ECG Challenge: A 58-year-old woman with a history of a previous myocardial infarction and mild mitral regurgitation presents to her cardiologist for a routine office visit. Her only medicines are aspirin and hydrochlorothiazide for mild hypertension. She is asymptomatic and her physical examination is remarkable for a grade 3/6 late systolic murmur heard best at the apex. The result of the physical examination is normal except for an irregular pulse. An ECG was obtained.
The rhythm is irregular with group beating (ie, 3 QRS complexes and a pause); the rhythm is therefore regularly irregular. The first and third QRS complexes of the group have the same morphology. They have an increased duration (0.12 s) and a morphology of a right bundle-branch block with an RSR′ in lead V1 (→) and a broad terminal S wave in leads I and V5 to V6 (↑). There is an extreme left axis between –30° and –90° (positive QRS complex in lead I and negative in leads II and aVF). In leads II and aVF, there is a QS complex, consistent with an old inferior wall myocardial infarction that is the cause for the very left axis and not a left anterior fascicular block. The QT interval is normal (420 ms and 400 ms when corrected for the prolonged QRS interval). There is a P wave (+) before the first QRS complexes of the 3 with a stable PR interval (0.18 s). The P wave is positive in leads I, II, aVF, and V4 through V6. This is therefore a sinus complex. There is a P wave before the third QRS complex (^). However, the P wave is negative in leads II, III, and aVF, and the PR interval is 0.20 s. Hence, this is not a sinus P wave, although the QRS complex that follows this P wave has the same morphology as the first QRS complex that is a sinus complex. The second or middle QRS complex (*) is wide (0.16 s) and has a morphology that is different from the first and third complexes. They have an indeterminate axis between –90° and ±180° (negative QRS complex in leads I and aVF). A wide QRS complex with an indeterminate axis results from direct myocardial activation and is seen with a paced QRS complex, a preexcited QRS complex (ie Wolff-Parkinson-White pattern) or a ventricular complex. This complex is not preexcited and it is not preceded by a pacemaker stimulus or a P wave; therefore, this is a premature ventricular complex. The negative P wave that follows this is a retrograde P wave attributable to ventriculoatrial conduction from the premature ventricular complex. The QRS complex that follows this retrograde P wave, ie the third QRS complex of the three, is therefore termed an echo complex or beat. An echo complex occurs when there is ventriculoatrial conduction resulting from the preceding QRS complex. This may happen when this preceding QRS complex does not have a P wave before it, therefore allowing for retrograde atrioventricular nodal conduction and atrial activation. This, therefore, occurs when there is a junctional complex, ventricular complex, or a ventricular paced complex. The retrograde activation of the atrium produces an impulse that can then reenter the atrioventricular node antegradely and conduct to the ventricles, resulting in ventricular activation. The echo complex will have the same morphology as the sinus complex, because ventricular activation is through the normal His-Purkinje system.
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