The correct diagnosis is sinus rhythm with 2:1 AV block due to Mobitz type II; ventriculophasic arrhythmia (Figure 2).
The rhythm is regular at a rate of 40 beats/min, although the fifth QRS complex (↑) is early. The QRS complexes are within the upper limits of normal (0.10 sec) from a minor intraventricular conduction delay. There is a physiologic left axis (about -30 degrees) with a positive QRS complex in lead I, biphasic QRS complex in lead II, and negative QRS complex in lead aVF.
P waves occur before each QRS complex (+), and the PR interval is stable (0.16 sec). A second nonconducted P (*) wave is seen after each QRS complex. It has the same morphology as the P wave before the QRS complexes. Hence, a sinus rhythm is present at a rate of 80 beats/min. As every other P wave is nonconducted, this is a second-degree AV block termed 2:1 block. It may be either Mobitz I (Wenckebach) or Mobitz II.
As indicated, the fifth QRS complex (↑) is early, and it is preceded by an on-time sinus P wave (^). The PR interval associated with this QRS complex is 0.16 sec, the same as all the other QRS complexes. Hence, the 2:1 AV block is a Mobitz type II.
In addition, the PP intervals differ slightly: The PP interval surrounding the QRS complex is slightly shorter (⊔) than the PP interval without the QRS complex (↔). This is termed ventriculophasic arrhythmia and is a normal physiologic finding. It may occur with either 2:1 AV block or complete heart block. There are three proposed causes:
With ventricular contraction there is pulsatile blood flow through the sinus node artery which enhances sinus node automaticity;
With ventricular contraction there is stretching of the right atrium that enhances sinus node automaticity; or
A baroreceptor effect due to ventricular contraction and a stroke volume.
Philip Podrid, MD, is an electrophysiologist, a professor of medicine and pharmacology at Boston University School of Medicine, and a lecturer in medicine at Harvard Medical School. Although retired from clinical practice, he continues to teach clinical cardiology and especially ECGs to medical students, house staff, and cardiology fellows at many major teaching hospitals in Massachusetts. In his limited free time he enjoys photography, music, and reading.
You can follow Dr Podrid on Twitter @PPodrid
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