ECG Challenge: A 58-year-old man with a history of hypertension being treated with amlodipine and hydrochlorthiazide and coronary artery disease being treated with a β-blocker and Imdur presents to his cardiologist for a routine physical examination. His pulse is noted to be irregular, and a 12-lead ECG is obtained.
The rhythm is irregular, but all of the long intervals (↔) are the same and the short intervals are the same (┌┐). The rhythm is therefore regularly irregular with group beating, 2 QRS complexes and a pause. The average rate is 60 bpm. All of the QRS complexes are the same. They have a normal duration (0.08 sec) and normal morphology. The axis is normal between 0° and +90° (positive QRS complex in leads I and aVF). The QT/QTc intervals are normal (400 ms/400 ms). There are no P waves seen before the first of the 2 QRS complexes. Hence, these are junctional complexes. There is a P wave (+) seen between the 2 QRS complexes, particularly in leads II, III, aVF, V3, and V5. The P wave is negative in leads II and aVF. Therefore it is not originating from the sinus node. It may be either originating from a low atrial focus, making the second complex a premature atrial complex which is occurring in a bigeminal pattern. More likely is a retrograde P wave resulting from the junctional complex. This retrograde P wave is followed by a supraventricular complex, and the RP and PR intervals are constant. The second QRS complex is termed an echo beat. Echo beats result from intact VA conduction that produces atrial activation. This atrial impulse may enter the AV node and be conducted antegradely to restimulate the ventricles. As there needs to be VA conduction for this to happen, an echo beat will only occur after a QRS complex that does not have a preceding P wave (ie, a junctional complex, ventricular complex, or ventricular paced complex). VA conduction may be a result of a retrograde impulse conducted to the atria via an overt or concealed accessory pathway, 1 of 2 (dual)AV nodal pathways or via a single AV nodal pathway given the right timing. The echo beat will always be supraventricular as it conducts to the ventricle via the AV node His-Purkinje system.
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- © 2016 American Heart Association, Inc.