A 15-year-old boy is brought to the emergency department by his father 10 minutes after falling into a frozen lake during ice fishing. He was in the water for less than 1 minute before his father managed to pull him out. On arrival, his clothes are still wet and he appears scared. His body temperature is 36.2°C (97.1°F), pulse is 102/min, blood pressure is 133/88 mm Hg. Which of the following mechanisms contributes most to maintaining this patient's core body temperature?

Involuntary muscular contractions

Increase in hypothalamic set point

Contraction of arrector pili muscles

A 37-year-old man is brought to the emergency department by a friend after he was found lying unconscious outside his front door. The friend reports that they were “pretty drunk” the previous night, and she had dropped her friend off at his home and driven off. When she came back in the morning, she found him passed out on the ground next to the doorstep. On arrival, he is conscious and cooperative. He reports feeling cold, with severe pain in his hands and face. He remembers having lost his gloves last night. His rectal temperature is 35.2°C (95.3°F), pulse is 86/min, respirations are 17/min, and blood pressure is 124/58 mm Hg. Examination shows decreased sensations over the distal fingers, which are cold to touch. The skin over the distal phalanges is cyanotic, hard, waxy, and tender, with surrounding edema. Laboratory studies are within the reference range. An x-ray of the chest and ECG show no abnormalities. Which of the following is the most appropriate next step in management?

Immersion of affected extremities in warm water

Intravenous administration of warmed crystalloid

Debridement of the affected tissue

Intra-arterial administration of tissue plasminogen activator

Intravenous administration of antibiotics

A cardiologist is studying how a new virus that infects the heart affects the electrical conduction system of the cardiac myocytes. He decides to obtain electrocardiograms on patients with this disease in order to see how the wave patterns and durations change over time. While studying these records, he asks a medical student who is working with him to interpret the traces. Specifically, he asks her to identify the part that represents initial ventricular depolarization. Which of the following characteristics is most consistent with this feature of the electrocardiogram?

Normal duration defined as less than 120 milliseconds

Elevated in patients with full thickness ischemic injury of the heart

Becomes peaked in states of hyperkalemia

Becomes prominent in states of hypokalemia

Normal duration defined as less than 200 milliseconds

A 41-year-old woman presents to the emergency room with chest pain. She has had progressive substernal chest pain accompanied by weakness and mild shortness of breath for the past 2 hours. Her past medical history is notable for poorly controlled systemic lupus erythematosus (SLE), Sjogren syndrome, and interstitial lung disease. She was hospitalized last year with pericarditis presumed to be from SLE. Her temperature is 98.6°F (37°C), blood pressure is 106/56 mmHg, pulse is 132/min, and respirations are 26/min. On exam, the skin overlying the internal jugular vein fills at 9 cm above the sternal angle and distant heart sounds are appreciated. There is no friction rub. She is given 1000cc of intravenous fluids with no appreciable change in her blood pressure. An electrocardiogram in this patient would most likely reveal which of the following findings?

ST elevations in leads II, III, and aVF

Wide QRS complexes with no P waves

An investigator is conducting a study on hematological factors that affect the affinity of hemoglobin for oxygen. An illustration of two graphs (A and B) that represent the affinity of hemoglobin for oxygen is shown. Which of the following best explains a shift from A to B?

Decreased serum 2,3-bisphosphoglycerate concentration

Increased hemoglobin γ-chain synthesis

Cardiac muscle serves many necessary functions, leading to a specific structure that serves these functions. The structure highlighted is an important histology component of cardiac muscle. What would be the outcome if this structure diffusely failed to function?

Failure of propagation of the action potential from the conduction system

Failure of potassium channels to appropriately open to repolarize the cell

Ineffective excitation-contraction coupling due to insufficient calcium ions

Inappropriate formation of cardiac valve leaflets

A 42-year-old man undergoes therapeutic hypothermia (target temperature 33°C/91.4°F) following cardiac arrest with return of spontaneous circulation. During the cooling phase, he develops shivering, which increases oxygen consumption and interferes with target temperature achievement. He is already on sedation and neuromuscular blockade is being considered. Evaluate the most appropriate management strategy considering both efficacy and safety.

