ECG of the month – April 2022

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Photo caption: Lead II ECG. Paper speed: 25mm/sec, 1cm = 1mV.

A 12-year-old female spayed Westie presents to your clinic for vomiting and diarrhoea. On auscultation, her rhythm is irregular. You perform an ECG to investigate. What is your rhythm diagnosis? What further tests or treatments are necessary?

Rhythm diagnosis

There is an underlying sinus rhythm, with an average heart rate of approximately 80 beats per minute. The 2^nd, 6^th, 9^th, 13^th and 16^th p waves appear “blocked” with no corresponding QRS complex. However, every QRS complex present is preceded by a P wave. The PR interval increases with each subsequent QRS complex until a blocked P wave is seen. Therefore, this is consistent with an underlying sinus rhythm with second degree atrioventricular (AV) block, Mobitz type 1.

Second degree AV block occurs when there are more P waves than QRS complexes. This occurs if a sinus impulse becomes interrupted on its way through the AV node and bundle of His to the ventricles. Unlike third-degree AV block where there is no association between P waves and QRS complexes, in second degree AV block the interruption is not complete, and therefore some impulses are normally conducted.

There are two forms of second degree AV block. Mobitz type 1 (aka Wenckebach) occurs where there is successive lengthening of the PR interval with each heartbeat until eventually a blocked P wave occurs. Mobitz type 2 occurs when there is a fixed, unchanging PR interval throughout the rhythm.

Next steps

Mobitz Type 1 second degree AV block is usually seen due to high vagal tone and requires no treatment. However, it may indicate that diseases resulting in high vagal tone (gastrointestinal, pulmonary, neurologic) are present. Mobitz Type 2 is likely due to disease of the AV node itself and is usually pathologic.

If it is difficult to determine if the PR interval varies with beats, an atropine response test may be performed. An atropine response test should also be considered if sedation or anesthesia is planned. A positive response is considered resolution of all AV block and the development of sinus tachycardia (generally >160 bpm). An incomplete or negative response points to disease of the conduction system itself.