Advanced Electrocardiography User manual
Spacelabs Medical: ADVANCED ELECTROCARDIOGRAPHY
ADVANCED
ELECTROCARDIOGRAPHY
Stanley 1. Anderson, MB
Department of Cardiology
Alfred Hospital
Prahran, Victoria, Australia 3181
Robert L. Burr, MSEE, PhD
University of Washington
Health Science Building
Nursing Research Office
Seattle, Washington 98195
W. Gregory Downs, BSE
Research Biomedical Engineer
Division of Cardiology
University Hospitals of Cleveland
Cleveland, Ohio 44106
Carol Jacobson, RN
Cardiovascular Clinical Specialist
Swedish Hospital Medical Center
Seattle, Washington 98104
Paul Lander, PhD
Assistant Professor of Medicine
The University of Oklahoma
Health Sciences Center
Oklahoma City, Oklahoma 73104
G. Ali Massumi, MD
Adult Cardiology
Texas Heart Institute
Houston, Texas 77030
David M. Mitvis, MD
Professor of Medicine
University of Tennessee
The Health Sciences Center
Memphis, Tennessee 38163
James C. Perry, MD
Associate in Pediatric Cardiology
Children’s Heart Institute
San Diego, CA 92123
Carlos Rizo-Patron, MD
Adult Cardiology
Texas Heart Institute
Houston, Texas 77030
This book is part of the SpaceLabs Medical Biophysical
Measurement Book Series for biomedical and clinical
professionals. The series is an educational service of
SpaceLabs Medical, a leading provider of patient
monitoring and clinical information systems.
0 SpaceLabs Medical, Inc., 1995
First printing, 1992
Second printing, 1995
All rights reserved.
No part of this book may be reproduced by any means
or transmitted or translated into a machine language
without the written permission of the publisher.
All brands and product names are trademarks of their
respective owners.
Published by SpaceLabs Medical, Inc.,
Redmond, Washington, U.S.A.
Printed in the United States.
ISBN O-9627449-3-X
TABLE OF CONTENTS
Spacelabs Medical: ADVANCED ELECTROCARDIOGRAPHY
Page
INTRODUCTION . . . . . . . . ..t........................................... 1
1.0
1.1
1.2
1.3
1.4
1.5
1.6
1.7
1.8
1.9
2.0
2.1
2.2
2.3
2.4
LEAD SYSTEMS ......................................
by Stanley T. Anderson, MB
Standard IL’-Lead Electrocardiogram ......................... 3
1 .I .1 Additional Leads ......................................... 3
1.12 Lead Problems ............................................ .7
1.1.3 Lead Presentation
Vectorcardiography .................................................. 11
Po2ar
Cf7rdiogrnphy ..................................................
Monitoriny: .............................................................. 13
1.4.1
Bedside
1.42 Exercise Testing ......................................... 13
1.4.3 Holter Monitoring
1.4.3.1 Continuous Monitoring ............ .15
1.4.3.2
Body Surface Mappil?g .............................................
Magnetocardiography .............................................. 17
Sij@Azleraged Electrocardiography ...................... 17
12.Lead Electrocardiogram Reconstruction ..............
Vectorcardiogram
....................................................... 13
Intermittent Monitoring.. .......... .15
Recorlstructiorl ............................ 19
........................................ 7
..................................... 15
3
11
15
17
CARDIAC RHYTHM
INTERPRETATION ................................
by Carol Jacobson, RN
Interpretation of Cardiac Rhythm Strips
Rkythms Originating in the Sinus Node
22.1 Normal Sinus
2.2.2 Sinus Bradycardia
2.2.3 Sinus Tachycardia
2.2.4 Sinus Arrhythmia ..................................... .25
2.2.5 Sinus Arrest..
Arrhythmias Originating in the Atria .................... .26
2.3.1 Premature Atria1 Complex ....................... 26
2.3.2 Wandering Atria1 Pacemaker
2.3.3 Multifocal Atria1 Tachycardia ................. .28
2.3.4 Atria1 Tachycardia and Paroxysmal
Atria1 Tachycardia
2.3.5 Atria1 Flutter .............................................. 29
2.3.6 Atria1 Fibrillation ...................................... .30
Arrhythmias
2.4.1 Premature Junctional Complex..
2.4.2 Junctional Rhythm
Originating
Rhythm .............................. 23
....................................
.................................... .24
.............................................. 25
.................................... .29
in the AVJunction.. ...... ..3 1
....................................
