Veterinary ECG Reference (Normal Intervals - Dog & Cat)
Lead II ECG reference values for dogs and cats. Heart rate, intervals, amplitudes, and mean electrical axis.
Heart rate (bpm)
| Population | Range |
|---|---|
| Adult dog (general) | 60–160 |
| Toy breed | 90–160 |
| Giant breed | 60–100 |
| Puppy | up to 220 |
Intervals (Lead II, 25 mm/s)
| Wave / interval | Range |
|---|---|
| P duration | ≤ 0.04 s (40 ms) |
| P amplitude | ≤ 0.4 mV |
| PR interval | 0.06–0.13 s (60–130 ms) |
| QRS duration | ≤ 0.06 s (small breeds 0.05) |
| R amplitude | < 3.0 mV (large breed); 2.5 (small) |
| QT interval | 0.15–0.25 s (varies with HR) |
| Mean electrical axis | +40 to +100° |
Reference values for lead II ECG at standard paper speed. Cat ECGs are usually run at 50 mm/s to better resolve the fast intervals. Always interpret in context of the patient and clinical picture.
Frequently asked questions
What is the normal QRS duration in dogs?
Small breed: < 0.05 s. Large breed: < 0.06 s. A wide QRS suggests ventricular enlargement, ventricular ectopy, or a conduction abnormality (bundle branch block).
Why are cat ECGs lower amplitude?
Smaller heart mass + thicker chest wall relative to heart size. Normal R wave is < 0.9 mV in cats vs up to 3.0 mV in some dogs. Tall R waves in cats suggest left ventricular hypertrophy (HCM).
What is sinus arrhythmia and when is it normal?
Cyclic variation in HR with respiration - normal and very common in dogs, particularly brachycephalic breeds. Pronounced in resting dogs; disappears with sympathetic drive. Almost never normal in cats - investigate cardiac disease.