What Do You Think 39

I reported this ECG recently.

I have no clinical information.

What do you think?

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The most obvious abnormality that hits the eye is a prominent delta (Δ) wave, particularly in lead II (red highlight).

This ECG appearance is referred to as pre-excitation and the relationship to atrial tachyarrhythmias was first reported in young adults by Wolff, Parkinson andWhite (WPW) in 1930. It is due to a congenital accessory pathway between the atria and ventricles, thus bypassing normal AV conduction and can be associated with a very small risk of sudden cardiac arrhythmic death.  

In the normal heart, the fibrous skeleton acts as an electrical barrier to conduction from atrium to ventricle, thus directing cardiac conduction to the ventricle via theAV node and conducting system. The AV node also acts as a gatekeeper delaying conduction via its refractory period, to allow optimal ventricular filling and this represents most of the PR interval on the ECG. A congenital, thin filamentous muscular breach of this fibrous skeleton is referred to as the bundle of Kent and is an accessory conduction pathway or bypass tract from atrium to ventricle. It may conduct to the ventricles faster than via the AV node, and thus on the ECG the PR interval is shortened. If the conducting system is bypassed, then a Δ wave appears prior to the commencement of the QRS. Pathways can be left and right sided, anterior, posterior, or septal. Accessory pathway conduction may be intermittent.

When compared to a conventional P-QRST, there is a:

• Short PR interval <120 ms, suggesting bypass of the AV node (red arrow).
• Δ wave or slurring of the initial QRS, denoting the passage of impulse through the accessory pathway (red highlight).
• The QRS is widened, often > 110 ms (blue arrow).
• Tall voltages (green arrow) and occasional Q waves (negative Δwave).
• ST/T wave changes, with the T wave usually discordant with the Δ wave.  (Remember abnormal depolarization results in abnormal repolarization).

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Here is a summary of the bypass tracts:

On the 12-lead ECG, the dipoleor axis of the Δwave, assists in determining the site of the accessory pathway.

With our case study,

This would be consistent with an antero-septal or mid septal pathway, adjacent to the AV node.

Another example:

The other abnormality on the ECG is atrial flutter.

The P waves are at a cycle length of 200 ms (300 bpm) (yellow highlight).

As with most cases of atrial flutter, the P waves are best seen in leads II, III and V1.

For the purpose of ECG diagnosis, atrial flutter is a tachyarrhythmia defined by an atrial rate of about 200 to 350 bpm and characterized by uniformly shaped, biphasic, saw-tooth oscillations, which often obliterate the iso-electric baseline.

There can also be conventional shaped P waves, without baseline obliteration

The ventricular response is usually 2:1 AV block with a ventricular rate of 150 bpm. Remember the golden rule: A narrow QRS tachycardia, rate ~150 bpm is atrial flutter with 2:1 block until proven otherwise”.

Atrial flutter can be life threatening when there is 1:1 conduction.

Atrial flutter with 2:1 conduction may degenerate into 1:1 conduction with an antiarrhythmic, which slows the flutter rate, but does not impair AV conduction. In our case with pre-excitation, there is 2:1 AV block, where 1:1 conduction might be expected. However, not all accessory pathways conduct at very high rates (lazy pathway) nor do all result in re-entry tachycardias.

Another explanation, however, needs to be considered.

Pre-excitation conduction maybe through an obscure bypass tract, which is actually below the AV node called a fasciculoventricular pathway. Most septal pathways are referred to as Mahaim or atrio-fascicular bypass tracts. The fasciculoventricular pathway is more distal with the proximal insertion immediately below the AV node in the AV bundle with the distal insertion into ventricular muscle.

In summary, with conduction through a fasciculoventricular pathway:

• The ECG appearance mimics the antero-septal or mid septal pathways.
• Not surprisingly, the Δ wave is smaller and the PR interval longer.
• There are other reported differences in V1.‍
• The pathway only conducts anterograde and thus re-entry tachyarrhythmias have not been reported.‍
• No need therefore to ablate.

In our case study, the Δ wave is prominent, but thePR interval is long. However, the PR interval with atrial flutter can be long and maybe this would explain the lengthening.

You make your own diagnosis, septal or fasciculoventricular pathway?

Harry Mond