.................. 21
................ .23
................. .27
............. .31
21
.24
32
2.5
2.6
2.7
3.0
3.1
3.2
Page
Supraventricular Tachycardia ................................. .33
Arrhythmias Originating in the Ventricles
2.6.1 Premature Ventricular Complex ............ .33
2.6.2 Ventricular Tachycardia
2.6.3 Ventricular Fibrillation ............................ ,35
2.6.4 Accelerated Ventricular Rhythm ............ .35
2.6.5 Ventricular Asystole ................................ .36
AV Blocks ................................................................ 37
2.7.1 First-Degree AV Block ............................. .37
2.7.2 Second-Degree AV Block ........................ .37
2.7.2.1 Mobitz Type I Second-Degree
AV Block (Wenckebach) ........... .37
2.7.2.2
2.7.3 High Grade AV Block
2.7.4 Third-Degree AV Block ........................... .40
Mobitz Type II Second-Degree
AV Block ..................................... .38
.............................. .39
............. .33
.......................... .34
ARRHYTHMIA DETECTION
ALGORITHMS.. ......................................... 41
by W. Gregory Downs, BSE
Typical Applications
Defection Algorithms
3.1.1 Dedicated Arrhythmia
Monitoring System..
3.1.2 Holter Monitoring
3.1.3 Other Electrocardiographic Monitors ... ..4 3
3.1.4 Automatic Implantable Cardioverter-
Defibrillator ............................................... .43
Signal Processing .................................................... .43
3.2.1
Noise Sources..
3.2.1.1 Power Line Interference
3.2.1.2 Muscle Artifact.. ......................... .4l
3.2.1.3 Electrode Contact Noise ............ .44
3.2.1.4 Baseline Wander ........................ .44
3.2.1.5
3.2.2 Noise Removal ......................................... .45
3.2.3 Noise Detection
3.2.3.1 Primary Issues in Noise
3.2.4 Sample Rate ............................................... 47
3.2.5 Transformations ....................................... .47
3.2.6 Beat Detection
3.2.7 Feature Extraction .................................... .51
3.2.7.1
3.2.7.2 Frequency Domain Features .... .51
3.2.8 Beat Classification ..................................... 51
3.2.8.1
of
Arrhythmia
............................................... 42
................................. .42
..................................... 42
.......................................... .43
(60
Hz or 50 Hz) ......................... .43
Noise From a Single Electrode .45
......................................... 45
Detection/Rejection .................... 47
........................................... .51
Time Domain Features .............. .51
Template Match (Correlation)
Algorithms ................................. .51
TABLE OF CONTENTS
3.3
3.4
4.0
4.1
4.2
4.3
4.4
4.5
4.6
5.0
5.1
5.2
3.3
5.4
Page
3.2.8.2 Feature Extraction (Cluster
3.2.8.3 Hybrid Algorithms
3.2.8.4 Rhythm Analysis
3.2.9 Pacemaker Spike Detection ...................... 55
3.2.10 Ventricular Fibrillation ............................. 55
3.2.11 Lead Selection ............................................
Alprifhm Verification
Current Trends in Arrhythmia Monitoring
3.4.1 Multiple Leads ...........................................
3.4.2 Improved Noise Rejection
3.4.3 ST Segment Monitoring ............................ 59
3.4.4 Incorporation of Other Parameters
3.4.5 P Wave Detection ...................................... 59
ST SEGMENT ANALYSIS
by David M. Mirvis, MD
Normal ST Segment and T
Basic Effects ofMyocardia1 Ischrmia
4.2.1 Hemodynamic Consequences of
Coronary Obstruction ............................... 63
4.2.2 Electrophysiologic Effects of
Myocardial Ischemia
4.2.3 Injury Currents .......................................... 63
Elecfrocurdic)~rapllic Efircts of
Myocardial lschemia ................................................ 64
4.3.1 DC-Coupled Amplifiers
4.3.2 AC-Coupled Amplifier ............................. 64
Recording E/ecfrocardiop’aphic Effects
of Myocardial Ischemia ........................................... .65
4.4.1 ECG Lead Systems
4.4.2 Amplifier and Monitor Systems .............
4.4.3 Analysis Systems ....................................... 67
Electrocardiographic Features
of Transient Ischemia ............................................... 68
Clinical Significance of Transient
ST Segment Depressiol7 .......................................... .68
Analysis) Algorithms ................. 53
....................
........................ 53
............................................. 57
........................
.............................
Wazv
........................
.................................
........................... 64
.................................... 65
.53
55
.57
............
.............
.59
........
.61
37
59
61
61
63
.67
PEDIATRIC
ELECTROCARDIOGRAPHY.. .....
by James C. Perry, MD
Heart Rate ................................................................ 69
Intervals and Leads
Cardiac Malposition
Effects of Congenital Hearf Defects .......................... 73
.................................................. 71
................................................. 71
.a
5.5
5.6
6.0
6.1
6.2
6.3
6.4
6.5
6.6
6.7
7.0
7.1
7.2
Page
Pediatric Arrhythmias..
5.5.1 Fetal Arrhythmias ....................................
5.5.2 Chaotic Atria1 Tachycardia..
5.5.3 Bradycardia in the Newborn.. .................
5.5.4 Developmental Aspects of WolffParkinson-White Syndrome and
Supraventricular
5.5.5 Junctional Ectopic
5.5.6 Permanent Junctional Reciprocating
Tachycardia ................................................ 81
5.5.7 Ventricular Tachycardia ........................... 83
5.3.8 Permanent Pacing Systems in Children.. 83
Pediatric Electrophysiology Studies ......................... 85
HEART RATE VARIABILITY..
by Robert L. Burr, MSEE, PhD
Physiologic Models for Heart Rate
Variability Analysis ................................................. 89
Heart Rate Definifion Problems ............................... 89
Which to Use: Heart Rate or Heart Period? ............
