Page 73 - Rapid Review of ECG Interpretation in Small Animal Practice, 2nd Edition

Answers 12, 13 ECG Cases

Answer 12
VetBooks.ir 1 ECG 12 shows atrial fibrillation (AF) and left ventricular enlargement.

2 • The heart rate ranges from 130 bpm to 272 bpm, with the average ~220 bpm. The rhythm is
irregularly irregular.
• There are no consistent P waves noted; however, despite the lack of P waves, the QRS complexes are
narrow and appear to be of supraventricular origin.
• The amplitude is 1.3 mV (normal <0.9 mV) indicating left ventricular enlargement. Note that the
caudal limb leads III and aVF, which are similar in orientation with lead II, also display QRS complex
amplitudes that are relatively increased, further supporting left ventricular enlargement.
• The irregularity, lack of P waves, and supraventricular QRS complexes are all findings consistent with
AF.
• In cats, AF is usually due to severe underlying heart disease, and further diagnostics such as
radiographs and echocardiography are indicated.
• Treatment of AF involves slowing the ventricular rate by decreasing conduction through the AV node
with medications such as beta-blockers, calcium-channel blockers, and/or digoxin.

Answer 13

1 ECG 13 shows sinus rhythm with a left bundle branch block (LBBB).
2 • The heart rate is 100 bpm. The QRS complexes look wide and bizarre, and the duration is prolonged
at 100 ms. The MEA is normal. The rhythm is sinus in origin as evidenced by the fact that there are
P waves preceding each QRS complex, with a regular PR interval of 140 ms. The P wave morphology
changes slightly with heart rate, indicating a wandering pacemaker. The PR interval is slightly
prolonged, consistent with first-degree AV block. The ST segment is depressed and slurred,
which is suggestive of myocardial hypoxia.
• The diagnosis of a LBBB is made based on sinus QRS complexes with a normal MEA and increased
duration of the QRS (>80 ms in a dog, >50 ms in a cat). The wide QRS duration is caused by
complete disruption of the left bundle branch and delayed depolarization of the left ventricle.
• The QRS complex morphology of LBBB mimics that of a VPC. The key feature discriminating
between LBBB and VPCs is the presence of P waves associated with each QRS, signifying the QRS as
supraventricular in origin. If the heart rate is very rapid, the P waves may be hidden in the preceding
T waves, and the rhythm can be difficult to discern from VT.
• LBBB almost never occurs by itself as a benign abnormality. Rather, it occurs secondary to left
ventricular myocardial disease (cardiomyopathy, mitral valve disease) or degenerative conduction
system disease. While the LBBB does not require treatment per se, the presence of underlying cardiac
disease results in a relatively poor prognosis. The finding of LBBB should prompt further diagnostics
such as echocardiography to search for an underlying cardiac disease.

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