8.1 Toxicology Emergencies, Toxidromes & Antidote Therapy

Toxicology Emergencies, Toxidromes & Antidote Therapy

Toxicological emergencies in critical care transport demand a rapid, systematic approach. Critical care paramedics frequently care for poisoned patients prior to definitive laboratory identification of the ingested substance. Management relies on physical examination, constellation of vital sign abnormalities, and toxidrome recognition to initiate life-saving resuscitation and targeted antidote administration.

Toxidrome Assessment & Differential Framework

A toxidrome is a constellation of clinical signs and symptoms associated with a specific class of toxic substances. Recognizing toxidromes allows immediate syndromic diagnosis and treatment initiation.

Cholinergic Crisis & Organophosphate Poisoning

Organophosphate insecticides and nerve agents (soman, sarin, VX) irreversibly inhibit acetylcholinesterase, leading to massive accumulation of acetylcholine at muscarinic and nicotinic receptors.

• Muscarinic Manifestations: Represented by the pneumonic SLUDGEM (Salivation, Lacrimation, Urination, Defecation, Gastrointestinal upset, Emesis, Miosis) or DUMBBBELLS (Diarrhea, Urination, Miosis, Bradycardia, Bronchospasm, Bronchorrhea, Emesis, Lacrimation, Lethargy, Salivation). Respiratory collapse from bronchorrhea ("killer Bs") and bronchospasm is the primary cause of death.
• Nicotinic Manifestations: Muscle fasciculations, weakness, flaccid paralysis, tachycardia, and hypertension.

Antidote Protocol:

1. Atropine: Competitive antagonist at muscarinic receptors. Administer 2 mg IV push, doubling the dose every 3 to 5 minutes (2 mg, 4 mg, 8 mg, 16 mg...) until bronchial secretions dry and bronchospasm resolves. Note: Atropine does not reverse nicotinic muscle paralysis or tachycardia; target end-point is clearing pulmonary secretions, not normal heart rate.
2. Pralidoxime (2-PAM): Reactivates acetylcholinesterase by removing the phosphate group from the enzyme's active site before irreversible "aging" occurs. Administer 1 to 2 g IV diluted in 100 mL 0.9% Normal Saline over 15 to 30 minutes, followed by a continuous infusion of 200 to 500 mg/hr.

Anticholinergic Delirium & Physostigmine

Anticholinergic toxicity results from competitive inhibition of acetylcholine at central and peripheral muscarinic receptors. Classic mnemonic: "Blind as a bat (mydriasis), mad as a hatter (delirium), red as a beet (flushed skin), hot as a hare (hyperthermia), dry as a bone (anhidrosis), bowel and bladder lose their tone (urinary retention, ileus)."

• Management: Supportive care, aggressive external cooling for hyperthermia, and IV benzodiazepines (lorazepam 2-4 mg or midazolam 2.5-5 mg) for agitation and seizures.
• Physostigmine: A tertiary amine acetylcholinesterase inhibitor that crosses the blood-brain barrier. Indicated for severe central anticholinergic delirium. Dosing: 0.5 to 2 mg IV slow push over 5 minutes. Contraindication: Strictly contraindicated in suspected Tricyclic Antidepressant (TCA) overdose, as it can precipitate refractory asystole and lethal conduction blunts.

Specific Antidotes: Acetaminophen, Cyanide & Carbon Monoxide

Acetaminophen (APAP) Toxicity

Acetaminophen overdose is the leading cause of acute liver failure in North America. At therapeutic doses, APAP is conjugated by glucuronidation and sulfation. In overdose, these pathways saturate, routing APAP to cytochrome P450 (CYP2E1), producing the toxic metabolite N-acetyl-p-benzoquinone imine (NAPQI). NAPQI depletes hepatic glutathione stores and causes centrilobular hepatic necrosis.

Acetaminophen (Overdose)
       │
       ├─► Glucuronide / Sulfate Pathways (Saturated)
       │
       └─► CYP2E1 Metabolism ──► NAPQI (Toxic Metabolite)
                                    │
                                    ├─► Glutathione Depletion
                                    │
                                    └─► Hepatocellular Necrosis & Acute Liver Failure

• Rumack-Matthew Nomogram: Used to assess hepatotoxicity risk after a single acute ingestion between 4 and 24 hours post-ingestion. APAP serum concentration is plotted against hours post-ingestion. If the level falls on or above the treatment line (starting at 150 mcg/mL at 4 hours), antidote therapy is indicated.
• N-Acetylcysteine (NAC): Acts as a glutathione precursor and substitute, directly binding NAPQI and restoring hepatic glutathione. Maximum efficacy is achieved when administered within 8 hours of acute ingestion.
• IV NAC Protocol (3-Bag Method):
  1. Loading Dose: 150 mg/kg IV in 200 mL D5W over 60 minutes.
  2. Second Dose: 50 mg/kg IV in 500 mL D5W over 4 hours.
  3. Third Dose: 100 mg/kg IV in 1,000 mL D5W over 16 hours.