Time- Wei,qhfing Versus Beat- Weighting of
Statistical Summaries .............................................
Heart Rafe Variability Measures .............................
6.5.1 Kleiger Global Standard Deviation
6.5.2 Magid Statistic ..........................................
6.5.3 SDANN ...................................................... 97
6.5.4 Ewing BB50, pNNSD, RMSSD ................
6.5.5 Frequency Versus Beatquency
6.5.6 Traditional Versus Autoregressive
Model-Based Spectral Analysis
Idenfification of Nonsinus Beats ............................. 106
Heart Rate Variability as a Measure
in Clinical Environment
............................................ 75
.75
....................
Tachycardia
Tachycardia
........................................ 109
.................. 81
...............
................
.75
.79
.81
.86
......
........
,101
............
.92
.95
.96
.96
.96
.97
.99
LATE POTENTIALS AND THE
ELECTROCARDIOGRAM
by Paul Lander, PhD
Recording the High-Resolufion
Electrocardiogram ................................................. ,111
7.1.1 Registration ............................................. 113
7.1.2 Amplification and Filtering
7.1.3 Sampling ................................................. 113
7.1.4 Isolation ....................................................
Signal Averaging ................................................... 115
7.2.1 Triggering
7.2.2 Signal Averaging Techniques
7.2.3 Noise Monitoring .................................... 119
................................................
................... 113
................ 118
,111
...........
115
117
TABLE OF CONTENTS
Spacelabs Medical : ADVANCED ELECTROCARDIOGRAPHY
7.3
7.4
7.5
8.0
8.1
8.2
8.3
Page
Time Domain Analysis
Electrocardiogram . . . . . . . . . . . . . . . . . . . . . . 121
7.3.1 Filtering . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 123
7.32 Vector Magnitude Transform . .._.......... 127
7.3.3 Automatic Measurements . .._____ 127
Inferprefafio~z of Lntc Potentials . .._..._._._..._........... 129
Frequency Domain Analysis of
the High-Resolution Ekcfrocardiogram . . .._........ 129
7.5.1 Techniques .._____....................................... 133
7.5.2 Spectrotemporal Mapping . . . . . . .._______..... 133
of
the HigIt-Resolufion
ELECTROPHYSIOLOGY 134
by G. Ali Massumi, MD
and Carlos Rizo-Patron, MD
Electrophysiology Equipment
Requirements . . . . . ..___._....._.....................................,. 135
8.1.1 Recording Devices . . . . . . . . . . . . . . . . . . . . . . . . 133
8.1.2 Stimulator for Cardiac Pacing _____..__.___,,, 133
Surfncc Elecfro~rams
Infracavifary Elecfrograrns . . . . . . . . . . . 137
. . . . . . . . . . . . . . . . . . . 137
Page
8.4
8.5
8.6
8.7
8.8
8.9
8.10
9.0
10.0
11.0
12.0
13.0
Programmed Sfinrnlution
Cardiac Mapping ................................................... 139
Radiofrequency Cafhefer Ablation
Transesophageal Pacing and Recording ................. 141
Cardioversion ......................................................... 143
Defibrillation .......................................................... 143
Implanfable Cardioverfer-Defibrillator .................. ,143
ABBREVIATIONS ................................ ,147
REFERENCES ........................................
ILLUSTRATION CREDITS .......... ,155
BIBLIOGRAPHY ....................................
GLOSSARY
...................................... ,137
.......................... 141
................................................
INDEX . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
149
156
162
166
Spacelabs Medical : ADVANCED ELECTROCARDIOGRAPHY
INTRODUCTION
In the last 20 years, we have seen remarkable innovations in the diagnosis and treatment of cardiac disorders. Many of these result from the continued development of medical diagnostic instrumentation,
particularly the improved interpretation and analysis of electrocardiograms. This book focuses on the
enhancements of the electrocardiogram and its recording systems and the computer-based applications
that have been developed over the past two decades.
Section 1.0 reviews the fundamentals of
applications. Vectorcardiography, exercise testing, and assorted monitoring techniques are also discussed.
Section 2.0 provides an overview of the electrical physiology of the heart and a guide to the interpre-
tation of rhythm from electrocardiographic monitors.
Section 3.0 focuses on algorithms for arrhythmia detection and how their rapid advancement in realtime monitoring applies to current medical trends. Arrhythmia detection has become easier for the clinician, but increased use of these systems requires an understanding of how signal processing, noise removal, and beat detection relate to the patient’s condition.
Another specialized medical application of the electrocardiogram is ST segment analysis. Section 4.0
discusses various aspects of ST segment analysis, including the effects of myocardial ischemia, coronary
blockage, and transient ischemia.
