ECG of the month – March 2022

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Six lead ECG. The top leads (I, II, III, aVR, aVL, aVF, V3) are all simultaneous recordings. The bottom, longer strip is a continuous recording of lead II. Paper speed: 25mm/sec, 1cm = 1mV.

A 4-month-old male entire Beagle presents to your clinic for routine vaccination, and a 4/6 left basilar systolic murmur is noted. As part of a cardiac screen you obtain the following ECG. What is your rhythm diagnosis? What would be your top differential for this puppy’s murmur? What are your next steps?

Rhythm diagnosis

The average heart rate is approximately 140 beats per minute. There is a p wave associated with every QRS complex and an appropriate PR interval. The QRS complex appears wide and bizarre (90ms) and is negatively deflected in lead two (characterized by a deep S wave). Therefore, this is a normal sinus rhythm conducted with aberrancy (otherwise known as a bundle branch block). Due to the negative deflection of the QRS in lead two, this ECG is consistent with a right bundle branch block.

Causes

In the normal conduction system, impulses start at the sinoatrial node, travel across the atria, through the atrioventricular node and bundle of His, then rapidly down the right and left bundle branches. If one of these bundles is disrupted or damaged, the impulse travels more slowly, resulting in widening of the QRS complex (since the x axis of the ECG is time). Bundle branch block can be differentiated from a rhythm of ventricular origin by the presence of a p wave associated with every beat; these impulses are still originating from the sinoatrial node but have a disruption further down in the conduction system.

Bundle branch block can be due to congenital defects, acquired structural heart disease, or degenerative/inflammatory conduction disorders. Given this puppy’s breed, murmur location and evidence of right bundle branch block (implicating that the right side of the heart is affected), pulmonic stenosis would be the top differential.

Next steps

No treatment is necessary for a sinus rhythm with a bundle branch block. However, as it is often a sign of underlying cardiac disease, further investigation (thoracic radiographs, echocardiography) is warranted.