing: Critical - must be given before aging

• Most effective within 12-24 hours of exposure
• May still provide some benefit up to 48 hours
• Soman (nerve agent): Aging in 2 minutes - oximes rarely effective

Controversy (PMID: 19342097):

• Eddleston 2009 RCT in Sri Lanka: Pralidoxime vs placebo in OP self-poisoning
  • No mortality benefit (25.3% vs 23.3%)
  • No benefit on mechanical ventilation
  • Possible increased mortality in some subgroups
• Possible explanations: Late presentation, already aged enzyme, inadequate dosing
• Current recommendation: Still use pralidoxime per WHO guidelines, especially for:
  • Nerve agent exposure
  • Early presentation (<12 hours)
  • Known OP with slow aging (not dimethyl compounds)
  • Nicotinic features (muscle weakness)

Adverse Effects:

• Hypertension (rapid infusion)
• Headache, dizziness
• Nausea, vomiting
• Visual disturbance
• Laryngospasm (rare)

Other Oximes:

• Obidoxime: Used in Europe; may be superior for dimethyl OPs
• HI-6: Specifically for nerve agents; not routinely available

BENZODIAZEPINES (PMID: 10475300)

Indication: Seizure control and prophylaxis; may be neuroprotective

Dosing:

• Diazepam: 10-20 mg IV; repeat every 5-10 minutes as needed
• Midazolam: 5-10 mg IV/IM; may use infusion for refractory seizures
• Lorazepam: 2-4 mg IV

Evidence: Animal studies show improved survival when benzodiazepines added to atropine + oxime

Consider prophylactic use in severe poisoning with nerve agents

Organ Support Strategies

Respiratory Support:

Mechanical Ventilation:

• Expect prolonged ventilation (days to weeks)
• Lung-protective strategy: Vt 6-8 mL/kg PBW, Plateau <30 cmH2O
• PEEP: Titrate to oxygenation (often need 8-12 cmH2O)
• High FiO2 initially; wean as secretions clear
• Frequent suctioning essential

Intermediate Syndrome (Day 2-4):

• May require reinstitution or prolongation of mechanical ventilation
• Supportive care until recovery (5-18 days)
• Monitor for aspiration pneumonia

Weaning:

• Do not wean until:
  • Cholinesterase levels recovering
  • No residual nicotinic weakness
  • Successful spontaneous breathing trial
• Consider tracheostomy if prolonged ventilation expected

Cardiovascular Support:

Bradycardia: Atropine is definitive treatment

Hypotension:

• Fluid resuscitation (cautious - pulmonary oedema risk)
• Noradrenaline if refractory
• Address underlying cause (hypovolaemia, cardiac dysfunction)

Arrhythmias (PMID: 12556948):

• QT prolongation common; avoid drugs that prolong QT
• Magnesium 2 g IV for torsades de pointes
• DC cardioversion for unstable arrhythmias

Renal Support:

AKI Causes: Hypotension, rhabdomyolysis, direct nephrotoxicity

Prevention:

• Maintain adequate perfusion pressure
• Aggressive fluid resuscitation if rhabdomyolysis
• Target urine output >1 mL/kg/hr

RRT Indication:

• Standard KDIGO indications
• Note: Dialysis does NOT remove OPs (large volume of distribution, protein binding)
• May support if renal failure develops

Nutritional Support:

Route: Enteral preferred once GI function returns

• May be delayed due to ileus, aspiration risk
• Parenteral nutrition if prolonged ileus

Target: 25-30 kcal/kg/day once stabilised

Ongoing ICU Care (Beyond 48 Hours)

Daily Management:

• Monitor for intermediate syndrome (day 2-4)
• Serial muscle strength assessments
• Forced vital capacity daily
• Wean atropine as tolerated
• Continue pralidoxime until clinical improvement

Intermediate Syndrome Monitoring (PMID: 18565562):

• Peak incidence: Days 2-4
• Clinical features:
  • Proximal limb weakness (cannot raise arms above head)
  • Neck flexion weakness (cannot lift head off pillow)
  • Cranial nerve palsies (ptosis, facial weakness)
  • Respiratory muscle weakness (decreasing FVC)
• Management: Supportive; mechanical ventilation; no specific antidote
• Recovery: 5-18 days

Complications Prevention:

• VTE prophylaxis: Enoxaparin once bleeding risk resolved
• Stress ulcer prophylaxis: PPI
• VAP prevention: HOB 30°, oral care, subglottic suction
• Glycaemic control: Target 6-10 mmol/L

ICU Liberation:

• Sedation minimisation once atropine requirements stable
• Daily SBT once intermediate syndrome excluded
• Early mobilisation when safe
• Tracheostomy consideration if ventilation >7 days anticipated

Australian-Specific Protocols

ANZICS-CORE Considerations:

• Report to National Poisons Centre (13 11 26)
• Notify public health unit for occupational exposure
• WorkSafe notification for occupational poisoning

Therapeutic Guidelines Australia:

• eTG Complete Toxicology section - Anticholinesterase Poisoning
• Follow PBS guidelines for pralidoxime access

Antidote Availability:

• Atropine: Widely available
• Pralidoxime: May need to source from hospital pharmacy or state supply; ensure adequate stock for mass casualty
• Contact Poisons Information Centre for advice on supply

Remote/Rural Considerations:

• Early retrieval for severe poisoning (RFDS, state retrieval services)
• Telemedicine toxicology consultation available
• Pre-hospital atropine by paramedics
• Stock adequate atropine and pralidoxime in regional centres

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Monitoring and Complications

ICU-Specific Monitoring

Daily Parameters:

• Vital signs: Continuous monitoring
• Fluid balance: Target euvolaemia (avoid fluid overload)
• Laboratory: Daily FBC, UEC, LFT, CK, cholinesterase levels
• ABG: Every 4-12 hours based on stability
• ECG: Daily QTc assessment