Pediatric electrocardiography requires special considerations by the clinician and the biomedical
equipment technician. Adult criteria do not apply to newborns, infants, or youngsters. Section 5.0 de-
scribes the particular exceptions and parameters that must be understood when assessing pediatric patients’ heart rate as well as cardiac anomalies, defects, and other problems. Biplane fluoroscopy, an essential correlate of pediatric electrophysiologic studies, is also reviewed.
Heart rate variability has become a major noninvasive monitoring parameter for the influence of the
nervous system on the human heart. Section 6.0 summarizes the physiologic models for heart rate variability studies and the mathematical considerations
S’ection 7.0 emphasizes how late potentials relate to the high resolution electrocardiogram, a product
of advances in computer technology. The mathematical variables and theoretical concepts that led to this
application are presented.
Slection 8.0 describes
they apply to the clinical monitoring of the human
physiology equipment as well as the interpretations of the resulting electrograms.
the
achievements of research and application studies in electrophysiology as
the
standard 12-lead electrocardiogram and leads for specific
that
apply to its use in patient monitoring.
heart.
This section reviews the current used for electro-
1.0
Spacelabs Medical: ADVANCED ELECTROCARDIOGRAPHY
An electrocardiographic lead is a pair of polar terminals connected to electrodes. The heart
approximates a double dipole layer, and the time-varying electrical field produced propagates to the surface of the body.
To reach the recording electrodes on the surface, the electrical field must pass through
various tissues. This results in differing intensity of signals produced at equidistant points
from the cardiac source. The display of electrical activity recorded, therefore, depends on
the site of electrode placement and the lead configuration.
1.1
Standamll2-Lead Electrocardiogram
The reference electrode is attached to the right leg. Leads I, II, and III are bipolar leads introduced by Einthoven.’ The augmented limb leads aVR, aVL, and aVF were introduced
by Goldberger, who found that, by removing the exploring electrode from Wilson’s central terminal, the amplitude increased on these “unipolar” limb leads.‘,” The precordial
leads, V,-V,, are “unipolar” with the electrode position on the torso following the convention of the American Heart Association.” A detailed discussion of leads is in an earlier
publication in the Biophysical Measurement series entitled
A. Rawlings.5 Figure 1.1 shows site placement of standard electrocardiographic leads.
The bipolar and the augmented limb leads approximate the frontal plane, while the
precordial leads approximate components of the horizontal plane (Figure 1.2). Thus, the
standard 12-lead recording largely describes the cardiac electrical forces in only two of the
three orthogonal planes. Einthoven’s rule outlines the mathematical relationship on the
bipolar leads, and the relationship of the augmented limb leads is easily calculated.’ From
any two of the six standard leads, the remaining four can be derived. Similar extrapolation
of the precordial leads can be made using any two as a subset. The ability to derive leads
from a lessor number has been used in the computer storage and analysis of electrocardiogram (ECG) data. The clinician, however, requires information from all 12 leads for clinical diagnosis and therapeutic management.
1.1.1 Additional Leads
Electroclzrdiugrapky
by Charles
Other leads also provide specific clinically significant information. However, they have
not been incorporated into current ECG recorders.
The Unipolar Precordial Lead: Lead V,R, recorded with the exploring electrode in
the position for V, but only on the right side, and V,R aid the diagnosis of right ventricular
infarction (Figure 1.3).h Mirror image precordial placement is required in dextrocardia.
Thus, in this situation, V,R is recorded in the same position as V,, V,R as V,, and the remainder in positions as V, to V, only on the right side of the chest. ?o facilitate assessment,
the polarity of lead I is reversed by changing the right and left arm electrodes. This results
in the appropriate transposition of leads II and III and of leads aVR and aVF (Figure 1.3).
Leads V;,V,, and V, are recorded in the same horizontal line as V, to V, at the posterior
axillary line (VJ, the angle of the scapula (V,), and over the spine (V,). These leads may be
useful in the diagnosis of posterior infarction (Figure 1.4).
Spacelabs Medical: ADVANCED ELECTROCARDIOGRAPHY
Occasionally, clinicians may wish to record in other lead positions, such as one
interspace higher. No accepted nomenclature exists to describe these leads. Therefore,
careful annotation should be made to avoid confusion, especially when serial tracings are
compared.
The Bipolar Precordial Lead: A lead, sometimes called the atria1 lead, is recorded
from the right of the sternum in the third interspace to the xiphoid process of the sternum
to aid detection of atria1 activity. Usually used for monitoring, an atria1 lead can provide
additional information in the differentiation of rhythm disturbances when combined with
the standard ECG (Figure 1.5).
In Europe, the Nehb leads are occasionally used to access atria1 activity. This presentation consists of three bipolar leads with the placement of the electrodes on the second rib
at the junction with the sternum, the posterior axillary line at the level of the apex of the
scapula, and on the left front of the chest at the level of the scapular apex.7
The Semi-Orthogonal Lead: The X, Y, and Z leads are available on some three-channel recorders and will be discussed later.
