ECG of the Month – January 2022

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

A 9-year-old Cockapoo presents to your clinic for exercise intolerance and bradycardia is noted on physical examination. An ECG is performed. What is your diagnosis? What are your next steps?

Diagnosis

This ECG is consistent with third degree atrioventricular block (AVB). Third degree AVB occurs when atrial impulses are not conducted to the ventricles; therefore, P waves are present but not reasonably related to QRS complexes. In this dog, the PR interval varies with every beat, and P waves are seen in the ST segment or superimposed onto T waves. The escape rhythm seen with third degree AVB may be ventricular as in this case (characterized by wide/bizarre complexes at a rate of approximately 20-40bpm) or junctional (narrow complexes with a rate of approximately 40-60bpm).

Third degree AVB occurs most commonly due to idiopathic degeneration of the AV node, though can also be caused by infection (endocarditis), inflammatory processes, or neoplasia.

Next steps

Dogs with third degree AVB should be referred emergently for pacemaker placement as they are at a significant risk of sudden death (even if they remain asymptomatic). Dogs with third degree AVB may also experience syncope, exercise intolerance, lethargy, or congestive heart failure.

Unfortunately, medical management of third degree AVB with drugs to increase heart rate (such as theophylline) is usually unsuccessful. This rhythm should never be treated with lidocaine, as it could suppress the escape rhythm and result in worsening bradycardia or ventricular arrest. If congestive heart failure is present, therapy should be initiated.

Interestingly, cats with third degree AVB behave somewhat differently than dogs. The ventricular escape rate in cats is higher (approximately 120bpm), and they are at a lower risk of sudden death. Therefore, unlike dogs, cats often do not require pacemaker placement unless evidence of hypoperfusion (typically renal) or clinical signs arise.