A 68-year-old man is brought to the emergency department after being found unconscious in his apartment during a cold snap. Core body temperature is 29°C. His ECG shows sinus bradycardia with a characteristic deflection at the QRS-ST junction (marked **A** in the diagram) and prolonged PR, QRS, and QT intervals. Which of the following best describes the pathophysiological basis for the ECG finding marked **A**?

Explanation

## Why option 1 is correct The Osborn (J) wave marked **A** is the pathognomonic ECG finding in hypothermia, appearing as a distinctive deflection at the QRS-ST junction (the "camel-hump" appearance). Its mechanism is directly attributable to progressive slowing of ion-channel kinetics and conduction velocity throughout the myocardium as core temperature drops below 35°C. This temperature-dependent slowing prolongs the action potential duration and critically alters the transmural voltage gradient between the epicardium and endocardium during phase 1 repolarization, producing the characteristic J wave. The height of the Osborn wave is roughly proportional to the degree of hypothermia. This finding, combined with bradycardia and prolonged intervals (PR, QRS, QT), is pathognomonic for hypothermia and reflects the fundamental effect of cold on myocardial electrophysiology (Harrison 21e Ch 471; Tintinalli 9e Ch 207). ## Why each distractor is wrong - **Option 2 (Acute STEMI)**: While acute myocardial infarction produces ST elevation, it does not produce the characteristic Osborn J wave at the QRS-ST junction. STEMI ST elevation is typically concave and occurs in specific coronary distributions; the Osborn wave is a distinct deflection appearing globally across the ECG in the setting of low core temperature, not acute coronary occlusion. - **Option 3 (Hyperkalemia)**: Although severe hypothermia can cause hyperkalemia (which produces peaked T waves), the finding marked **A** is specifically the Osborn J wave, not peaked T waves. Hyperkalemia is a secondary consequence of severe hypothermia, not the primary mechanism of the J wave itself. Peaked T waves (option B in the diagram) are a separate ECG finding. - **Option 4 (Early repolarization)**: Early repolarization is a benign normal variant seen in young, athletic individuals and is not associated with the clinical context of severe hypothermia, bradycardia, prolonged intervals, and altered mental status. Early repolarization does not explain the systemic ECG changes seen in this patient. **High-Yield:** Osborn J wave = pathognomonic for hypothermia; mechanism = temperature-dependent slowing of ion channels + altered epicardial-endocardial voltage gradient during phase 1 repolarization; height proportional to degree of cooling. [cite: Harrison 21e Ch 471; Tintinalli Emergency Medicine 9e Ch 207]