A 72-year-old man is brought to the emergency department after being found unresponsive on a park bench during winter. His core temperature is 28°C (rectal), heart rate 38 bpm, blood pressure 90/50, and GCS 8. The 12-lead ECG shows sinus bradycardia with prolonged PR interval (240 ms), widened QRS (130 ms), prolonged QT interval, and a characteristic positive deflection at the J point appearing as a hump-shaped notch best seen in leads II and V4–V6. Which of the following best describes the structure marked **A** in the diagram and its clinical significance in this patient?

Explanation

## Why Osborn waves are correct The structure marked **A**—the positive deflection at the J point (junction of QRS and ST segment) appearing as a hump-shaped notch in lateral precordial (V4–V6) and inferior (II, III, aVF) leads—is the **Osborn wave** (also called J wave, hypothermic hump, or camel-hump wave). This is the pathognomonic ECG hallmark of moderate-to-severe hypothermia. According to Harrison 21e and AHA/ERC guidelines, Osborn waves typically appear when core temperature drops below 32°C and increase in size and prominence as temperature falls further. The mechanism involves a voltage gradient between epicardium (transient outward current Ito) and endocardium during repolarization, which is accentuated by hypothermia. In this patient with a core temperature of 28°C (moderate-to-severe hypothermia), the presence of Osborn waves, sinus bradycardia, prolonged conduction intervals, and hypotension are all consistent with the expected ECG and hemodynamic findings of moderate-to-severe hypothermia. ## Why each distractor is wrong - **Brugada pattern**: While Brugada syndrome also features J waves, it presents as a coved ST elevation specifically in precordial leads V1–V3 in normothermic patients and is a genetic channelopathy associated with sudden cardiac death risk. This patient's J waves are diffuse (II, V4–V6), occur in the context of profound hypothermia, and are reversible with rewarming—not a fixed Brugada pattern. - **Hyperkalemia-induced peaked T waves**: Hyperkalemia produces tall, peaked (tented) T waves, not the characteristic hump-shaped Osborn waves at the J point. While severe hypothermia can cause electrolyte derangements, the specific ECG finding described (positive deflection at J point) is diagnostic of Osborn waves, not hyperkalemia. - **Anterior STEMI with ST elevation in V1–V4**: Although the patient has ST-segment changes, the finding is a J-point deflection (Osborn wave) in lateral and inferior leads (V4–V6, II), not anterior ST elevation in V1–V4. Moreover, the clinical context (unresponsiveness in winter, core temperature 28°C, profound bradycardia, hypotension) is diagnostic of hypothermia, not acute coronary syndrome. **High-Yield:** Osborn waves (J waves at the J point) are the ECG hallmark of moderate-to-severe hypothermia (typically <32°C), best seen in lateral precordial and inferior leads, and increase in amplitude with progressive temperature decline—they are reversible with rewarming and should prompt gentle handling and extracorporeal rewarming strategies. [cite: Harrison 21e—Hypothermia; AHA ACLS Special Circumstances]