Page 113 - Small Animal Internal Medicine, 6th Edition

CHAPTER 4 Cardiac Arrhythmias and Antiarrhythmic Therapy 85

Atrial Fibrillation Slow HR, no CHF

Uncontrolled HR, CHF
VetBooks.ir HCM cat

Treat CHF; PO digoxin, or Diltiazem*, treat PO digoxin or Monitor
if HR still treat CHF CHF; add PO
uncontrolled digoxin
HR not HR or
HR not
controlled controlled controlled

Add β-blocker Intensify therapy; HR Resting HR Try to convert–
or diltiazem or try sotalol or controlled > 130/min amiodarone, or
amiodarone diltiazem, or
sotalol, or other
drug, see text;
or electrical
cardioversion
Continue Continue digoxin, PO
digoxin & diltiazem (or β-blocker),
CHF therapy & CHF therapy

*Either cautiously IV, or PO loading dose
FIG 4.4
A therapeutic approach to atrial fibrillation. See Table 4.2 for drug doses and text for
more information. AF, Atrial fibrillation; CHF, congestive heart failure; HCM, hypertrophic
cardiomyopathy; HR, heart rate.

less than that of verapamil (which should not be used in and control ventricular rate. The potential myocardial
++
CHF) or an IV β-blocker, although esmolol could be cau- depressant effects of β- and Ca -blockers warrant caution
tiously tried if necessary because of its short half-life. Oral in patients with myocardial failure. Concurrent pimobendan
digoxin at maintenance doses could be used for initial heart therapy or allowing 1 to 2 days of digoxin therapy before
rate control (see Table 3.3), especially in dogs without CHF. adding or increasing the dose is recommended. Digoxin is
However, twice the eventual oral maintenance dosage can be not used in cats with HCM and AF; a β-blocker or diltiazem
given for 1 to 2 days for more rapid effect, and this strategy is used instead. Amiodarone and sotalol are other agents that
can be used for dogs with CHF as well. Although IV digoxin can be used for heart rate control in dogs. Occasionally, con-
has been used, it is generally avoided because of toxicity version to sinus rhythm occurs in response to amiodarone or
concerns. Acidosis and hypoxemia associated with severe even diltiazem therapy. In experimental dog models, fish oil
pulmonary edema could increase myocardial sensitivity to administration has reduced the occurrence of AF.
digoxin-induced arrhythmias; furthermore, rapid IV digoxin When AF develops in patients that also have ventricular
++
administration can promote peripheral vasoconstriction by preexcitation, AV nodal blocking drugs (Ca -blockers,
increasing central sympathetic output. If IV digoxin must be digoxin, and possibly β-blockers) should not be used because
used, for lack of other options, it must be given slowly and they can paradoxically increase the ventricular response rate.
in small doses. When dobutamine or dopamine infusion is Amiodarone is recommended in these cases; sotalol or pro-
necessary to support myocardial function during acute CHF cainamide also can be used.
management (see p. 63 and Box 3.1), a β-blocker usually is Electrical cardioversion of AF is successful in some
avoided. Diltiazem (cautiously IV, or PO) is recommended, animals, although not usually those with marked atrial
but if unavailable, a PO loading dose of digoxin could be enlargement. Use of biphasic current delivery combined with
used. Ultimately, the combination of both diltiazem and amiodarone (or other drug) therapy may increase effective-
digoxin typically is used because this has been shown to ness. However, most animals with significant underlying
control heart rate in AF better than either agent alone. heart disease revert to AF.
Digoxin alone does not fully control the heart rate in
many animals with AF. Increases in sympathetic tone from Lone Atrial Fibrillation
CHF, exercise, or excitement can override the vagal effect of AF sometimes develops in large or giant-breed dogs without
digoxin on AV conduction. Either diltiazem or a β-blocker cardiomegaly or other evidence of structural heart disease.
can be added PO, initially at a conservative dose and then This is known as “lone AF.” AF with a slow ventricular
titrated upward as needed to adequately slow AV conduction response rate can be an incidental finding in such dogs. AF