Electrocardiography
by John H. Dirckx, M.D.
T
he language of electrocardiography, one of the most
fundamental and widely used diagnostic procedures in
cardiology, is full of arcane and elusive terms and
abbreviations. For persons outside the medical profession, a
major obstacle to mastering that language is a lack of understanding of the basic principles of electrocardiography, the
kinds of information it provides about the heart, and the way
that information is used in diagnosis.
The following survey of the history, theory, and practice
of electrocardiography explains basic concepts and defines
related terms, and concludes with a glossary of additional
terms and abbreviations not discussed in the text.
History
Electrocardiography was developed by the Dutch physician and physiologist Willem Einthoven around 1900.
Einthoven was looking for a way to supplement available
methods of gathering diagnostic information about the heart
such as feeling the pulse, listening with a stethoscope, percussing the thorax, and using the newly developed x-ray.
Earlier investigators had shown that the action of the heart
generates small, transitory electrical waves that can be
detected by applying an appropriate instrument to the outer
surface of the body.
Einthoven devised a highly responsive voltage detector
called a string galvanometer to measure the electrical activity
of the heart. In order to make a permanent record of the
rapidly shifting electrical potentials, he arranged for the movements of the galvanometer needle to deflect a beam of light as
it fell on a moving strip of photographic film. When the film
was developed it showed a linear tracing whose up-and-down
deviations corresponded to electrical events in the cardiac
cycle.
Einthoven derived his name for the tracing of cardiac
electrical activity from three Greek words that should be well
known to every transcriptionist. Following Dutch and German
practice, he spelled it Elektrokardiogramm, and that is the
source of the K in EKG, still widely used today as an alternative to ECG. In modern parlance, an electrocardiograph is
a machine, and an electrocardiogram is the tracing it makes.
Nerve and muscle tissue, including heart muscle and the
highly specialized conducting system of the heart, consists of
bundles or sheets of microscopic fibers, each of which is a living cell. A constant chemical pumping action of the cell membrane tends to keep it polarized—that is, the outside of the cell
is electrically positive with respect to the inside. The travel of
an impulse along the course of a nerve or muscle fiber is a
wave of depolarization, followed a fraction of a second later
by a return to normal polarity (repolarization).
You may not understand this fully (does anybody?), but
you should at least be aware that it is entirely different from
the flow of electrical current through a wire.
It is, of course, impossible to measure or record the passage of an electrical impulse through a single fiber of heart
muscle from outside the body. What the electrocardiograph
actually detects, with electrodes placed at strategic sites on the
body surface, is the mass or net effect of all the electrical
activity going on in the heart from one instant to the next, as
damped and diffused by the body tissues intervening between
the heart and the electrodes.
Voltage is a measure of the difference in electrical potential between two points—for example, the positive and negative terminals of a battery. You can’t test the voltage of a
battery by applying a meter to only one of its terminals.
Similarly, the electrocardiograph is always testing the difference of potential between two points.
Einthoven’s original equipment was primitive and cumbersome by comparison with what is available today, and his
methods were correspondingly crude. The subject sat with
both arms and the left leg immersed in large jars of saline
solution, which served as detecting electrodes. By setting his
machine to record potential differences between each pair of
electrodes in turn, Einthoven produced three bipolar tracings,
each “looking at” the heart from a different angle. Although
the jars of saline were long ago replaced by metal electrodes
applied to the skin of the arms and the left leg, the standard
limb leads still retain the designations Einthoven gave them:
Lead I – Right Arm, Left Arm
Lead II – Right Arm, Left Leg
Lead III – Left Arm, Left Leg
(Mnemonic: The Roman numeral gives the number of
Ls.)
The difference of potential between the two electrodes is
constantly changing because waves of depolarization and
repolarization are constantly moving through the heart with
respect to the electrodes, which are fixed in position. By convention, one of the two electrodes of each bipolar limb lead
is called the recording electrode. When a wave of depolarization or repolarization passing through the heart moves toward
the recording electrode and away from the other electrode, the
tracing shows an upward deflection from the baseline. When
John H. Dirckx, M.D., Electrocardiography, The SUM Program Cardiology Transcription Unit, 2nd ed.
©2011, Health Professions Institute, www.hpisum.com
a wave of depolarization or repolarization moves away from
the recording electrode and toward the other electrode, the
tracing shows a downward deflection from the baseline. If a
wave of electrical activity moves transversely, keeping the
same distance from both electrodes, the tracing will show no
deflection.
