Clinical Electrocardiography

B L Chia

• Publisher: WSPC
• Year: 2015
• eBook ISBN: 9789814723282
• Edition: 4
• Topic: Medicine
• Subtopic: Cardiology

Since the publication of the 3rd Edition 17 years ago, progress has been truly remarkable in all areas of electrocardiography. Five of the seven chapters in this 4th Edition have been extensively revised. Out of the total of 125 ECGs, 65 are new. Each ECG is crisp and clear and all the abnormalities are highlighted by arrows for easy recognition and understanding. This book is the culmination of the author's 45 years of experience in the teaching of electrocardiography to coronary care unit nurses, medical undergraduates, postgraduates, interns, residents, senior residents and above. As the title of the book implies, the approach to the subject has been entirely from the viewpoint of a clinician. Hence, clinical-electrocardiographic correlations have been emphasized simply and succinctly throughout the text.

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Since the publication of the 3rd Edition 17 years ago, progress has been truly remarkable in all areas of electrocardiography. Five of the seven chapters in this 4th Edition have been extensively revised. Out of the total of 125 ECGs, 65 are new. Each ECG is crisp and clear and all the abnormalities are highlighted by arrows for easy recognition and understanding. This book is the culmination of the author's 45 years of experience in the teaching of electrocardiography to coronary care unit nurses, medical undergraduates, postgraduates, interns, residents, senior residents and above. As the title of the book implies, the approach to the subject has been entirely from the viewpoint of a clinician. Hence, clinical-electrocardiographic correlations have been emphasized simply and succinctly throughout the text.

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Readership: Medical undergraduates, doctors, nurses and cardiology registrars.

CHAPTER 1

THE NORMAL ELECTROCARDIOGRAM

Fig. 1.1Electrodes are attached to the body surface to record an ECG. The right leg electrode functions solely as a ground to prevent alternating current interference and is a non-recording electrode. (Abbreviations: RA = right arm, LA = left arm, RL = right leg, LL = left leg).

An electrocardiogram or ECG is the recording of the electrical activity generated by the cells of the heart which reaches the surface of the body. This is achieved by attaching electrodes which are simply disposable paste-on disks or metal plates used to detect the electrical currents of the heart. These electrodes are placed on (1) the limbs – the right and left arms (both above the wrist) and the right and left legs (both above the ankles) and (2) at specific points on the anterior chest wall. All these electrodes are connected to the ECG recording machine via electrical wires. The electrode which is attached to the right leg functions solely as a ground to prevent alternating current interference (Fig. 1.1) and is therefore a non-recording electrode.

Fig. 1.2Diagram showing the positions of the electrode when recording the different left and right-sided chest leads.
1 to 6 = leads V₁ to V₆.
MCL = mid-clavicular line,
AAL = anterior axillary line,
MAL = mid-axillary line,
V₃R and V₄R = right-sided chest leads. See text.

From the 3 recording limb electrodes and the 6 recording chest electrodes, the following 12-lead ECG is obtained.

The 12-LEAD ELECTROCARDIOGRAM

The 12-lead electrocardiogram (ECG) consists of the following leads

(1) Six Limb (Extremity) Leads

The six limb leads (which are all in the frontal plane) consist of 3 bipolar leads – lead I (between the right and left arm), lead II (between the right arm and the left leg), lead III (between the left arm and the left leg), and 3 augmented unipolar leads – leads aVR (right arm), aVL (left arm) and aVF (left leg) (a = augmented).

(2) Six Unipolar Chest Leads

These are designated as V leads. There are six V leads (from V₁ to V₆) depending on where the electrode is placed on the chest (Fig. 1.2). Lead V₁ is recorded with the electrode on the fourth intercostal space just to the right of the sternum, and lead V₂ on the fourth intercostal space just to the left of the sternum. Lead V₃ is recorded at a position exactly mid-way between leads V₂ and V₄. Lead V₄ is recorded on the fifth left intercostal space in the mid-clavicular line. Leads V₅ and V₆ are recorded at the same horizontal level as lead V₄, with lead V₅ in the left anterior axillary line and lead V₆ in the left mid-axillary line.

Apart from the above conventional 12 leads, other leads such as right-sided chest leads V₃R, V₄R, V₅R and V₆R are recorded in positions which correspond to leads V₃, V₄, V₅ and V₆ respectively, except that the electrodes are now placed on the right side of the chest instead of on the left. These leads (especially lead V₄R) are particularly useful for the diagnosis of right ventricular infarction.

