Ventricular Arrhythmias in Dogs

Chapter 172


Ventricular Arrhythmias in Dogs




Ventricular arrhythmias are encountered frequently in canine patients. The clinical setting ranges from single ventricular premature complexes in an asymptomatic dog to life-threatening ventricular tachycardia requiring immediate treatment. The decision whether to initiate antiarrhythmic therapy is contingent on clinical signs and the severity and underlying cause of the arrhythmia. Since ventricular arrhythmias in dogs can occur either as a consequence of primary cardiac disease or secondary to other systemic illness, successful management often requires a thorough diagnostic workup.



Diagnosis


Impulses originating from the ventricle are recognized by the presence of abnormally wide QRS complexes with large T waves typically displaying a polarity opposite that of the QRS complex. Because the impulse originates from an ectopic focus in the ventricle, it is not consistently associated with a preceding P wave. Ventricular premature complexes (VPCs) or ventricular tachycardia (VT) is seen on the electrocardiogram (ECG) as impulses originating from the ventricle and occurring at a faster heart rate than the sinus rate. Determining the most likely underlying cause of the rhythm disturbance has practical importance because it affects whether therapy is needed, whether there is a risk of sudden death, and whether any treatment is likely to be short-lived or lifelong.


The most malignant ventricular arrhythmias typically occur in dogs with significant underlying cardiac disease, such as cardiomyopathy, valvular heart disease, or myocarditis. These animals are more likely to have VT at rapid heart rates (>200 beats/min), which reduces cardiac output, causes clinical signs such as syncope or weakness, and has the potential to culminate in fatal ventricular fibrillation or asystole. Boxers and English bulldogs with arrhythmogenic right ventricular cardiomyopathy, Dobermans with dilated cardiomyopathy, and German shepherds with inherited ventricular arrhythmias are at particularly high risk of sudden death from ventricular arrhythmias. Malignant VT also may result from myocardial hypoxia in diseases that cause ventricular hypertrophy, such as subaortic stenosis or pulmonic stenosis.


Single VPCs and less malignant VT often are associated with noncardiac illness. Although the precise mechanism for these rhythm disturbances may not be known, the clinical associations are well established. For example, splenic masses and other abdominal neoplasms, gastric dilatation and volvulus, trauma, hypoxia, and systemic inflammatory states such as immune-mediated cytopenias or pancreatitis often are related to the incidental finding of ventricular arrhythmias. Ventricular rhythms at relatively slower heart rates (<150 beat/min) are unlikely to affect cardiac output, cause clinical signs, or result in sudden death, and therefore treatment usually is not necessary. These “slow VT” or accelerated idioventricular rhythms often occur at heart rates similar to the sinus rate and resolve without antiarrhythmic therapy once the underlying systemic illness has been resolved.



Diagnostic Approach


The clinical importance of ventricular arrhythmias depends on the underlying cause. In breeds with a high incidence of primary cardiac causes of arrhythmias (e.g., boxers and Doberman pinschers), single VPCs, although unlikely to be causing clinical signs, raise suspicion of underlying cardiomyopathy. In these animals, additional diagnostic tests, such as echocardiography, thoracic radiography, or 24-hour ambulatory ECG (Holter) monitoring, are performed routinely to corroborate the diagnosis, assess the risk of future congestive heart failure, and evaluate the need for antiarrhythmic therapy. In other breeds of dogs with less risk of underlying cardiomyopathy and in dogs with hearts of normal size and function, a search for noncardiac causes of ventricular arrhythmias is indicated, and further diagnostic testing might include complete blood count and blood chemistry studies, infectious disease titers, serum cardiac troponin I level, abdominal ultrasonography, and thoracic radiography. If intermittent arrhythmias are suspected, Holter monitoring generally is recommended to determine the presence and malignancy of any arrhythmias and their relationship to any clinical signs that might be noted.

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Jul 18, 2016 | Posted by in PHARMACOLOGY, TOXICOLOGY & THERAPEUTICS | Comments Off on Ventricular Arrhythmias in Dogs

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