ECG Challenge: A 59-year-old woman has a history of lupus with renal involvement for which she is taking glucocorticoids. She has no known heart disease. She presents to her rheumatologist with concerns of intermittent palpitations that first began ≈1 month ago. She was referred to a cardiologist and an ECG was obtained.
The ECG shows a regular rhythm with 2 different rates. The initial portion of the ECG shows a regular rhythm with a rate of 72 bpm. The second part of the ECG shows a regular rhythm at a rate of 160 bpm. All the QRS complexes (except for the 8th QRS complex; ↓) have the same morphology. The QRS complex duration is normal (0.08 s), and there is a normal morphology and a normal axis between 0° and +90° (positive QRS complex in leads I and aVF). The QT/QTc intervals are normal (320/350 ms). The initial QRS complexes (ie, complexes 1–7) are preceded by a P wave (+) with a stable PR interval (0.14 s). The P wave is positive in leads I, II, and aVF. Therefore, this is likely a normal sinus rhythm. There are P waves also seen before each of the QRS complexes in the second part of the ECG (complexes 9–19). These are most obvious in lead V1 (*). The P wave has a morphology that is different than the sinus P wave. In the comparison of the QRS complex in V1 during the tachycardia with the sinus QRS in V1, it can be seen that there is a small positive deflection at the end of the QRS complex (▼) not seen on the sinus complex. This is a second P wave, and the P waves are regular at a rate of 320 bpm. Therefore, the underlying rhythm is atrial flutter, and there is a 2:1 atrioventricular (AV) block (or AV conduction) present. Atrial waveforms at this rate can also be seen between the last 2 QRS complexes (▲); the last 2 QRS complexes have a slightly longer RR interval (↔) or slower rate (150 bpm), although there is still 2:1 AV conduction. This is the result of antegrade-concealed conduction (ie, some of the atrial impulses conduct through the AV node to activate the ventricles, some of the atrial impulses are blocked and do not penetrate the AV node, whereas other atrial impulses partially penetrate the AV node and are concealed within the node, rendering it partially refractory). As a result, the next atrial impulse, if able to conduct through the AV node, will be conducted at a slightly slower conduction velocity.
The 8th QRS complex (↓) is premature, and there is a P wave (↑) preceding this complex. The P-wave morphology is different from that of the sinus P wave; this is a premature atrial complex. This QRS complex has a wide duration (0.12 s) and a morphology of a right bundle-branch block (RBBB). Although it might be considered that the RBBB is rate related, it can be seen that the RR intervals during the atrial flutter (└┘) are shorter than the RR interval between the sinus complex and this premature complex (rate 130 bpm; ┌┐). Therefore, if this were a rate-related RBBB, all of the QRS complexes during the atrial flutter (rate 160 bpm) would have a RBBB. However, it can be seen that the premature atrial complex is preceded by a long–short RR interval. Therefore, the RBBB aberrancy is the result of an Ashman phenomenon that is attributable to rate-related changes in His-Purkinje refractoriness, which are normal and physiological. When the heart rate is slower (long RR interval), His-Purkinje refractoriness increases, whereas, with a faster heart rate (short RR interval), His-Purkinje refractoriness shortens. When there is an abrupt change in heart rate (ie, going from slow [long RR interval] to faster [short RR interval]), refractoriness may not adjust immediately, and, hence, 1 or several QRS complexes may be conducted with a RBBB. Most often, the aberrated complex will have a RBBB because the refractoriness of the right bundle is slightly longer than that of the left bundle.
Please go to the journal’s Facebook page for more ECG Challenges: http://goo.gl/cm4K7. Challenges are posted on Tuesdays and Responses on Wednesdays.
- © 2014 American Heart Association, Inc.