Cyanide Toxicity

Cyanide is a potent cellular toxin encountered in industrial exposures, residential structure fires (burning synthetic polymers), and sodium nitroprusside infusions. Cyanide binds ferric iron (Fe3+) in cytochrome c oxidase (complex IV) of the mitochondrial electron transport chain, completely arresting aerobic ATP production. Cells switch to anaerobic metabolism, generating severe metabolic lactic acidosis despite abundant oxygen delivery.

• Clinical Presentation: Profound lactic acidosis (lactate > 10 mmol/L), elevated central venous oxygen saturation (SvO2 > 90% due to cellular inability to extract oxygen), headache, confusion, seizures, cardiovascular collapse, and sudden arrest.
• First-Line Antidote: Hydroxocobalamin (Cyanokit): Hydroxocobalamin contains a cobalt ion that binds cyanide with higher affinity than cytochrome oxidase, forming non-toxic cyanocobalamin (Vitamin B12), which is excreted in urine. Dosing: 5 g IV reconstituted in 200 mL 0.9% Normal Saline infused over 15 minutes (a second 5 g dose may be given for persistent instability). Causes transient reddish discoloration of skin, mucous membranes, and urine, and interferes with co-oximetry and colorimetric lab tests.
• Second-Line Antidote: Sodium Thiosulfate: Provides a sulfur donor for the enzyme rhodanese to convert cyanide into non-toxic thiocyanate. Administer 12.5 g IV (50 mL of 25% solution) over 10 to 20 minutes.
• Note: Nitrite reagents (Amyl nitrite / Sodium nitrite) form methemoglobin to bind cyanide, but are avoided in smoke inhalation casualties due to worsening hypoxia and carboxyhemoglobinemia.

Carbon Monoxide (CO) Poisoning

Carbon monoxide is a colorless, odorless gas produced by incomplete combustion. CO binds hemoglobin with an affinity 200 to 250 times greater than oxygen, forming carboxyhemoglobin (COHb). This severely reduces oxygen-carrying capacity and shifts the oxyhemoglobin dissociation curve to the left (Haldane effect), inhibiting tissue oxygen unloading.

• Diagnostic Pitfall: Standard pulse oximetry (SpO2) cannot distinguish oxyhemoglobin from carboxyhemoglobin, yielding falsely normal SpO2 readings even in lethal poisoning. Co-oximetry (blood gas or RAD-57 pulse co-oximetry) is mandatory.
• Half-Life & Oxygen Therapy:
  • Room Air (21% FiO2): COHb half-life is approximately 320 minutes (5.5 hours).
  • High-Flow 100% FiO2 (Normobaric): COHb half-life decreases to 80 minutes.
  • Hyperbaric Oxygen (HBO at 2.5–3.0 ATA): COHb half-life decreases to 20 to 30 minutes.
• Hyperbaric Oxygen (HBO) Indications:
  1. Carboxyhemoglobin level > 25% (or > 15% in pregnant patients due to fetal hemoglobin CO affinity).
  2. History of syncope or loss of consciousness.
  3. Acute focal neurological deficits, altered mental status, or coma.
  4. Acute myocardial ischemia or dysrhythmias.
  5. Persistent symptoms despite 4 hours of normobaric 100% FiO2.

Cardiotoxic Ingestion & Tricyclic Antidepressant (TCA) Protocols

Beta-Blockers (BB) vs. Calcium Channel Blockers (CCB)

Overdoses of beta-blockers and calcium channel blockers cause profound myocardial depression, vasodilation, and refractory shock. Differentiating between the two in the pre-hospital or transport environment relies on blood glucose assessment.

