Page 123 - Rapid Review of ECG Interpretation in Small Animal Practice, 2nd Edition

Answer 50 New General-Level ECG Cases

Answer 50
VetBooks.ir 1 The ventricular rate is 150 bpm. The P waves do not appear to be related to the QRS complexes. At the

beginning of the recording the P waves (arrows) are in front of the QRS complexes but “drift” closer
to, and then into and past, the QRS complexes in the middle and end of the recording (ECG 50b).
Thus, there is no relationship between the P wave and QRS complexes indicating that two different
independent pacemakers are driving the atria and ventricles. In the case of the atria, it is most likely
that the sinus node is controlling the atrial rate whereas the normal QRS morphology suggests that a
pacemaker located in or near the AV nodal junction is controlling the ventricles at a rate similar to the
sinus node. The ECG is best described as isorhythmic atrioventricular dissociation.
2 Isorhythmic atrioventricular dissociation is typically benign as the junctional rate approximates the
normal sinus rate. Treatment of isorhythmic AV dissociation is typically not required. In the authors’
experience, the rhythm most commonly occurs in cats and is seen during anesthesia, in cases of
systemic disease, and, occasionally, in cases of underlying heart disease.

50b
I

II

III

aVR

aVL

aVF

110