Theoretically, pimobendan may increase the rate of intestinal digoxin absorption.15 In our clinical experience, the coadministration of pimobendan with digoxin has neither increased paired serum digoxin concentrations
nor resulted in concentrations within the upper 40th percentile of our reference range. We seldom add digoxin to pimobendan
therapy except in the face of atrial fibrillation in dogs with advanced dilated cardiomyopathy. Based on serial Holter recordings
in our patients with atrial fibrillation, pimobendan does not seem to markedly attenuate the effects of digoxin on reducing
atrioventricular conduction. In boxers and Doberman pinschers with advanced dilated cardiomyopathy treated at the University
of Georgia Veterinary Teaching Hospital with pimobendan, digoxin, an ACE inhibitor, and furosemide, the ventricular response
rates have usually been below 140 beats/min for more than 85% of the approximate 24-hour Holter recording time. In addition,
the combination of pimobendan with amiodarone or mexiletine or both has, in our clinical experience, been well-tolerated in
boxers and Doberman pinschers with advanced dilated cardiomyopathy and severe ventricular arrhythmias.
Pimobendan has not been evaluated in pregnant and lactating dogs. Administer pimobendan to pregnant or lactating dogs only
if the potential benefits outweigh the potential risks. Although adverse effects are uncommon, polyuria, polydipsia, vomiting,
diarrhea, and inappetence are possible. A dose-related sinus tachycardia can result,4 and as with any strong inotropic agent, ventricular tachyarrhythmias may develop or worsen while pimobendan is administered.16 Ventricular tachyarrhythmias are of particular concern in Doberman pinschers and boxers but could occur in any dog with
advanced dilated cardiomyopathy. Pimobendan's effect on myocytes—conserved energy demand with small increases in intracellular
calcium concentration—may reduce the likelihood of a proarrhythmic effect,9 but additional studies are warranted.
In our experience with advanced dilated cardiomyopathy, pimobendan's durability as a positive inotropic agent in dogs may
not be as good as reported in human studies. Thus, premature administration of pimobendan to patients with only mild to moderate
myocardial failure caused by dilated cardiomyopathy could result in decreased effectiveness later in the course of the disease.
It is for this reason that we recommend initiating pimobendan for patients with advanced cardiomyopathy.
At the University of Georgia Veterinary Teaching Hospital, we have prescribed pimobendan for more than 100 dogs and have followed
each patient's progression. We have found pimobendan to be virtually free of overt adverse effects in dogs with either dilated
cardiomyopathy or mitral valve degeneration. Mild diarrhea may have been attributable to pimobendan in one dog. We have not
evaluated the potential effects of pimobendan on insulin metabolism, increased mitral regurgitation volume, or myocardial
USE IN CATS
The efficacy of pimobendan in cats has not been reported. Hypertrophic cardiomyopathy is the most common cardiomyopathy in
cats, and pimobendan is contraindicated. Although the manufacturer has not conducted any trials to determine the safety of
pimobendan in cats, we have administered pimobendan in 10 cats with idiopathic dilated cardiomyopathy or advanced restrictive
cardiomyopathy. We use a dosage of 0.6 mg/kg given orally every 12 hours. We have not encountered clinically relevant adverse
effects. In our experience, when added to ace inhibitor and furosemide therapy in cats with restricted cardiomyopathy, pimobendan
appeared to have no efficacy.
THE FUTURE OF PIMOBENDAN
Pimobendan has never been licensed in the United States to treat congestive heart failure in either dogs or people. However,
pimobendan has been approved for veterinary use and has been used extensively in some countries of Europe by veterinarians
for more than six years. Pimobendan studies in dogs are being conducted in the United States. We have been using pimobendan
at the University of Georgia Veterinary Teaching Hospital for about four years, and clients have been pleased with the results.
Currently, approval to obtain pimobendan for individual dogs can be requested from the fda Center for Veterinary Medicine.
Pimobendan is an inodilator used to treat overt or impending congestive heart failure in dogs. Preliminary studies, although
limited, have either demonstrated or suggested a favorable influence when pimobendan is used as adjunctive therapy (e.g. with an ace inhibitor, furosemide, digoxin) in patients with advanced dilated cardiomyopathy or mitral valve disease.12,13 Pimobendan is well-tolerated in dogs and has a favorable adverse effects profile. Pimobendan should not be used as a replacement
for, but rather as cotherapy with, other cardiac drugs to enhance the quality of life in dogs with overt or impending congestive
Justin D. Thomason, DVM, DACVIM (small animal internal medicine)
Tiffany K. Fallaw, BS, RVT
Clay Calvert, DVM, DACVIM (small animal internal medicine)
Department of Small Animal Medicine and Surgery
College of Veterinary Medicine
University of Georgia
Athens, GA 30605