Trend Monitoring:

• Cholinesterase levels: Track recovery (RBC AChE may take weeks)
• Muscle strength: Serial MRC assessment for intermediate syndrome
• Forced vital capacity: Daily bedside spirometry
• Atropine requirements: Track infusion rate and bolus needs

Safety Monitoring:

• QTc monitoring: Avoid QT-prolonging drugs
• Temperature: Hyperthermia with atropine toxicity
• Pupil size: Mydriasis indicates atropinisation
• Bowel sounds: Ileus with atropine

Complications

Early Complications (First 24-48 hours):

Respiratory Failure:

• Incidence: 30-50% of severe cases
• Risk factors: High-dose ingestion, delayed presentation, bronchorrhoea
• Presentation: Hypoxia, hypercapnia, respiratory distress
• Prevention: Early intubation, aggressive atropine
• Management: Mechanical ventilation, lung-protective strategy

Aspiration Pneumonia:

• Incidence: 20-30%
• Risk factors: Altered consciousness, vomiting, bronchorrhoea
• Presentation: Fever, purulent secretions, CXR infiltrates
• Management: Antibiotics per hospital protocol; tracheal aspirate culture

Seizures:

• Incidence: 10-30% of severe cases
• Risk factors: CNS penetration of OP, hypoxia
• Presentation: Generalised tonic-clonic seizures; may be subtle if paralysed
• Prevention: Early benzodiazepines in severe poisoning
• Management: Benzodiazepines first-line; phenobarbital second-line; avoid phenytoin (less effective)

Cardiac Arrhythmias (PMID: 12556948):

• Incidence: 20-40%
• Types: Bradycardia, QT prolongation, torsades de pointes, VT, VF
• Risk factors: Hypoxia, electrolyte abnormalities, certain OPs
• Prevention: Correct hypoxia, electrolytes; atropine for bradycardia
• Management: Standard ACLS; magnesium for torsades

Late Complications (Beyond 48 hours):

Intermediate Syndrome (PMID: 18565562):

• Incidence: 10-40% of severe cases
• Onset: 24-96 hours post-exposure
• Presentation: Proximal muscle weakness, neck flexion weakness, respiratory failure
• Risk factors: Severe poisoning, inadequate treatment, lipophilic OPs
• Management: Supportive; mechanical ventilation; no specific treatment
• Prognosis: Recovery over 5-18 days with supportive care

OPIDN (Organophosphate-Induced Delayed Neuropathy) (PMID: 8093958):

• Incidence: <1% (specific OPs only)
• Onset: 2-4 weeks post-exposure
• Presentation: Distal motor weakness, sensory loss, ascending paralysis
• Risk factors: TOCP, certain pesticides (chlorpyrifos, methamidophos)
• Management: Supportive; physiotherapy; may have incomplete recovery
• Prevention: Not prevented by atropine or oximes

ICU-Acquired Complications:

• VAP: Common with prolonged ventilation
• CLABSI: Line infections
• ICU-acquired weakness: Compounded by OP effects
• Delirium: Multifactorial; atropine may contribute
• Pressure injuries: Prolonged immobility

Iatrogenic Complications:

• Atropine toxicity: Hyperthermia, tachyarrhythmias, agitation, urinary retention
• Pralidoxime adverse effects: Hypertension, visual disturbance
• Prolonged paralysis: If suxamethonium inadvertently used

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Prognosis and Outcome Measures

Mortality

Short-Term Outcomes:

• ICU mortality: 5-20% in developed settings with aggressive treatment (PMID: 21131908)
• Hospital mortality: 10-25%
• 28-day mortality: 15-30%

Long-Term Outcomes:

• 90-day mortality: Similar to hospital mortality (most deaths occur early)
• 1-year mortality: Additional 5-10% from complications
• Delayed mortality from intermediate syndrome: 5-10%

Comparison by Setting:

• Developed countries (Australia, Europe): 5-15% mortality
• Developing countries (South Asia, Africa): 20-70% mortality
• Difference due to ICU access, ventilator availability, antidote supply

Morbidity

Functional Recovery:

• Return to baseline function: 70-80% at 6 months
• Return to work: 60-70% at 1 year
• Persistent neurological deficits: 10-20%

Neuropsychiatric Sequelae (PMID: 23800979):

• Cognitive impairment: 10-30%
• Depression/anxiety: 20-40%
• PTSD: 10-20% (especially deliberate self-poisoning)
• Extrapyramidal symptoms: Rare

OPIDN (if occurs):

• Incomplete recovery common
• May have permanent disability
• Requires prolonged rehabilitation

Prognostic Factors

Good Prognostic Factors:

• Early presentation (<6 hours)
• Carbamate (rather than OP) poisoning
• Prompt decontamination and antidote therapy
• Mild initial severity (POP score <4)
• Normal cholinesterase levels on admission (less severe)
• Young age

Poor Prognostic Factors (PMID: 26188042):

• Respiratory failure requiring intubation on admission
• Seizures
• Coma (GCS <8)
• Severe metabolic acidosis (pH <7.1)
• QTc prolongation >500 ms
• Very low cholinesterase levels (<10% of normal)
• Delayed presentation (>24 hours)
• High-dose ingestion
• Lipophilic OP (prolonged absorption)
• Need for vasopressor support
• Elderly age
• Pre-existing comorbidities

Scoring Systems

Peradeniya Organophosphorus Poisoning (POP) Scale (PMID: 11239329):

• Score 0-3: Mild (mortality <5%)
• Score 4-7: Moderate (mortality 10-20%)
• Score 8-11: Severe (mortality 30-50%)

APACHE II:

• Higher scores predict mortality
• Score >20 associated with >50% mortality

SOFA Score:

• Useful for tracking multi-organ dysfunction
• Delta SOFA in first 48 hours predicts outcome

Australian/NZ Outcome Data

ANZICS CORE/APD Data:

• Limited specific data on OP poisoning due to relative rarity
• Case reports and small series from rural centres

Indigenous Health Outcomes:

• May have delayed presentation due to remote location
• Access challenges for specialist toxicology care
• Importance of early retrieval and telemedicine consultation

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Progressive Difficulty Assessments

Basic Level (Foundation Knowledge)

Question 1: Mechanism

Q: Explain the mechanism by which organophosphates cause toxicity.

