Page 243 - Clinical Small Animal Internal Medicine

21 Supraventricular Arrhythmias 211

Prognosis Syncopal episodes are rarely witnessed in cats with AV
VetBooks.ir Myocardial damage usually progresses, extending to block and dyspnea is a more frequent chief complaint
by owners of cats presented with AV block, whether
the ventricles. Most animals succumb within 12–18
months after initial diagnosis, despite pacemaker congestive heart failure is present or not.
implantation.
Diagnosis
Atrioventricular Block Electrocardiography
There are three types of AV block. With first‐degree
AV block, all the atrial impulses are conducted to
Etiology/Pathophysiology
the ventricles, but the PR interval is prolonged on the
Causes of AV block include excessive vagal tone, AV ECG (PR >130 ms in dogs, >90 ms in cats). Second‐
node fibrosis, inflammation or infiltration, and poten­ degree AV block is diagnosed when some P‐waves are
tially drug toxicity (calcium channel blockers, beta‐ not followed by a QRS complex on the surface ECG.
blockers, or digoxin). Age‐related fibrodegenerative Second‐degree AV block is said to be high grade when
changes in the AV node is the most common cause of AV the number of atrial impulses that fail to be conducted
block in dogs. (P‐waves not followed by a QRS complex) to the ven­
tricles are more than the number of impulses that are
conducted (P‐waves followed by a QRS complex).
Epidemiology Third‐degree or complete AV block is characterized
Atrioventricular block represents between 60% and 80% by an absence of conducted P‐waves to the ventricles.
of the indications for permanent pacemaker implanta­ The ECG displays independent atrial and ventricular
tion in dogs. The prevalence of clinically significant AV activities (Figure 21.6).
block is much lower in cats than dogs. When AV block Cardiac output is dramatically reduced. In response,
does occur in cats, it is typically associated with cardio­ the atrial rate, which is under adrenergic tone, is ele­
myopathy. Fortunately, feline escape rhythms are vated. Electrical activation of the ventricles is dependent
typically faster than canine (80–120 bpm) and pacing on an escape rhythm beyond the site of block. In dogs,
therapy is usually not required. the QRS complexes are generally wide and bizarre at
rates around 20–60 bpm. In cats, the escape rhythm is
usually seen as narrow QRS complexes, indicating their
Signalment origin in the area of the His bundle, and it occurs at a rate
of 80–140 bpm.
Atrioventricular block is more common in geriatric pets.
Most dogs are above 10 years of age at the time of diag­ Finally, the ventricular rate is regular, unless ventricu­
nosis. Labradors, cocker spaniels, West Highland white lar premature beats originating from ischemic areas of
terriers, beagles, German shepherds, springer spaniels, myocardium are present.
Cavalier King Charles spaniels, and dachshunds are
overrepresented. Cats with high‐degree second‐degree Echocardiography
and complete AV block are typically older than 10 years An echocardiogram is indicated to identify concomi­
of age. There is no evidence of breed or sex predisposi­ tant structural cardiac disease, which is frequent in
tion in this species. cats with complete AV block. It is also essential to
identify cardiac neoplasia that can infiltrate and dis­
rupt the AV nodal tissue, to assess systolic function in
History and Clinical Signs the presence of myocarditis, and to determine the

The hemodynamic effect of AV block depends on the degree of volume overload secondary to chronic
rate of ventricular contraction. However, one‐third of bradycardia.
cats, and occasionally dogs, with advanced AV block may
show no apparent signs, and bradycardia is detected on Other Methods
physical examination. More commonly, animals show Serum cardiac troponin I can be used to assess the degree
signs of anorexia, lethargy, exercise intolerance, disori­ of myocardial damage and raise a suspicion of myocardi­
entation, episodic weakness, and syncopal episodes. tis. Although a mild elevation of plasma cardiac troponin
Some animals develop right‐sided congestive heart I level is common in dogs with complete AV block,
failure and, less commonly, left‐sided congestive heart marked increases suggest myocarditis as the cause for
failure if the bradycardia is left untreated. the AV block.