Other Leads: In the diagnosis of broad complex tachyarrhythmias, the use of unipolar
or bipolar esophageal recordings in association with standard leads facilitates identification of separate atria1 and ventricular activity (Figure 1.6). Post cardiac surgery epicardial
electrodes are often placed to assess pacing in the postoperative period. These electrodes
can record either unipolar atria1 or ventricular electrograms, or can combine this information into a bipolar electrogram.
1 .I .2 Lead Problems
Misplacement of electrodes is the most commonly recognized problem associated with
the limb leads. Reversal of the arm leads causes inversion of lead I, with reversal of II and
III, and reversal of leads aVR and aVL. The components of lead II or III may be reversed,
or all three leads may be rotated clockwise or counterclockwise producing specific patterns that are important to recognize to avoid false interpretations? Mispositioning of the
exploring chest electrode high on the precordium or the reversal of leads can make interpretation difficult, particularly when serial comparisons are necessary.
The electrodes may be placed on any part of the arms or the left leg as long as they are
below the shoulders in the former and below the inguinal fold anteriorly and the gluteal
fold posteriorly in the Iatter.9 When it is not possible to place the electrodes accordingly,
such as with an amputation or severe bums, another more proximal placement should be
used.
I. 1.3 Lead Presentation
The standard X&lead presentation commonly uses limb leads in the order I, II, III, aVR,
aVL, aVF, and the precordial leads V, through Vg. This is done either by grouping the
leads into subsets of three displayed horizontally or in groups of six displayed vertically.
Fumagalli introduced the concept of presenting aVR as - aVR and a more logical sequencing of the standard leads.lO This presentation, subsequently popularized by
Cabrera and represented in the American literature by Dower and colleagues, offers a way
to use the available information more readily and turns the aVR from a relatively ignored
lead into a very useful one.iill*
7
Spacelabs Medical : ADVANCED ELECTROCARDIOGRAPHY
1.2
Vectorcamliography
Electrical activity radiates from the heart in all directions. Thus, a record of it in three
planes that are at right angles to each other should contain more information than the
standard surface recording. The recording of electrical activity in three planes requires the
use of leads that represent the frontal, horizontal, and sagittal planes and is known as
vectorcardiography. When the lead configuration closely approximates this situation, the
leads are said to be orthogonal. For convenience of lead application (due to a reluctance to
abandoning the 12-lead ECG tracing), a semiorthogonal system was developed.lh
A semiorthogonal system includes mutual perpendicular leads in three planes by the
use of the Frank lead system with the placement of the electrodes as in Figure 1.7. Resistors are placed in the circuit to correct for the magnitude of the vectors.‘” The head (H)
electrode is usually positioned on the back of the neck, but can be placed on the forehead.
in males, the A, C, E, I, and M electrodes are positioned in the fourth intercostal space at
the left midaxillary line (A), midway between A and E (C), over the sternum (El, the right
midaxillary line (I), and the spine (Ml. Some lead adjustment may be required in females.
The level of the fifth interspace may be used to facilitate the simultaneous recording of the
12-lead ECG.
Willems and co-workers, utilizing a large series of tracings comparing the Frank leads
with 12-lead recordings, concluded that “the conventional 12-lead ECG is as good as the
vectorcardiogram (VCG) for the differential diagnosis of seven main entities”, and “the
classification results show in a quantitative way that both lead systems contain equivalent
information.“” Advantages of the VCG in comparison with the standard ECG have been
well documented in selected diagnostic categories and will be considered in the discussion of the reconstructed VCG.
A less commonly used lead system was introduced by McFee and Parungao, who described it as an axial-lead system for orthogonal-lead electrocardiography.‘5 A comparative study showed no significant diagnostic differences of this system when compared
with the 12-lead tracing.‘(’
Semiorthogonal or hybrid systems were basically designed to allow the simultaneous
recording of the 12-lead ECG with X, Y, and Z leads with the latter not being true orthogonal. They add a vectorial approach to the 12-lead without the addition of many electrodes
and can be readily positioned. The system, designed by Macfarlane, uses two electrodes in
addition to the standard 12, has one electrode placed in the V,R position, and the other on
the back.” An alternative system positions the electrodes in the left and right axillas to
produce lead X (Figure 1.81.”
1.3
Polar Camliography
Polar cardiography graphically displays the magnitude and direction of the heart vector in relation to time. The lead system has not been defined. Currently, dedicated polar cardiographic recorders are no longer required since the tracing is derived, using computers, from more conventional information.
1.4 Monitoring
1.4.1 Bedside
Monitoring is most commonly used in patients with coronary artery disease in whom
rhythm disturbances occur with a high frequency. Monitoring can be performed in a coronary care unit, an intensive care unit, an operating room, or in transit to one of these areas.