A: Organophosphates bind to and phosphorylate the serine residue at the active site of acetylcholinesterase (AChE), irreversibly inhibiting the enzyme. This prevents hydrolysis of acetylcholine (ACh), causing ACh to accumulate at:

1. Muscarinic receptors (parasympathetic effector organs)
2. Nicotinic receptors (autonomic ganglia, neuromuscular junction)
3. Central nervous system synapses

The accumulated ACh causes overstimulation then desensitisation of these receptors.

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Question 2: Clinical Features

Q: List the clinical features of organophosphate poisoning using the SLUDGE/BBB mnemonic.

A:

• S: Salivation (excess saliva production)
• L: Lacrimation (excessive tear production)
• U: Urination (urinary incontinence)
• D: Defecation (diarrhoea, faecal incontinence)
• G: GI upset (abdominal cramps, nausea)
• E: Emesis (vomiting)
• B: Bradycardia
• B: Bronchospasm
• B: Bronchorrhoea (excessive bronchial secretions)

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Question 3: Antidotes

Q: Name the two main antidotes for organophosphate poisoning and describe their mechanisms.

A:

1. Atropine:

   • Competitive antagonist at muscarinic receptors
   • Reverses muscarinic effects (secretions, bronchospasm, bradycardia)
   • Does NOT affect nicotinic effects

2. Pralidoxime (2-PAM):

   • Oxime that reactivates phosphorylated AChE
   • Cleaves OP-enzyme bond, regenerating active enzyme
   • Must be given before enzyme "aging" (within 24-48 hours)
   • Primarily affects nicotinic symptoms

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Question 4: Intermediate Syndrome

Q: Define intermediate syndrome in organophosphate poisoning.

A: Intermediate syndrome is a distinct clinical phase occurring 24-96 hours after organophosphate exposure, characterised by:

• Proximal muscle weakness
• Neck flexion weakness
• Respiratory muscle weakness leading to respiratory failure
• Cranial nerve palsies

It occurs despite initial recovery from the cholinergic crisis and is due to persistent AChE inhibition at nicotinic receptors. Recovery occurs over 5-18 days with supportive care.

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Intermediate Level (Applied Knowledge)

Question 1: Case-Based Scenario

Stem: A 52-year-old farmer is brought to ED after collapsing while spraying pesticides. He has profuse salivation, miosis, HR 48, RR 8, SpO2 78%, widespread crackles on auscultation.

ABG: pH 7.18, PaCO2 65, PaO2 48, HCO3 21, Lactate 4.5

Q1: Interpret this ABG. (3 marks)

A1:

• Acidaemia (pH 7.18) (1 mark)
• Mixed respiratory (PaCO2 65) and metabolic (lactate 4.5) acidosis (1 mark)
• Severe hypoxaemia (PaO2 48) - Type II respiratory failure (1 mark)
• Expected HCO3 for acute respiratory acidosis: 24 + (0.1 × 25) = 26.5; actual 21 suggests concurrent metabolic acidosis

Q2: Outline your immediate management. (5 marks)

A2:

1. Airway protection: Intubate immediately (GCS likely reduced, copious secretions, respiratory failure) - avoid suxamethonium, use rocuronium (1 mark)
2. Decontamination: Remove clothing, copious skin irrigation, staff PPE (1 mark)
3. Atropine: 2 mg IV immediately, repeat every 3-5 minutes until secretions controlled (1 mark)
4. Pralidoxime: 30 mg/kg bolus then 8-10 mg/kg/hr infusion (1 mark)
5. Supportive care: Mechanical ventilation, IV fluids, cardiac monitoring, urinary catheter (1 mark)

Q3: What investigations would you order? (3 marks)

A3:

1. Cholinesterase levels (RBC AChE and plasma BuChE) (1 mark)
2. ECG (QT prolongation, arrhythmias) (1 mark)
3. CXR (aspiration, pulmonary oedema), electrolytes, CK, ABG, lactate (1 mark)

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Question 2: Atropine Titration

Q: A patient with severe OP poisoning has received 20 mg atropine over 1 hour. HR is 120 bpm but still has copious bronchial secretions and rhonchi on auscultation. What is your approach?

A:

• Continue atropine boluses (the endpoint is SECRETION CONTROL, not heart rate)
• Tachycardia may be due to hypoxia, catecholamine release (nicotinic effect), or atropine - but presence of ongoing secretions indicates inadequate atropinisation
• Double the dose (4 mg boluses every 3-5 minutes) until:
  • Clear chest auscultation (no crackles/rhonchi)
  • Dry axillae
  • Systolic BP >80 mmHg
• May require 100-1000+ mg total in severe poisoning
• Once controlled, start maintenance infusion (10-20% of loading dose per hour)

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Question 3: Pralidoxime Controversy

Q: Discuss the evidence for and against pralidoxime use in organophosphate poisoning.

