Page 158 - Feline Cardiology

Chapter 11: Hypertrophic Cardiomyopathy 157

where there may be blockade of β-1 and β-2 (bronchial phy is imperative. Side effects of atenolol are relatively
smooth muscle) receptors. Since β-2 receptors stimulate infrequent, and may include lethargy, weakness, and
bronchodilation, blockade of β-2 receptors with non- exacerbation of cough or wheeze in asthmatic cats; gas-
specific β-blockers (e.g., propranolol) may trigger or trointestinal side effects are extremely rare. Selective beta
exacerbate an asthmatic crisis in cats with asthma. blockers are not contraindicated for patients with dia-
Therefore, nonselective (concurrent β-1 and β-2 recep- betes mellitus, contrary to unfounded statements made
tor) beta blockers are contraindicated in cats with in drug formularies regarding their effect on insulin
asthma, and cats with bronchial disease should be mon- resistance or prolonged physiologic response to a hypo-
itored for signs of exacerbated bronchoconstriction glycemic crisis (Majumdar 1999).
even on selective beta blockers (e.g., atenolol), especially
at higher doses. The every-8-hour dosing requirement
of propranolol precludes its use as chronic therapy in Calcium Channel Blocker Therapy
cats with HCM, given the availability of atenolol as a q Cardiomyopathies
12 h or q 24 h drug. Subjectively, propranolol is respon- Diltiazem is a nondihydropyridine (i.e., not vascular
sible for a greater number of adverse drug reactions in selective) calcium channel blocker used in HCM. It has
cats than is atenolol. Atenolol has a high margin of mild negative inotropic effects (less than verapamil) and
safety in the cat, including inadvertent overdoses. is used as an antihypertrophic agent in HCM, as well as
In addition to negative inotropic effects, beta blockers for treatment of supraventricular arrhythmias. Calcium
are also negative chronotropic drugs, which may effec- channel blockers improve early diastolic relaxation by
tively reduce tachycardia and lessen the influence of reducing isovolumic relaxation time (Lorell et al. 1982).
tachycardia on elevated diastolic filling pressure. This Calcium channel blockers also may reduce LV end-
may be important in cats with sustained sinus tachycar- diastolic pressure and cause a downward shift of the LV
dia (HR >220 bpm) or in cats with other more malig- diastolic pressure volume relation, suggesting improved
nant tachyarrhythmias. There is a potential for periods LV distensibility (Lorell et al. 1982). In apparent contra-
of marked tachycardia that may contribute to develop- diction, a study measured LV pressure- volume relations
ment of congestive heart failure in cats with diastolic in human patients with LV hypertrophy before and after
dysfunction from HCM. These periods may occur in intravenous esmolol (beta blocker) and/or verapamil
situations of extreme stress or physical exertion, such as (calcium channel blocker) and found unaltered late dia-
being chased by another animal, so beta blockers may stolic compliance with both drugs (Kass et al. 1993).
blunt the extreme sinus tachycardia during these stress- This study helps illustrate that chamber compliance is
ful situations. Beta blockers address the sympathetic determined by passive structural elements (i.e., hyper-
potentiation of both supraventricular and ventricular trophy and fibrosis) rather than active processes. Even if
tachyarrhythmias, either of which may be present in early diastolic relaxation is improved, increased chamber
asymptomatic cats with HCM. stiffness has a large impact on overall diastolic dysfunc-
Short-term administration of atenolol for planned tion. Early diastolic relaxation was found to be improved
stressful events such as a plane trip may be considered in a small study of 40 people with HCM treated with
in cats that otherwise would not be given chronic life- verapamil. At baseline, 70% of patients had evidence of
long therapy. The theory is to lessen the magnitude of diastolic dysfunction based on reduced peak filling rate
stress-induced tachycardia, which can be a trigger to and time to peak filling rate from gated radionuclide
worsen diastolic function and precipitate heart failure in angiograms (Bonow et al. 1981). After 1 week of oral
some cats with HCM. Atenolol (6.25 mg) may be admin- verapamil, only 30% had evidence of diastolic dysfunc-
istered q 12 h–24 h for 5–7 days prior to the event, con- tion. Verapamil increased peak filling rate and reduced
tinued for 2–3 days after the end of the stressor, and then time to peak filling rate (Bonow et al. 1981). Improvement
weaned off. Ideally the atenolol is dosed 1–3 hours prior of diastolic function by verapamil was associated with
to the stressful event. This technique is recommended improved exercise capacity (Bonow et al. 1985). Another
by some cardiologists, but there is no proven benefit. acute study of intravenous diltiazem in people with
Beta blockers may be safely used concurrently with HCM showed that it reduced asynchronous LV filling
other medications including methimazole or amlodip- and improved global diastolic parameters (Ito et al.
ine in cats with hyperthyroidism or systemic hyperten- 2004). Like beta blockers, calcium channel blockers may
sion, respectively. However, beta blocker therapy should also relieve myocardial ischemia.
not be used in substitution for methimazole or amlo- There are four different forms of diltiazem: standard
dipine in cats, as direct treatment of any underlying oral formulation (30 mg), two sustained release formu-
systemic disease that can contribute to cardiac hypertro- lations, and an intravenous preparation. Whether