The left parasternal window should remain available for the possible use of an external
defibrillator and to allow easy access for clinical examination of the heart. Thus, Marriott
and Fogg designed a modified bipolar lead (MCL,).‘” The neutral, or ground, electrode is
placed under the outer aspect of the right clavicle, the positive electrode in the position of
V,, and the negative electrode near the left shoulder (Figure 1.9). This configuration usually permits good visualization of atria1 activity. Since alternate precordial positions are
sometimes needed, bipolar leads with the positive electrode placed near the apex or on
the lower left rib cage can be used.
1.4.2 Exercise Testing
Spacelabs Medical: ADVANCED ELECTROCARDIOGRAPHY
Monitoring the standard 12-lead ECG usually recorded during exercise is not practical because of motion artifact introduced into the limb leads. Mason and Likar recommended
moving the limb leads centrally, with little effect on the 12-lead recording.?” They also
moved the right arm electrode to the right infraclavicular fossa, medial to the deltoid insertion and 2 cm below the medial end of the clavicle, the left arm electrode to a similar
position below the left clavicle, the left leg electrode midway between the costal margin
and the iliac crest in the anterior axillary line, and the right leg electrode on the midthigh.
Subsequently, the use of the right leg electrode positioned in the region of the right iliac
fossa has become standard (Figure l.lO).*’
Significant differences between the standard ECG and the 12 leads recorded by the
above methods before and during exercise suggests that such tracing should be labelled as
“torso positioned” or “nonstandard.“?’ A study comparing the recommended lead positions with those of the standard 12-lead ECGs showed that inferior and posterior infarcts
were lost in 69% and 31% of the recordings, respectively. Use of electrodes placed on the
proximal portions of the right and left arm have produced 12-lead ECGs more closely resembling the standard tracing.‘? Subsequent investigation has shown that differences
along the left arm were accentuated relative to those along the right arm and, along the
left leg, an anterior site showed less deviation than did a more lateral site?”
Ease of placement and reduction of motion artifact has led to the popular use of bipolar precordial recordings during exercise. Usually the positive electrode is positioned at
the V5 level, the negative electrode being in a similar position on the right of the chest
(CC5), on the manubrium (CM5), on the head (CH5), on the right arm (CR5), or the right
shoulder (CS5). The CM5 position is less sensitive than the V, or the CC5 and has a more
negative J point and a more positive slope? When a single lead is used, then V, is the
most sensitive for the detection of ischemia. However, the failure to detect ischemia with
this lead alone is not specific (Figure 1.11).2h
The use of the Frank orthogonal system has not found a significant following.‘” Thus,
body surface potential mapping is experimental at presentz7
13
1.4.3 Holter Monitoring
1.4.3.1 Continuous Monitoring
Two-channel continuous bipolar recordings are commonly used to facilitate interpretation
and to obtain some information should an electrode become detached. The American
Heart Association recommends that a V,-type lead with a positive electrode be located in
the fourth right intercostal space 2.5 cm from the sternal margin, and the negative elec-
trode over the lateral one-third of the left infraclavicular fossa.2H Then, a V,-type lead
would accompany the positive electrode in the fifth left intercostal space at the anterior
axillary line, the negative electrode being posterior 2.5 cm below the inferior angle of the
right scapula. The ground electrode should be placed in the lateral one-third of the right
infraclavicular fossa, but its positioning is not crucial since it is not used in the recording
(Figure 1.12).
Alternative lead positioning particularly aids in the detection of ischemia by incorpo-
rating an aVF-like lead. 2y~3” The generation of a third bipolar lead by alternating the recording in the second channel using a switching device has been described.30 Use of such a sys-
tem correlated with ischemic changes detected by a 12-lead exercise test. The electrodes in
this system are placed between the standard V, position and the upper manubrium sternum to produce a bipolar V,-like lead. A bipolar V,-like lead is attached between the standard V, position and the upper manubrium sternum and an aVF-like bipolar lead is located between the ninth rib in the anterior axillary line and the upper manubrium sternum. In this system, the positive electrode is switched to a ground electrode so that the V,
and aVF leads alternate (Figure 1.13). The use of an esophageal lead in association with a
surface lead has been reported.“’
Three-channel recorders are also available. The ground lead should be placed on the
lower sternum or on the lower rib cage on the right. The positioning of the positive electrodes includes the bipolar electrodes to the left anterior axillary line (CM5), to the left
midaxillary line on the lowest rib (aVF-like), and to the left sternal border at the junction of
the fifth rib (CM2). The negative eIectrode for each lead is placed on the upper manu-
brium sternum (Figure 1.14).32
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1.4.3.2 Intermittent Monitoring
The time interval between episodic “palpitations” may be such that the standard Holter
recordings, even if repeated, may fail to capture significant events. Easy and quick application of electrodes is essential. This may be achieved by use of hand-held electrodes (lead
I) or by the application of a small device to the chest with feet electrodes. The tracings recovered do not have a precise lead equivalent, but do diagnostically confirm the presence
of and type of rhythm disturbance. The electrodes usually incorporate a mechanism for
activating the recorder.