A:

Evidence FOR pralidoxime:

• Reactivates AChE before aging, restoring enzyme function
• Addresses nicotinic effects (weakness, paralysis) that atropine cannot
• WHO and ATSDR recommend use
• Pawar 2006 study (PMID: 16461906): Continuous infusion showed reduced atropine requirements, improved outcomes
• Clear benefit in nerve agent exposure

Evidence AGAINST:

• Eddleston 2009 RCT (PMID: 19342097): No mortality benefit in Sri Lankan OP self-poisoning (n=235)
• Possible increased mortality in some subgroups
• Buckley meta-analysis (PMID: 21131908): No clear evidence of benefit

Current position:

• Still recommended by WHO
• Consider early use (<24 hours) especially for:
  • Nerve agent exposure
  • Early presentation before aging
  • Nicotinic symptoms (weakness)
  • OPs with slow aging
• May have less benefit in late presentation with already-aged enzyme

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Exam Level (CICM Second Part Standard)

See SAQ Practice and Viva Questions sections below.

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SAQ Practice

SAQ 1: Severe Organophosphate Poisoning

Time Allocation: 10 minutes
Total Marks: 20

Stem: A 45-year-old male farmer is brought to ICU after being found unconscious in a shed where he had been mixing pesticides. His wife reports he was using "Parathion" (an organophosphate insecticide). On arrival:

Observations:

• GCS: 6 (E1V2M3)
• HR: 52 bpm
• BP: 85/50 mmHg
• RR: 8 breaths/min
• SpO2: 72% on 15L O2 via non-rebreather
• Temperature: 36.2°C
• Pupils: Bilateral 1mm, reactive

Examination:

• Profuse oral secretions requiring continuous suctioning
• Widespread crackles and rhonchi bilaterally
• Fasciculations visible over deltoids and thighs

ABG (FiO2 1.0 via NRB):

• pH: 7.15
• PaCO2: 72 mmHg
• PaO2: 45 mmHg
• HCO3: 18 mmol/L
• Lactate: 6.2 mmol/L

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Question 1.1 (8 marks)

Outline your immediate management of this patient in the first 30 minutes.

Question 1.2 (6 marks)

The patient is intubated and ventilated. Describe your atropine dosing strategy, including endpoints and expected total dose.

Question 1.3 (6 marks)

Discuss the role of pralidoxime in this patient, including timing, dosing, and the evidence base.

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Model Answer

Question 1.1 (8 marks total)

Immediate Life-Saving Interventions (First 30 minutes):

A - Airway and Breathing (3 marks):

• Immediate intubation: GCS 6, copious secretions, respiratory failure
• RSI with rocuronium 1.2 mg/kg (AVOID suxamethonium - prolonged paralysis) (1 mark)
• Pre-oxygenation limited by secretions; consider apnoeic oxygenation
• Aggressive suctioning before and after intubation
• Ventilator settings: Vt 6-8 mL/kg, RR 14-16, PEEP 8, FiO2 1.0 initially (1 mark)
• Atropine 1-2 mg IV should be given during/before intubation to reduce secretions (1 mark)

C - Circulation (2 marks):

• IV access × 2 large bore
• Fluid bolus 500-1000 mL crystalloid for hypotension (1 mark)
• Cardiac monitoring, arterial line insertion
• Treat bradycardia with atropine (will give for secretions anyway) (1 mark)

Decontamination (2 marks):

• Staff PPE: Gloves, gown, mask, eye protection (1 mark)
• Remove all clothing, place in sealed bags
• Copious skin and hair washing with soap and water (1 mark)

Antidotes (1 mark):

• Atropine: Immediate IV boluses as above
• Pralidoxime: 30 mg/kg (approximately 2 g) IV over 15-30 minutes (1 mark)

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Question 1.2 (6 marks total)

Atropine Dosing Strategy:

Initial Dosing (2 marks):

• Start with 2 mg IV bolus (higher dose given severity) (1 mark)
• Repeat 2-4 mg every 3-5 minutes, doubling dose if no response
• Continue until endpoints achieved (1 mark)

Endpoints of Atropinisation (2 marks):

• Clear chest auscultation (no crackles/rhonchi from secretions) - PRIMARY ENDPOINT (1 mark)
• Heart rate >80 bpm
• Dry axillae
• Systolic BP >80 mmHg
• NOT pupil dilation (delayed/unreliable)
• NOT tachycardia >120 (indicates over-atropinisation) (1 mark)

Expected Total Dose and Maintenance (2 marks):

• Severe poisoning: May require 100-500 mg in first few hours (cases up to 1000+ mg reported) (1 mark)
• Once controlled: Maintenance infusion at 10-20% of loading dose per hour (e.g., if 100 mg loading, 10-20 mg/hr)
• OR intermittent boluses every 1-4 hours
• Continue until 12-24 hours without need for top-ups (1 mark)

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Question 1.3 (6 marks total)

Role of Pralidoxime:

Mechanism and Rationale (2 marks):

• Nucleophilic attack on phosphorylated AChE, cleaving OP-enzyme bond (1 mark)
• Reactivates AChE before aging occurs
• Addresses nicotinic effects (weakness, paralysis) not covered by atropine (1 mark)

Timing and Dosing (2 marks):

• Must give before aging (24-48 hours for parathion) (1 mark)
• Loading: 30 mg/kg IV over 15-30 minutes (max 2 g)
• Maintenance: 8-10 mg/kg/hr infusion OR 30 mg/kg Q4-6H
• Continue for 24-48 hours after last atropine dose (1 mark)

Evidence Base (2 marks):

• WHO recommends pralidoxime for all OP poisoning
• Eddleston 2009 RCT (PMID: 19342097): No mortality benefit in Sri Lankan OP self-poisoning; possible harm in some subgroups (1 mark)
• Limitations: Late presentation, inadequate dosing, already-aged enzyme
• Still give early (<24 hours) particularly for nicotinic symptoms and known slow-aging OPs
• This patient: Early presentation (<6 hours), severe nicotinic features - likely to benefit (1 mark)

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Common Mistakes:

• Using suxamethonium for RSI
• Titrating atropine to heart rate rather than secretion control
• Not giving adequate atropine doses (being too timid)
• Forgetting decontamination and staff safety
• Not recognising pralidoxime time-dependency

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SAQ 2: Intermediate Syndrome

Time Allocation: 10 minutes
Total Marks: 20

Stem: A 38-year-old female was admitted to ICU 3 days ago following deliberate self-poisoning with an organophosphate pesticide. She required intubation and mechanical ventilation for 48 hours due to respiratory failure from bronchorrhoea. She was extubated yesterday and has been maintaining adequate oxygen saturation on room air.