CCB Overdose ──► Blockade of L-type Ca2+ Channels in Pancreas ──► Inhibited Insulin Release ──► Hyperglycemia
BB Overdose  ──► Blockade of Beta-2 Receptors in Liver       ──► Inhibited Gluconeogenesis ──► Hypoglycemia / Normal

Pharmacological Antidote Bundle:

1. Calcium Chloride 10%: Restores extracellular-to-intracellular calcium concentration gradient. Administer 1 to 2 g (10 to 20 mL) IV push over 5 minutes via central line (or Calcium Gluconate 10% 3 to 6 g via peripheral line). Repeat every 10 to 20 minutes up to 3 to 4 doses.
2. Glucagon: Binds Gs protein-coupled receptors bypassing blocked beta-receptors, increasing intracellular cyclic AMP (cAMP) and enhancing inotropy and chronotropy. Administer 5 to 10 mg IV slow push over 3 to 5 minutes. If heart rate and BP improve, initiate a continuous IV infusion at 2 to 5 mg/hr.
3. High-Dose Insulin Euglycemia Therapy (HIET): Primary metabolic therapy for severe CCB and BB shock. In shock states, cardiac myocytes shift from fatty acid metabolism to carbohydrate (glucose) oxidation, but CCB/BB toxicity impairs insulin secretion and glucose uptake. High-dose insulin acts as a potent inotrope.
   • Loading Dose: 1 unit/kg IV regular insulin bolus co-administered with 0.5 g/kg Dextrose (50% Dextrose 1 mL/kg).
   • Maintenance Infusion: Continuous regular insulin infusion at 0.5 to 1.0 units/kg/hr titrated up to 2.0 to 10.0 units/kg/hr. Infuse Dextrose 10% or 50% to maintain blood glucose between 100 and 200 mg/dL. Monitor serum potassium hourly and replate to maintain K+ ≥ 3.5 mEq/L.

Digoxin Toxicity & DigiFab Calculation

Digoxin inhibits the myocardial membrane Na+/K+-ATPase pump, increasing intracellular sodium and secondary intracellular calcium accumulation. Acute toxicity causes severe hyperkalemia (> 5.5 mEq/L), whereas chronic toxicity is exacerbated by hypokalemia, hypomagnesemia, and renal failure.

• Clinical Presentation: Nausea, vomiting, confusion, visual xanthopsia (yellow-green halos around lights), sinus bradycardia, AV block, junctional escape rhythms, and bidirectional ventricular tachycardia (pathognomonic).
• DigiFab (Digibind) Neutralization: Digoxin-specific Fab fragments bind free serum digoxin, rapidly reversing toxicity.
  • Calculation based on Known Ingestion Amount: $\text{Number of Vials} = \frac{\text{Total Ingested Digoxin (mg)}}{0.5 \text{ mg/vial}}$
  • Calculation based on Steady-State Serum Level: $\text{Number of Vials} = \frac{\text{Serum Digoxin Concentration (ng/mL)} \times \text{Weight (kg)}}{100}$
  • Empiric Administration: If dose and serum levels are unknown, administer 10 to 20 vials IV for acute cardiac arrest or life-threatening ventricular dysrhythmias.

Tricyclic Antidepressant (TCA) Overdose

TCA overdose (amitriptyline, nortriptyline, imipramine) produces toxic effects through four major pharmacological mechanisms:

1. Fast Sodium Channel Blockade (INa): Slows phase 0 depolarization in cardiac conduction tissue, leading to QRS widening, right bundle branch block, and lethal ventricular arrhythmias (VT/VF).
2. Alpha-1 Adrenergic Blockade: Causes profound peripheral vasodilation and refractory shock.
3. Central & Peripheral Anticholinergic Effects: Mydriasis, tachycardia, hyperthermia, delirium, ileus.
4. GABA-A Receptor Antagonism: Triggers rapid-onset central nervous system depression and seizures.

Diagnostic Electrocardiogram (ECG) Criteria:

• QRS Duration > 100 ms: Predicts increased risk of seizures (34% risk).
• QRS Duration > 160 ms: Predicts increased risk of ventricular dysrhythmias (50% risk).
• Lead aVR Morphology: Terminal R wave > 3 mm in lead aVR, or terminal R/S ratio > 0.7 in aVR, reflecting delayed right ventricular depolarization.

Primary Antidote Protocol: Sodium Bicarbonate

• Indications: QRS duration > 100 ms, terminal R wave in aVR > 3 mm, hypotension unresponsive to fluid boluses, or ventricular dysrhythmias.
• Mechanism: Increases extracellular sodium concentration, overriding competitive fast sodium channel blockade, and alkalinizes serum pH (target 7.45–7.55), which increases protein binding of TCA and uncouples the drug from cardiac sodium channels.
• Bolus Dosing: Administer 1 to 2 mEq/kg IV push (8.4% Sodium Bicarbonate) over 1 to 2 minutes. Repeat every 3 to 5 minutes until QRS narrows (< 100 ms) and hypotension resolves.
• Continuous Infusion: Mix 3 ampules (150 mEq) of Sodium Bicarbonate in 1 L D5W and infuse at 150 to 250 mL/hr, titrating to maintain arterial pH 7.45 to 7.55.