Page 131 - Rapid Review of ECG Interpretation in Small Animal Practice, 2nd Edition

Answer 54 New Advanced-Level ECG Cases

Answer 54
VetBooks.ir 1 The heart rate during the majority of the recording is 275 bpm. The QRS complex is narrow and

positive in lead II and appears to be of supraventricular origin. Thus, supraventricular tachycardia is
the most likely ECG diagnosis. There are small negative deflections in the ST segment of each QRS
complex that are likely a retrograde P wave (P’, arrows) and best seen in leads II and III. Toward the
end of the recording, the supraventricular tachycardia is suddenly terminated and a normal sinus beat
occurs after a pause (see Box). The presence of the P’ wave and its close proximity to the preceding
QRS complex is highly suggestive of a re-entrant mechanism due to an accessory pathway between
the atrium and ventricle. These types of supraventricular arrhythmias are most commonly detected
in Labrador Retrievers and are the result of an abnormal band of conduction tissue that provides a
second connection between the atrium and ventricle in addition to the AV node (see boxed text below).
2 The rapid heart rate during supraventricular tachycardia can cause weakness, exercise intolerance, and
syncope. Treatment consists of blocking the re-entrant circuit at the AV node using calcium channel
blockers, beta-blocker, or digoxin, and at the accessory pathway using sodium-channel or potassium-
channel blockers. Catheter-based radiofrequency ablation of the accessory pathway can also be
performed but is limited to a small number of centers worldwide.

Re-entrant supraventricular tachycardia due to accessory pathways
(A) Accessory pathways involve conduction tissue between the atrium and ventricle independent of the AV
node. During the most common type of supraventricular tachycardia associated with accessory pathways,
an impulse travels from the atrium across the AV node and into the ventricle (1), resulting in a normal
appearing QRS complex (2). The ventricular action potential is then able to conduct from the ventricle
back into the atrium, using the accessory pathway, which depolarizes the atrium in a retrograde fashion
and produces a retrograde (i.e., negative) P wave within the ST segment of the preceding QRS complex
(3). The impulse then re-enters the AV node (4) and the cycle repeats itself producing a supraventricular
tachycardia (B). The re-entry circuit involves both the AV node and the accessory pathway, and these two
areas represent potential targets to disrupt and terminate the tachycardia using antiarrhythmic drugs or
catheter-based radiofrequency ablation.

A QRS
P’ T wave
4 3
1
P’ AV node Accessory
Atrium pathway

Ventricle

2

QRS
B

P’ P’ P’ P’

118