The Unofficial Guide to ECGs - E-Book (eBook)

Unofficial Guide to ECGs - E-Book

Ali B.A.K. Al-Hadithi, Alex Hobson, Senthil Kirubakaran, Zeshan Qureshi (Herausgeber)

eBook Download: EPUB

2022 | 1. Auflage
250 Seiten
Elsevier Health Care - Lehrbücher (Verlag)
978-0-323-93283-7 (ISBN)

The unique and award-winning Unofficial Guides series is a collaboration between senior students, junior doctors and specialty experts. This combination of contributors understands what is essential to excel on your course, in exams and in practice - as well as the importance of presenting information in a clear, fun and engaging way. Packed with hints and tips from those in the know, when you are in a hurry and need a study companion you can trust, reach for an Unofficial Guide.

The Unofficial Guide to ECGs is a straightforward yet complete guide to interpreting ECGs and easily translating this knowledge to clinical contexts.

Edited by experts in cardiology, the book systematically goes through how an ECG is performed and what every part of the ECG tracing represents. It covers common conditions associated with ECG changes and explains the underlying pathophysiology of why these changes occur. An extensive collection of 40 clinical cases with fully annotated ECGs and 120 MCQs are included to allow readers to practise ECG interpretation.

Primarily aimed at medical students, this new book will also be useful for all those who may need to brush up on ECGs, including junior doctors, nurses, paramedics, physicians' associates and advanced nurse practitioners.

Clearly defined and systematic structure that is accessible to new learners

Explanations for every component of an ECG tracing and their changes to aid understanding and memorisation

40 cases with real-life ECGs, ranging in difficulty from beginner to advanced, and clinically-focused questions for the reader to test themselves

Real-life fully annotated ECG images to clearly highlight features of ECG changes

120 MCQs with detailed explanations of correct and incorrect answers alongside annotated and unannotated ECGs

A handy quick reference guide at the end of book covers ECG basics and important changes for rapid revision on the wards

1: ECG Basics

What are ECGS?

An electrocardiogram (ECG) is a recording of the heart`s electrical activity. It is derived from electrodes placed on the skin which measures small changes in voltage due to depolarisation and repolarisation of cardiac myocytes. Just like all muscle cells, contraction of heart myocytes is caused by depolarisation, so every time the heart beats, there is a wave of electrical charge spreading through the heart.

The Utility of ECGS

ECGs are an indispensable, non-invasive tool that constitute part of the basic clinical work-up. They provide information on the function of the cardiac electrical system which can be affected by a wide range of cardiac and non-cardiac pathologies (Box 1.1). In terms of cardiac pathology, ECGs can offer evidence of electrical as well as structural pathology. Furthermore, they are key to diagnosing arrhythmias and myocardial infarction.

Normal Pacemakers and Heart Conduction

An understanding of cardiac anatomy and electrophysiology is key for effective interpretation of ECGs.

Box 1.1 Pathologies that produce ECG changes

• Electrical conditions
1. • Cardiac arrhythmias
2. • Conduction block
3. • Channelopathies
4. • Pacemakers
• Structural pathologies
1. • Ventricular hypertrophy
2. • Atrial dilatation
• Myocardial ischaemia and infarction
• Electrolyte abnormalities
1. • Potassium (hypokalaemia/hyperkalaemia)
2. • Calcium (hypocalcaemia/hypercalcaemia)
3. • Magnesium (hypomagnesaemia)
• Pulmonary disease, e.g. pulmonary embolism
• Metabolic, e.g. digoxin toxicity
• Hypothermia

Cardiac Anatomy

The heart is normally made up of four chambers: the left and right atria and the left and right ventricles (Figs 1.1 and 1.2). The right atrium and ventricle are separated by the tricuspid valve; the left atrium and ventricle are separated by the mitral valve. The left and right sides of the heart are divided by the septum, a wall of muscular and connective tissue. In the ventricular septum, there are vital components of the heart’s conduction system: the bundle of His, the left and right bundle branches and the Purkinje fibres. Since the left and right side normally depolarise and repolarise simultaneously, we can often consider both atria as a single unit, and both ventricles as a single unit, for the purpose of ECG interpretation.

Electrical Conduction

The sinoatrial node (SAN) in the wall of the right atrium initiates depolarisation in the heart due to its inbuilt pacemaker activity. This wave of depolarisation spreads through the atria, causing them to contract. Electricity is unable to pass from the atria to the ventricles except at the atrioventricular node (AVN), which acts to slow down conduction between the atria and the ventricles to make sure the ventricles do not contract until the atria have fully contracted, allowing the atrium to push blood into the ventricle.

Fig. 1.1 Anatomy of the heart – chambers and valves. Illustration of the four chambers: left and right atria, and left and right ventricles. Their associated blood vessels are also shown (blue for right-sided vessels, red for left-sided vessels). Source: Aehlert, B. (2009) Anatomy and physiology. In: ECGs Made Easy, 4th ed. Maryland Heights, MO: Elsevier.

