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Electrocardiography
Anna R.M. Gelzer, DVM, PhD, DACVIM (Cardiology), DECVIM-CA (Cardiology)
and Marc S. Kraus, DVM, DACVIM (SAIM, Cardiology), DECVIM-CA (Cardiology)

Department of Clinical Sciences & Advanced Medicine‐Philadelphia, School of Veterinary Medicine, University of Pennsylvania, Philadelphia, PA, USA

Cardiac arrhythmias are defined as variations of the car- using electrodes attached to standardized locations on
diac rhythm from normal sinus rhythm. Some cardiac the limbs and chest wall. The electrical signals are ampli-
arrhythmias are benign and clinically insignificant and fied by the electrocardiograph machine and displayed on
require no specific therapy whereas others can manifest special graph paper in voltage and time. The limb leads
with a variety of clinical signs, such as episodic weak- are defined as RA = right arm, LA = left arm, and LL =
ness or collapse (syncope), exercise intolerance or inap- left leg. The output from each electrode pair (differential
propriate panting/restlessness or sudden death (i.e., recording) is referred to as a lead and numbered with
ventricular fibrillation). In order to diagnose the nature the Roman numerals I, II, and III. The limb leads form
of an arrhythmia and determine therapeutic strategies, the points of what is known as Einthoven’s triangle
it is imperative to acquire an electrocardiogram (ECG). (Figure 17.1). An imaginary line connecting the two elec-
trodes is the lead axis. Each lead looks at the heart from
a different angle within the animal’s frontal plane.
Indications for ECG Recordings
The Six Limb Leads
Document arrhythmias detected by auscultation.
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Evaluate a patient presenting for syncope (fainting) or There are six limb leads which depict the cardiac depo-
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episodic weakness. larization in the frontal plane. Leads I, II, and III are
Heart rate and rhythm monitoring during anesthesia. called bipolar limb leads because they record the electri-
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Heart rate and rhythm monitoring in critically ill patients. cal activity between two limb electrodes (Table 17.1).
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Monitoring changes in rate and rhythm following drug Using the same electrodes as leads I, II, and III, the uni-
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administration. polar limb leads also measure the electrical activity
Assessing changes in ECG morphology and heart rate between two terminals, but the recording electrode is
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due to electrolyte imbalances associated with extracar- always positive and the negative terminal (called Wilson’s
diac disease or drug toxicities. central terminal) is made up of the sum of the electrodes
Detection of pericardial disease. attached to the right arm, left arm, and left leg. These uni-
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Identifying conduction disturbances or axis shifts polar limb leads view the heart from different angles (or
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associated with myocardial hypertrophy or dilation. vectors) than the bipolar leads, because the negative elec-
Ambulatory ECG recording (Holter) for long‐term trode is zeroed out, allowing the positive electrode to
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monitoring of intermittent arrhythmias. become the “exploring” electrode. The unipolar limb leads
generate a very low voltage signal, so need to be amplified
by the ECG machine (augmented). These limb leads are
ECG Lead Terminology named after their positive electrode, located on the Left
and Acquisition Technique arm (aVL), the Right arm (aVR), and the left Foot (aVF),
where the “aV” stands for augmented voltage (Figure 17.2).
Electrocardiography is used to record electric potentials Together with leads I, II, and III, augmented limb leads
generated by the heart muscle during each cardiac cycle. aVR, aVL, and aVF form the basis of the hexaxial refer-
These potentials are detected on the surface of the body ence system (see Figure 17.7) which is used to calculate

Clinical Small Animal Internal Medicine Volume I, First Edition. Edited by David S. Bruyette.
© 2020 John Wiley & Sons, Inc. Published 2020 by John Wiley & Sons, Inc.
Companion website: www.wiley.com/go/bruyette/clinical