Any one lead can supply information about the rate and
rhythm of ventricular contractions, and may also show abnormalities of impulse formation or conduction or evidence of
ischemia or infarction. But a single lead can provide only limited data about the size, shape, and position of the heart, and
may not show any sign of disease affecting parts of the heart
other than those facing the recording electrode.
Suppose you wanted to find out something about a house
that is completely surrounded by a high wall. If you could find
a narrow chink somewhere in the wall to peer through, you
could get an idea of what the house looks like, but it would
be only a very limited idea. Finding two more chinks at equal
distances around the periphery would give you view of two
additional aspects of the house and enable you to form a rudimentary three-dimensional concept of its size and shape.
Einthoven chose the three electrode positions because
they formed a roughly equilateral triangle with the heart at the
center. Unencumbered by modern electrophysiologic theory
and forced to work within the constraints of nineteenth-century technology, he devised an exquisitely simple and effective system that is still valid today and still the basis of all
electrocardiography. For his contribution, he received the
Nobel Prize for Medicine or Physiology in 1924.
Building on Einthoven’s pioneer work, later researchers
refined electrocardiographic equipment and devised the unipolar limb leads and precordial leads to supply further information about the heart by “looking at” it from additional vantage
points.
The three unipolar limb leads are obtained by simply
changing the electrical hookup between the electrodes attached
to the subject and the galvanometer in the machine. Instead of
measuring the difference in potential between the recording
electrode and one other electrode (as is done for leads I, II,
and III), the unipolar limb leads compare the potential of the
recording electrode against that of all the other electrodes
together. (In modern practice this includes a fourth electrode
on the right leg, which is never used as a recording electrode.)
Because the electrical sum of these three other electrodes
is zero, the recording electrode picks up a smaller voltage
than in the standard limb leads, and deflections in the tracing
are therefore smaller. In order to make these deflections comparable in amplitude to those of the standard limb leads, the
unipolar limb leads are “augmented” by the ECG machine.
All this means is that the machine automatically increases the
sensitivity of its galvanometer while these leads are being
recorded.
Whereas the recording axis of each bipolar lead is fixed
by the positions of the other two electrodes, the recording
electrode of a unipolar lead registers a true vector. (A vector
is a quantity having both magnitude and direction.) In the following nomenclature, a stands for augmented and V for vec-
tor. The third letter in each abbreviation refers to the site of
the recording electrode.
aVR – Right Arm
aVL – Left Arm
aVF – Left Leg (Foot)
For reasons too complex to discuss here, the unipolar limb
leads provide electrical views of the heart from vantage points
lying midway between those given by the standard bipolar
limb leads—three more chinks in the wall, at the most advantageous intervals.
The precordial or chest leads are obtained by placing
electrodes on the chest in front of the heart. The standard electrode positions for the six chest leads occur at approximately
equal intervals around the anterior chest starting from the
right sternal border and ending in the left midaxillary line.
These too are vector leads, with the recording electrode balanced against all four of the limb electrodes together. They
are numbered V1 through V6.
A standard 12-lead electrocardiogram (Figure 1) consists
of the standard limb leads, I, II, and III; the augmented bipolar limb leads, aVR, aVL, and aVF; and precordial leads V1
through V6, in that order. The bipolar and unipolar limb leads
taken together give a composite view of the heart in the frontal
plane, as it appears in a standard PA chest x-ray. The precordial leads yield a composite view of the heart in the transverse plane, as it is seen in a CT scan or MRI projection.
The modern ECG machine features solid state circuitry
and incorporates many technical advances that make it relatively simple to take a tracing and that eliminate most of the
problems that once plagued the electrocardiographer, such as
wandering baselines and interference from fluorescent lights.
Ordinarily an ECG is performed with the subject lying
supine in bed or on an examining table (Figure 2). The technician attaches an electrode to each forearm and each leg, just
above the ankle, and connects each electrode to a correspondingly marked wire leading into the machine. Electrodes
are small disks or cups of highly conductive, corrosion-resistant metal that are held in place with elastic bands or spring
clips or by suction. To ensure good electrical contact, a film
of gel or paste containing common salt or another suitable
electrolyte may be applied. Alternatively a thin strip of fabric
impregnated with electrolyte may be used. A single suctiontype electrode, having its own wire, is used to record chest
leads, and is moved from one position to the next between
tracings. For some applications (stress testing, monitoring
during resuscitation or in an acute-care setting) disposable
electrodes with adhesive backing may be applied.
