ECG measures the electrical activity at the body surface and graphically represents depolarization and repolarization of cardiac muscle. It provides information about heart rate and intracardiac conduction and is essential for the diagnosis of abnormal cardiac rhythms. Its results may also suggest the presence of chamber enlargement, myocardial or pericardial disease, certain electrolyte imbalance, some drug toxicities … So, although it may be scary to some of us, “non-cardiologists”, it is an important tool, so don’t be too afraid to start using it. Remember, practice makes perfect 😉

The normal appearance of the P-QRS-T complex, which is the basic unit of the ECG, is probably known to us all. The first step to interpreting ECG is knowing, what waves of the complex represent. This helps us understand, why heart rhythm disorders and conduction disturbances make certain changes in the ECG recording.

  • P wave: the first upward (positive) deflection on the ECG, which represents depolarization (activation) of atrial muscle; because of the relatively small atrial muscle mass, the electrical changes (and P wave) are also small
  • PR interval: time from onset of atrial muscle activation and travelling of the depolarization wave through the AV node, the bundle of His, bundle branches and Purkinje fibres
  • QRS complex: depolarization of ventricular muscle; although individual parts of the QRS complex can be identified, it is easier to think of it as a whole
    • Q wave: a small downward (negative) deflection, caused by depolarization of the ventricular septum; it always precedes the R wave
    • R wave: a large positive wave, created by the depolarization of the ventricular myocardium (large muscle mass); it may or may not be preceded by the Q wave
    • S wave: a small negative wave following the R wave, caused by depolarization of the basilar portions of ventricles
  • ST segment: the period between ventricular depolarization and repolarization
  • T wave: ventricular muscle repolarization (preparation for the next stimulus); the T wave can be positive, negative or biphasic
  • QT interval: total time od ventricular depolarization and repolarization

ECG recording should be done in a quiet environment to calm the animal, ideally with the patient lying in right lateral recumbency on a non-conducting surface. If the patient is uncooperative or in respiratory distress, most information about heart rhythm can be gained in sternal recumbency or sitting/standing position, but the complex amplitude and orientation will be more variable. Body position is less important in cats. If needed, the animal is gently restrained to avoid motion artefacts. Trembling, shaking, panting or purring of the animal will result in baseline artefacts. Gently holding the mouth to stop panting and placing a hand on the chest or holding the limbs of a trembling animal may help. Chemical restraint should be avoided, as it affects the rate and rhythm of the heart!

Front limb electrodes are placed at the elbows (at the flexor angle or caudal and just dorsal to the elbow). Rear limb electrodes are placed at the stifles (above or below, on the dorsal aspect) or the flexor angle of the hocks. Ensure good contact of the electrodes with the skin, using an ECG gel or alcohol. Pay attention, that the electrodes do not touch each other.

A consistent approach to the ECG interpretation is recommended. Systematically check the recording from left to right. If it is difficult to read, find the recognisable part and start from there. At the start, one should know the paper speed (usually 25 or 50 mm/sec), leads used and calibration (at standard calibration 1 cm equals 1 mV). Then the heart rate is calculated, and heart rhythm is identified.

Scan the ECG for irregularities and check, if the complexes are complete. Evaluate the relationship between the P waves and QRS complexes. Are they related? Is every P wave followed by a QRS complex? Or do the QRS complexes seem to appear without connection to the P waves? Compare the lengths of PR intervals. Are they equal?

Identify the morphology of the individual waveforms. Check for the presence of the P waves and differences in their shape (are they unusually tall or broad?), abnormalities of the QRS complexes (are they too broad or tall, do they have an unusual shape?), possible elevation or depression of the ST segments and shape of the T waves (are they abnormally large, peaked?).

Finally, the individual waveforms are measured and compared to normal values. Deviations from normal values can indicate the presence of chamber enlargement or conduction abnormalities.

At the end: keep in mind, that unfortunately, ECG alone cannot give us full information about the condition of the heart. It is often poorly related to its mechanical function and does not provide information about the severity of heart disease. A normal ECG does not necessarily mean a normal heart, and when ECG is abnormal, it is important to determine the clinical significance of abnormality and if treatment is indicated or not.


Nelson R. W., Couto C. G.: Small Animal Internal Medicine, 4th Ed. Mosby Elsevier, 2009

Martin M.: Small Animal ECGs; An introductory guide, 2nd Ed. Blackwell Publishing, 2007

Fuentes V. L., Johnson L. R., Dennis S.: BSAVA Manual of Canine and Feline Cardiorespiratory Medicine, 2nd Ed. BSAVA, 2016 (reprinted)