Beta blockersBeta blockers play an important role in the management of cardiomyopathies, especially hypertrophic cardiomyopathy and hypertrophic
obstructive cardiomyopathy. Their role in dilated cardiomyopathy, restrictive cardiomyopathy, and intermediate cardiomyopathy
is usually limited to control of arrhythmias, specifically supraventricular tachycardias (atrial fibrillation, atrial tachycardia,
and frequent atrial premature contractions) or ventricular tachycardia.
In hypertrophic cardiomyopathy, beta blockers slow down the heart rate which prolongs diastole, an important goal in management
of this disease. This increases coronary blood flow and thereby reduces myocardial ischemia.
Oxygen requirements are also reduced by reducing heart rate, contractility, systolic myocardial wall stress, and a variety
of other factors. The arrhythmia control achieved also may increase life span, which is uncommon amongst antiarrhythmic drugs.
In hypertrophic obstructive cardiomyopathy, beta blockers reduce dynamic outflow tract obstruction.
The predominant adverse side effect of these drugs is the result of decreased cardiac output, that is, exacerbation of heart
failure or hypotension. This is less of a concern with hypertrophic cardiomyopathy but needs to be considered in cats with
the other forms of cardiomyopathy. Using a low dose initially and then gradually increasing the beta blocker is the preferred
way to use this group of medications. Abrupt discontinuation should be avoided. Commonly used beta blockers include atenolol
(6.25-12.5 mg once to twice daily) and propranolol (5-10 mg two to three times daily). Atenolol is a selective beta 2 blocker,
which may decrease the likelihood of bronchoconstriction as could in theory occur with non-selective beta-blocker therapy.
The use of calcium channel blockers is usually reserved for hypertrophic cardiomyopathy. In other forms of cardiomyopathy
it is used as an antiarrhythmic drug, whereby it is most effective in managing supraventricular tachycardias (atrial fibrillation
and atrial tachycardia). Calcium channel blockers work to decrease heart rate, improve diastolic filling and mildly decrease
cardiac output (which decreases myocardial oxygen consumption). These are all positive effects in hypertrophic cardiomyopathy.
There are a variety of preparations available.
Regular diltiazem (1 mg/kg three times daily) can be used but must be given frequently. Sustained release products such as
Dilacor XR (30 to 60 mg once daily per cat) or Cardizem CD (10 mg/kg once daily) are easier to use in cats since they need
to be given less frequently.
Digoxin is used in some cases of dilated and restrictive cardiomyopathy, especially if right-sided heart failure is present.
Unfortunately, digoxin toxicity is common and is especially pronounced if renal dysfunction or hypokalemia is present. Digoxin
should not be used in cats that are anorectic or dehydrated. Dosage recommendations are as follows; cats weighing 2 to 4 kg
receive one-fourth of a 0.125 mg tablet every 2 days, for cats 4 to 6 kg one-fourth of 0.125 mg tablet every 24 to 48 hours,
and for cats > 6 kg one-fourth of a 0.125 mg tablet every 24 hours, occasionally every 12 hours. It is best to use the lower
dosages initially and recheck blood digoxin levels in 10 days and best at 8 hours post-pill. Monitoring for signs of digoxin
toxicity is vital. Any signs of anorexia or GI problems warrant discontinuation of the digoxin and evaluating for toxicity
by repeating a digoxin level.