Recently, leads V₇, V₈ and V₉ have been reported to be valuable in the diagnosis of posterior myocardial infarction. These 3 leads are recorded at the same horizontal level as lead V₆, with the electrode being placed in the left posterior axilliary line for lead V₇, left mid-scapular line for lead V₈ and at the left border of the spine for lead V₉. Although useful, they have not proven to be popular and are not done frequently.

CALCULATION OF HEART RATE

There are 2 methods of calculating the heart rate: (1) Box Counting Method (2) QRS Counting Method.

Box Counting Method

The ECG is normally recorded at a speed of 25 mm/sec. The horizontal distance between 1 large box on the ECG paper recorded at this speed represents 0.20 sec. Since this distance spans the length of 5 small boxes, each small box therefore represents 0.04 sec.

A simple way to calculate the heart rate is to divide 300 by the number of large boxes between 2 consecutive beats if the rhythm is regular. For example, the following are the heart rates corresponding to the number of large boxes in between 2 consecutive beats.

Heart Rate	Number of Large Boxes between 2 Consecutive Beats
300	1 (300/1)
150	2 (300/2)
100	3 (300/3)
75	4 (300/4)
60	5 (300/5)
50	6 (300/6)
43	7 (300/7)
38	8 (300/8)

The rationale for the above calculation is as follows. There are 300 fifths of a second in 1 minute (5 × 60). One fifth of a second (i.e. 0.20 sec) is represented by 1 large box. Therefore, the heart rate is conveniently calculated by dividing 300 by the number of large boxes between 2 consecutive beats. The Box Counting Method is accurate when the rhythm is regular, but inaccurate when it is irregular.

QRS Counting Method

This method for counting an average heart rate is accurate when the rhythm is regular or irregular (e.g. atrial fibrillation). At the normal recording speed of 25 mm/sec, the recording time of most 12-lead ECG recordings is 10 seconds (Fig. 1.3). Simply counting the number of QRS complexes within this 10 second interval and multiplying it by 6 will give you the average heart rate in beats/min.

Fig. 1.3Calculation of heart rate by QRS counting method. The number of QRS complexes in 10 sec is 13. Therefore the heart rate is 13 × 6 = 78 beats per minute. Using the Box counting method – the number of large boxes between 2 consecutive beats is 4. Therefore the heart rate is 300 ÷ 4 = 75 beats per minute. The extreme left shows ECG calibration − 1 millivolt signal = 10 mm vertical amplitude (see text).

Calculating QRS Axis

The QRS axis is defined as the mean QRS vector in the frontal plane. It is determined using the hexaxial reference system which is described below.

The Einthoven’s equilateral triangle is formed by leads I, II and III (Fig. 1.4). A triaxial reference system is obtained by redrawing the Einthoven’s triangle so that leads I, II and III intersect at a common central point (Fig. 1.4). This is done simply by sliding lead I downwards, lead II to the right and lead III to the left.

Fig. 1.4(Left hand panel) Einthoven’s equilateral triangle. (Right hand panel) Converting Einthoven’s equilateral triangle to a triaxial reference system. (Abbreviations: RA = right arm, LA = left arm, LL = left leg, Ld I = lead I, Ld II = lead II, Ld III = lead III) (see text).

A similar triaxial reference system can be obtained, this time by using the 3 augmented unipolar leads aVR, aVL and aVF (Fig. 1.5). Finally, the 2 triaxial reference systems can be combined to obtain the hexaxial reference system that shows the relationship of all the 6 limb leads (Figs. 1.6 and 1.7).

Fig. 1.5Triaxial reference system obtained from leads aVR, aVL and aVF (see text).

Fig. 1.6Hexaxial reference system obtained from the previous 2 triaxial reference systems (see text).

Fig. 1.7Hexaxial reference system diagram that is used clinically.

When calculating the axis of the heart, it is useful to assess all the limb leads (i.e....

Table of contents

• Cover page
• Title page
• Copyright page
• Dedication
• Foreword
• Preface
• Acknowledgements
• CONTENTS
• 1. The Normal Electrocardiogram
• 2. Ischaemic Heart Disease
• 3. Miscellaneous Conditions
• 4. Cardiac Arrhythmias
• 5. Supraventricular Arrhythmias
• 6. Ventricular Arrhythmias
• 7. Bundle Branch Block, Hemiblock and Atrioventricular (AV) Block
• Index