Page 157 - Feline Cardiology

156 Section D: Cardiomyopathies

that beta-blockade confers by slowing the heart rate and concentric hypertrophy, or to treat concurrent supra-
prolonging diastole to increase passive LV filling (Maron ventricular or ventricular tachyarrhythmias. The most
et al. 2003). Negative inotropic effects and reduction of common, affordable, well-tolerated beta blocker used in
myocardial oxygen demand may reduce microvascular feline medicine is atenolol, which is a selective B-1
myocardial ischemia. Troponin I is a cardiac isoenzyme receptor antagonist. Atenolol is highly bioavailable
that increases when there is cardiomyocyte damage or (90%), has a half-life of 3.5 hours in the cat, and beta
ischemia, and was shown to decrease over 1 to 4 weeks blocking effects last for at least 12 hours but less than
of atenolol therapy in five asymptomatic cats with HCM 24 hours, so many cardiologists feel that it should ideally
(Côté 2007). These results indicate that atenolol may be given every 12 hours (Quinones et al. 1996). The
reduce myocardial damage or ischemia in this small lowest strength tablet size of atenolol available is 25 mg.
Cardiomyopathies continued investigation. Beta blockers are also effective 12 h, starting typically at 6.25 mg PO q 12 h and then
Doses range from 6.25 mg PO 24 hr to 12.5 mg PO q
number of asymptomatic cats with HCM and warrant
up-titrated to achieve a heart rate in the exam room of
in reducing LVOT obstruction from SAM of the mitral
valve, and, in cats, are superior to calcium channel block-
<160–180 bpm (cardiologists’ opinions vary regarding
optimal target heart rate; ideally, a limit of ∼140 bpm
ers for doing so (Wey and Kittleson 2000). Preliminary
results from a prospective, blinded, placebo controlled
of lethargy, decreased appetite, or behavioral changes).
study evaluating the effects of atenolol versus diltiazem could be achieved, without inducing any clinical signs
in asymptomatic cats with HCM revealed that atenolol In one approach, atenolol is started at 6.25 mg PO q
had modest effects (improved one variable of diastolic 12 h, because pharmacologic studies in cats have dem-
function and slightly reduced septal wall thickness), and onstrated that q 12 h dosing is optimal for sustained
diltiazem had no effects on diastolic function, left atrial beta blockade, and the cat is rechecked a week later
size, LV hypertrophy, or SAM of the mitral valve. In (Quinones et al. 1996). The sympathetic stimulation of
contrast, a small, open-label, randomized, prospective a visit to the veterinary hospital serves as a stress test
clinical trial found that diltiazem reduced LV free wall for assessing beta blocker efficacy and duration of effect.
and interventricular septal thickness from 9 mm ± If the heart rate at recheck remains >160–180 bpm,
0.5 mm to 6 mm ± 0.6 mm after 6 months of therapy, there is not adequate beta blockade, so the dose may be
improved symptoms, and shortened LV relaxation time increased to 12.5 mg PO q 12 h and the heart rate
index after 3 and 6 months of treatment in 12 cats with rechecked a week later. Alternatively, clinical experience
HCM (Bright et al. 1991). In summary, treatment of suggests that in some cats, duration of the beta blocker
asyptomatic cats with HCM using atenolol or diltiazem effect of atenolol approaches 24 hours and once-daily
has produced conflicting results with regard to myocar- dosing may be appropriate in these cats. In this
dial effects, and no information is available with regard approach, atenolol is administered q 24h in the evening,
to effects on disease progression or survival. and a daytime recheck is done after 7–14 days of ongoing
treatment, corresponding to >12 hours postpill. A pre-
Beta Blocker Therapy treatment baseline heart rate of 180 beats/minute that
Beta-blocking drugs have undergone a resurgence in is <160–180 beats/minute after treatment suggests
popularity for treating feline cardiovascular disease ongoing adequate beta blockade, whereas persistent
since the 1970s and 1980s. Originally, propranolol (beta tachycardia suggests inadequate beta blockade warrant-
blocker) and diltiazem (calcium-channel blocker) were ing an increase in dosing to q 12h. The increase from q
used, but both require every-8-hour dosing, which is 24h to q 12h dosing in this manner has been associated
very challenging for lifelong therapy in cats. Subsequently, with further reductions in circulating cTnI concentra-
atenolol, a beta blocker, which is administered q 12– tions in asymptomatic feline HCM (Côté 2007).
24 h in the cat, was demonstrated to be safe through Assessment of murmur intensity is also important, as
repeated anecdotal use in cats and is now the beta often the beta blocker is used for treatment of signifi-
blocker of choice in this species. Conversely diltiazem, cant SAM of the mitral valve. Murmur intensity and
administered once daily as a sustained-release prepara- heart rate should decrease on beta blocker therapy.
tion made for the human, but not feline, gastrointestinal Follow-up echocardiographic quantification of the
and hepatobiliary systems, is associated with a 10–20% severity of SAM may be done, especially in cats with
rate of systemic complications and is now used infre- moderate to severe SAM, to help guide therapy and
quently in cats. determine whether the current dose of the beta blocker
The potential targets of beta blocker therapy are to effectively reduced the pressure gradient or whether a
decrease severity of SAM of the mitral valve (if moder- higher dose is needed. The selectivity of atenolol for
ate or severe), to decrease severity of left ventricular blocking β-1 receptors diminishes at higher doses,