ECG Challenge: A 64-year-old woman with a history of hypertension for which she is taking metoprolol, diltiazem, and hydrochlorthiazide presents to her primary care physician with complaints of fatigue that have been present for 2 to 3 days. Her blood pressure is normal, but her pulse rate is noted to be slow. An ECG is obtained.
There is a regular rhythm at a rate of 38 bpm. The QRS complex has an increased duration (0.12 sec) with a right bundle-branch block morphology (ie, there is an RSR' in lead V1 [←] and a terminal broad S wave in leads I and V5–V6 [→]. The axis is normal between 0° and +90° (positive QRS complexes in leads I and aVF). There are minor ST-T wave changes in leads V1–V3, consistent with the right bundle-branch block. The QT/QTc intervals are normal (520/415 ms and 500/400 ms when the prolonged QRS complex duration is considered). There is a P wave before each QRS complex (+) with a stable PR interval (0.20 sec). The P waves are positive in leads I, II, aVF, and V4–V6; hence this is a sinus rhythm. After each QRS complex there is second P wave (^), with the same morphology. It is not followed by a QRS complex (ie, it is nonconducted). The presence of an occasional nonconducted P wave defines second degree AV block, and there is a repeating pattern of every other P wave being nonconducted. Because it cannot be established whether this is a Mobitz type I or II second degree AV block, this is termed 2:1 AV block. The average sinus rate is 72 bpm (normal sinus rhythm), but the PP intervals are not completely regular. There is a repeating pattern to the PP intervals. The PP interval surrounding the QRS complex (└┘) is slightly shorter (0.80 sec) than the PP interval without the QRS complex (0.88 sec; ↔). This is termed ventriculophasic arrhythmia, which may be seen with 2:1 AV block or complete heart block. It is attributable to changes in sinus node automaticity related to ventricular contraction and may be the result of:
sinus node artery pulsatile blood flow with ventricular contraction which enhances sinus node automaticity
stretch of the right atrium with ventricular contraction which enhances sinus node automaticity
changes in carotid sinus baroreceptor inputs into sinus node automaticity
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- © 2014 American Heart Association, Inc.