This morning, nursing staff are concerned as she appears weaker and is having difficulty swallowing.

Observations:

• GCS: 15
• HR: 92 bpm
• BP: 125/78 mmHg
• RR: 24 breaths/min
• SpO2: 94% on room air
• Temperature: 37.2°C

Examination:

• Clear chest, no secretions
• Unable to lift head off pillow
• Proximal arm weakness 3/5 bilaterally
• Proximal leg weakness 3/5 bilaterally
• Distal strength preserved 5/5
• Bulbar weakness with pooling of saliva (not excessive secretions)
• Forced vital capacity: 12 mL/kg (was 25 mL/kg yesterday)

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Question 2.1 (6 marks)

What is the most likely diagnosis? Outline the pathophysiology.

Question 2.2 (8 marks)

How would you manage this patient?

Question 2.3 (6 marks)

Discuss the prognosis and any other delayed syndromes the patient may develop.

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Model Answer

Question 2.1 (6 marks total)

Diagnosis (2 marks):

• Intermediate syndrome following organophosphate poisoning (2 marks)
• Distinct clinical entity occurring 24-96 hours after OP exposure

Pathophysiology (4 marks):

• Occurs despite initial recovery from cholinergic crisis (1 mark)
• Due to persistent AChE inhibition at nicotinic receptors at the neuromuscular junction (1 mark)
• Leads to:
  • Accumulation of ACh at nicotinic receptors
  • Prolonged depolarisation and receptor desensitisation
  • Neuromuscular transmission failure (1 mark)
• Affects predominantly:
  • Proximal muscles (shoulder, hip girdle)
  • Neck flexors (cannot lift head - classic sign)
  • Respiratory muscles (intercostals, diaphragm)
  • Cranial nerves (bulbar weakness, facial weakness, ptosis) (1 mark)

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Question 2.2 (8 marks total)

Immediate Assessment (2 marks):

• Serial forced vital capacity monitoring every 2-4 hours (FVC 12 mL/kg is concerning - <15 mL/kg predicts need for ventilation) (1 mark)
• Single breath count (normal >25)
• Arterial blood gas (for CO2 retention)
• Prepare for reintubation (1 mark)

Airway Management (2 marks):

• High likelihood of requiring reintubation given declining FVC and bulbar weakness (1 mark)
• Consider early elective intubation rather than waiting for crisis
• RSI with rocuronium (avoid suxamethonium - may still have reduced pseudocholinesterase) (1 mark)

Supportive Care (2 marks):

• Mechanical ventilation as needed (may be prolonged - 5-18 days)
• Nutritional support (enteral preferred once GI function adequate)
• DVT prophylaxis
• Early rehabilitation/physiotherapy when able (1 mark)
• Consider tracheostomy if ventilation expected >7-10 days (1 mark)

Pharmacological Management (2 marks):

• No specific pharmacological treatment for intermediate syndrome (1 mark)
• Continue atropine if still having any cholinergic symptoms
• Pralidoxime: Controversial benefit at this stage (enzyme likely already aged); may continue if was started early but unlikely to reverse intermediate syndrome (1 mark)

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Question 2.3 (6 marks total)

Prognosis for Intermediate Syndrome (3 marks):

• Recovery expected over 5-18 days with supportive care (1 mark)
• Most patients make full neurological recovery
• Main risk is mortality from respiratory failure if not recognised/ventilated (1 mark)
• Duration of ventilation: Median 7-14 days
• Overall mortality of intermediate syndrome: 10-20% (primarily from complications of prolonged ICU stay) (1 mark)

Other Delayed Syndromes (3 marks):

OPIDN (Organophosphate-Induced Delayed Neuropathy) (2 marks):

• Onset: 2-4 weeks post-exposure
• Presents with distal motor weakness, sensory loss, ascending paralysis
• Due to inhibition of neuropathy target esterase (NTE), not AChE (1 mark)
• Specific OPs implicated (TOCP, chlorpyrifos, methamidophos)
• May have incomplete recovery; requires prolonged rehabilitation (1 mark)

Neuropsychiatric Sequelae (1 mark):

• Cognitive impairment (10-30%)
• Anxiety, depression (20-40%)
• PTSD (especially in deliberate self-poisoning) (1 mark)

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Common Mistakes:

• Attributing weakness to recurrent cholinergic crisis (intermediate syndrome has NO muscarinic features)
• Expecting pralidoxime to treat intermediate syndrome
• Not monitoring FVC and missing respiratory deterioration
• Confusing intermediate syndrome with OPIDN (different timing and mechanism)

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Hot Case Scenarios

Hot Case 1: Severe OP Poisoning Day 1

Setting: ICU Bed 5
Duration: 20 minutes (10 min assessment + 10 min discussion)
Equipment: Ventilator, monitors, IV pumps, charts available

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Actor/Simulator Briefing (Not given to candidate):

Patient Details:

• Age: 55 years
• Gender: Male
• Admission diagnosis: Organophosphate poisoning - agricultural exposure
• Day of ICU stay: Day 1 (12 hours post-exposure)

History (available from nurse if asked):

