Ease Into Prep with a USMLE^® Step 2 CK Question of the Day

The correct answer is A. Peaked T waves are associated with significant hyperkalemia. In this patient with chronic renal insufficiency, metabolic acidosis has led to acute hyperkalemia. As a result of the lowered pH, the extracellular concentration of protons increases, thereby increasing the H+/K+ antiports on the cell surface. These antiports then drive protons into the cells and potassium into the extracellular space. ECG changes with hyperkalemia indicate an increased risk for cardiac arrhythmia and therefore the hyperkalemia should be immediately corrected. Calcium gluconate should be administered to decrease membrane excitability.

Hypocalcemia causes prolonged QT intervals (choice B). The QT interval is the time difference between ventricular depolarization and repolarization. Since the QT interval depends on the heart rate, the corrected QT interval (QTc) is often used. The correction factor incorporates the interval between consecutive P waves. Hypocalcemia causes tetany rather than muscle weakness.

ST depression (choice C) would be seen in cardiac ischemia. Although patients with peripheral vascular disease commonly have coronary artery disease as well, there is no indication in this case of an acute coronary syndrome.

T wave inversion (choice D) is another indication that the patient may be undergoing an ischemic event.

U waves (choice E) are seen in hypokalemia. If an ECG shows these changes, the risk of an arrhythmia is significant, and the hypokalemia must be corrected immediately with either oral or intravenous potassium.