Page 45 - Rapid Review of ECG Interpretation in Small Animal Practice, 2nd Edition

Treatment of Arrhythmias

used antiarrhythmic BBs. Digoxin has a mild In most cases, ventricular rate control via slowing of
vagomimetic effect and can indirectly contribute
AV node conduction with diltiazem and or digoxin is
VetBooks.ir to prolonging AV node conduction time; the primary goal (Table 4.1). For clinically unstable
dogs (i.e., those with overt weakness, syncope, or
however, its antiarrhythmic utility is primarily
limited to the treatment of AF. congestive heart failure [CHF]) and/or those with
2. Atrial or ventricular myocardium: HR >250 bpm, IV therapy to slow the ventricular
Depolarization in these tissues is sodium- response quickly, using diltiazem IV bolus, followed
channel driven, and repolarization primarily by a constant-rate infusion and subsequent titration
involves the potassium channel. To treat to oral medications, is often performed.
arrhythmias that originate from the atrial In clinically stable dogs with AF, oral therapy
and ventricular myocardium, sodium-channel is started using diltiazem and digoxin. Extended-
blockers, potassium-channel blockers, or release diltiazem (Dilacor-XR) has the advantage of
combinations thereof are often used in twice daily dosing over diltiazem modified release,
conjunction with BBs. The sodium-channel which requires 3 times daily dosing. An effect on HR
blockers commonly used for treatment of typically occurs within a few hours of oral dosing.
arrhythmias in dogs are lidocaine (IV only), Only mild effects on contractility are seen with
mexiletine, and, rarely, procainamide. The most diltiazem when given at prescribed doses. Diltiazem
important potassium-channel blocker is sotalol, is often co-administered with digoxin. Atenolol can
which also has BB properties. Amiodarone, also be used, but due to its potential to decrease
which is predominantly a potassium-channel contractility, particularly in dogs with poor systolic
blocker but also has potent sodium-channel function or in heart failure, initial doses should be
blocker and some CCB and BB activity, is relatively low and then subsequently titrated as
occasionally used for arrhythmias that are needed. The electrocardiographic goal of chronic
particularly difficult to control. AF therapy is a 24-hour-average heart rate of
<125 bpm as determined by post-treatment Holter
TREATMENT OF ARRHYTHMIAS recordings. Rate control for AF in cats is attempted
Treatment of supraventricular arrhythmias with dilacor-XR monotherapy (for drug dosages,
Supraventricular arrhythmias (SVA) include rhythms see Table 4.1). Due to poor bioavailability the oral
that originate in the sinus node, atrial tissue, and AV dose is much higher in cats than in dogs.
junction. Physiologic sinus tachycardia must be dif- Dogs with AF but little to no underlying heart
ferentiated from SVA and is typically caused by sys- disease (often referred to as “primary” or “lone”
temic disease states associated with fever, anemia, AF) are candidates for electric cardioversion of AF
heart failure, adrenergic medications, or anxiety and to sinus rhythm while under anesthesia.
pain. As such, correcting the underlying cause can re- Electrical cardioversion refers to a synchronized
sult in normalization of the heart rate. delivery of an electrical shock to the heart, which
The decision regarding how and when to treat a depolarizes the majority of the cardiac muscle at
supraventricular arrhythmia is based on presence one time. The myocardium becomes temporarily
and frequency of the clinical signs, such as fainting inexcitable, causing disruption of the AF, thus
or co-existing congestive heart failure. Emergency promoting the return of sinus rhythm. Shock
management using intravenous drugs may be delivery must be timed with the QRS complex
indicated before a 24-hour Holter recording can be (i.e., synchronized) to avoid delivery during the
obtained. Both diltiazem and esmolol are available vulnerable T wave segment, which could induce
in an IV formulation, allowing emergency treatment ventricular fibrillation. Cardioversion units will
of very rapid supraventricular arrhythmias. possess a synchronization feature (SYNC) that
See Table 4.1 for drugs commonly used in the must be used with the supplied ECG leads. Newer
treatment of supraventricular arrhythmias. defibrillators deliver biphasic shocks, which improve
effectiveness at lower energy levels compared with
Atrial fibrillation (AF) the older monophasic units.
Treatment of AF largely depends on the heart rate Dogs with primary AF typically have lower
(i.e., ventricular response rate). Conversion of AF ventricular response rates than dogs with underlying
to sinus rhythm with antiarrhythmic drugs is rarely heart disease and are often asymptomatic. One
achieved in dogs and never even attempted in cats. advantage of cardioversion even in cases of

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