Electrocardiograms in selected species

Abstract

The electrocardiogram, or ECG, is the recording of the electrical activity of the heart from electrodes placed on the surface of the body. The recorded electrical activity results from the depolarization of the individual cardiomyocytes that give rise to small extracellular potential changes. When two electrodes (referred to as a lead) are attached to the body surface, a circuit is formed, and by using an amplifier and a voltage meter, the changes in voltage between these two electrodes can be measured. When these signals are recorded continuously as voltage against time the (bipolar) ECG is formed. In every lead, one electrode is designated the positive side and the remaining electrode(s) as the negative side. By definition, all deflections toward the positive pole are recorded as positive, or upward, deflections and those away from this pole are recorded as negative, or downward, deflections. The movement of electrical charges in the heart is a three-dimensional event, and the net resulting charge at any moment in the cardiac cycle can be represented by a vector, with both a direction and a magnitude. This cardiac vector is represented by an arrow that describes these characteristics: the direction is represented in the shaft’s direction, the polarity with the arrow head and the magnitude with the shaft’s length. The surface ECG is therefore the projection of the cardiac vector in a plane determined by the position of the electrodes. For example, if the electrodes are placed on both front limbs and the left hind limb the ECG measured between these electrodes corresponds to the frontal plane leads I, II, and III.