Rapid Review of ECG Interpretation

131 Pages·2005·2.58 MB·English

by  Azeem

Rapid Review of
ECG
Interpretation
Tariq Azeem
MBBS, MRCP
Specialist Registrar in Cardiology
Glenfield Hospital
Leicester, UK
Michael Vassallo
MD, FRCP, DGM, MPhil, PhD
Consultant Physician
The Royal Bournemouth Hospital
Bournemouth, UK
Nilesh J Samani
BSc, MD, FRCP, FACC, FMedSci
Professor of Cardiology
University of Leicester and Glenfield Hospital
Leicester, UK
MANSON
PUBLISHING
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Contents
Preface 4
Abbreviations 4
Section 1: Principles of electrocardiography 5
Explaining the terminology 5
The electrodes 5
The leads 6
The P-QRS-T complex 8
Action potential 8
The cardiac conducting system 8
Components of the P-QRS-T complex 8
The 12 lead ECG 10
Limb leads 10
Chest (precordial) leads 12
Section 2: Comprehensive assessment of the ECG 13
Rate 13
Calculating the heart rate 14
Rhythm 14
The cardiac axis 14
The AP axis 15
Calculating the QRS axis 15
Axis deviation around the vertical axis 18
The P wave 19
The PR interval 19
The Q (q) wave 20
The QRS complex 20
The ST segment 22
ST segment elevation 22
ST segment depression 23
The T wave 24
The U wave 24
The QT interval 25
Section 3: Cases 27
Classification of cases 127
Index 127
4 Rapid Review of ECG interpretation
Preface
The electrocardiogram (ECG) is a commonly used common ECG abnormalities are provided. The
investigation in clinical practice. A working know- second part of this book deals with more practical
ledge of the electrocardiogram is essential for the aspects of electrocardiography. By discussing
practising clinician. This book is intended as a 50 real case histories that illustrate all the major
succinct and enjoyable primer for junior doctors ECG abnormalities, it is hoped a more practical
and students, including candidates preparing for approach to learning the ECG is provided.
both undergraduate and postgraduate examina-
tions. The first section explains some of the basic
principles of electrocardiography and includes a Tariq Azeem
framework for the systematic evaluation of the Michael Vassallo
ECG. Tables listing the differential diagnosis of Nilesh J Samani
Abbreviations
ACC/AHA American College of L left
Cardiology/American Heart Association LAD left axis deviation
AF atrial fibrillation LDH lactate dehydrogenase
ALT alanine amino transferase LFT liver function tests
AP anteroposterior LGL Lown–Ganong–Levine (syndrome)
ASD atrial septal defect LVH left ventricular hypertrophy
ATP adenosine triphosphate MCV mean cell volume
AV atrioventricular MI myocardial infarction
AVNRT atrioventricular nodal re-entrant Na+ sodium ion
tachycardia NSTEMI non-ST elevation myocardial infarction
AVRT atrioventricular re-entrant tachycardia QTc corrected QT interval
BBB bundle branch block R right
BP blood pressure RAD right axis deviation
bpm beats per minute RV right ventricle
CCU coronary care unit RVH right ventricular hypertrophy
CK/CKMB creatinine kinase/creatinine kinase SA sinoatrial
MB fraction STEMI ST segment elevation myocardial
CSH carotid sinus hypersensitivity infarction
CSM carotid sinus massage SVT supraventricular tachycardia
CSS carotid sinus syndrome TFT thyroid function tests
CT computed tomography TSH thyroid stimulating hormone
CXR chest X-ray U&E urea and electrolytes
ECG electrocardiogram VQ (scan) ventilation perfusion (scan)
FBC full blood count VSD ventricular septal defect
Hb haemoglobin VT ventricular tachycardia
ICD implantable cardiac defibrillator WPW Wolff–Parkinson–White (syndrome)
K+ potassium ion
5
Section 1
Principles of electrocardiography
Explaining the terminology
The electrodes
The ECG is recorded by applying electrodes to the
limbs and to different parts of the praecordium.
There are four limb and usually six chest electrodes
(V1–V6). Occasionally additional chest electrodes
are used to obtain more information regarding the
right ventricle and posterior aspects of the heart.
The position of the commonly used electrodes is
given in Table 1. These positions are standardized
and are shown in (1) and (2). These electrodes are
used to generate the leads.
Table 1 Standard positions of the
electrodes
Electrodes Standard position
Limb R arm
L arm
1Position of limb electrodes.
R leg (earth)
L leg
Chest
V1 4th intercostal space, just
right of the sternum
V2 4th intercostal space, just
left of the sternum
V3 Mid-way between V2 and V4
V4 5th intercostal space in the
mid-clavicular line
V5 At the same level as V4 in
V1 V2
the anterior axillary line
V3
V6 At the same level as V5 in V6
V5
the mid-axillary line V4
Optional leads
V7 At the same level as V6 but
in the posterior axillary line
V3R Equivalent position of V3
on the right side
V7R Equivalent position of V7
on the right side
2 Position of chest electrodes.
6 Section 1 Principles of electrocardiography
The leads the central terminal) is made up of the sum of the
An important concept to understand is that the electrodes attached to the right arm, left arm, and
leads assess the heart’s electrical activity from left leg. The sum of these three limb electrodes is
particular viewpoints and should not be confused at all times equal to zero potential. The voltage
with the electrodes or connecting wires that record using this method is rather low and therefore
this electrical activity. A lead is obtained from two needs to be augmented, hence the term
electrodes attached to the body. One of the ‘augmented’ or ‘a’ unipolar leads.
electrodes is labelled as positive and the other
negative. The imaginary line joining the two The chest (precordial)leads
electrodes is the lead axis. For any electrical activity The chest electrodes generate the chest leads V1,
in the heart, the deflection in that lead will point V2, V3, V4, V5, and V6. The chest leads
upwards if the overall balance of the activity is measure the potential between these electrodes
moving towards the positive terminal and and the central terminal. These view the heart in
downward pointing if it is moving away from the the horizontal plane (5).
