Page 167 - Problem-Based Feline Medicine

10 – THE CAT WITH TACHYCARDIA, BRADYCARDIA OR AN IRREGULAR RHYTHM 159

TRAUMATIC
● Ventricular arrhythmias (ventricular premature beats, ventricular tachycardia) (p 164)
Ventricular arrhythmias can be seen in cases following severe trauma to the chest. These are usu-
ally secondary to traumatic myocarditis. The majority of the cases do not have symptoms related
to the arrhythmia. Periods of tachycardia (rates above 300 bpm) may cause lethargy or syncope
BRADYARRHYTHMIAS
DEGENERATIVE

● Atrioventricular block (p 169)
Several degrees of atrioventricular block are due to His bundle degeneration, vacuolization and fibrosis.
Bradyarrhythmias cause clinical signs of lethargy, weakness and syncope. Depending on the
degree of the bradyarrhythmia, many cats may not show clinical signs and usually are an inciden-
tal finding.
METABOLIC
● Sinus bradycardia (p 168)
Sinus bradycardia can result from hyperkalemia, hypokalemia, hypoglycemia, acidosis and
hypothermia or be drug-induced. In addition to the signs related to the underlying disease, cats
may present with lethargy, weakness, anorexia and syncope. Heart rate is <120 beats/min.

● Atrial standstill (p 168)
Atrial standstill can result from hyperkalemia primarily associated with urethral obstruction or
from long-standing cardiac disease, most commonly dilated cardiomyopathy. In addition to the
signs related to the underlying disease, cats may present with lethargy, weakness, anorexia and
syncope.
TOXIC
● AV block** (p 169)
AV block can be drug-induced with digitalis, beta-blockers and calcium-channel blockers. In addi-
tion to the signs related to the underlying disease, cats may present with lethargy, weakness,
anorexia and syncope.

INTRODUCTION Alterations of electrical impulse formation occur sec-
ondary to instability of the resting membrane potential
and are usually the result of diseased myocardial cells.
MECHANISMS?
● If the electrical impulse forms earlier than expected, a
The normal electrical impulse is generated in the SA premature beat is generated. If the electrical impulse
node and spreads through the atria via the Bachmann forms later than expected, a pause in the cardiac
bundles, continues down through the AV node, His bun- rhythm is seen.
dle, bundle branches and Purkinje fibers and finally
Alterations of electrical impulse conduction occur sec-
reaches the ventricles.
ondary to primary diseases of the conduction system.
Arrhythmias are caused by two basic abnormalities, These either result in block of conduction of the electri-
disorders of impulse formation and disorders of cal wave, or alterations that facilitate the perpetuation
impulse conduction. of the electrical wave in a circuit (re-entry).