ECG Challenge: A 67-year-old man recently underwent radical neck surgery for pharyngeal cancer. Three days later, he developed severe muscle twitching and spasms and sharp chest pain. An ECG was obtained.
The rhythm is regular at a rate of 78 bpm. There is a P wave (+) before each QRS complex with a stable PR interval (0.16 second). The P waves are positive in leads I, II, aVF, and V4 through V6. Hence, this is a normal sinus rhythm. The QRS complex duration is normal (0.08 second) and has a normal axis (between 0° and 90°, positive QRS complex in leads I and aVF). The QRS complex has a normal morphology, although there is poor R-wave progression in leads V1 and V2. This is likely a normal variant, although an old anteroseptal myocardial infarction should be considered. The QT/QTc intervals are prolonged (480/540 milliseconds). There are 2 different patterns for QT prolongation. The first is delayed repolarization in which the long-QT interval is the result of a long ST segment (^) and the T wave is normal in duration. This pattern is seen with either hypocalcemia or hypomagnesemia, which results in a prolongation in phase 2 of the action potential and hence a delay in the onset of repolarization. This type of QT prolongation is not associated with any cardiac abnormalities; specifically, it is not associated with torsade des pointes. The second pattern is called prolonged repolarization in which there is a widening of the T wave (prolongation in the T-wave duration). This is often the result of a slow efflux of potassium during phase 3 of the action potential, which prolongs repolarization or the refractoriness of the membrane. This is a long-QT pattern that is associated with the development of torsade des pointes, or a long-QT syndrome. The long-QT interval in this situation may be acquired, a result of therapy with a drug that interferes with the efflux of potassium during repolarization (eg, the class III antiarrhythmic drugs), or congenital, a result of a channelopathy that causes a prolongation in membrane refractoriness. In this ECG, the pattern is one of delayed repolarization as there is a long ST segment (^) and a normal T wave. The patient likely developed hypocalcemia as a result of hypoparathyroidism due to the recent neck surgery.
- © 2013 American Heart Association, Inc.