After attaching the electrodes, the technician makes a
tracing by setting the recording paper in motion and selecting
each of the twelve leads in turn by moving a dial. The paper
drive is stopped between chest leads to permit shifting of the
electrode from one position to the next. The paper moves at
a constant speed under a needle or stylus that inscribes a continuous line showing fluctuations in potential. The recording
surface of the paper is coated with a thin film of white or gray
John H. Dirckx, M.D., Electrocardiography, The SUM Program Cardiology Transcription Unit, 2nd ed.
©2011, Health Professions Institute, www.hpisum.com
Figure 1
12-Lead Electrocardiogram
Source: Wikipedia Commons
Figure 2
ECG Electrode Placement
Source: Wikipedia Commons
wax. Under this wax, the paper is a solid color, usually black
or red. The stylus, which is electrically heated, melts the wax
at the point where it touches the paper, allowing the background color to show through. This eliminates the need for an
inked stylus.
ECG paper is sold in rolls, 100 feet or 150 feet in length.
The width of the paper varies from 48 to 63 mm, depending
on the brand of machine. For a standard tracing the paper
moves forward beneath the stylus at a rate of 25 mm/sec. The
paper drive can also be set to run at 50 mm/sec to expand the
tracing and permit finer analysis of small deviations or to produce a more informative tracing when the heart rate is very
rapid and complexes are close together.
To facilitate measurement of time and voltage, the paper
has a grid imprinted on top of the wax film. The larger
squares of the grid are 1 cm on each side, and each of these
squares is subdivided into 25 smaller squares 2 mm on a side.
At a paper speed of 25 mm/sec, each larger square thus represents 0.2 sec, and each smaller square represents 0.04 sec.
In a standard ECG tracing, an upward or downward deflection of 2 cm (2 large squares) corresponds to a difference in
potential of 1 millivolt (mV). When voltages are very high,
the machine can be set at half standard (1 cm = 1 mV) so
that the line traced by the stylus doesn’t run off the paper.
Every tracing includes a record of the deviation from the
baseline that is obtained by presenting a difference of potential of exactly 1 mV to the galvanometer while the paper is
running but the galvanometer is disconnected from the subject. This standardization wave, inserted by the technician at
the touch of a button, verifies the sensitivity setting of the
machine at the time the tracing was made. The machine also
includes a device for identifying which lead is being run by
marking the edge of the moving paper with a code consisting
of dots and dashes.
When the tracing is completed, the technician cuts it into
strips, trimming each lead to a standard size for mounting on
John H. Dirckx, M.D., Electrocardiography, The SUM Program Cardiology Transcription Unit, 2nd ed.
©2011, Health Professions Institute, www.hpisum.com
a printed form. Most of these strips are 10-12 cm in length,
corresponding to a recording time of 2 sec and typically
including 2-3 cardiac cycles. The strip of lead II, however, is
30 cm in length, corresponding to a recording time of 6 sec.
Because this is done to provide a broader overview of cardiac
rhythm than is possible with a 2-sec tracing, the longer lead
II strip is called a rhythm strip. In cutting strips for mounting, the technician makes a point of including any abnormal
complexes or runs of abnormal rhythm.
The form on which the strips are mounted is printed on
stiff paper and provides a labeled box for each lead. Each box
is coated with adhesive under a protective peel-away strip to
facilitate mounting. The form provides space for identifying
information about the patient and for salient points of the medical history, including current medicines. It also has spaces for
entering measurements taken from the tracing and for the
physician’s interpretation.
In modern practice, various modifications and variations
of the standard ECG are used for specialized applications. An
esophageal lead, using a swallowed electrode, can explore the
posterior surface of the heart electrically, and at surgery electrodes can be applied directly to the heart. A Holter monitor
is a compact ECG machine that is worn by a patient for 24
hours or longer to provide continuous sampling of cardiac
electrical activity. By this means, intermittent runs of arrhythmia can be documented and characterized, and ECG findings
can be correlated with events such as meals, effort, chest pain,
and palpitation.
In exercise electrocardiography (stress testing) the subject
performs a measured amount of work on a treadmill while a
continuous tracing of standard limb lead II is monitored to
detect any indication of myocardial ischemia or arrhythmia. In
the electrophysiology laboratory, an ECG records changes in
heart rhythm and function induced by the injection of chemicals into the circulation and by the application of artificial pacing to the heart.
