Contributors: Serena Brownlie, Peter Darke, Phil Fox, Mark Rishniw

 Species: Feline   |   Classification: Miscellaneous

Introduction

  • 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 onto 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 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 myocardial electrical activity and cardiac rhythm.
  • Under certain circumstances, the ECG may be useful in the assessment of certain electrolyte and acid-base disorders.

ECGs do NOT measure myocardial contractility.

Connecting the ECG leads

  • By convention, the small animal ECG is recorded with the patient in right lateral recumbency ECG: positioning - lateral recumbency .

If dyspnea is evident then sternal recumbency  ECG: positioning - sternal recumbency may be adopted instead. Voltages are different in this position so use a different reference range.

  • In a conventional ECG recording with Einthoven's triangle, electrodes are attached to each of the patient's legs, above the elbow and 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 between the electrode attached to either the right front (RF), left front (LF), or left hind (LH) legs.
  • Recordings are taken of electric potential between electrodes attached to different legs (leads). There are six 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:
    • Lead 1 records the potential between RF (-ve) and the LF (+ve).
    • Lead 2 records the potential between RF (-ve) and the LH (+ve).
    • Lead 3 records the potential between LF (-ve) and the LH (+ve).
    • Lead aVR records the potential between RF (+ve), LF and the LH (0).
    • Lead aVL records the potential between LF (+ve), RF and the LH (0).
    • Lead aVF records the potential between LH (+ve), RF and the LF (0).
  • A prolonged lead 2 recording is commonly used for the evaluation of cardiac rhythm and for the measurement of various ECG parameters (rhythm strip).
  • The controls of the ECG machine readily enable the user to switch from measuring one lead to another, to calibrate the response to 1 mV (usually 1 cm) and adjust the recording paper speed (typically 25 and/or 50 mm/sec).

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 usually 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.