Hypothermia

1. THE 2-MINUTE PHYSIOLOGY (Rapid Pathophysiology)

• The Temperature Threshold: Accidental hypothermia is strictly defined as an involuntary drop in core body temperature below 35°C (<95°F).
• Hematologic Breakdown: As core temperature falls, the enzymatic processes governing the clotting cascade fail, directly inducing severe coagulation dysfunction and decreased platelet aggregation. This creates a lethal triad component, vastly complicating hemorrhage control in trauma patients.
• Myocardial Irritability: Profound cooling impairs cardiac pacemaker cells and conduction velocity, leading to profound bradycardia and prolonged PR, QRS, and QT intervals. More critically, severe hypothermia drastically lowers the fibrillation threshold, causing the myocardium to become highly irritable; minimal mechanical stimulation can precipitate sudden ventricular dysrhythmias and cardiac arrest.

2. THE BEDSIDE ACTION PLAN (Rapid ER Management)

• The 60-Second Pulse Check: Because severe hypothermia induces profound, life-sustaining bradycardia, standard 10-second pulse checks are insufficient. If there are no obvious vital signs, you must perform a full 60-second breathing and pulse check (or utilize a cardiac monitor) prior to initiating CPR to avoid triggering ventricular fibrillation.
• Handling & Positioning: Employ minimal, cautious movements when moving the patient from the EMS stretcher to the bed to avoid mechanical induction of ventricular arrhythmias.
• Targeted Rewarming:
• Immediately halt ongoing heat loss by removing wet clothing and drying the patient.
• For moderate to severe hypothermia, initiate active external and minimally invasive internal rewarming simultaneously: utilize forced air blankets, warmed intravenous fluids, and hot packs.
• Target active heat application specifically to the upper trunk, and strictly avoid applying heat to the head.
• Cardiac Arrest Resuscitation: If the patient is in ventricular fibrillation/pulseless VT, you may attempt up to three doses of epinephrine and three defibrillation attempts; if the patient remains unresponsive to these initial interventions, further shocks and medications may be withheld while continuing CPR and active rewarming until the core temperature reaches at least 32°C.

3. THE DIAGNOSTIC GRID (Differential Diagnosis & Workup)

• The "Can't-Miss" Secondary Mimics: Always consider underlying pathologies that precipitate "secondary hypothermia": Sepsis, profound hypoglycemia, myxedema coma, toxidromes (alcohol, sedatives), and occult trauma (intracranial hemorrhage or spinal cord injury).
• Prioritized Diagnostic Workup:
• Core Temperature: Obtain an immediate, continuous core temperature (esophageal or rectal).
• Metabolic & Renal: Point-of-care glucose is mandatory for every patient. Order a basic metabolic panel, serum lactate, and arterial blood gas (ABG) to assess for severe acidosis.
• Muscle Breakdown: Measure serum creatine kinase (CK) to evaluate for concurrent cold-induced rhabdomyolysis, frostbite, or compartment syndrome.
• The Lethal Biomarker (Potassium): In cases of hypothermic cardiac arrest, obtaining a STAT serum potassium is critical. A serum potassium level $\ge$ 12 mmol/L is an objective marker of irreversible cellular death and indicates that resuscitation is futile.

4. THE VISUAL BOARD (ECG / POCUS / Imaging)

• The Electrocardiogram (ECG): Obtain an ECG for all moderate to severe hypothermia patients. Look explicitly for J waves (Osborn waves)—a positive deflection at the J-point. Expect to see prolongation of all elements of the PQRST complex. Atrial fibrillation (with a slow ventricular response) and sinus bradycardia are the most common dysrhythmias.
• Radiography: A chest radiograph (CXR) should be ordered to evaluate for aspiration, pneumonia, or pulmonary edema.
• Neuroimaging: Consider a non-contrast CT of the head if the patient has an altered mental status out of proportion to their core temperature, to rule out an underlying stroke or traumatic brain injury causing the exposure.

5. THE SCORING MATRIX (Risk Stratification & Guidelines)

Emergency disposition and resuscitation efforts hinge on specific physiologic criteria:

• ECMO/CPB Triage Criteria: Transport to a specialty center capable of Extracorporeal Membrane Oxygenation (ECMO) or Cardiopulmonary Bypass (CPB) is indicated if the patient is in cardiac arrest with a core temperature < 32°C AND a serum potassium < 12 mmol/L.
• High-Risk Transfer Criteria: Hemodynamically intact patients require immediate transfer to a higher level of care if they exhibit prehospital cardiac instability, strictly defined as: Systolic Blood Pressure < 90 mm Hg, ventricular dysrhythmias, or a core temperature < 28°C.
• Avalanche Resuscitation Termination Rule: You may terminate CPR in an avalanche victim if the burial time exceeded 35 minutes AND the airway is packed with snow (confirming death by asphyxiation prior to cooling).

6. THE DANGER ZONE (Pitfalls & Critical Actions)

• The Lethal Pulse Check Pitfall: Pitfall: Performing a standard 10-second pulse check, finding no pulse, and immediately starting chest compressions on a patient with a heart rate of 12 bpm. Critical Action: You must assess breathing and central pulses for a full 60 seconds to avoid precipitating VFib with unnecessary chest compressions.
• The Jostling Pitfall: Pitfall: Vigorously transferring or rolling the severely hypothermic patient. Critical Action: Handle the patient with extreme caution; the cold myocardium is uniquely susceptible to mechanically induced ventricular fibrillation.
• The Premature Termination Pitfall: Pitfall: Calling a code on a hypothermic patient simply because standard ACLS has failed. Critical Action: "Nobody is dead until they are warm and dead." You must continue rewarming the patient to $\ge$ 32°C before declaring death, unless the serum potassium is $\ge$ 12 mmol/L.

7. MCQ MASTERCLASS (Written Exam Tips)

• Buzzword: "J-waves" or "Osborn waves" on an ECG. Exam Answer: Systemic hypothermia.
• Buzzword: "Serum Potassium of 14 mmol/L" in a hypothermic cardiac arrest patient. Exam Answer: Terminate resuscitation (ECMO/CPB is futile).
• Distractor: An option may suggest administering 5 doses of epinephrine and immediately shocking a VFib arrest 3 times in a row for a patient at 26°C. Correction: Cold hearts are refractory to drugs and electricity. Guidelines recommend up to 3 doses/shocks, then withholding further standard ACLS therapies until the patient is actively rewarmed above 30-32°C.
• Distractor: An option may suggest applying heating pads directly to the patient's head. Correction: Guidelines explicitly state to apply heat to the upper trunk and "do not apply heat to head".

8. THE BOARDROOM SCRIPT (OSCE & Oral Board Tips)

• The Initial Resuscitation: "Given this patient's core temperature of 26 degrees Celsius, my immediate priority is to handle the patient with extreme caution to prevent mechanically inducing ventricular fibrillation. Before initiating chest compressions, I will mandate a full 60-second check for central pulses and respiratory effort."
• The Rewarming Orders: "I will initiate active external and internal rewarming. Please remove all wet clothing, apply a forced-air warming blanket to the upper trunk, and administer intravenous fluids warmed to 40 degrees Celsius. Ensure we do not apply heat to the head."
• The ECMO Disposition: "Because this patient is in hypothermic cardiac arrest, I am ordering a STAT serum potassium and an ABG. If the potassium is less than 12 mmol/L, I will maintain high-quality CPR and immediately initiate transfer to an ECMO-capable center for extracorporeal rewarming."