The ECG signal is readily converted to a visual display
on a cathode ray tube (CRT) for continuous monitoring and
immediate interpretation. The ECG signal can be digitized and
transmitted by radio waves or telephone wires for recording
and interpretation at a site remove from the patient’s location.
Telemetry (radio transmission of an ECG signal from a monitor worn by the patient to a nearby receiving station) allows
maximum mobility of patients in a step-down coronary care
unit. Computer interpretation of ECGs has reached an
advanced stage of accuracy.
contractions, even if all nerves to the heart are severed. In
medical jargon, the SA node is almost universally called simply “the sinus.”
The electrical impulse generated by the SA node triggers
a wave of depolarization that spreads over both atria, causing
them to contract. When this impulse reaches the atrioventricular (AV) node, which is another mass of specialized tissue located low in the right atrium, it is picked up and
transmitted down the atrioventricular bundle (bundle of
His), a tract of specialized conducting fibers, to the septum
between the two ventricles. In the septum the bundle divides
into right and left bundle branches, of which the left further
divides into anterior and posterior fascicles. All of these tracts
of conducting tissue eventually break up into a network of
Purkinje fibers, which penetrate the walls of the ventricles.
The outward passage of the wave of depolarization through
the muscular ventricular walls causes them to contract.
The electrical events of the normal cardiac cycle cause a
series of characteristic deflections, or waves, to appear in the
ECG tracing. It is important to remember that the ECG
detects electrical activity, not muscular contraction or movement. Figure 3 shows the waves of the normal ECG and indicates the cardiac event to which each corresponds.
Not all of these waves necessarily appear in every lead.
Moreover, some of them typically vary in amplitude (size)
and polarity (above or below the baseline) from one lead to
another. For example, the P wave is always upright in leads
I, II, and aVF, as it appears in Figure 2. In lead aVR, however, it is always inverted (dips below the baseline). In V1,
the chest lead whose electrode faces the right side of the heart,
Figure 3
Waves of the Normal ECG
Basic Electrophysiology of the Heart
The conducting system of the heart (see Figure 1) consists of highly specialized tissue capable of initiating and transmitting the electrical impulses that cause the heart to beat. The
pacemaker of the heart is the sinoatrial (SA) node, a small
nubbin of tissue in the upper part of the right atrium.
Although nerves of the autonomic nervous system and circulating substances such as epinephrine and many drugs can
affect the rate and rhythm of the heart, the SA node continues to function as a pacemaker, stimulating regular cardiac
John H. Dirckx, M.D., Electrocardiography, The SUM Program Cardiology Transcription Unit, 2nd ed.
©2011, Health Professions Institute, www.hpisum.com
the S wave it typically larger (goes farther from the baseline)
than the R wave. (This is designated an rS pattern.) As the
chest leads progress around the heart to its left side, this relation gradually changes. In V3 or V4 the R and S waves will
be found to be approximately equiphasic (that is, the R wave
goes about as far above the baseline as the S wave goes below
it—an RS pattern), while in V6 the R wave predominates (Rs).
In addition, the waves of the ECG can vary in amplitude,
polarity, and shape as a reflection of various cardiac abnormalities. For example, inverted T waves in lead I or lead II
usually indicate myocardial ischemia (deficient blood supply
to heart muscle). A deep, wide Q wave in any lead is generally evidence of myocardial infarction. The total absence of P
waves from all leads indicates that the sinoatrial node is not
functioning as a pacemaker.
The principal uses of electrocardiography are in identifying and analyzing abnormalities in heart rate and rhythm,
abnormalities in the size or shape of the heart or one or more
of its chambers, and transitory or permanent effects of impairment in the coronary circulation. In addition, electrocardiography is of value in diagnosing pericarditis, in verifying the
function of an artificial pacemaker, and in assessing various
systemic and metabolic disorders that affect heart function.
The ECG in Disorders of Rhythm and Conduction
As a preliminary to interpreting an ECG, the physician
determines the following values directly from the tracing and
enters them on the mounting or report form:
Atrial rate: The number of P waves/min.
Ventricular rate: The number of R waves/min.
(With normal cardiac function these are, of course, identical.)
PR interval: The time elapsed between the beginning of
the P wave and the beginning of the R wave, representing the
interval between the beginning of atrial depolarization and the
beginning of ventricular depolarization.
QRS interval: The time duration of a typical QRS complex, which represents the whole process of ventricular depolarization.
