Small Animal ECGs

An Introductory Guide

Mike Martin, Mike Martin

• Publisher: Wiley-Blackwell
• Year: 2015
• Print ISBN: 9781118409732
• eBook ISBN: 9781118409725
• Edition: 3
• Topic: Medicine
• Subtopic: Veterinary Medicine

Small Animal ECGs: An Introductory Guide provides all the information veterinarians need when using electrocardiography techniques for the first time.

• An ideal introduction to veterinary electrocardiography written in a very easy to understand way, for what can be a daunting subject

• The author is RCVS Recognised Specialist in Veterinary Cardiology and regularly speaks on this subject

• Full of examples of colour ECG tracings, as well as colour illustrations to explain arrhythmias

• Covers techniques that can readily be used in first opinion small animal practice

• Includes new chapters on mechanisms of supraventricular arrhythmias, accelerated idioventricular rhythm and use of Holters

Part 1
Understanding the electricity of the heart and how it produces an ECG complex

1. Chapter 1: What is an ECG?
2. Chapter 2: The electricity of the heart
3. Chapter 3: Rhythms of sinus origin

Chapter 1
What is an ECG?

An electrocardiograph (ECG), in its simplest form, is a voltmeter (or galvanometer) that records the changing electrical activity in the heart by means of positive (+ve) and negative (−ve) electrodes (Fig. 1.1). Electrocardiography is the process of recording these changing potential differences.

Figure 1.1 An ECG records the ‘electricity of the heart’.

A +ve and a −ve electrode can be placed almost anywhere on, or in, the body to record electrical changes. One of the most common and simplest methods is to place these electrodes on the limbs of animals – referred to as a body surface limb ECG recording. However, if monitoring an ECG during anaesthesia or echocardiography, for example, it is sufficient to place the –ve electrode anywhere cranial to the heart (e.g. the forelimb or neck) and the +ve electrode caudal to the heart (e.g. the hind limb, abdomen or flank). Electrodes can also be placed on the chest (precordial chest ECG recording – commonly used in humans) or inside the cardiac chambers (used in electrophysiological studies). This book focuses on the conventional limb ECG recording, which is the method most commonly used in veterinary medicine for clinical diagnosis of arrhythmias.

Chapter 2
The electricity of the heart

Electrical coordination of atrioventricular contraction

For the heart to function efficiently as a ‘circulatory pump’, it must have a coordinated contraction: the two atria contracting and passing blood into the two ventricles, followed by contraction of the ventricles, pumping blood into the aorta and pulmonary artery; that is, there must be a coordinated atrioventricular (AV) contraction. In order for the cardiac muscle cells to contract, they must first receive an electrical stimulus. It is this electrical activity that is detected by an ECG.

The electrical stimulus must first depolarise the two atria. Then, after an appropriate time interval, it must depolarise the two ventricles. The heart must then repolarise (and ‘refill’) in time for the next stimulus and contraction. Additionally, it must repeatedly do so, increasing in rate with an increase in demand and conversely slowing at rest.

Formation of the normal P–QRS–T complex

All cells within the heart have the potential to generate their own electrical activity; however, the sinoatrial (SA) node is the fastest part of the electrical circuit to do so and is therefore the ‘rate controller’, termed the pacemaker. The sinus node rate is, therefore, the dominant pacemaker (over the other cells in the heart) by being the fastest and by a mechanism termed overdrive suppression. The rate of the SA node is influenced by the balance in the autonomic tone, that is, the sympathetic (increases rate) and parasympathetic (decreases rate) systems.

The electrical discharge for each cardiac cycle (Fig. 2.1) starts in the SA node. Depolarisation spreads through the atrial muscle cells. The depolarisation wave then spreads through the AV node; however, it does so at a relatively slower rate, creating a delay. Conduction passes through the AV ring (from the atria into the ventricles) through a narrow pathway called the bundle of His. This then divides in the ventricular septum into left and right bundle branches (going to the left and right ventricles). The left bundle branch divides further into anterior and posterior fascicles. The conduction tissue spreads into the myocardium as very fine branches called Purkinje fibres.

Figure 2.1 Illustration of the heart's electrical circuit. SA – sinoatrial; AV – atrioventricular; RA – right atrium; LA – left atrium.

Formation of the P wave

The SA node is therefore the start of the electrical depolarisation wave. This depolarisation wave spreads t...

Table of contents

• Cover
• Title Page
• Dedication
• Copyright
• Table of Contents
• Preface to third edition
• About the author
• Acknowledgements
• Abbreviations
• Part 1: Understanding the electricity of the heart and how it produces an ECG complex
• Part 2: Abnormal electricity of the heart
• Part 3: More advanced electrocardiography
• Part 4: Management, clinical significance and treatment of arrhythmias
• Part 5: Recording and interpreting ECGs
• Further reading
• Index
• End User License Agreement