Fig. 1.2 Anatomy of the heart – vessels. The left anterior descending artery supplies the anterior two-thirds of the interventricular septum (septal perforators), anterior and lateral wall of the left ventricle (LV: diagonal branches) and sometimes part of the right ventricle (RV).Circumflex (Cx) supplies the LV lateral (anterolateral marginal branches) and posterior walls, and occasionally its inferior aspect (posterior LV arteries: 15% of patients) and the posterior septum.The right coronary artery (RCA) supplies the RV wall, and usually the posterior septum and inferior (diaphragmatic) wall of the LV (posterior LV arteries; 85% of people). The RCA is ‘dominant’ (as opposed to the Cx) if it gives rise to the posterior descending coronary artery and the posterior left ventricular arteries. Source: Bersten, A., Handy, J. M. (2018). Acute cardiac syndromes, investigations and interventions. In: Oh’s Intensive Care Manual, 8th ed. Elsevier.

After a short pause at the AVN, electricity moves down the septum through the bundle of His, into the bundle branches and then the Purkinje fibres all the way to the apex of the heart before moving up the ventricular walls (Fig. 1.3). This causes the ventricles to contract, and then relax as the cells repolarise. After a certain amount of time (depending on what the heart rate is), the SAN will depolarise again, and the whole process will repeat.

ECG Paper Basics

ECG paper conventionally shows recordings over a period of 10 s. The horizontal axis represents time moving from left to right. The vertical axis represents the level of electrical activity (voltage) (Fig. 1.4).

Fig. 1.3 Electrical conduction of the heart. Normal cardiac impulses start in the sinoatrial node and then get passed to the atrioventricular node and down the His–Purkinje fibres. Source: Elsevier collection – Cardiovascular Electrical conduction of the heart.

The ECG is printed on a grid at a rate of 25 mm/s; each small square is 1 mm in length while larger squares are 5 mm. Therefore, each small square represents 40 ms of time and larger squares represent 200 ms.

The amplitude (voltage) of electrical activity is represented by the vertical axis. Conventionally, 1 mV is represented by 10 mm. A rectangular calibration pulse of 1 mV is produced at the edge of the page to calibrate the amplitude of recordings.

ECG OF a Single Heart Beat

Several important ECG waves and intervals occur in a single heart beat that directly relate to the cardiac conduction described above (Figs 1.5 and 1.6).

P wave		First wave (usually upwards deflection) before the QRS complex	Atrial depolarisation. Abnormalities in the P wave can tell us about the size and function of the atria

QRS complex	Q	Downwards deflection preceding the first positive deflection (R wave, as defined below)	The narrow width of the QRS complex depends on speed of depolarisation through the ventricles via the His–Purkinje fibres

R	First positive deflection

S	Downwards deflection immediately following the R wave

T wave		First wave (usually upwards deflection) after the QRS complex	Ventricular repolarisation

PR interval		Beginning of the P wave to the beginning of the QRS complex	Represents the time from atrial to ventricular depolarisation

PR segment		End of the P wave to the beginning of the QRS complex	Time for depolarisation to conduct from the atria to the ventricles

Table Continued

ST segment		The end of the QRS complex to the beginning of the T wave	Represents the beginning of ventricular repolarisation. The junction between the end of the QRS complex and the start of the ST segment is termed the J-point. Abnormalities can indicate cardiac ischaemia and/or infarction

QT interval		Start of the QRS complex to the end of the T wave	Represents the time it takes for the ventricles to depolarise and then repolarise

Fig. 1.4 ECG paper axes. The ECG is recorded on graph paper divided into millimetre squares, with darker lines marking 5-mm squares.(A) Time is presented horizontally and voltage vertically on ECG tracings. With a paper speed of 25 mm/s, a large box represents 200 ms of time and a small box represents 40 ms. Most ECG recordings are done over a total period of 10 s.(B) A calibration pulse (left side of image) represents 1 mV (10 mm amplitude) of electrical activity. Source: Goldberger, A., Goldberger, Z., Shvilkin, A. (2017). ECG basics. In: Goldberger’s Clinical Electrocardiography: A Simplified Approach, 9th ed. Elsevier.

Electrode Placement

An ECG trace is recorded by placing several sticky electrodes on various parts of the body. Measuring electrical activity between electrodes allows us to produce ‘leads’ that are placed at specific anatomical locations (Figs 1.7 and 1.8). In order to obtain a 12 lead ECG, 10 electrodes are placed at specific locations to give a comprehensive...

Erscheint lt. Verlag	20.12.2022
Mitarbeit	Herausgeber (Serie): Zeshan Qureshi
Sprache	englisch
Themenwelt	Medizinische Fachgebiete ► Innere Medizin ► Kardiologie / Angiologie
ISBN-10	0-323-93283-5 / 0323932835
ISBN-13	978-0-323-93283-7 / 9780323932837

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