positive terminal. There are two types of leads: the By convention, the standard ECG is composed
limb leads and the chest leads. of the above 12 leads so that all ECGs have the
same format wherever they are recorded.
The limb leads However, additional electrodes can be used to
The limb leads assess the electrical activity of the view other areas of the heart, such as the right
heart in the frontal plane. There are six limb leads: ventricle (V3R) or posterior surface of the heart
LI, LII, LIII, aVR, aVL, and aVF. The positive and (V7, V8, V9) (5).
negative terminals of the common leads are given in One should now be able to understand the
Table 2 and their relation to each other is shown in anatomical relationship of the ECG leads to the
(3) and (4). heart and the terms used in interpreting the
The LI, LII, and LIII leads are referred to as ECG. The inferior leads are LII, LIII, and aVF
bipolar leads as they directly assess the activity because they look at the heart from the inferior
between two of the limb electrodes. LI measures aspect. The lateral leads are LI and aVL. The
the potential difference between the right and left anterior leads are V1–V6, which are often further
arms, LII between the right arm and the left leg, divided into anteroseptal (V1–V4) and lateral
and LIII between the left leg and left arm. (V5–V6). V1 and V3R (if recorded) assess the
The aVR, aVL, and aVF leads are referred to as right ventricle.
unipolar leads, although they still measure the
electrical activity between two terminals. However,
in their case, the negative terminal (referred to as
Table 2The positive and negative terminals of the limb leads
Bipolar limb leads (frontal plane)
Lead I R arm (-) to L arm (+)
Lead II R arm (-) to L foot (+)
Lead III L arm (-) to L foot (+)
Augmented unipolar limb leads (frontal plane)
Lead aVR R arm (+) to common terminal (-)
Lead aVL L arm (+) to common terminal (-)
Lead aVF L foot (+) to common terminal (-)
7
LI
aVR aVL
aVR aVL
(-150°) (-30°)
LI
(0°)
– +
– –
LIII aVF LII
LII LIII
(+120°) (+90°) (+60°)
aVF
+ +
3The limb leads of the ECG. 4The view point each limb lead has of
the heart.
V9
V8
V7
V6
V5
V4
V3R V3
V1 V2
5Transverse view of the chest and heart showing
the relationship of the chest leads to each other.
8 Section 1 Principles of electrocardiography
The P-QRS-T complex The impulse then travels down the atrioventricular
Action potential bundle of His to reach the ventricles. In the normal
The P-QRS-T complex (6) is generated through heart the atria and the ventricles are well insulated
the propagation of an action potential through the by fibrous tissue to ensure that the only way
cardiac cells (7). At rest the cell has a negative electrical impulses travel down to the ventricles is
potential primarily due to the activity of the sodium through this route.
pump, which extrudes positively charged Na+ ions The bundle of His divides into right and left
from the cells. A slight reduction in the resting bundle branches that depolarize the right and left
potential due to the imminent arrival of the action ventricles respectively. The left branch further
potential from an adjacent cell results in a transient divides into anterior and posterior branches. The
large increase in permeability for Na+ ions and depolarization of the ventricles produces the QRS
abrupt depolarization. This is followed by an active complex. After the ventricular muscles are
two-phase process of repolarization, during which depolarized there is a time period where they cannot
the Na+ ions are again actively extruded from the respond to further electrical stimuli. This is the
cells to restore the resting potential. During part of refractory period. This can be absolute when there
the repolarization phase the cell is refractory to can be no response no matter how strong the
further stimulation. In the meantime, the depolar- stimulus, or relative when there can be a response to
ization wave has moved to the next cell. The elec- a stronger than normal stimulus. This refractory
trical impulse is usually generated in the sinoatrial period is represented on the ECG by the ST
(SA) node, which has specialized cells that segment. The muscle subsequently repolarizes to
spontaneously depolarize. The rate of depolariza- revert to its resting electrical state. This process is
tion of these cells is controlled by neural input from represented by the T wave. The U wave represents
the sympathetic and vagal nerves. the period of greatest excitability of the ventricles.
Its origin is unclear but it may represent the
The cardiac conducting system repolarization of the interventricular septum or the
When an electrical impulse is generated by the SA slow repolarization of the ventricles. Note that atrial
node it moves through the cardiac conducting repolarization is not normally visible because it is
system. The conduction system is a network of submerged in the QRS complex.
modified cardiac muscle cells that transmit the
cardiac impulse through the vascular myocardium Components of the P-QRS-T complex
in a manner that ensures the synchronized action of The normal ECG (6) therefore comprises a P wave,
the heart. A diagram of the components of the QRS complex, ST segment, and T wave. There may
conduction system is shown in (8). The P-QRS-T be another terminal deflection termed the U wave. A
complex is generated as the electrical impulse is summary of the cardiac events generating the
conducted down this system. components of the ECG complex is given in Table 3
The impulse starts by depolarization of the SA (see page 10).
node. This event is too weak to register on the
surface ECG. The impulse then travels
through the atria via the internodal tracts,
depolarizing them and generating a P
wave on the surface ECG. The impulse RR interval
reaches the atrioventricular (AV) node in
the atrioventricular junction. There is a QRS complex
delay in the conduction of the impulse at
this point contributing to the PR interval.
ST T wave
segment
P wave U wave
PR QRS
QT
6The components of the normal ECG.