ECG Challenge: A 64-year-old woman with a history of hypertension treated with a β-blocker and angiotensin-converting enzyme inhibitor presents to her primary care physician for a routine physical examination. She is asymptomatic, although she does state that she has intermittently felt her pulse and noted it to be irregular.
The rhythm is irregular, but there is a repeating pattern of long (↔) and short RR intervals (└┘). Therefore the rhythm is regularly irregular, with an average rate of 90 bpm. There appears to be group beating with 3 QRS complexes (triplet) and then a pause. There is no organized P wave before any of the QRS complexes, but there are rapid and regular atrial waveforms seen in leads II, III, and aVF (^). They are uniform in morphology, amplitude, and interval and have a rate of 300 bpm. They are negative in leads II and aVF. These are atrial flutter waves. As the atrial flutter waves are negative in leads II and aVF, this is typical (or isthmus dependent) atrial flutter. The regularly irregular rhythm is attributable to varying degrees of conduction through the AV node (ie, 2:1 alternating with 4:1 AV block [or conduction]). There are 2 different QRS complex morphologies noted. The first and third QRS complexes of the triplet (▼) are the same. They have a normal duration (0.08 sec), morphology, and axis between 0° and +90° (positive QRS complex in lead I and aVF). There are nonspecific ST-T wave changes in leads I, aVL, and V5 through V6. The QT/QTc intervals are normal (320/390 ms). The second or middle QRS complex of the triplet (+) has a prolonged duration (0.14 sec) and a right bundle-branch block (RBBB) morphology with an RSR in lead V1 and broad terminal S waves in leads I and V5 through V6. Although it might be considered that the RBBB is rate related, it can be seen that the RR interval between the first and second QRS complex (that has a RBBB morphology) is the same as the interval between the second and third QRS complex (└┘). Therefore, if this was a rate related RBBB both the second and third QRS complexes of the triplet would have a RBBB. However, only the middle QRS complex has the RBBB, and it is noted that it follows a long-short RR interval. Therefore, the RBBB aberrancy is the result of an Ashman’s phenomenon that is attributable to rate-related changes in His-Purkinje refractoriness, which are normal and physiological. When there is a slowing of the heart rate (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 as the refractoriness of the right bundle is slightly longer than that of the left bundle.
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- © 2014 American Heart Association, Inc.