Page 148 - Rapid Review of ECG Interpretation in Small Animal Practice, 2nd Edition

New Holter Monitoring ECG Cases Answer 61

VetBooks.ir Electrical cardioversion of atrial fibrillation has similarities to electrical defibrillation of ventricular fibrillation
(see Case 25). Similar to ventricular defibrillation, cardioversion of atrial fibrillation is accomplished by
delivering an electrical shock to the heart, thereby depolarizing the majority of the cardiac muscle. The
myocardium becomes temporarily inexcitable, causing disruption of the arrhythmia reentry circuits and
promoting the return of sinus rhythm. Unlike ventricular defibrillation, cardioversion of atrial fibrillation is
specifically timed with the QRS complex so as to avoid shock delivery during the T wave and the relative
refractory portion of the cardiac cycle, which can induce ventricular fibrillation.
Defibrillators suitable for cardioversion of atrial fibrillation have capability to be set to a SYNC mode so
the synchronizing circuit within the defibrillator will detect the patient’s R or S waves. Newer defibrillation
units deliver biphasic shocks, which are effective at lower energy levels as compared with monophasic
shocks. During cardioversion of atrial fibrillation, the SHOCK button is pressed and held, and the unit
automatically discharges during the next detected R or S wave. When in SYNC mode, the unit displays
arrow markers that the operator can examine to ensure accurate identification of QRS complex. For
a biphasic defibrillator, the recommended initial shock energy for an external cardioversion of atrial
fibrillation using paddles or patches is 0.5–3 J/kg body weight. If the first shock is not effective, additional
single shocks of increasing energy are delivered. Shock delivery is extremely painful and the patient
with atrial fibrillation has to be anesthetized. The dog is placed in dorsal or lateral recumbency and self-
adhesive defibrillation pads or handheld defibrillation paddles are applied on opposites sides of the chest
following application of conductive paste or gel.

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