• Farmer found collapsed in paddock while spraying crops
• Product identified as "Chlorpyrifos" (organophosphate)
• Dermal and inhalational exposure
• No known suicide intent - accidental exposure
• PMH: Type 2 diabetes, hypertension
• Medications: Metformin, lisinopril

Examination Findings:

• General: Intubated, sedated, no spontaneous movement
• Airway: ETT in situ, copious secretions requiring frequent suctioning
• Breathing: Bilateral coarse crackles and rhonchi; ventilator: SIMV, Vt 450, RR 16, PEEP 10, FiO2 0.6
• Circulation: HR 62 bpm, BP 105/65 mmHg (on noradrenaline 5 mcg/min), warm peripheries
• Disability: RASS -4 (propofol and fentanyl), pupils 2mm bilateral
• Exposure: Mildly diaphoretic, no visible fasciculations currently

Charts/Data Available:

• Flowsheet: Atropine infusion 6 mg/hr, pralidoxime infusion 500 mg/hr, noradrenaline 5 mcg/min
• Recent ABG (FiO2 0.6): pH 7.32, PaCO2 48, PaO2 95, HCO3 22, Lactate 2.1
• Bloods: Na 139, K 4.2, Cr 98, WCC 14.2, Hb 135, Plt 245, CK 2450
• RBC cholinesterase: 15% of normal
• ECG: Sinus rhythm, QTc 440 ms
• CXR: Diffuse bilateral infiltrates consistent with pulmonary oedema/aspiration

Current Management:

• Ventilator: SIMV, Vt 450, RR 16, PEEP 10, FiO2 0.6
• Infusions: Atropine 6 mg/hr, pralidoxime 500 mg/hr, propofol 100 mg/hr, fentanyl 50 mcg/hr, noradrenaline 5 mcg/min
• Fluids: 0.9% saline 80 mL/hr
• Total atropine given so far: 180 mg

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Candidate Task:

"You are the ICU registrar. This 55-year-old farmer was admitted 12 hours ago with organophosphate poisoning from agricultural exposure. Please assess the patient and present your findings to the consultant. You have 10 minutes to examine and review the charts, then 10 minutes for discussion."

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Expected Performance:

Assessment Phase (10 minutes) - 15 marks

History (3 marks):

• Collateral from nurse: Product identification (chlorpyrifos - OP), estimated exposure time, atropine and pralidoxime doses given, current infusion rates
• Review admission details: Circumstances, severity on arrival
• Past medical history: Diabetes (affects recovery), hypertension
• Current concerns: Ongoing secretions requiring frequent suctioning

Examination (10 marks):

• Airway (1 mark): ETT position (check CXR), copious secretions
• Breathing (2 marks): Ventilator settings, chest auscultation (crackles indicating ongoing secretions vs resolving), SpO2, review ABG
• Circulation (2 marks): HR, BP, vasopressor requirement, perfusion, ECG review (QTc)
• Disability (2 marks): Sedation level (RASS), pupil size (2mm suggests reasonable atropinisation), GCS if sedation held
• Exposure (1 mark): Temperature, diaphoresis, fasciculations, rash
• Charts/Data Review (2 marks): Atropine total dose and current rate, pralidoxime dose, cholinesterase levels, CK (rhabdomyolysis), ABG trend

One-Minute Summary (2 marks): "This is a 55-year-old farmer admitted 12 hours ago with severe organophosphate poisoning from chlorpyrifos exposure. He is currently intubated and ventilated on Day 1. He is receiving atropine infusion 6 mg/hr (total 180 mg given) and pralidoxime infusion 500 mg/hr. He requires low-dose noradrenaline and has ongoing secretions requiring frequent suctioning. Key issues are: 1) Ongoing secretions suggesting inadequate atropinisation, 2) Low RBC cholinesterase at 15%, 3) Elevated CK suggesting rhabdomyolysis, 4) Risk of intermediate syndrome developing. Plan is to increase atropine to control secretions, continue pralidoxime, monitor for intermediate syndrome."

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Discussion Phase (10 minutes) - 15 marks

Q1: "He still has copious secretions despite 180 mg atropine. What would you do?" (3 marks)

Expected Answer:

• Increase atropine infusion by 50-100% (from 6 mg/hr to 9-12 mg/hr)
• Give additional boluses (4-8 mg) to achieve control faster
• Target: Clear chest on auscultation - if still crackles/rhonchi, not adequately atropinised
• Doses up to 500-1000 mg may be required in severe poisoning
• Monitor for signs of over-atropinisation (fever, severe tachycardia >140)

Q2: "The RBC cholinesterase is 15% of normal. What is the significance and when will it recover?" (3 marks)

Expected Answer:

• 15% indicates severe poisoning (correlates with synaptic AChE inhibition)
• RBC ChE has half-life of 120 days (reflects RBC lifespan)
• Full recovery takes approximately 90 days (requires synthesis of new RBCs with new AChE)
• Serial measurements useful to confirm trend of recovery
• Plasma cholinesterase recovers faster (synthesised by liver) but less specific

Q3: "This patient is Aboriginal. How does this affect your management and communication?" (3 marks)

Expected Answer:

• Involve Aboriginal Health Worker (AHW) or Aboriginal Liaison Officer (ALO)
• Family-centered decision making - may need extended family involvement
• Cultural safety considerations: Sorry business, kinship obligations
• Consider health literacy when explaining condition and prognosis
• Ensure interpreter if needed
• May be from remote area - plan for follow-up after discharge
• Higher rates of comorbidities (diabetes, CKD) may affect recovery

Q4: "He develops proximal weakness on Day 3 - what are you concerned about?" (3 marks)

Expected Answer:

