Cardiac Arrhythmia
An abnormal heart rhythm — too fast, too slow, or irregular. Arrhythmias range from clinically insignificant incidental findings to life-threatening emergencies requiring immediate intervention.
A cardiac arrhythmia is any deviation from the normal sinus rhythm of the heart. The normal canine heart rate is 60-140 beats per minute (varying by size and fitness), with a regular rhythm generated by the sinoatrial (SA) node — the heart’s natural pacemaker. Arrhythmias occur when the electrical system that coordinates heartbeats malfunctions.
Classification
By Rate
- Tachyarrhythmias: abnormally fast heart rates. Include supraventricular tachycardia (SVT), atrial fibrillation, and ventricular tachycardia.
- Bradyarrhythmias: abnormally slow heart rates. Include sick sinus syndrome, atrioventricular (AV) block, and atrial standstill.
By Origin
- Supraventricular: originating above the ventricles (atria, AV node). Generally less immediately dangerous than ventricular arrhythmias, though not always benign.
- Ventricular: originating in the ventricles. Ventricular tachycardia and ventricular fibrillation are the most immediately life-threatening arrhythmias.
By Clinical Significance
- Benign: clinically insignificant, no treatment needed (e.g., respiratory sinus arrhythmia — a normal variation in dogs)
- Hemodynamically significant: arrhythmia impairs cardiac output, causing weakness, syncope, or exercise intolerance
- Malignant: risk of sudden cardiac death (e.g., sustained ventricular tachycardia, ventricular fibrillation)
Breed Predispositions
- Boxer: arrhythmogenic right ventricular cardiomyopathy (ARVC) causing ventricular arrhythmias. Can cause sudden death in apparently healthy dogs.
- Doberman Pinscher: dilated cardiomyopathy frequently presents with ventricular arrhythmias before clinical heart failure.
- German Shepherd: inherited ventricular arrhythmias
- Cavalier King Charles Spaniel: mitral valve disease leading to atrial fibrillation in advanced cases
Diagnosis
- Cardiac auscultation: irregular rhythm, abnormal rate, or pulse deficits detected on physical examination
- Electrocardiogram (ECG): the definitive diagnostic tool for arrhythmia classification
- Holter monitor: 24-hour ambulatory ECG recording that captures intermittent arrhythmias missed on in-clinic ECG
- Echocardiogram: evaluates structural heart disease underlying the arrhythmia
Treatment Approach
Treatment depends on the arrhythmia type, underlying cause, and hemodynamic impact. Not all arrhythmias require treatment. Antiarrhythmic drugs (sotalol, mexiletine, lidocaine, diltiazem), pacemaker implantation (for severe bradyarrhythmias), and treatment of the underlying heart disease are the primary therapeutic tools.