1.5 Body Surface Mapping
On the surface of the body, the potential field reflects the complexity of the currents and
exhibits multiple maxima, minima, pseudopods, saddles, niches, and so on. These fea-
tures, which convey information on the location and time sequence of the electrophysi-
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1.6
1.7
ological events of the heart, are often located in areas not explored by the
trocardiographic leads.“” Body surface mapping has developed as a research tool and has
not been widely accepted as a clinical tool.
The number of recording sites on the chest varies from 12 (i.e., 4 by 3), to 242, the
distance between the electrodes being related to the number of electrodes and whether or
not recording is extended on to the posterior aspects of the chest.%35 Since such
in number and position exists, detailed features of each system must be selected on an
individual basis. The value of continued use of this technique, as shown in a recent study,
lies in the understanding gained about the relationship between pathology and the genera-
tion of the electrical signals recorded in the standard 12-lead ECG.36
12
classical elec-
variation
Magnetocamliography
The magnetic field of the heart was first detected in 1963.37 The magnetic signals are weak,
particularly since the exploring magnet is not directly applied to the surface of the heart.
The use of magnetic signals has not yet reached a clinically applicable stage.
Signal-Averaged Electrocardiography
The detection of ventricular late potentials using high-resolution or signal-averaged electrocardiography has had prognostic significance for the development of ventricular
arrhythmias.“8 A modified or hybrid orthogonal lead system, which is as sensitive as body
surface mapping, is most commonly used. 39 The standard lead configuration recommended in the time domain is an XYZ system with the X-lead electrodes placed in the
fourth intercostal space in both midaxillary lines, the Y-lead electrodes on the superior aspect of the manubrium and the upper left leg or left iliac crest, and the Z-lead electrodes in
the V, position and directly posterior from V, on the left side of the vertebral column (Figure l.l5).“O Comparison with bipolar precordial leads of various types has shown a more
prolonged QR!S with the XYZ system and the detection of more abnormal measurements
with
the
bipolar precordial leads.“’
The results of high-resolution electrocardiography are lead-dependent. Accordingly,
criteria and approaches established with one lead system may not be applicable to other
systems. The Frank leads and modified uncorrected orthogonal leads have been used in
the frequency domain. Additional studies are required to determine the optimal lead system.“O
1.8
12-Lead Electrocardiogram Reconstruction
The standard 12-lead ECG remains the most commonly used for cardiac investigation because of the simplicity of the equipment required, the short amount of time needed to obtain the tracing, the amount of information recorded, and the relatively low cost of the
procedure. This does not devalue other expressions of cardiac electrical activity. These offer solutions or potential solutions to signs of disturbed pathology, particularly in relation
to time, such as exercise electrocardiography, Holter monitoring, or fixed monitoring. The
contributions that have and will be made by the more research-orientated techniques cannot be underemphasized.
17
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Because of the usefulness of the standard ECG, attempts have been made to recon-
struct the electrical information present in other forms of recording, The clinical applica-
tions of accurately reproducing such recordings would be of major value in exercise electrocardiography and in monitoring situations in which a potentially changing pattern of
the QRS or the ST-T wave occurs over a relatively short period of time.
The electrical activity recorded at the surface depends on the geometry and resistive
properties of the passive volume conductors between the source of the activity and the
site of recording. Considering the anatomical position of the heart as assessed by magnetic
resonance imaging, shifts of only 0.5 cm in relation to the V, lead have been shown to alter
the reconstructed ECG, the magnitude of the error relating to the activation sequence.“*
The effect of alteration of the limb lead positions on the standard ECG has been discussed
in the section on exercise testing with clear differences observed on the left arm electrode
compared with the right arm electrode and with the anterior electrode compared with lateral positioning on the left leg.?”
The problems of ECG reconstruction are of considerable magnitude. Even with slight
variations occurring in the anatomical surface relationship over time, individual variations in body contour, nonuniformity of the passive volume conductors, and the recording used for derivation may not contain all the information. Reconstruction from orthogonal XYZ leads has shown minor differences between the derived tracing and the 12-lead
ECG. The former correlates better with the clinical situation, and significant differences in
amplitude do not influence patient treatment.““fM
An example of the potential value of ECG reconstruction relates to the time-dependent changes in the ST segment seen during brief coronary occlusion. This situation
shows that changes determining the true magnitude and extent of the ST segment in the
12-lead ECG, as conventionally recorded, need to be established.32
1.9
Vectorcamliogram Reconstruction
Vectorcardiography has proved a very useful tool, particularly in the understanding of
the QRS complex. Chou, in reviewing the value of vectorcardiography, indicates that it is
more reliable than the ECG in the diagnosis of atria1 enlargement and right ventricular
hypertrophy.@ In addition, it is more sensitive than the ECG in the diagnosis of myocardial infarction, particularly inferior myocardial infarction. M,43 Using criteria developed in
association with the ECG and recorded with a three-channel recorder so that important
features of the vector QRS loop can be predicted, Warner and colleagues showed improved ability to diagnose inferior myocardial infarction.46
Reconstruction of the VCG from the standard 12-lead ECG, rather than recording both
separately, has merit in selected cases with considerable economic savings while retaining
the benefits of the information from ECG waveforms and providing additional diagnostic
data. Edenbrandt and Pahlm compared three methods of VCG reconstruction and concluded that the inverse transformation matrix of Dower to be the best method of syntheses.4i Subsequently, this method has been shown to be comparable to a regression technique.M The ability to derive additional information and the enhancement of electrocar-
diographic understanding by use of VCGs reconstructed from the standard ECG avoid
the need for additional leads. Such methods could become accepted practice in selected
cases if incorporated into current ECG systems.
