ECG Challenge: Slow Pulse on Routine Exam

Philip J. Podrid, MD

Disclosures

April 18, 2022

The correct diagnosis is sinus rhythm, complete heart block, junctional escape

Figure 2.

Discussion

There is a regular rhythm at a rate of 48 beats/min. The QRS complex duration is normal (0.08 sec), with a normal morphology and axis between 0° and 90° (positive QRS complex in leads I and aVF).

The QT/QTc intervals are normal (440/390 msec). P waves occur (+) but with no relationship to the QRS complexes; ie, there are variable PR intervals. This represents AV dissociation.

The P waves are positive in leads I, II, aVF, and V4-V6, indicating an underlying sinus rhythm. Most of the PP intervals are constant (⊔), with a rate of 75 beats/min.

There are two etiologies for atrioventricular (AV) dissociation: complete (third-degree) heart block, in which the atrial rate is faster than the rate of the QRS complexes, or an accelerated lower pacemaker (ie, junctional or ventricular), in which the atrial rate is slower than the rate of the QRS complexes. This is therefore complete heart block and the escape rhythm is junctional.

The etiology of the escape rhythm is based on the QRS complex morphology, not the rate of the escape rhythm. The morphology in this case is normal. Three longer PP intervals occur (↔). Each of these longer PP intervals is associated with a QRS complex (third, fourth, and sixth; [v]) that has a slightly different morphology due to a negative deflection that is seen after these QRS complexes (^). This represents a retrograde P wave, resulting from VA conduction associated with the junctional complex. Because of retrograde atrial activation, the sinus node is reset, accounting for the long PP interval. Although there is complete AV block, VA conduction may be seen in up to 40% of cases. This may be due to:

  • Different antegrade and retrograde AV nodal conduction parameters;

  • Dual AV nodal pathways, one of which is capable of VA conduction; or

  • Presence of a concealed bypass tract, which only conducts in a retrograde conduction.

 The most likely explanation is a concealed bypass tract.

 In addition, two premature P waves (*) have a morphology different from the sinus P waves. These are premature atrial complexes that are not conducted — that is, blocked premature atrial complexes.

Philip J. 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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