Contributors: Serena Brownlie, Peter Darke, Mark Rishniw

 Species: Canine   |   Classification: Miscellaneous


  • An electrocardiogram (ECG), is a recording of the electrical potential generated within the heart during the cardiac cycle.
  • It is a measurement of voltage (amplitude), against time.
  • The electrical potential generated during the cardiac cycle is measured by connecting electrodes (attached to specific body sites), to a galvanometer and the changes in electrical potential with time are recorded on to graph paper.
  • The cardiac cycle is usually initiated by an electrical impulse from the sinus node and a wave of depolarization and subsequent repolarization spread throughout the cardiac myocardium.
  • The ECG machine measures the sum of this electrical activity, as detected by the surface electrodes, against time and, providing that the electrodes are placed correctly, an examination of the subsequent ECG tracing will provide useful information on cardiac structure and arrhythmias.
  • Under certain circumstances the ECG may be useful in the assessment of some electrolyte and acid-base disorders.

Connecting the ECG leads

  • By convention, the small animal ECG is recorded with the patient in right lateral recumbency.
    If dyspnea is evident then sternal recumbency may be adopted instead. Voltages are different in this position so use a different reference range.
  • In a conventional ECG recording, electrodes are attached to each of the patient's legs at or just below either the elbow or stifle joints respectively.
  • The electrode attached to the right hind leg acts as the ground or earth connection.
  • The ECG machine is then able to display the sum of the electrical potential that results when measurements taken from the electrode attached to either the right front (RF), or left front (LF), or left hind (LH), legs is compared with the measurements taken from either one or both of the two remaining electrodes.
  • This comparison of measurements between electrodes attached to different legs is termed a 'lead recording' and there are 6 possible combinations to choose from.
  • The lead recordings are given the titles - 1, 2, 3, aVR, aVL and aVF:
    • Leads 1, 2, and 3 are known as the standard bipolar leads.
    • Leads aVR, aVL and aVF are known as the augmented unipolar limb leads.
  • Thus:
    • A lead 1 recording compares the RF (-ve) with the LF (+ve).
    • A lead 2 recording compares the RF (-ve) with the LH (+ve).
    • A lead 3 recording compares the LF (-ve) with the LH (+ve).
    • A lead aVR recording compares the RF (+ve) with the LF and LH (0).
    • A lead aVL recording compares the LF (+ve) with the RF and LH (0).
    • A lead aVF recording compares the LH (+ve) with the RF and LF (0).
  • The lead 2 recording is commonly used for the evaluation of cardiac rhythm and for the measurement of various ECG parameters.
  • The controls of the ECG machine readily enable the user to switch from measuring one lead to another.

Interpreting the ECG

  • When a normal lead 2 ECG tracing is examined, the pattern of electrical potential associated with a single cardiac cycle may be broken down into a number of different regions or 'waves'.
  • By convention, these are the P, Q, R, S and T waves respectively.
  • The Q, R and S waves are often grouped together for ease of description and are referred to as the QRS complex.
  • These waves are produced by the electrical activity during various stages of the cardiac cycle.
  • Thus:
    • P wave is produced by atrial depolarization.
    • QRS complex by ventricular depolarization.
    • T wave by ventricular repolarization.