Page 59 - Rapid Review of ECG Interpretation in Small Animal Practice, 2nd Edition

Answers 1, 2 ECG Cases

Answer 1
VetBooks.ir 1 ECG 1 shows sinus rhythm with low amplitude QRS complexes and electrical alternans.

2 • The heart rate is ~145 bpm. The P wave amplitude is normal. The QRS amplitude in all leads is
<1.0 mV which is unusual for a large breed dog. The R wave height varies slightly with every other
beat, most easily appreciated in leads II and aVF. Beat to beat variation in QRS complex amplitude or
configuration is called electrical alternans.
• Low amplitude QRS complexes and electrical alternans are most commonly found in association
with pericardial effusion. These findings are not extremely sensitive, but fairly specific for pericardial
effusion. Dampening of the electrical signal by the pericardial effusion decreases the QRS amplitude,
and swinging of the heart in the fluid-filled pericardial sac is considered the origin of the electrical
alternans.
• Large volumes of pericardial effusion impair cardiac filling, resulting in cardiogenic shock
(pericardial tamponade), and require pericardiocentesis and often intravenous fluid administration.
In this patient, pericardial effusion was confirmed by echocardiography and a mass was visualized
originating at the right atrium. The right atrial mass is most likely a hemangiosarcoma, which is
commonly diagnosed in older, large breed dogs with pericardial effusion.

Answer 2

1 ECG 2 shows either SVT or sinus tachycardia.
2 • The average heart rate is 200 bpm. The rhythm is regular.
• There is a P wave (blue arrow), QRS complex (black arrow), and T wave (green arrow) for every
beat.
• The rapid heart rate causes the P wave to be partially obscured by the preceding T wave.
• The QRS complex morphology is normal indicating a supraventricular origin, which could be ectopic
(SVT) or originating from the sinus node (sinus tachycardia).
• Differentiation between the two rhythms can sometimes be difficult. SVT is characterized by sudden
(paroxysmal) stopping and starting of the tachycardia, electrical alternans (cyclical variation in
the amplitude of the QRS complex), changes in P wave morphology, persistence despite sedation
or alleviation of patient stress, and underlying myocardial or valvular disease. In contrast, sinus
tachycardia is usually associated with gradual acceleration and deceleration of the tachycardia,
disappearance when patient stress is alleviated, and normal underlying heart function.
• Labrador Retrievers are particularly prone to SVT and may require additional diagnostic procedures,
such as echocardiography or 24-hour ambulatory (Holter) monitoring, as well as medical therapy to
slow the heart rate (i.e., beta-blockers, calcium-channel blockers, digoxin).

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