The physician then determines the mean electrical axis of
the heart in the frontal plane, usually called simply the electrical axis or still more simply the axis. This is an imaginary
line representing the direction along which the mean or maximum electrical activity passes down through the heart during
each cardiac cycle. The axis as projected in the frontal plane
is determined very simply by a glance at the first six leads of
the ECG. The lead whose recording electrode is most nearly
in line with the electrical axis will show the highest R waves,
while the lead whose recording electrode is most nearly at a
right angle to the electrical axis will show the lowest R waves.
The electrical axis is expressed in degrees, using as an
arbitrary baseline a perfectly horizontal line of zero degrees.
Imagine a front view of the heart projected on a clock dial. If
an electrical axis paralleling the position of an hour hand
pointing at 3 o’clock is defined as 0° degrees, then one pointing at 6 o’clock would be 90°. The electrical axis of the normal heart is between 0° and 90°.
A heart whose electrical axis lies above the horizontal 0°
line (for example, one with an axis of minus 60°) is said to
show left axis deviation, and a heart whose electrical axis lies
to the right of the vertical 90° line (for example, one with an
axis of 150°) is said to show right axis deviation. Significant
axis deviation indicates either abnormality in the shape of the
heart (left or right ventricular hypertrophy) or some deflection
of the normal depolarizing wave by a block somewhere in the
conducting system.
The rotation of the heart in the horizontal plane (that is,
around an imaginary vertical axis) is determined by examination of the precordial leads. Here the interpretation is somewhat more complex. Right ventricular preponderance causes
clockwise rotation (with the heart imagined as being viewed
from below) and left ventricular proponderance causes counterclockwise rotation.
The physician next determines the rhythm shown in the
tracing. The subject of heart rhythm and its abnormalities is
exceedingly complex, and can only be surveyed here. A number of terms referring to arrhythmias and conduction defects
are defined in the glossary at the end of this section.
The rhythm of the heart, as paced by the sinoatrial node,
is called regular sinus rhythm (RSR) or normal sinus
rhythm. A common variant of this is a slight increase in heart
rate with inspiration and a slight decrease with expiration,
known as sinus arrhythmia. All other variations from regular
sinus rhythm are abnormal, although some of these are benign
and of no clinical significance.
The term arrhythmia denotes any deviation from regular
sinus rhythm, and by convention also includes bradycardia
(heart rate below 60/min) and tachycardia (heart rate above
100/min), even with regular sinus rhythm. These are known
as sinus bradycardia and sinus tachycardia. An arrhythmia can
occur with a normal pulse rate (60-100/min), with bradycardia (bradyarrhythmia), or with tachycardia (tachyarrhythmia).
An arrhythmia can be present with an apparently regular
pulse, with an irregular but patterned pulse (regular irregularity), or with an entirely chaotic pulse (irregular irregularity).
An arrhythmia can be continuous, affecting the entire 12-lead
ECG, or it can be intermittent, causing variations only occasionally throughout the tracing, or even perhaps affecting only
one QRS complex in one lead.
Arrhythmias are broadly divided into disorders of
impulse formation and disorders of impulse conduction. In
order to understand disorders of impulse formation, one must
be aware that, under certain circumstances, parts of the heart
other than the sinoatrial node (“sinus”) can display automaticity—that is, can generate electrical impulses capable of
triggering ventricular contraction. Such an ectopic focus can
be anywhere in the walls of the atria or ventricles. Impulses
can arise, simultaneously or alternately, from two or more
ectopic foci.
John H. Dirckx, M.D., Electrocardiography, The SUM Program Cardiology Transcription Unit, 2nd ed.
©2011, Health Professions Institute, www.hpisum.com
The conducting system itself (AV node, AV bundle,
Purkinje fibers) cannot generate ectopic impulses. Impulses
arising in the immediate vicinity of the AV node are often
called nodal, but the preferred term is junctional (referring to
the junction between the AV node and the bundle of His). An
ectopic focus anywhere in the heart may discharge at relatively long intervals, causing only transitory disturbances in a
basically normal rhythm. On the other hand, an ectopic focus
may discharge at a sufficiently rapid rate to become the actual
pacemaker of the heart, usurping the function of the SA node.
An ectopic impulse arising from an atrial or junctional
focus passes down the bundle of His and causes normal ventricular depolarization, with a normal QRS complex appearing in the ECG. However, an impulse arising from an ectopic
focus in a ventricle does not travel through the conduction system in normal fashion. Spreading across the ventricular muscle by deviant pathways, it causes a wide, bizarre QRS
complex in the ECG.