• Intermediate syndrome - occurs 24-96 hours post-exposure
• Assessment: Neck flexion strength, proximal limb power, forced vital capacity
• May require reintubation if FVC declining (<15 mL/kg)
• No specific treatment - supportive care
• Expected recovery over 5-18 days
• This is NOT recurrence of cholinergic crisis (no muscarinic features)

Q5: "The pralidoxime trial showed no benefit. Should we stop it?" (3 marks)

Expected Answer:

• Eddleston 2009 trial showed no benefit in Sri Lankan agricultural OP poisoning
• However, this patient presented early (<12 hours) - enzyme may not yet be fully aged
• Chlorpyrifos has relatively slow aging (24-48 hours)
• WHO still recommends pralidoxime
• I would continue for at least 24-48 hours
• Monitor clinical response (muscle strength, cholinesterase levels)

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Marking Criteria (Total 30 marks):

Pass/Fail:

• Pass: ≥20/30, no domain scored 0
• Fail: <20/30 or critical safety error (e.g., not recognising need for more atropine)

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Hot Case 2: Intermediate Syndrome

Setting: ICU Bed 8
Duration: 20 minutes

Scenario: Day 4 post OP poisoning, patient extubated 24 hours ago, now developing proximal weakness and declining FVC.

Follow similar structure as Hot Case 1, focusing on:

• Recognition of intermediate syndrome
• Assessment of respiratory reserve (FVC, single breath count)
• Decision for reintubation
• Supportive management
• Prognosis discussion with family

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Viva Questions

Viva 1: Agricultural Organophosphate Exposure

Stem: "A 48-year-old farmer is brought to the emergency department after collapsing while spraying pesticides. He has profuse salivation, miosis, bradycardia, and widespread fasciculations."

Duration: 12 minutes (2 min reading + 10 min discussion)

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Opening Question:

"What are your immediate concerns about this patient?"

Expected Answer (2-3 minutes):

• Airway: Copious secretions - risk of aspiration and airway compromise
• Breathing: Bronchorrhoea and bronchospasm cause hypoxic respiratory failure - leading cause of death
• Neurological: Coma, seizures from CNS ACh accumulation
• Cardiac: Bradycardia can progress to asystole; QT prolongation and arrhythmias
• Staff safety: Risk of secondary contamination - need PPE
• Diagnosis: Clinical features consistent with organophosphate poisoning (cholinergic toxidrome)

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Follow-up Question 1 (2-3 minutes):

"Explain the mechanism of toxicity and the concept of 'aging'."

Expected Answer:

• OPs bind to serine residue at AChE active site, forming phosphorylated enzyme
• This inhibits ACh hydrolysis → ACh accumulates at muscarinic, nicotinic, and CNS synapses
• Aging: Phosphorylated enzyme undergoes spontaneous dealkylation (loss of alkyl group)
• Creates irreversible bond that cannot be broken by oximes
• Aging time varies by OP:
  • "Soman: 2 minutes (oximes rarely effective)"
  • "Sarin: 5 hours"
  • "VX: >40 hours"
  • "Most pesticides: 24-48 hours"
• After aging, recovery requires synthesis of new AChE (weeks)
• This is why pralidoxime must be given early

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Follow-up Question 2 (2-3 minutes):

"How would you manage the airway and what precautions are needed for RSI?"

Expected Answer:

• Likely needs immediate intubation given secretions, respiratory failure, altered consciousness
• Decontamination first if dermal exposure: Staff PPE, remove clothing, wash skin
• RSI considerations:
  • AVOID SUXAMETHONIUM - plasma cholinesterase inhibited, will cause prolonged paralysis (hours to days)
  • Use rocuronium 1.2 mg/kg
  • Reduced induction agent doses (propofol 1 mg/kg or ketamine 1 mg/kg)
  • Give atropine 1-2 mg IV before induction (reduces secretions, prevents vagal response)
  • Aggressive suctioning before and after intubation
  • Prepare for difficult airway (secretions obscuring view)
• Post-intubation: Generous PEEP for secretions, frequent suctioning

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Follow-up Question 3 (2-3 minutes):

"Discuss the evidence for pralidoxime use."

Expected Answer: Evidence FOR:

• Reactivates AChE if given before aging
• WHO and ATSDR recommend use
• Pawar 2006 study: Continuous infusion reduced atropine requirements
• Clear benefit for nerve agent exposure

Evidence AGAINST:

• Eddleston 2009 RCT (PMID: 19342097): No mortality benefit in Sri Lankan OP self-poisoning
• Meta-analysis inconclusive

Current recommendation:

• Still give per WHO guidelines
• Best evidence for early use (<24 hours), nerve agents, and nicotinic features
• May be less effective for late presentation or dimethyl OPs (fast aging)
• In this farmer presenting early, I would give pralidoxime 30 mg/kg bolus then 8-10 mg/kg/hr infusion

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Examiner's Expected Level:

Pass:

• Recognises life-threatening cholinergic crisis
• Prioritises airway and secretion control
• Knows to avoid suxamethonium
• Understands aging concept
• Balanced view on pralidoxime controversy

Fail:

• Would use suxamethonium for RSI
• Titrates atropine to heart rate
• No understanding of aging
• Unable to describe mechanism
• Unsafe airway management

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Viva 2: Nerve Agent Mass Casualty

Stem: "There has been a suspected chemical terrorism incident at a shopping centre. Multiple casualties are presenting with miosis, secretions, and seizures. Initial reports suggest a 'nerve agent' exposure."

Duration: 12 minutes (2 min reading + 10 min discussion)

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Opening Question:

"What nerve agents might be involved and how do they differ from pesticides?"

