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200 Section 3 Cardiovascular Disease

arrhythmias, usually within 72 hours. Dogs with single Ventricular premature depolarizations can occur in
VetBooks.ir VPCs and underlying myocardial disease may or may not patterns such as ventricular bigeminy (every other beat is
a VPC) and ventricular trigeminy (every third beat is a
need to be treated. More often, the presence of VPCs in
a patient with underlying heart disease indicates the
with severity or potential malignancy of a ventricular
need to further investigate the arrhythmia (e.g., with VPC). This type of pattern, however, does not correlate
24‐hour ambulatory ECG [Holter] analysis) to determine rhythm. More important characteristics of potentially
if more serious arrhythmias are occurring at other times dangerous ventricular arrhythmias include the rate of
(Figure 20.1). firing of a VPC and the morphology of a ventricular
An example would be a Doberman pinscher with rhythm. If a VPC fires rapidly and occurs close to the
dilated cardiomyopathy, who is already receiving therapy peak of the preceding T‐wave (so‐called R‐on‐T phe-
but is found to have a newly discovered ventricular nomenon), depolarization during this time may predis-
arrhythmia. Immediate introduction of antiarrhythmic pose to ventricular fibrillation (Figure 20.4).
therapy in this case may or may not be appropriate and Ventricular rhythms can be monomorphic (one
further evaluation with Holter monitoring would be rec- morphology) or polymorphic (several different mor-
ommended. Patients with VPCs do not show clinical phologies), with the polymorphism generally considered
signs associated with only single premature beats. Even to imply multiple origins or pathways for the arrhythmia,
with ventricular tachycardia, if the rate of the ventricular thus increasing the potential for degeneration into a
is below 200–220 bpm, no clinical signs may be present. malignant arrhythmia (Figure 20.5).
The rate at which a ventricular tachycardia will cause A ventricular couplet is two consecutive VPCs, a
clinical signs will depend on many underlying factors ventricular triplet is three consecutive VPCs, and ven-
such as rate, underlying myocardial function, species, tricular tachycardia is >3 consecutive VPCs occurring at
and/or breed. Typically, however, when the rate of the a rate of >170–180 bpm. Ventricular tachycardia is
ventricular tachycardia is rapid (greater than 250 bpm), considered sustained if it lasts >30 seconds and nonsus-
signs of poor cardiac output will be present and syncope tained if it lasts <30 seconds. A ventricular rhythm that is
may occur. Clinical signs will also be influenced by the less than approximately 140–170 bpm is not truly a
degree of underlying heart disease. Thus, the patient tachycardia and is therefore best termed an accelerated
with systolic dysfunction and ventricular arrhythmias idioventricular rhythm (sometimes referred to as slow
may exhibit clinical signs at a slower ventricular rate than ventricular tachycardia) (Figure 20.6).
a patient without underlying myocardial disease and Accelerated idioventricular rhythms are seen com-
ventricular tachycardia. Patients with more serious ven- monly in hospitalized patients in association with noncar-
tricular arrhythmias (ventricular tachycardia, ventricu- diac diseases as mentioned in the beginning of the chapter.
lar fibrillation) and compromised/diminished myocardial These are often benign, the rate is too slow to cause clinical
function may have a history of exercise intolerance, signs and they often do not require therapy.
shortness of breath, syncope or near syncope.

Therapy
Diagnosis
Prior to discussing specific therapeutic interventions, it is
Two basic ECG characteristics of VPCs are that they important to review the main objectives of treating a ven-
are premature in their timing (earlier than the sinus tricular arrhythmia: (1) to improve hemodynamics in order
beat) and they are wide and bizarre in morphology to alleviate clinical signs and (2) to prevent sudden death.
compared to the sinus beat (Figures 20.2 and 20.3). When making the decision to treat or not, it is impor-
They appear wide and bizarre because VPCs originate tant to attempt to determine whether the arrhythmia is
below the bundle of His and therefore cannot take causing hemodynamic compromise. A healthy heart can
advantage of the specialized conduction system. They tolerate many types of VA whereas a diseased heart
therefore must depolarize the ventricles muscle cell to cannot. Therefore, the presence or absence of underlying
muscle cell. This is relatively slow and produces a wide cardiac disease is an important factor in this decision.
and bizarre complex. QRS complexes can be positive or Evaluation of the peripheral pulse quality, mucous mem-
negative in lead II, depending on the site of origin of the brane color, attitude of the patient, and arterial blood
VPC, and they have no relationship to the P‐waves. pressure can be helpful in determining whether hemody-
Because depolarization occurs in an abnormal fashion, namics are being affected. Sustained ventricular tachy-
repolarization is also abnormal and there is typically a cardia at a high rate is more likely to cause hemodynamic
large and bizarre T‐wave which is opposite in polarity compromise than an idioventricular rhythm or a ven-
to the VPC. tricular tachycardia firing at slower rates.