In quadripeds, the magnitude and direction of electrocardiographic vectors determined from limb leads can be vastly altered by changes in the position of muscular attachments of the shoulder girdle to the thorax.Įach pair of limbs should be held in parallel and limbs should not be allowed to contact one another. Noises from clinical activity and other animals may significantly affect a patient’s heart rate and rhythm. The ECG should be recorded in an area as quiet and distraction-free as possible. With ECG machines that utilize four electrodes, the electrode placed on the right hind leg is the ground (it is not part of any of the leads). The net depolarization moving through the ventricles (green arrow) is normally oriented toward the left hind leg (the positive pole of lead II) in dogs and cats, and therefore the QRS complex is predominantly positive in lead II. In lead III the left foreleg (LA) is negative and the left hindleg (LL) is positive. In lead II the right foreleg (RA) is negative and the left hind leg (LL) is positive. In lead I, the right foreleg (RA) is negative and the left foreleg (LA) is positive. The three bipolar frontal plane leads (I, II and III) and Einthoven’s triangle (red). We will be discussing primarily rhythm diagnosis.įigure 1. For the purposes of this overview, we’ll focus on lead II, a bipolar lead in which the right arm (RA-the right foreleg in veterinary patients) is negative and the left leg (LL-or left hind leg), is positive (Figure 1). A single lead would provide information on only one dimension of current flow. Standard electrocardiographic leads are used to create multiple angles to assess the waveforms that travel through the three-dimensional heart. Electrical impulses with a net direction perpendicular to the positive electrode will not generate a waveform or deflection at all and are said to be isoelectric.
Electrical impulses with a net direction toward the positive electrode will generate a positive waveform or deflection, and those directed away from the positive electrode will generate a negative waveform or deflection.Īs the angle between the lead axis and the direction of the activation wave increases, the ECG deflection in that lead becomes smaller. The orientation of a lead with respect to the heart is called the lead axis. A lead consists of the electrical activity measured between a positive electrode and a negative electrode.
The bipolar triaxial lead system we use today was developed by Dutch physiologist Willem Einthoven in the early 20th century, along with the P-QRS-T terminology that describes the ECG waveform complexes. It’s also important to remember that the ECG may be normal even in the face of advanced cardiovascular disease. However, the ECG does not record cardiac mechanical activity, so it does not yield information regarding cardiac contractility.
The ECG may also yield useful information regarding chamber dilation and hypertrophy. It is the most important test to perform in animals with an auscultable arrhythmia (other than sinus arrhythmia in dogs). The electrocardiogram (ECG) is a valuable diagnostic test in veterinary medicine and is easy to acquire. Specific waveforms represent stages of myocardial depolarization and repolarization. It is a record of the average electrical potential generated in the heart graphed in voltage and time. Electrocardiography is the recording at the body surface of electrical activity in the heart.