Expected Answer (2-3 minutes):

• G-agents: Sarin (GB), soman (GD), tabun (GA) - volatile, rapid onset
• V-agents: VX - persistent, more toxic, slower onset
• Differences from pesticides:
  • More potent (µg quantities lethal vs grams for pesticides)
  • Faster onset (seconds to minutes)
  • "Faster aging: Soman ages in 2 minutes (oximes ineffective)"
  • Inhalational exposure common (vs dermal/oral for pesticides)
  • "Same mechanism: AChE inhibition"
  • "Same treatment principles: Atropine, oximes, benzodiazepines"

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Follow-up Question 1 (2-3 minutes):

"How would you organise your ICU response to a mass casualty incident with nerve agent exposure?"

Expected Answer: Command and Control:

• Activate hospital Major Incident Plan
• ICU leads clinical response for critical patients
• Establish command structure (Medical Commander, Nursing Commander)

Decontamination:

• Casualties must be decontaminated BEFORE entering hospital
• Staff PPE mandatory - nerve agents can cause secondary exposure
• Establish decontamination area outside ED

Triage:

• Expectant: Apnoeic, no pulse after atropine
• Immediate: Severe respiratory distress, seizures, unconscious
• Delayed: Moderate symptoms, ambulatory
• Minor: Miosis only, mild symptoms

Resources:

• Stock adequate atropine (may need hundreds of ampoules per patient)
• Pralidoxime less effective for soman (aging in 2 minutes)
• Benzodiazepines for seizures - consider prophylactic
• Prepare for surge ventilator capacity

Communication:

• Contact Poisons Information Centre
• Notify Public Health
• Coordinate with ambulance, police, fire services

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Follow-up Question 2 (2-3 minutes):

"A patient arrives with seizures, apnoea, and profuse secretions. What is your immediate management?"

Expected Answer: Immediate Actions (in parallel):

1. Atropine: 2-4 mg IV immediately, repeat every 3-5 minutes until secretions controlled
2. Diazepam: 10 mg IV for seizures (benzodiazepines neuroprotective in OP/nerve agent poisoning)
3. Airway: BVM ventilation initially; intubate once atropine working (AVOID suxamethonium)
4. Pralidoxime: 30 mg/kg IV (though less effective for soman due to rapid aging)

Military Mark 1 Kit (if available):

• Autoinjector with atropine 2 mg + pralidoxime 600 mg
• Used for immediate self/buddy treatment in military setting

Ongoing:

• Atropine infusion to maintain dry secretions
• Benzodiazepine infusion if seizures recurrent
• Full decontamination when stable
• ICU admission for mechanical ventilation

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Follow-up Question 3 (2-3 minutes):

"How would you protect your staff from secondary contamination?"

Expected Answer: PPE Requirements:

• Minimum: Nitrile gloves (double), impermeable gown, N95 respirator, eye protection
• Ideal: PAPR or full CBRN suit for highly contaminated patients
• Remove contaminated PPE carefully to avoid self-exposure

Decontamination:

• All patients decontaminated OUTSIDE clinical areas
• Remove clothing (90% of decontamination)
• Copious water irrigation of skin
• Dedicated decontamination teams

Environment:

• Negative pressure rooms if available
• Good ventilation
• Limit personnel in contaminated areas

Monitoring:

• Staff monitoring for symptoms (miosis, secretions)
• Low threshold for prophylactic atropine if exposure suspected
• Occupational health follow-up for all exposed staff

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Examiner's Expected Level:

Pass:

• Understands nerve agents are same mechanism as pesticides
• Can organise ICU response to mass casualty
• Prioritises decontamination and staff safety
• Knows treatment is atropine + oxime + benzodiazepine
• Aware of soman's rapid aging

Fail:

• No understanding of nerve agent differences
• Cannot prioritise in mass casualty setting
• Ignores staff safety/decontamination
• Would delay treatment for investigations

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Interactive Elements

[INTERACTIVE: ABG Interpreter]

Case: Severe OP Poisoning

ABG Results:

• pH: 7.18
• PaCO2: 68 mmHg
• PaO2: 52 mmHg (FiO2 0.6)
• HCO3: 19 mmol/L
• Base Excess: -8
• Lactate: 5.2 mmol/L
• Na: 140, Cl: 104, K: 4.8

Step-by-Step Analysis:

1. Is this acidaemia or alkalaemia? Acidaemia (pH 7.18 < 7.35)

2. What is the primary disorder? Respiratory acidosis (elevated PaCO2 68 mmHg)

3. Calculate anion gap: (Na - (Cl + HCO3)) Anion gap = 140 - (104 + 19) = 17 mmol/L Elevated (normal 8-12) - high anion gap metabolic acidosis

4. Is there appropriate compensation? Expected HCO3 for acute respiratory acidosis: 24 + (0.1 × 28) = 26.8 Actual HCO3: 19 - lower than expected Indicates concurrent metabolic acidosis

5. Calculate A-a gradient: ((FiO2 × 713) - (PaCO2/0.8)) - PaO2 A-a gradient = ((0.6 × 713) - (68/0.8)) - 52 = (428 - 85) - 52 = 291 mmHg Markedly elevated - VQ mismatch/shunt

6. Final interpretation: Mixed respiratory and metabolic acidosis with elevated anion gap (lactic acidosis from hypoxia, seizures) and severe hypoxaemia with elevated A-a gradient (consistent with bronchorrhoea causing shunt and VQ mismatch)

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[INTERACTIVE: ECG Library]

Case: OP Poisoning ECG

ECG Features to Identify:

• Sinus bradycardia (HR 48) - muscarinic effect
• Prolonged QTc (520 ms) - risk of torsades
• ST depression in lateral leads - hypoxic changes
• U waves - hypokalaemia from vomiting/diarrhoea

Clinical Significance:

• Bradycardia responds to atropine
• QT prolongation: Avoid QT-prolonging drugs, correct electrolytes, consider magnesium
• Monitor for ventricular arrhythmias

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