ECG interpretation starts with recognizing normal sinus rhythm. Key arrhythmias: sinus arrhythmia (normal in dogs), atrial fibrillation, VPCs, AV blocks. Emergency: VT with hemodynamic compromise, VF, severe bradycardia.
Normal sinus rhythm (NSR) originates from the sinoatrial (SA) node. On ECG: regular P waves (upright in lead II), each followed by a QRS complex, with a consistent PR interval. Heart rate is within normal range for the species: dogs 60-120 bpm (larger dogs slower), cats 120-180 bpm. NSR indicates the electrical conduction system is functioning normally.
Sinus arrhythmia is a normal finding in dogs. It is a rhythm irregularity in which the R-R interval varies with the respiratory cycle: heart rate increases during inspiration and decreases during expiration. This is due to changes in vagal tone with respiration. P waves and QRS complexes are normal; only the rhythm is irregular. Sinus arrhythmia is a sign of healthy vagal tone and is not treated. In cats, sinus arrhythmia is less common and may warrant closer evaluation.
Atrial Fibrillation
Atrial fibrillation (AF) is a common arrhythmia in dogs, especially large breeds with underlying heart disease (e.g., dilated cardiomyopathy). The atria fibrillate rather than contract in an organized way. On ECG: irregularly irregular rhythm, no P waves (replaced by f waves or a wavy baseline), and variable R-R intervals. Ventricular rate may be rapid (150-250 bpm) or controlled. AF reduces cardiac output and may cause or worsen heart failure. Treatment goals: rate control (digoxin, diltiazem, beta-blockers) and management of underlying disease. Cardioversion is rarely used in veterinary medicine.
Ventricular Premature Complexes
Ventricular premature complexes (VPCs, or PVCs) originate from the ventricles rather than the SA node. On ECG: wide, bizarre QRS complexes that appear early, often without a preceding P wave. The QRS morphology differs from the sinus beats. Occasional VPCs may be benign; frequent VPCs, runs of ventricular tachycardia (VT), or VPCs in a patient with syncope or heart failure warrant treatment. Antiarrhythmics (lidocaine, sotalol, mexiletine) may be used. Underlying causes (myocardial disease, electrolyte imbalance, toxin) should be addressed.
AV blocks occur when conduction from atria to ventricles is delayed or blocked. First-degree AV block: prolonged PR interval; every P wave conducts. Usually benign. Second-degree AV block: some P waves do not conduct. Mobitz Type I (Wenckebach) shows progressive PR prolongation before a dropped beat; often benign. Mobitz Type II shows sudden non-conduction without PR prolongation; more serious. Third-degree (complete) AV block: no P waves conduct; atria and ventricles beat independently. Ventricular escape rhythm is slow (20-40 bpm in dogs). Pacemaker is typically required.
Emergency Arrhythmias: Ventricular tachycardia with hemodynamic compromise (weak pulse, collapse); ventricular fibrillation (chaotic—defibrillation required); severe bradycardia (complete AV block with syncope); rapid AF with heart failure. Treatment: lidocaine for VT, atropine or pacing for bradycardia, rate control for AF. Always assess perfusion and treat the patient, not just the ECG.
Practice ECG interpretation with the ECG Reader AI and discuss findings with the Cardiology Specialist.
- NSR—regular P-QRS, normal rate (dogs 60-120, cats 120-180 bpm).
- Sinus arrhythmia—normal in dogs; R-R varies with respiration.
- AF—irregularly irregular, no P waves; rate control + underlying disease.
- VPCs—wide bizarre QRS; treat if frequent, VT runs, or syncope/heart failure.
- Emergency—VT with compromise, VF, severe bradycardia, rapid AF with CHF.