ECG Challenge: A 45-year-old man with chest pain that is atypical for angina is referred for an exercise test. He exercises for almost 10 minutes and stops for fatigue. After exercise, his heart rate remains rapid for ≈6 minutes, and while he is monitored during recovery, there is an abrupt slowing of his heart rate as seen in the ECG.
The first part of the ECG shows a regular rhythm at a rate of 120 bpm. There is a P wave before each QRS complex (+) with a stable PR interval (0.14 second). The P wave is positive in leads I, II, aVF, and V4 through V6. This is therefore a sinus tachycardia. The QRS complex duration is normal (0.10 second), and there is a normal morphology. There are narrow but deep Q waves in leads II, III, and aVF (^); they are normal. The axis is normal between 0° and +90° (positive QRS complex in leads I and aVF). The QT/QTc intervals are normal (280/395 milliseconds). The sinus tachycardia abruptly terminates (↔) to a regular rhythm at a rate of 72 bpm. The QRS complexes have the same duration, morphology, and axis as those during the tachycardia. The QT/QTc intervals are also the same. There is a P wave before each of these QRS complexes (*) with a stable PR interval (0.14 second). The PR interval and the P wave morphology are identical to what was present during the sinus tachycardia. Although the initial rhythm appears to be a sinus tachycardia followed by a normal sinus rhythm, sinus tachycardia gradually slows and does not abruptly terminate. Therefore, this is not a sinus tachycardia but is called sinus node reentry. This is an actual arrhythmia that results from a reentrant circuit that involves the sinus node and the tissue around the node. It has on occasion been thought to represent a type of a reentrant atrial tachycardia from a circuit near the sinus node. However, the P waves and PR intervals are identical in every lead during the sinus tachycardia and the normal sinus rhythm, suggesting that they are originating from the same location.
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- © 2015 American Heart Association, Inc.