ECG Challenge: A 72-year-old man with a history of chronic obstructive pulmonary disease presents with an increase in shortness of breath that began 2 weeks ago after experiencing an upper respiratory infection presenting with a cough productive of yellowish sputum, pleuritic chest pain, and a low grade fever with temperatures of up to 100°. He did not seek medical attention, but because of worsening shortness of breath he presented to the emergency room. On physical examination he has bilateral rhonchous sounds and wheezing. A chest x-ray showed evidence of chronic obstructive pulmonary disease with hyperaeration and fibrous streaking. His O2 saturation was 88% on room air. He was placed on supplemental O2 via a nasal cannula at 4 L/min. He received albuterol therapy via a nebulizer. Shortly after his first treatment he was noted to have a transient increase in heart rate and an ECG was obtained.
The first 4 QRS complexes have a rate of 100 bpm. Each QRS complex is preceded by a P wave (+) that is positive in leads I and II, and there is a PR interval of 0.14 sec. The third QRS complex is slightly premature and is preceded by a P wave with a different morphology (*). This is a sinus tachycardia, and there is a single premature atrial complex (ie, the third complex). The fifth QRS complex (^) is also premature and is preceded by a premature P wave (▼), which is identified by the positive waveform noted at the end of the QRS complex. This positive waveform is not seen in the initial sinus QRS complexes. The PR interval associated with this premature atrial complex is 0.20 sec, which is longer than the sinus PR interval. There is another P wave after this QRS complex (↓) and the PR interval with this P wave is also 0.20 sec. The rate of these two premature P waves (▼,↓) is 210 bpm, which is identical to the rate of the tachycardia that follows. After this premature atrial complex there is a regular tachycardia at a rate of 210 bpm. It can be seen that there are P waves associated with each QRS complex, most obvious in leads V1 through V3 (↑), and the PR interval is 0.20 sec, identical to the PR interval of the fifth complex, which is the first complex of the tachycardia. Importantly, the PR interval associated with the tachycardia (ie, 0.20 sec) is longer than the PR interval when the rate is slower (ie, 0.14 sec). This is not a sinus tachycardia which would have the same or a slightly shorter PR interval when compared to a slower rate. Therefore this is an atrial tachycardia. All of the QRS complexes of the tachycardia, except for the first 9 (▲), are identical to the initial sinus complexes. They have a normal duration (0.08 sec) and morphology. There is an extreme left axis between -30° and -90° (positive QRS complex in lead I and negative QRS complex in leads II and aVF with an rS morphology). There is therefore a left anterior fascicular block. The first 9 QRS complexes of the tachycardia are wide (duration=0.12 sec), and they are aberrated with a morphology of a right bundle branch block (ie, they have a broad R wave in lead aVR). The aberration is not rate related, because the rate of these complexes is identical to the rate of the narrow complexes that follow. The right bundle branch block is likely a result of the Ashman phenomenon because there is a long–short RR interval that precedes these aberrated complexes. The Ashman phenomenon is attributable to normal physiologic rate-related changes in the refractory period of the His-Purkinje system. 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 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. The presumed reason for the persistence of the right bundle branch block for several QRS complexes is that there is retrograde penetration of the right bundle from the impulse coming from the left bundle; this maintains the longer refractory period of the right bundle, maintaining the right bundle branch block aberrancy for several cycles.
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- © 2014 American Heart Association, Inc.