Rapid Review of ECG Interpretation in Small Animal Practice

Mark Oyama, Marc S. Kraus, Anna R Gelzer, Mark A Oyama

• Publisher: CRC Press
• Year: 2019
• eBook ISBN:
  9780429627903
• Topic: Medicine
• Subtopic: Veterinary Medicine

The standard electrocardiogram (ECG) is an indispensable, safe, and inexpensive test to assess dogs and cats with heart disease. This bestselling user-friendly book discusses the principles of electrocardiography, then systematically explores the evaluation of the ECG, including determination of heart rate, measurement of intervals, derivation of mean electrical axis, and criteria for atrial/ventricular enlargement or hypertrophy. At the core of this book is an extensive series of ECG cases for the reader to work through: practice makes perfect.

New to this edition:

• Instructions on how to obtain an ECG
• A new chapter on the treatment of the most common clinically important ECG arrhythmias
• A second new chapter on 24-hour ECG (Holter) monitoring
• A handy one-page reference guide of important ECG values and diagrams, which can be downloaded from the book's webpage for easy reference
• 15 new ECG cases have been added to the original 46 cases, with a selection of advanced cases geared toward readers craving more challenging topics such as electrical cardioversion and pacemaker function.

This updated edition will further aid veterinarians in their quest to better interpret the ECG. It provides information in an appealing, accessible, and easy to use format that fits with the busy lives of veterinary practitioners.

Section 1

Principles of Electrocardiography

The electrocardiogram (ECG) is a graphical record of electric potentials generated by the heart muscle during each cardiac cycle. These potentials are detected on the surface of the body using electrodes attached to the limbs and chest wall, and are then amplified by the electrocardiograph machine and displayed on special graph paper in voltage and time. The ECG serves to characterize arrhythmias and conduction disturbances.

Indications for ECG recordings

•Evaluating arrhythmias and heart rate disturbances detected on auscultation.

•History of syncope (fainting) or episodic weakness.

•Cardiac monitoring during anesthesia.

•Cardiac monitoring in critically ill patients.

•Monitoring changes in rate and rhythm due to drug administration.

•Assessing changes in ECG morphology and heart rate due to electrolyte imbalances associated with extracardiac disease or drug toxicities.

•In addition, the ECG may also be helpful to identify anatomical changes due to myocardial hypertrophy or dilation, and detect pericardial disease. However, echocardiography has largely replaced the ECG for these indications due to its superior sensitivity.

ECG lead terminology

In order to record an ECG waveform, a differential recording is made between two electrodes, placed on different points on the body. One of the electrodes is labeled positive, and the other negative. The positions of the electrodes on the body are standardized (Fig. 1.1) and defined as RA = right arm, LA = left arm, and LL = left leg. The output from each electrode pair (differential recording) is referred to as a lead and numbered with the Roman numerals I, II, and III. These leads are called limb leads.

Fig. 1.1 The standardized positions of the electrodes on the body are defined as RA = right arm, LA = left arm, and LL = left leg. The output from each electrode pair is referred to as a lead and numbered with the Roman numerals I, II, and III.

The limb leads form the points of what is known as Einthoven’s triangle (Fig. 1.2). The positive pole for lead I is on the left arm, the positive pole for lead II on the left leg and the positive pole for lead III is on the left leg. An imaginary line connecting the two electrodes is the lead axis. Each lead looks at the heart from a different angle within the animal’s frontal plane. There are 12 standard ECG leads used in veterinary medicine, which provide information regarding the magnitude of the electrical activity of the heart and the direction of a moving depolarization wavefront in multiple orientations. A wavefront traveling toward the positive terminal of a lead results in a positive deflection of the ECG in that lead. When a wavefront travels away from the positive electrode, a negative deflection occurs. A lead axis in parallel to the direction a wavefront is moving results in a large deflection, while a lead axis perpendicular to the direction of a moving wavefront results in a small (or no) deflection on the ECG.

Fig. 1.2 The limb leads (I, II, III) form the Einthoven triangle.

The limb leads

There are six limb leads which assess the cardiac depolarization in the frontal plane. A modified version of the frontal plane in the dog is shown in Figure 1.3. Leads I, II, and III directly record the electrical activity between two limb electrodes (Table 1.1) and are thus referred to as bipolar limb leads. The unipolar (augmented) limb leads use the same electrodes as lead I, II, and III, and also measure the electrical activity between two terminals, but the recording electrode is always positive and the negative terminal is made up of the sum of the electrodes attached to the right arm, left arm, and left leg (Fig. 1.4). These limb leads are named after their positive electrode, located on the left arm (aVL), the right arm (aVR), and the left foot (aVF), where the “a” stands for augmented, and “V” stands for vector (Fig. 1.5). Together with leads I, II, and III, augmented limb leads aVR, aVL, and aVF form the basis of the hexaxial reference system, which is used to calculate the heart’s electrical axis in the frontal plane. The positive and negative terminals of the six limb leads are listed in Table 1.1.

Fig. 1.3 The limb leads display the cardiac depolarization in the frontal plane. The chest leads display the depolarization in the transverse plane.

Fig. 1.4 The augmented (unipolar) limb leads use the same electrodes as leads I, II, III, but the recording electrode is always positive and the negative terminal is made up of the sum of the electrodes attached to the right arm, left arm, and left leg.

Fig. 1.5The augmented limb leads are named after their positive electrode, located on the left arm (aVL), the right arm (aVR), and the left foot (aVF), where the “a” stands for augmented, and “V” stands for vector.

Table 1.1 The positive and negative terminals of the six ECG limb leads

Bipolar limb leads		Augmented unipolar limb leads
I	R arm (–) to L arm (+)	aVR	R arm (+) to common terminal (–)
II	R arm (–) to L foot (+)	aVL	L arm (+) to common terminal (–)
III	L arm (–) to L foot (+)	aVF	L foot (+) to common terminal (–)

The precordial or chest leads (V leads)

The chest leads view the heart’s electrical activity in the transverse plane (see Fig. 1.3, p. 2). This complements the information regarding the electrical fields gained from the six limb leads. The chest leads are termed V (voltage) leads. They are considered to be unipolar with the positive exploring electrode placed on the chest (precordium). The electrodes for the chest leads (V1, V2, V3, V4, V5, and V6) are placed directly on the chest (Fig. 1.6). In dogs, V1 is placed to the right of the sternum at the 5th intercostal space (ICS). The 6th ICS is used for all the left sided leads V2-V6. V2 is placed just to the...

Table of contents

• Cover
• Half Title
• Title Page
• Copyright Page
• Contents
• Preface
• Authors
• Abbreviations
• Section 1: Principles of Electrocardiography
• Section 2: Evaluation of the Electrocardiogram
• Section 3: Approach to Evaluating Arrhythmias
• Section 4: Treatment of Arrhythmias
• Section 5: Holter Monitoring
• ECG Cases
• Reference Guide
• Index