ECG Challenge: A 76-year-old man with a nonischemic cardiomyopathy is admitted to the hospital because of progressive weight gain and worsening peripheral edema. He notes that he has gained 12 pounds over the past 2 weeks. He states that he has been compliant with all of his medications, but does admit to dietary indiscretion.
ECG A shows that the first 5 QRS complexes are regular at a rate of 48 bpm. There is a P wave before each of these QRS complexes (+) with a stable PR interval (0.24 s). The P wave is positive in leads I, II, aVF, and V4 to V6. Hence, this is a sinus bradycardia with a first-degree atrioventricular (AV) block or prolonged AV conduction. The QRS complex duration is increased, but there is a beat-to-beat variation in the width (ie, 0.12 and 0.14 s). The wider QRS complex has a typical right bundle branch block morphology with an RSR′ in lead V1 (→) and a broad terminal S wave in leads I and V4 to V6 (←). Although the narrower QRS complex has a morpholpogy that resembles a right bundle branch block, there is no terminal S wave in leads I or V5-V6. Importantly, with a bundle branch block (right or left), the QRS complex width is constant and does not change with rate. However, with an intraventricular conduction delay there may be a change in QRS complex width with rate. The presence of 2 different QRS complex widths means that the narrower QRS complex, which also has a right bundle branch block morphology, is not actually a right bundle branch block, but rather an intraventricular conduction delay to the right ventricle. The QT/QTc intervals are slightly prolonged (520/470 ms) but are normal when the prolonged QRS complex duration is considered (480/430 ms). Because the QT interval includes the QRS complex and the ST segment and T wave, as well, prolongation of the QRS complex duration needs to be considered when establishing the QTc interval. The amount of widening of the QRS complex that is above the normal width needs to be subtracted from the QT interval measurement before the QT is corrected for heart rate.
Complexes 6 and 7 (*) are wide but have a morphology that is different from the other QRS complexes. In addition, there are no P waves preceding these complexes, but there are P waves seen after within the ST segment (▲). These P waves are on time and have the same PP interval as the other P waves (↔). Therefore, these are the sinus P waves that are dissociated from the QRS complexes. Therefore, these are premature ventricular complexes, and 2 in a row represent a couplet.
ECG B shows a repeating pattern of 4 QRS complexes followed by a pause. The first 3 of the QRS complexes (*) resemble the premature ventricular complexes seen on ECG A. There are no P waves seen before any of these QRS complexes. The fourth QRS complex (+) has a right bundle-branch block morphology and resembles the sinus complexes on ECG A; it is preceded by a P wave (^) that is negative in leads II and aVF. It can be seen that there are also P waves following the ventricular complexes; the P waves are in the T waves, altering their morphology (▲). This is most obvious in leads II, III, aVF, aVL, and V4 to V5. The P waves are negative in leads II and aVF. They are therefore retrograde, indicating ventriculoatrial conduction from the ventricular complex. Also noted is a progressive increase in the RP interval (↔; ie, retrograde or ventriculoatrial Wenckebach [0.24, 0.36, 0.40 s]). The retrograde P wave after the third ventricular complex results in the QRS complex that has the right bundle-branch block and that is therefore conducted through the AV node and His-Purkinje system. This represents an echo beat. Echo beats can occur whenever there are QRS complexes without a preceding P wave (ie, with a junctional complex, ventricular complex, or ventricular paced complex) and intact ventriculoatrial conduction resulting from these QRS complexes. The retrograde activation of the atrium may result from impulse activation from the ventricle or lower part of the AV junction back to the atria via an overt or concealed bypass tract, 1 of 2 AV nodal pathways (ie, when there are dual AV nodal pathways present) or given a critical timing, via 1 AV nodal pathway. After the retrograde atrial activation there may be antegrade conduction through the AV node and His-Purkinje system to restimulate the ventricles, resulting in the echo beat. In this case, retrograde conduction back to the atria is via the AV node, because this is the only part of the conduction system that manifests Wenckebach conduction.
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- © 2014 American Heart Association, Inc.