ECG Challenge: An 80-year-old man with a history of a nonischemic dilated cardiomyopathy and stable hemodynamics presents to his cardiologist for a routine physical examination. His pulse is noted to be slow, and an ECG is obtained.
The first part of the ECG shows a regular rhythm at a rate of 34 bpm. The last 3 QRS complexes have a regular rate of 60 bpm (└┘). All of the QRS complexes are identical with an increased duration (0.16 second) and have a morphology of a right bundle-branch block with a broad R wave in lead V1 (→) and a broad terminal S wave in leads I and V5 and V6 (←). The axis is extremely left between −30° and −90° (positive QRS complex in lead I and negative QRS complex in leads II and aVF). The 2 reasons for an extreme left axis are an old inferior wall myocardial infarction with a deep Q wave in leads II and aVF and a left anterior fascicular block with an rS morphology in leads II and aVF. This is a left anterior fascicular block. The presence of a right bundle-branch block and a left anterior fascicular block is called bifascicular block. The QT/QTc intervals are normal (560/420 and 500/380 milliseconds when corrected for the prolonged QRS complex duration). There is a P wave before each QRS complex (+) with a stable PR interval (0.22 second; ^). The P wave is positive in leads I, II, aVF, and V4 through V6. Hence, this is a sinus rhythm with a first-degree atrioventricular (AV) block or first-degree AV conduction delay. A second P wave of the same morphology, without an associated QRS complex (nonconducted P wave), can be seen after each QRS complex (*). The presence of an occasional on-time but nonconducted P wave is defined as a second-degree AV block. Because every other P wave is nonconducted, this is 2:1 AV block. This may be either a Mobitz I or a Mobitz II. The presence of bifascicular disease and a first-degree AV block has often been referred to as trifascicular disease. This is further supported by the presence of 2:1 AV block. However, the cause of the first- and second-degree AV block with 2:1 AV conduction may be a conduction abnormality of either the AV node or the His-Purkinje system. Hence, it cannot be called trifascicular block or disease unless the cause of the first- and second-degree AV block is established. In addition, the PP intervals during the 2:1 AV block are not constant. There is a repeating pattern with the PP interval surrounding the QRS complex (┌┐) being shorter than the PP interval without a QRS complex (↔). This is called ventriculophasic arrhythmia, and it may be seen whenever there is 2:1 AV block or complete heart block. This is attributable to the fact that ventricular contraction results in an acceleration of the sinus rate as a result of enhancement of sinus node impulse generation as a result of pulsatile blood flow through the sinus node artery with ventricular contraction, enhancement of sinus node impulse generation as a result of stretch of the right atrium with ventricular contraction, or enhancement of sinus node impulse generation as a result of a changes in baroreceptor activity
The reason for the 2:1 AV block cannot be established unless there is a pattern of conduction, that is, 2 sequentially conducted P waves. There is a P wave before each of the last 3 QRS complexes (+) with a stable PR interval (0.22 second) that is the same as all of the other PR intervals (^). No nonconducted P waves are seen, and the PP interval is constant (└┘). Hence, there is 1:1 AV conduction. Because all of the PR intervals are the same, the 2:1 AV conduction is Mobitz type II.
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- © 2015 American Heart Association, Inc.