An arrhythmia due to an ectopic focus at or above the
junctional level (and hence accompanied by normal QRS complexes) is called supraventricular. Common supraventricular
arrhythmias are premature atrial contractions (which are really
premature ventricular contractions triggered by an ectopic
focus at the atrial level); paroxysmal atrial tachycardia (PAT),
a sudden episode of rapid heartbeat due to an abnormal pacemaking focus in the atrial wall; atrial flutter, in which an atrial
focus firing regularly at a rate of 200-400/min becomes the
pacemaker; and atrial fibrillation (“A fib”), with ectopic atrial
impulses occurring irregularly at a rate of 400-1000/min. In
atrial flutter, P waves are replaced in the ECG by so called F
waves, each representing an impulse originating in the ectopic
atrial focus. Because these F waves occur at regular intervals,
they commonly form a sawtooth configuration. In atrial fibrillation, the more rapid and less regular firing of the ectopic
atrial focus is represented in the ECG by a correspondingly
irregular series of deflections called f waves (lowercase f).
Just as there is a physical limit to the number of times per
second that you can strike a computer key, there is a limit to
the rate at which the ventricles of the heart can respond to a
series of electrical impulses. For about 0.1 second after a
wave of depolarization passes over the ventricles, they are in
an electrically refractory state—that is, they will not respond
to any electrical stimulus, no matter how strong. This means
that the maximum pulse rate is around 200/min. If impulses
from a supraventricular ectopic pacemaker are arriving at the
ventricles at a rate much higher than this, some of them will
come during the refractory period of the myocardium. These
impulses will cause neither ventricular depolarization nor ventricular contraction. Often there is a simple ratio between the
number of impulses generated (P waves, F waves, or f waves)
per minute and the number of ventricular responses (QRS
complexes, pulse rate). If the atrial rate is 240/min and the
pulse rate is 120/min, this would be called atrial flutter with
a 2:1 (“two to one”) ventricular response.
Ventricular arrhythmias are those that originate from
one or more ectopic foci in the ventricles and are accompa-
nied by wide, bizarre QRS complexes indicating aberrant
depolarization of ventricular muscle. Commonly recognized
ventricular arrhythmias are premature ventricular contractions (PVCs); ventricular tachycardia (VT, “V tach’), a sudden episode of rapid pulse with absent P waves and
abnormal QRS complexes; and ventricular fibrillation, a
very rapid, chaotic, ineffectual twitching of the myocardium
that is rapidly fatal unless converted to a normal rhythm by
electrical defibrillation.
The presence of an anomalous accessory pathway in the
conducting system allows cardiac impulses to advance through
the heart by an aberrant route. The classical example of a conduction disorder involving an accessory pathway is WolffParkinson-White (WPW) syndrome. In this relatively
common condition, an accessory band of muscle fibers, the
bundle of Kent, connects the atria directly to the ventricles.
Impulses from the SA node pass through this bundle, bypassing the normal pathway. This results in early ventricular depolarization (pre-excitation). The PR interval is abnormally
short, and the R wave, instead of rising sharply, is slurred and
prolonged in what is called a delta wave.
Persons with Wolff-Parkinson-White syndrome are subject to various arrhythmias, particularly paroxysmal atrial
tachycardia. The development of this arrhythmia in WPW
syndrome involves transmission of cardiac impulses over both
the normal pathway and the accessory pathway. Usually the
impulse travels antegradely (forward) by the normal path and
then retrogradely (backward) by the bundle of Kent. On arriving at the atria, the impulse reenters the normal conducting
system and is again transmitted antegradely through it.
Repeated travel of the impulses in a circular path is called circus movement, and the resulting tachyarrhythmia is called a
re-entrant tachycardia.
When, as described above, the impulse moves in the normal forward direction through the conducting system, and
backward over the accessory pathway, the resulting circus
movement and tachycardia are called orthodromic. A reentrant tachycardia in which impulses travel forward by the
accessory pathway and backward through the normal conducting system is called antidromic.
The term heart block refers to any delay or obstruction
to the passage of an impulse through the conducting system of
the heart. Since the passage of a depolarizing wave through
the conducting system is not reflected in the ECG, the existence and location of a block must be inferred from indirect
evidence. Heart blocks are classified according to location as
sinoatrial (between the sinoatrial node and the atria), atrioventricular (at the atrioventricular node), and intraventricular
(in the bundle of His or one or more of its branches).