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2.0 CARDIAC RHYTHM INTERPRETATION
The heart consists of two main types of cells: muscle cells and conduction cells. Atria1 and
ventricular muscle cells are responsible for contraction of the heart’s chambers. Specialized conduction cells function to initiate and spread the electrical impulse through the
heart. The electrical impulse generated in the conduction system stimulates the muscle
cells to contract.
Depolarization refers to the electrical excitation of the heart resulting from the flow of
ions across the membrane of cardiac cells. This wave of excitation spreads from cell to cell
through the conduction system and into the muscle cells, providing the signal for them to
contract.
Repolarization returns the heart to its electrical resting state,
across the cardiac cell membrane. Once
larization.
The refractory period is the amount of time after depolarization when the heart cannot
respond to another stimulus. Cardiac cells must repolarize before they can depolarize
again. The refractory period occurs in two phases: (1) the absolute refractory period immediately following depolarization during which the heart cannot respond to another
stimulus and (2) the relative refractory period following the absolute refractory period
during which the heart can respond to a stronger than normal stimulus but with abnormally slow conduction.
Automaticity describes the ability of certain parts of the heart to initiate an impulse
without an external stimulus, or spontaneously depolarize. Conductivity refers to propa-
gation of an impulse from cell to cell within the heart. Contractility means the ability of
cardiac muscle cells to shorten, or contract, in response to the electrical stimulus. Aberrant
conduction refers to abnormal conduction of the impulse through the ventricles.
An arrhythmia is any cardiac rhythm that is not normal sinus rhythm at a normal rate.
Arrhythmias can arise from the atria, AV node, or ventricles. Or, they can occur when
conduction of the impulse from the atria to
the
heart is repolarized it can again undergo depo-
the
ventricles becomes abnormal.
again
due to ion flow
2.1
Interpretation of Camliac Rhythm Strips
An electrocardiogram (ECG) is a graphic recording of the electrical activity produced by
depolarization and repolarization of the heart. The ECG is recorded on standard graphic
ECG paper divided into small and large boxes. The horizontal axis of ECG paper mea-
sures time and the vertical axis records voltage. The standard system uses 25 mm / set as
the dimensional unit. Each small box (lmm x lmm) on the horizontal equals 0.04 second;
one large box (consisting of five small boxes) equals 0.20 second. Vertically, each small box
equals 0. ImV, one large box (five small boxes) equals 0.5 mV. Marks in the top margin of
most ECG paper divide it into 3-second time periods.
A rhythm strip should be analyzed in an organized manner to aid in arrhythmia interpretation. The following steps are suggested:
Regularity: Determine if
to calculate heart rate. If the rhythm appears irregular, determine if the irregularity is random or patterned (that is, repetitive groups of beats separated by pauses).
the
rhythm is regular or irregular. This information is needed
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Rate: Heart rate can be obtained from the ECG strip by several methods. If the rhythm
is regular, any of the following three methods can be used (Figure 2.1). Calculate atria1 rate
in the same way, using P waves instead of R waves:
1. Count the number of small boxes between two R waves and divide that number into
1500 (since there are 1500 small boxes in a l-minute strip of ECG paper).
2. Count the number of large boxes between two R waves and divide that number into
300 (since there are 300 large boxes in a l-minute strip of ECG paper).
3. If the rhythm is regular or irregular, count the number of R-R intervals in a 6-second
strip and multiply that number by 10.
P Waves: Locate I’ waves and determine if they all look alike and if they have a consis-
tent relationship to QRS complexes (that is, one P wave before every QRS; two or more I’
waves before each QRS; or random occurrence of I’ waves relative to QRS complexes).
PR Interval: Measure the PR interval of several complexes in a row to determine if it is
of normal duration and consistent for all complexes. A normal PR interval is 0.12 to 0.20
second.
QRS Width: Measure the QRS complex and determine if it is normal or wide. A nor-
mal QRS width is 0.04 to 0.10 second.
2.2
Rhythms Originating in the Sinus Node
2.2.1 Normal Sinus Rhythm
The sinus node normally fires at a regular rate of 60 to 100 beats per minute (bpm). The
impulse spreads from the sinus node through the atria and to the AV node, where it encounters a slight delay before it travels through the bundle of His, right and left bundle
branches, and Purkinje fibers into the ventricle. Figure 2.2 presents the ECG characteristics
of normal sinus rhythm:
Figure 2.2- Normal sinus rhythm.
Rhythm: Regular.
Rate: 60 to 100 bpm.
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