Page 244 - Clinical Small Animal Internal Medicine

212 Section 3 Cardiovascular Disease

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(b)

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(c)

Figure 21.6 ECG strips of dogs with atrioventricular block. (a) First‐degree atrioventricular block: there is a prolongation of the PR interval
(180 ms, recording speed at 50 mm/s) and all the P‐waves are associated with a QRS complex. (b) Second‐degree atrioventricular block:
the third and sixth P‐waves (arrows) are not followed by a QRS complex, indicating an intermittent failure of the impulse to propagate
to the ventricles. (c) Third‐degree atrioventricular block: there is complete dissociation between the P‐waves and the QRS complexes.
The ventricles are activated by an escape rhythm at a rate of 37 bpm (recording speed at 50 mm/s).

Therapy Most dogs will usually die of noncardiac‐related causes,
unless concurrent progressive degenerative valvular dis­
Medical Therapy ease or cardiomyopathy is present.
Sympathomimetic chronotropes increase heart rate by Pacemaker implantation may not be required in the vast
beta‐adrenergic stimulation. Agents with beta‐2 effects majority of cats with complete AV block. Indeed, these ani­
cause systemic vasodilation, whereas drugs with associ­ mals have a median survival greater than one year without
ated alpha stimulation cause vasoconstriction. Dopamine pacemaker therapy. Finally, the presence of underlying
(5–10 μg/kg/min IV) and dobutamine (2–10 μg/kg/min structural heart disease or congestive heart failure at the
IV) may contribute to an increase in heart rate and time of diagnosis may not significantly alter their prognosis.
systolic function. They are usually administered as a con­
tinuous rate infusion and the dose is increased to effect.
They are particularly indicated in the management of Atrial Fibrillation
beta‐blocker overdose. Isoproterenol, a pure beta‐ago­
nist, improves conduction in the AV node and the His– Etiology/Pathophysiology
Purkinje system, which may result in the partial or
complete resolution of AV block. It may also increase the Atrial fibrillation is characterized by uncoordinated
rate of a ventricular escape rhythm in complete AV block, atrial activation from multiple simultaneous electrical
but usually with limited success. It is administered as a wavelets resulting in inadequate mechanical contraction
continuous rate infusion and its dose adjusted to effect. and an irregular ventricular response rate. Atrial dilation
However, it causes a significant decrease in diastolic is a risk factor for developing AF. However, it commonly
blood pressure via beta‐2 stimulation. Finally, respiratory occurs in large‐breed dogs with only mild to moderate
and metabolic acidosis decrease its effectiveness. atrial dilation, and it is rarely present in small‐breed
dogs with degenerative mitral valve disease despite
Pacemaker Therapy extreme atrial dilation. It is also rarely diagnosed in cats.
Permanent ventricular pacemaker implantation is the treat­ Fibrosis, inflammation, and autonomic imbalances are
ment of choice for dogs and cats with clinical AV block. other important contributors to the initiation of AF.

Prognosis Epidemiology
In dogs, following permanent pacemaker implantation, In dogs, AF is one of the most common types of tachyar­
estimated survival at one year is approximately 85%. rhythmias that require treatment. Most dogs with AF