Page 222 - Clinical Small Animal Internal Medicine

190 Section 3 Cardiovascular Disease

positive inotrope and forms part of standard “quadruple target heart rate. Overly aggressive combinations of
VetBooks.ir therapy” for CHF in dogs along with ACEI, furosemide, digoxin, BB, and calcium channel blockers can produce
bradycardia. Some cardiologists recommend BB during
and spironolactone. The recommended dose in dogs is
0.5 mg/kg/day divided into two doses given 12 hours
tion and titration of BB is regarded as easier in the
apart. Pimobendan has also been shown to prolong time the preclinical phase of DCM in dogs. Successful institu-
to CHF and death in asymptomatic Doberman pinschers preclinical phase as opposed to starting BB after deve-
with echocardiographic evidence of reduced systolic lopment of CHF. Sudden withdrawal of BB can result
function and ventricular enlargement consistent with in reflex tachycardia so that dosing should continue
preclinical DCM. In humans, positive inotropes exacer- even if CHF eventually occurs. However, the dosage may
bate cardiac arrhythmias that can lead to increased risk require adjustment based on clinical symptoms, blood
of sudden death, and use of pimobendan in Doberman pressure, and heart rate. Limited studies in dogs indicate
pinschers with preclinical DCM and severe ventricular that titration of BB is relatively well tolerated by patients,
arrhythmias has not been specifically evaluated. but the long‐term effect on cardiac function and sur-
In general, pimobendan is well tolerated. The most vival is unknown.
common adverse reaction is anorexia and diarrhea that In general, BB should only be introduced in patients
often resolves at lessened dosage. Pimobendan should be free of active CHF as their negative inotropic properties
used with caution in disorders with a ventricular outflow can acutely reduce cardiac function and exacerbate clini-
tract obstruction such as congenital subvalvular aortic cal signs. When used in dogs with DCM, BB are typically
stenosis or hypertrophic obstructive cardiomyopathy. titrated upward over 4–8 weeks. The initial prescribed
Pimobendan does not have an approved use in cats dosage of BB is as little as 1/10th of the target dose and
with CHF, but has been occasionally used in cats with titrated upward based on heart rate and blood pressure.
heart failure. Pimobendan is not recommended in the The authors will often temporarily increase the dose of
preclinical stages of feline cardiomyopathy or in cats concurrent furosemide while introducing BB in patients
with left ventricular outflow tract obstruction. with advanced disease. Gradual reduction in the dosage
Pimobendan is most often used in cats with systolic over several weeks is recommended if a decision is made
myocardial dysfunction that is associated with dilated to cease treatment with chronic BB. Systemic hypoten-
cardiomyopathy as well as end‐stage hypertrophic car- sion can be seen when BB are used concomitantly with
diomyopathy (HCM), restrictive cardiomyopathy vasodilatory drugs such as ACEI or pimobendan. BB use
(RCM), and arrhythmogenic right ventricular cardiomy- in the preclinical phase of MMVD is not recommended.
opathy cases. Most reports of pimobendan use in cats BB are commonly used to slow heart rate in cats with
utilize the same dosing protocol that is used in dogs HCM or RCM, especially if left ventricular outflow tract
(0.25 mg/kg BID). The overall safety and efficacy of obstruction due to systolic anterior motion of the mitral
pimobendan in cats requires further study. valve is present (see Chapter 27). BB are usually not pre-
scribed to cats with active CHF; rather, they are used in
Beta‐Adrenergic Blockers the preclinical or stable heart failure phases. Typically,
Beta‐adrenergic blockers (BB) are part of standard care BB are titrated to achieve in‐hospital heart rate less than
in humans with systolic dysfunction. Chronic adminis- 160 bpm in cats. Monitoring of heart rate, systolic blood
tration of BB such as carvedilol or metoprolol results in pressure, and severity of left ventricular outflow tract
improved contractility, myocardial protection from cir- obstruction is recommended in cats with HCM.
culating catecholamines, and improved survival. The various BB are classified according to their selec-
Whether or not BB benefit veterinary patients with heart tivity for different beta‐receptors. Additionally, there are
disease or failure is unknown as there are relatively lim- differences involving lipophilicity, metabolism, antiar-
ited data compared to ACEI and pimobendan. The three rhythmic effects, and vasodilatory properties amongst
most common clinical situations for which BB are typi- the various BB. The most common BB used in veterinary
cally considered are rate control of atrial fibrillation cardiology include atenolol, carvedilol, metoprolol, and
wherein BB are co‐administered with digoxin, in the pre- sotalol.
clinical or stable heart failure phase of DCM in dogs, and Atenolol is a beta‐1‐specific antagonist dosed at
in cats with HCM or RCM, with special consideration 0.25–1 mg/kg BID in dogs and 6.25–12.5 mg/cat q12–
for cats with dynamic left ventricular outflow tract 24h in feline patients. Atenolol is commonly used in the
obstruction. treatment of HCM in cats and congenital subvalvular
Beta‐adrenergic blockers are often used in conjunc- aortic stenosis in dogs. Carvedilol is a nonspecific BB,
tion with digoxin to treat persistent supraventricular acting on both beta‐1 and beta‐2 receptors, and has
arrhythmias such as atrial fibrillation (see Chapter 21). adjunctive alpha‐1‐adrenergic blocking effects leading to
The BB dose can be gradually up titrated to achieve vasodilation. Carvedilol is metabolized in the liver and