Page 195 - Small Animal Internal Medicine, 6th Edition

CHAPTER 8 Myocardial Diseases of the Cat 167

reduced by half during the acute episode. The most common azotemia, electrolyte disturbances, and other complications
indication for starting atenolol in a cat that previously has is warranted.
VetBooks.ir experienced CHF is rate control in AF; it also can be useful Prognosis
for suppressing other tachyarrhythmias.
Diltiazem also has theoretical benefits for cats with severe
including the speed with which the disease progresses, the
LV hypertrophy, although it likewise has not been shown to Several factors influence the prognosis for cats with HCM,
improve survival, and side effects can be problematic in occurrence of thromboembolic events and/or arrhythmias,
++
some cats. Its Ca -blocking effect can modestly reduce heart and the response to therapy. Asymptomatic cats with only
rate and contractility (which reduces myocardial O 2 demand). mild to moderate LV hypertrophy and atrial enlargement
Diltiazem promotes coronary vasodilation and may have a often live with no clinical signs for many years. Median sur-
positive effect on myocardial relaxation. Longer-acting dil- vival time for all asymptomatic cats after diagnosis of HCM
tiazem products are more convenient for chronic use, is approximately 5 years. Cats with marked LA enlargement
although the serum concentrations achieved can be variable. and more severe hypertrophy appear to be at greater risk for
Diltiazem ER (or XR; Dilacor), dosed at one half of an inter- CHF, thromboembolism, and sudden death. Median sur-
nal (60-mg) tablet from the 240-mg capsule q12h, or Cardi- vival time for cats with CHF is between 1 and 2 years,
zem CD, compounded and dosed at 10 mg/kg q24h, have although this varies greatly with individual response to
been used most often. Similar to atenolol, the most common therapy and patient compliance with medication administra-
reason for initiating diltiazem therapy in a cat with previous tion. The prognosis is worse for older cats and cats with
CHF is rate control in AF. Occasionally, a β-blocker might severe LA enlargement, severe LV hypertrophy, LV or LA
be added to diltiazem therapy (or vice versa) to further systolic dysfunction, AF, and/or refractory CHF. Cats with
reduce heart rate in cats with AF. However, care must be low or high body weight may have a worse prognosis than
taken to prevent bradycardia or hypotension in animals those with normal weight. Thromboembolism confers a
receiving this combination. guarded prognosis (see p. 224) and recurrence of thrombo-
The negative chronotropic drug ivabradine is another embolism is common.
pharmacologic option that could prove helpful in controlling
heart rate in cats with HCM. Ivabradine is a selective “funny”
current (I f ) inhibitor. The I f is important in sinus node (pace- SECONDARY MYOCARDIAL
maker) function. Activation of the I f current increases mem- HYPERTROPHY
+
+
brane permeability to Na and K , thereby increasing the
slope of spontaneous phase 4 (diastolic) depolarization in Myocardial hypertrophy is a compensatory response to
sinus node cells, which increases the heart rate. Preliminary certain identifiable stresses or diseases. Marked LV wall and
studies have shown ivabradine to produce dose-dependent septal thickening and CHF can occur in some of these cases,
heart rate reduction with minimal adverse effects. Specific mimicking idiopathic HCM. Secondary causes should there-
recommendations await further study. fore be ruled out whenever LV hypertrophy is identified
Long-term management of cats with CHF also includes before making a diagnosis of idiopathic HCM. The most
therapy to reduce the likelihood of arterial thromboembo- common causes of secondary myocardial hypertrophy in
lism (see Chapter 12). Dietary sodium restriction is recom- cats are hyperthyroidism and systemic hypertension; less
mended if the cat will accept such a diet, but it is more frequent causes include subaortic stenosis, hypersomatotro-
important to forestall anorexia. pism (acromegaly), and infiltrative myocardial diseases.
Evaluation for hyperthyroidism is indicated in cats with
CHRONIC REFRACTORY CONGESTIVE myocardial hypertrophy or CHF older than 6 years of age.
HEART FAILURE Hyperthyroidism alters cardiovascular function by its direct
Refractory pulmonary edema or pleural effusion is difficult effects on the myocardium and through the interaction of
to manage. Moderate to large pleural effusions should be heightened sympathetic nervous system activity and excess
treated by thoracocentesis. Various medical strategies may thyroid hormone on the heart and peripheral circulation.
help slow the rate of abnormal fluid accumulation. These Cardiac effects of thyroid hormone include myocardial
include increasing the dosage of furosemide (up to ≈4 mg/ hypertrophy and increased heart rate and contractility. The
kg q8h), maximizing the dosage of an ACEI, adding or metabolic acceleration that accompanies hyperthyroidism
increasing dosage of pimobendan (up to ≈0.5 mg/kg q8h), causes a hyperdynamic circulatory state characterized by
using diltiazem or a β-blocker for greater heart rate control increased cardiac output, oxygen demand, blood volume,
if AF or other tachyarrhythmias are present, adding spi- and heart rate. Systemic hypertension can further stimu-
ronolactone, or using an additional diuretic (e.g., hydro- late myocardial hypertrophy. Manifestations of hyperthyroid
chlorothiazide; see Table 3.3). Spironolactone has been heart disease often include a systolic murmur, hyperdynamic
reported to cause facial pruritus and excoriations in some arterial pulses, a strong precordial impulse, sinus tachycar-
cats. Digoxin could be considered for additional heart dia, and various arrhythmias. Criteria for LV enlargement or
rate control or for cats with severe systolic dysfunction; hypertrophy often are found on ECG, thoracic radiographs,
however, toxicity can occur easily. Frequent monitoring for or echocardiogram. Signs of CHF develop in approximately