Page 95 - Rapid Review of ECG Interpretation in Small Animal Practice, 2nd Edition

Answers 28, 29 ECG Cases

Answer 28
VetBooks.ir 1 ECG 28 shows a normal sinus rhythm and first-degree AV nodal block.

2 • The heart rate is 70 bpm. The P waves and QRS complexes are normal in morphology. The PR
interval is 160 ms (normal: 60–130 ms) and consistent with a diagnosis of first-degree AV block.
• First-degree AV block has no clinical sequelae unless it progresses to a more advanced form (i.e.,
second- or third-degree AV block). The exact etiology of AV block is often undetermined but can
be due to high resting vagal tone, degeneration or infiltration of the AV node, or infectious or
inflammatory disease. A first-degree AV block can also be an incidental finding in a healthy dog.
Drugs that reduce or slow AV nodal conduction such as beta-blockers, digoxin, or calcium-channel
blockers can exacerbate this conduction delay and result in more advanced forms of AV block.
Opioids increase vagal tone and it might be best to avoid these drugs during the anesthesia protocol.

Answer 29

1 ECG 29 shows atrial fibrillation (AF).
2 • The average heart rate is ~210 bpm. The rhythm is irregularly irregular and there are no P waves.
The QRS complexes are narrow and of supraventricular morphology. The baseline displays small
atrial fibrillatory waves (f waves).
• AF usually occurs in the presence of advanced cardiac disease, such as mitral valve disease or dilated
cardiomyopathy. In these instances, the ventricular rate is typically rapid, thus requiring medications
(e.g., digoxin, diltiazem, and/or atenolol) to slow conduction through the AV node and slow the
rate. Spontaneous conversion back to sinus rhythm is rare. In some large breed dogs, AF can occur
without any identifiable underlying cardiac disease (lone AF). In this instance, the ventricular rate is
usually slow and might not require medication. Conversion of lone AF back to sinus rhythm using
electric cardioversion (see ECG 5, p. 50) or antiarrhythmic medications (amiodarone) is possible.
• The key points are the irregularity of the ventricular rhythm, lack of P waves, the normal morphology
of the QRS complexes indicating a supraventricular origin, and the rapid ventricular rate.

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