Page 168 - Problem-Based Feline Medicine

160 PART 3 CAT WITH SIGNS OF HEART DISEASE

Abnormal conduction of the electrical impulse can be Class II anti-arrhythmics are beta-blockers. This group
either blocked, choose an alternative pattern of conduc- antagonizes the action of cathecolamines and is most
tion (aberrantly conducted) or re-enter previously excited useful when increased sympathetic tone is present. Most
tissue (re-entry). common examples include propranolol (non-selective)
and atenolol (beta-1 selective). They are contraindicated
An arrhythmia should always be suspected whenever an
for cats with symptomatic respiratory disease.
irregularity in cardiac rhythm or arterial pulse is
detected. Class III anti-arrhythmics block the potassium chan-
nels. This group hyperpolarizes the cells making it
more difficult to reach threshold, and they prolong the
WHERE? refractory period. The most common used drug is
Arrhythmias can occur wherever there is excitable sotalol. Sotalol, besides class III properties, also has
cardiac tissue in the heart. class II properties.
Arrhythmias that originate in the atria, interatrial Class IV anti-arrhythmics block the calcium channels.
septum, or above the AV node are termed supraven- These drugs inhibit transmembrane calcium transport
tricular. and the most pronounced effects are at the sinoatrial and
atrioventricular nodes. The most common example is
Arrhythmias that originate in the ventricles or anywhere diltiazem.
below the bifurcation of the His bundle are termed ven-
tricular.

DISEASES CAUSING TACHYCARDIA
WHAT?
OR AN IRREGULAR RHYTHM
The most common arrhythmia is ventricular premature
contractions (VPCs).
SINUS TACHYCARDIA***
Cats with cardiac arrhythmias are generally suspected
of having myocardial disease, even if radiographic and Classical signs
echocardiographic findings are within normal limits.
● Heart rate greater than 220 beats per
Cardiomyopathies are the most common form of car- minute.
diac disease associated with arrhythmias. ● Strong precordial impulse.
● Loud S1 heart sound.
The diagnosis of atrial fibrillation in a cat is almost
invariably associated with the presence of severe left
atrial enlargement.
Pathogenesis
Most cats with a cardiac arrhythmia are asymptomatic.
This arrhythmia originates from the sinoatrial node, the
Ventricular arrhythmias in young cats with severe left normal pacemaker of the heart.
ventricular hypertrophy may be associated with a high
It results from increased sympathetic tone. The
risk of sudden death.
causes of increased sympathetic tone include stress,
fever, pain, metabolic disease (hyperthyroidism),
TREATMENT hypovolemia, thromboembolic disease, shock and
anemia.
Most cats do not require specific therapy with anti-
arrhythmics unless they are symptomatic.
Class I anti-arrhythmics block the sodium channels and Clinical signs
result in decreased excitability of the diseased cells. The
most common examples are procainamide and lidocaine In the majority of patients, clinical signs are related to
(high risk of toxicity). the underlying disease.