What Do You Think 64

This ECG is from Pantea, CardioScan Melbourne.

Thanks Pantea.

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27-year old female with recent onset of tiredness whilst jogging.

Her mother was diagnosed with systemic lupus erythematosus during pregnancy.

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ECG two minutes into jogging.

What do you think?

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Many of you will know the answer just reading the second line.

The diagnosis is sinus tachycardia (110 bpm) with isolated congenital AV block, There is 3:2 Wenckebach as well as 2:1 AV block.

Congenital AV (heart) block may occur in the developing foetus as a result of structural congenital heart disease called complex congenital AV block, whereas, if present in a structurally normal heart, it is isolated congenital AV block. The latter is often seen as a complication of maternal auto-immune disorders and in particular, systemic lupus erythematosus (SLE) and is referred to as autoimmune AV block. A pregnant woman with SLE, whether symptomatic or not,  who has anti-Ro/SS-A and/or anti-La/SS-B autoantibodies may passively transfer these antibodies across the placenta and in the developing heart can result in selective damage to the foetal cardiac conduction system at the AV junction. There may be other cardiac manifestations of SLE seen afterbirth, called cardiac neonatal lupus.

In the foetus/neonate with AV block, an inappropriate slow or irregular heart rate with pauses may be present before or at any time after birth. Although the neonate may have congenital complete AV block at birth requiring permanent pacing, it may not develop until much later in life and even then the patient may not be symptomatic having  with exertion, respectable physiologic heart rate changes from the junctional focus.In the interim, these patients may demonstrate Wenckebach AV block on the ECG, which may become symptomatic with exertion.  

With complete AV block, the subsidiary pacemaker is junctional with a narrow QRS.

Sinus rhythm (80 bpm) with complete AV dissociation and a junctional rate 46 bpm.  

Conducted beats if present, are near-identical in configuration to the junctional focus.  

Congenital complete AV block with a narrow junctional pacemaker. AV conduction occurs when the P wave falls in the supernormal phase of conductivity, which is at the end of the previous T wave (see What do you think 60).

The ECG in the case study shows both 3:2 and 2:1 AV block. Whenever 2:1 AV block is seen on the ECG, there is concern that this may be Mobitz type II AV block. What is not understood is that 2:1 AV block is frequently part of the Wenckebach spectrum.

Varying Wenckebach AV block sequences during a Holter monitor study. Over a short period of time, 4:3, 3:2 and 2:1 sequences are seen.

Can Wenckebach and Mobitz typeII block occur together?  

Although reported in the literature, these two forms of second degree AV block are usually from different sites in the cardiac conduction system and the aetiologies differ so the combination would be very rare. Therefore, when 2:1 AV block is seen on a Holter monitor, other sequences of Wenckebach AV block should be searched for.

In summary:

Wenckebach AV block

• Progressive lengthening of the PR interval with a dropped beat terminating the sequence.
• Predominantly AV nodal.
• Usually narrow QRS.

Mobitz type II block.

• PR interval remains constant with dropped beat(s).
• Distal conducting system.
• Often broad QRS unless intra-His.

On Holter monitoring, the most common presentation of Wenckebach AV block is vagal mediated. It is seen in about 6% of all Holter monitor studies, generally nocturnal and at all ages, but particularly in the young. It is frequently associated with sinus slowing during the sequence and pause.

In the elderly, Wenckebach AV block may reflect AV nodal degenerative disease. The features are:

• Established first degree AV block.
• Wenckebach AV block at any time.
• If distal conduction tissue disease, a bundle branch block is present.

Sinus rhythm, first degree AV block with the shortest PR interval 260 ms (yellow highlight) and Wenckebach sequences including 2:1 AV block, at any time (red highlight). Note that the P-P intervals containing a QRS (860 ms) are shorter than the P-P intervals with no QRS (960 ms).

This is ventriculophasic sinus arrhythmia which is physiologic and often overlooked when interpreting ECGs. It is simply variations in the sinus rate, depending on whether there are QRS complexes embedded between sinus cycles. The changes in the sinus rate are seen in about 50% of cases of second degree or complete AV block and is reflex mediated through intracardiac pressures and volumes.

Another example of Wenckebach AV block in an elderly patient with associated marked first degree AV block (440 ms).

The normal physiologic upper atrial rate limit that the AV node will conduct to the ventricle is about 200bpm, although higher in the young. Thus an atrial tachycardia of about 180 bpm in an adult may conduct 1:1 to the ventricle, whereas an atrial flutter at 300bpm, conducts with 2:1 AV block.

With AV nodal disease, Wenckebach AV block may occur at lower levels and seen with exertion.

An elderly patient with a resting bundle branch block demonstrates Wenckebach AV block (red highlight)during exertion with a sinus rate of only 110 to 120 bpm. This block may be eitherAV nodal or infra-nodal.  

With our case study, the aetiology is different again. The patient is young, and the cause of the progressive block is a congenital developmental malformation in the proximal conducting system as a result of antibody damage during the developing foetal heart.  

Harry Mond