13.2 Environmental Emergencies, Toxicology & Overdose

13.2 Environmental Emergencies, Toxicology & Overdose

Toxicological ingestions and environmental injuries represent high-yield USMLE Step 2 CK emergency scenarios. Rapid identification of clinical toxidromes, prompt institution of specific antidotes, and systematic management of thermal and altitude-related exposures are crucial to prevent irreversible organ damage or death.

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Clinical Approach to the Poisoned Patient

Initial management of any overdose begins with securing the airway, breathing, and circulation (ABCs). Hypoglycemia and hypoxia must be ruled out immediately in any patient presenting with altered mental status via rapid capillary blood glucose measurement and pulse oximetry. Gastrointestinal decontamination with activated charcoal ($1 \text{ g/kg}$, max $50 \text{ g}$) is effective only if administered within 1 hour of ingestion of a non-caustic, non-hydrocarbon substance in a patient with an intact, uncompromised airway.

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Major Toxidromes Comparison

Recognizing toxidromes enables rapid targeted therapy prior to receiving formal laboratory toxicology screens:

(DUMBELS mnemonic for Cholinergic excess: Diarrhea, Urination, Miosis, Bradycardia/Bronchospasm, Emesis, Lacrimation, Salivation).

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High-Yield Specific Antidotes Table

The following table provides essential specific reversal agents tested on USMLE Step 2 CK:

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TCA and Acetaminophen Overdose Protocols

Tricyclic Antidepressant (TCA) Toxicity

TCA overdose (e.g., amitriptyline) produces anticholinergic toxicity, $\alpha_1$-adrenergic blockade (hypotension), and inhibition of cardiac fast sodium channels.

• ECG Hallmark: QRS duration $> 100 \text{ ms}$ predicts seizures; QRS $> 160 \text{ ms}$ predicts ventricular arrhythmias (vtach/vfib). Terminal $R$ wave in lead aVR $> 3 \text{ mm}$.
• Management: Intravenous Sodium Bicarbonate ($1\text{--}2 \text{ mEq/kg}$ bolus) is indicated for QRS prolongation $> 100 \text{ ms}$ or hypotension. Hypertonic $\text{Na}^+$ overcomes sodium channel blockade, while serum alkalinization increases protein binding of the drug.

                    TCA OVERDOSE MANAGEMENT ALGORITHM
                    
           Suspected TCA Ingestion + Altered Mental Status
                                 |
                                 v
                     Obtain 12-Lead ECG + ABCs
                                 |
         +-----------------------+-----------------------+
         |                                               |
  QRS > 100 ms OR                                QRS < 100 ms +
  Ventricular Arrhythmia / SBP < 90              Hemodynamically Stable
         |                                               |
         v                                               v
  Administer IV Sodium Bicarbonate Bolus           Supportive Care + 
  Target Serum pH 7.45 - 7.55                      Continuous ECG Monitoring (6 hrs)

Acetaminophen Toxicity

Acetaminophen toxicity is mediated by the accumulation of the toxic metabolite N-acetyl-p-benzoquinone imine (NAPQI) when hepatic glutathione stores become depleted ($> 70%$ reduction).

• Nomogram Evaluation: Serum acetaminophen level is drawn at 4 hours post-ingestion and plotted on the Rumack-Matthew Nomogram.
• Treatment: N-acetylcysteine (NAC) (oral or IV) is curative if given within 8 hours. Treatment should be started immediately without waiting for lab levels if presentation is delayed $> 8 \text{ hours}$ after a known toxic ingestion ($> 7.5 \text{ g}$ or $> 150 \text{ mg/kg}$).

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Environmental Emergencies

Heat Illness: Heat Exhaustion vs. Heat Stroke

• Heat Exhaustion: Core body temperature $< 40^\circ\text{C}$ ($< 104^\circ\text{F}$) with intact neurological function (diaphoresis, headache, nausea, tachycardia). Treated with rest in a cool environment and oral fluid replenishment.
• Exertional Heat Stroke: Core body temperature $> 40^\circ\text{C}$ ($> 104^\circ\text{F}$) accompanied by central nervous system dysfunction (confusion, ataxia, seizures, coma) and anhydrosis/diaphoresis. Complications include rhabdomyolysis, DIC, hepatic necrosis, and ARDS.
  • Treatment: Immediate aggressive rapid cooling. Cold-water immersion is the gold standard for exertional heat stroke; evaporative cooling (mist and fans) is preferred for non-exertional heat stroke.

Thermal Burns and Fluid Resuscitation

Burn severity is calculated using the Rule of Nines for Second (partial-thickness) and Third (full-thickness) degree burns: Head ($9%$), Each Arm ($9%$), Anterior Trunk ($18%$), Posterior Trunk ($18%$), Each Leg ($18%$), Perineum ($1%$).

• Parkland Formula: Calculates total fluid required in the first 24 hours:

$\text{Fluid Requirement (mL)} = 4 \text{ mL} \times \text{Weight (kg)} \times \text{\% TBSA (2nd/3rd degree burns)}$

• Administration Schedule: Give $50%$ of the calculated volume over the first 8 hours (from time of burn), and the remaining $50%$ over the next 16 hours using Lactated Ringer's solution.