Page 101 - Rapid Review of ECG Interpretation in Small Animal Practice, 2nd Edition

Answers 33, 34 ECG Cases

Answer 33
VetBooks.ir 1 ECG 33 shows sinus arrhythmia, first-degree AV nodal block, and low amplitude QRS complexes.

2 • The heart rate is ~80 bpm. There is a prolonged PR interval of 140 ms, which is indicative of first-
degree AV block. The amplitude of the R waves is <1.0 mV in leads I, II, III, and aVF, which is
unusual for a medium or large breed dog.
• Conditions associated with low amplitude QRS complexes include a variety of cardiac and
extracardiac causes. Pericardial effusion is the most common cardiac cause of low amplitude QRS
complexes. Other causes include cardiomyopathy, cardiac neoplasia, constrictive pericarditis, and
myocarditis.
• Extracardiac causes are due to the pathology of organs and tissues surrounding the heart. Causes
include peritoneopericardial diaphragmatic hernia, pneumomediastinum, pneumothorax, pneumonia,
and obesity. This patient had a peritoneopericardial diaphragmatic hernia that was surgically
corrected as it was causing clinical signs of exercise intolerance. The first-degree AV block is an
incidental finding and of no clinical consequence.

Answer 34

1 ECG 34 shows sinus rhythm and right bundle branch block (RBBB).
2 • The heart rate is 150 bpm. The duration of the QRS is prolonged at 80 ms (normal: <40 ms).
The QRS complexes in leads I, II, III, and aVF are negative and the MEA is shifted to the right at
–120°. This shift in MEA occurs because conduction is blocked down the right bundle, so that the
depolarization travels rapidly down the left bundle branch activating the left ventricle first, followed
by right ventricular depolarization. The right ventricular depolarization is prolonged due to slower
conduction from myocyte to myocyte instead of the rapidly conducting Purkinje system. This results
in the late and dominant S wave in the aforementioned leads. There is a notch in the QRS complex,
which is commonly seen in cases of RBBB.
• RBBB occurs either as a consequence of conduction system disease in the right bundle branch or
can be an incidental finding in a healthy animal, particularly in dogs. Rarely, RBBB is associated
with severe right ventricular hypertrophy secondary to a pressure overload condition such as
pulmonic stenosis. More commonly, such conditions produce a right axis shift but without excessive
prolongation of conduction time.
• Due to the delayed activation of the right ventricle, RBBB may cause a prolonged right ventricular
ejection time, thus resulting in delayed closure of the pulmonic valve, which may be ausculted as a
split second heart sound.

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