ECG Abnormalities in Canine Dilated Cardiomyopathy: What Veterinarians Should Watch For

Dilated cardiomyopathy (DCM) is one of the most important acquired myocardial disorders in dogs and is considered the second most common acquired cardiac disease in canine patients¹. Apart from structural cardiac changes, DCM is frequently associated with significant electrical abnormalities that can directly influence prognosis, therapeutic decisions, and long-term patient stability. 

Electrocardiographic abnormalities have been reported in 55% to 91% of dogs affected with DCM^1,2. Commonly reported abnormalities include atrial fibrillation, atrial premature complexes, ventricular premature complexes (VPCs), and ventricular tachyarrhythmias¹. Among these, atrial fibrillation remains one of the most clinically relevant arrhythmias in dogs with advanced myocardial disease³. 

A clinical evaluation of 52 dogs diagnosed with DCM highlighted how frequently ECG abnormalities accompany the disease. Labrador Retrievers formed the majority of affected cases, followed by Dobermans, Rottweilers, German Shepherds, Golden Retrievers, Beagles, Pugs, and non-descript breeds. Most dogs presented with signs commonly associated with congestive heart failure including dyspnea, cough, ascites, limb edema, murmurs, and gallop rhythms¹. 

Sinus Tachycardia Was the Most Common ECG Finding 

Pre-therapeutic ECG evaluation revealed sinus tachycardia in 34.61% of affected dogs, making it the most commonly observed abnormality. In dogs with DCM, reduced cardiac output activates compensatory sympathetic stimulation, leading to increased norepinephrine release in an attempt to maintain circulatory function¹. 

Although compensatory initially, persistent tachycardia increases myocardial oxygen demand and may worsen myocardial stress over time. Clinically, affected dogs often demonstrate¹: 

• Tachypnea  

• Reduced exercise tolerance  

• Weak pulse quality  

• Progressive fatigue  

Because stress-related tachycardia can mimic cardiac tachyarrhythmias during examination, ECG interpretation should always be correlated with echocardiographic findings and overall clinical status. 

Atrial Fibrillation Carries Major Clinical Significance 

Atrial fibrillation was identified in 15.38% of dogs and represented the most common arrhythmia observed. This supraventricular arrhythmia is characterized by rapid and disorganized atrial activity resulting in loss of coordinated atrial contraction and compromised ventricular filling³. 

Clinically, these dogs often present with: 

• Rapid and irregular heartbeat  

• Variable pulse strength  

• Pulse deficits  

• Exercise intolerance  

• Progressive respiratory distress 

Management primarily focused on ventricular rate control using diltiazem and digoxin therapy. Diltiazem slows atrioventricular conduction, while digoxin exerts antiarrhythmic effects through sympathetic inhibition and parasympathomimetic activity¹. 

Interestingly, follow-up ECG evaluation demonstrated effective heart rate control in all monitored dogs. Some reverted back to sinus rhythm, while others maintained controlled atrial fibrillation. This reinforces an important practical point in veterinary cardiology; successful management does not always require complete rhythm conversion. 

Ventricular Arrhythmias Should Never Be Ignored 

Ventricular premature complexes were identified in a smaller subset of dogs, including one case of ventricular bigeminy. These arrhythmias originate from ectopic ventricular pacemakers and may indicate progressive myocardial electrical instability¹. 

Sotalol therapy resulted in effective suppression of ventricular ectopic activity during follow-up monitoring. Because ventricular arrhythmias may predispose dogs to sudden clinical deterioration, early ECG identification remains clinically important. 

Additional ECG Findings with Diagnostic Value 

Several dogs also demonstrated ST-segment coving, suggestive of myocardial hypoxia or subendocardial ischemia¹. Poor cardiac output combined with sustained tachycardia likely contributed to these changes. 

Low-amplitude QRS complexes were observed in dogs with pleural or pericardial effusion secondary to congestive heart failure. Such findings highlight the importance of correlating ECG abnormalities with thoracic imaging and echocardiographic findings. 

Clinical Takeaway 

In canine DCM, electrocardiographic abnormalities are more than incidental findings. They often reflect disease severity and may directly influence prognosis and treatment response. Routine ECG evaluation helps identify arrhythmias early, monitor therapeutic efficacy, and guide long-term management strategies in dogs affected with myocardial disease. 

References 

1. Saikrishna KS, Jeyaraja K, Vairamuthu S, Shafiuzama M, Selvaraj P. Electrocardiographic abnormalities in canine dilated cardiomyopathy and their management. https://krishikosh.egranth.ac.in/server/api/core/bitstreams/cec902a6-ec35-4ad4-a0dc-f2eaaf0f969f/content 

2. Freid KJ, Freeman LM, Rush JE, Cunningham SM, Davis MS, Karlin ET, Yang VK. Retrospective study of dilated cardiomyopathy in dogs. Journal of veterinary internal medicine. 2021 Jan;35(1):58-67. https://academic.oup.com/jvim/article-pdf/35/1/58/66649809/jvim15972.pdf 

3. Pedro B, Fontes-Sousa AP, Gelzer AR. Canine atrial fibrillation: Pathophysiology, epidemiology and classification. The Veterinary Journal. 2020 Nov 1;265:105548. https://doi.org/10.1016/j.tvjl.2020.105548