Atrioventricular blocks are of three degrees. In firstdegree block the PR intervals are prolonged, reflecting a
delay of impulse transmission, but every impulse gets through
to the ventricles and causes a beat. In second-degree block,
some impulses are transmitted and some are not. Often there
is a simple ratio between the rate of impulse formation (P
waves/min) and the pulse. If the atrial rate is 90 and the ven-
John H. Dirckx, M.D., Electrocardiography, The SUM Program Cardiology Transcription Unit, 2nd ed.
©2011, Health Professions Institute, www.hpisum.com
tricular rate is 60 (two of every three impulses getting through
and the third one being dropped), this would be called a 3:2
(“three to two”) block. Two kinds of second-degree block can
be distinguished electrocardiographically. In a Mobitz type I
or Wenckebach AV block, the PR intervals becomes progressively longer from one cycle to the next until at length a beat
is dropped. In a Mobitz type II AV block, the PR interval
remains constant. In third-degree block (complete heart
block), no impulses are transmitted between the atria and the
ventricles.
Third-degree block does not necessarily result in stoppage
of the heart. Even in the presence of sinus arrest (complete
cessation of impulse formation at the SA node), some ectopic
focus in the atria or the ventricles often spontaneously takes
over the pacemaker function. With a junctional (often called
“nodal”) rhythm the pulse is 50-60 and QRS complexes are
normal. With an idioventricular rhythm (one arising from an
ectopic pacemaker at the ventricular level) the pulse is 40-50
and QRS complexes are wide and bizarre.
An intraventricular block affects the bundle of His or one
or more of its branches. Intraventricular blocks that can be
diagnosed by analysis of the ECG include right bundle branch
block (RBBB), left bundle branch block (LBBB), left anterior
fascicular block, (left anterior hemiblock), and left posterior
fascicular block (left posterior hemiblock).
In right bundle branch block, the cardiac impulse is conducted normally to the left ventricle but not to the right ventricle. Depolarization of the right ventricle is delayed until the
impulse spreads to it from the left ventricle. In left bundle
branch block the situation is exactly reversed. In either case,
the QRS complex is widened because of the anomalous pattern of ventricular depolarization. Bundle branch block is classified as incomplete or complete on the basis of the width of
the QRS complexes.
The ECG in Coronary Artery Disease
In modern medical practice, electrocardiography receives
its widest application in identifying, localizing, classifying,
and monitoring the effects of coronary artery disease on the
heart. The adverse consequences of a reduction in coronary
blood flow occur in three well-defined levels or grades of
severity, each with its typical ECG finding.
Ischemia: The metabolic needs of the myocardium are
not being fully met, but no irreversible damage has taken
place. The patient may have chest pain (angina pectoris). The
ECG shows inverted T waves, typically symmetrical and
peaked or arrowhead-shaped, reflecting the influence of
altered tissue chemistry on ventricular repolarization. In stress
testing, transitory ST segment depression indicates myocardial
ischemia brought on by exercise.
Injury: Cell damage has occurred, but healing is still
possible. Levels of cardiac enzymes (troponins, CK-MB) in
the serum are elevated. The ECG shows elevation of ST segments, reflecting an abnormal response of dying tissue to the
ventricular depolarizing wave.
Infarction: Irreversible damage to myocardial tissue has
occurred. If the patient survives, the infarcted area will be
replaced by noncontractile scar tissue. Infarcted tissue is biologically dead and electrically inert. An ECG facing an
infarcted zone detects no electrical activity from it, but the
electrode may “look through” the infarct to pick up electrical
activity from the opposite side of the heart. This is the origin
of the large Q waves typically present in leads facing the
infarct.
An acute infarct is surrounded by a zone of injury and
this in turn is surrounded by a zone of ischemia. Hence in
acute myocardial infarction the ECG shows Q waves, elevated
ST segments, and inverted T waves all at the same time. As
the infarct evolves, the ST segments progress from a sloping
configuration to a coved or rounded shape, upwardly convex,
and eventually return to normal. Q waves, however, persist
throughout life, and may serve as an indication of an “old”
myocardial infarction, that is, one that occurred months or
years ago.
By studying the pattern of changes in the various leads,
it is possible to determine the location of a myocardial infarction with considerable accuracy. Commonly recognized locations are anterior, anterobasal, anterolateral, anteroseptal,
apical, inferior (diaphragmatic), posterobasal, posteroinferior,
posterolateral, and posteroseptal.