Increase sedation and add surface counter-warming of extremities before neuromuscular blockade

Administer meperidine alone to reduce shivering threshold

Abandon therapeutic hypothermia due to complications

Use only mechanical restraints to prevent movement

Immediate neuromuscular blockade without additional measures

Hypothermia pathophysiology for USMLE Step 2 CK: High-Yield Physiology Lessons

Hypothermia Staging - The Big Chill

Progression of clinical signs as core body temperature drops. Management is guided by the specific stage.

⭐ Osborn (J) waves on ECG are a hallmark of moderate hypothermia (28-32°C). They are positive deflections at the J point, most prominent in precordial leads.

Pathophysiology - The Cold Cascade

Initial Compensation: Cold exposure triggers peripheral vasoconstriction (shunting blood to the core) and hypothalamic-mediated shivering to ↑ heat production.
- Sympathetic nervous system activation → ↑ catecholamines, ↑ metabolic rate.
Decompensation & Failure: As core temperature drops further, compensatory mechanisms fail.
- Shivering ceases at ~32°C (90°F), leading to a rapid decline in core temperature.
- Progressive ↓ in metabolic rate, oxygen consumption, and cardiac output.
- Myocardial irritability increases, leading to bradycardia and arrhythmias (e.g., atrial fibrillation).

ECG: Osborn J waves in hypothermia

⭐ The Osborn J wave, a positive deflection at the J point on an ECG, is a characteristic finding in hypothermia, typically seen at core temperatures below 32°C (90°F).

Systemic Effects - When Organs Freeze Over

Cardiovascular:
- Initial tachycardia → progressive bradycardia.
- Arrhythmias: Atrial fibrillation is common. Ventricular fibrillation risk ↑ significantly below <28°C.
- ECG: Osborn J wave (positive deflection at QRS-ST junction).
Neurologic:
- Progressive ↓ in consciousness, confusion, lethargy → coma.
- ↓ Cerebral metabolic rate; EEG may become flatline (mimicking brain death).
Renal & Metabolic:
- Initial "cold diuresis" due to impaired ADH action.
- ↓ Insulin secretion & ↑ insulin resistance → Hyperglycemia.
Hematologic:
- ↑ Blood viscosity & hematocrit (due to plasma volume loss).
- Coagulopathy: Impaired enzyme function in the clotting cascade.

⭐ Clinical Pearl: Coagulation studies (PT/PTT) are often performed at a standard 37°C in the lab, which can falsely normalize results in a hypothermic patient. Clotting factors are temperature-dependent and dysfunctional at low body temperatures, leading to a high risk of bleeding despite "normal" labs.

ECG Findings - The J Wave's Debut

J wave (Osborn wave): Pathognomonic finding. A positive deflection at the J-point (QRS-ST junction), most prominent in precordial leads.
Progression:
- Sinus bradycardia
- Interval prolongation (PR, QRS, QT)
- Arrhythmias (atrial fibrillation, ventricular fibrillation)

⭐ The amplitude of the J wave is directly proportional to the degree of hypothermia.

ECG showing Osborn J waves in hypothermia

High‑Yield Points - ⚡ Biggest Takeaways

Hypothermia is a core body temperature <35°C (95°F).

Initial compensation involves intense peripheral vasoconstriction and shivering to generate heat.

Cessation of shivering is a key sign of severe hypothermia and failing compensatory mechanisms.

Leads to progressive CNS depression, moving from confusion to coma.

High risk of cardiac arrhythmias (e.g., atrial fibrillation, ventricular fibrillation), with the characteristic Osborn (J) wave on EKG.

Beware of rewarming shock from rapid temperature correction.

Continue reading on Oncourse

CONTINUE READING — FREE

or get the app

App Store|Google Play