John H. Dirckx, M.D., Electrocardiography, The SUM Program Cardiology Transcription Unit, 2nd ed.
©2011, Health Professions Institute, www.hpisum.com
Glossary of ECG Terms
ECG Transcription Tips
Degree Sign: Use the degree sign (°) for angular measurements (for example, the electrical axis of the heart):
Dictated: The electrical axis is minus thirty degrees.
Transcribed: The electrical axis is -30°.
Do not use the degree sign for degrees of heart block.
D: There is a first degree heart block with a PR interval
of point two six seconds.
T: There is a first-degree heart block with a PR interval
of 0.26 sec.
Interventricular/Intraventricular Conduction Defect:
All heart blocks below the AV node are intraventricular—that
is, the site of the block is within the ventricles. But such
blocks can also be called interventricular, since the bundle of
His and its branches travel through the interventricular septum, between the ventricles. Transcribe what you hear, but be
aware that in most contexts interventricular conduction defect
and intraventricular conduction defect are synonymous.
Normal Axis Deviation: Some physicians habitually
refer to the electrical axis of the heart as its “axis deviation,”
apparently thinking of the axis as a deviation, by so many
degrees, from the 0° reference line. Because deviation implies
abnormality, the phrase “normal axis deviation” is a contradiction in terms, like “healthy hypertension” or “benign
shock.” When it is dictated, transcribe as “normal axis.”
ST and T wave changes: Abnormalities in the ST segment and the T wave are cardinal signs of myocardial
ischemia or injury. Physicians constantly use the phrase “ST
and T wave changes” to refer broadly to these abnormalities.
Although this phrase can and should be transcribed verbatim,
remember that it means “ST segment and T wave changes,”
not “ST (wave) and T wave changes.” (There is no ST wave.)
asystole
activity.
Cardiac standstill; absence of ventricular
atrioventricular (AV) dissociation The presence of
two independent pacemakers in the heart, the normal one in
the SA node and another in the ventricles, of which only
the latter produces cardiac contractions.
bigeminy Regular 1:1 alternation of normal cardiac
contractions and ectopic beats, usually of ventricular origin.
Since the ectopic beat comes a little earlier than the
expected normal beat, the pulse consists of paired, twinned,
or coupled beats: * * * * * * * * Also called coupling.
capture 1. The successful transmission of a sinus
impulse to the ventricles in spite of, or at the end of, a
period of AV dissociation. 2. The successful transmission
of the impulses of an artificial pacemaker to the ventricles.
coupling Same as bigeminy.
current of injury In electrophysiology this term refers
to the small electric charge generated by damaged tissue. In
electrocardiography it nearly always refers to ST segment
elevation as a sign of coronary insufficiency and myocardial injury.
depressed, depression A positive (upright) wave of
the ECG is said to be depressed or to show depression
when it is not as far above the baseline as it would normally
be expected to be. For some parts of the ECG (for example, the ST segment), depression may proceed below the
baseline. Compare inverted.
digitalis effect A group of electrocardiographic
changes caused by administration of cardiac glycosides
(digoxin, digitoxin) and including bradycardia, first- or second-degree AV block, frequent or coupled premature beats
(see bigeminy), and a characteristic distortion of the ST
segment.
electrical alternans Jargon for “the ECG equivalent
of pulsus alternans.” Pulsus alternans is a pulse in which
every other beat is stronger. Electrical alternans is an ECG
finding in which every other R wave is taller.
escape beat A beat triggered by an ectopic focus that
is able to “escape” the usual pacemaking dominance of the
SA node because of disturbance in the SA node or the conducting system.
John H. Dirckx, M.D., Electrocardiography, The SUM Program Cardiology Transcription Unit, 2nd ed.
©2011, Health Professions Institute, www.hpisum.com
extrasystole Simply an extra systole (premature heartbeat), but correctly written as one word.
fusion beat Ventricular activation by the simultaneous
arrival at the ventricles of depolarization waves from two
independent pacemakers. See parasystole.
intrinsicoid deflection After inscribing the R wave (or
the S wave in an rS complex), the ECG stylus falls back to
the baseline because, for a split second, there is no electrical activity to record. Early researchers working with electrodes applied directly to the heart in experimental animals
called this return to the baseline an intrinsic deflection. The
corresponding deflection in a standard ECG tracing is
called intrinsicoid. The intrinsicoid deflection is delayed or
slurred in bundle branch block.
inverted, inversion An ECG wave is said to be
inverted when it goes below the baseline, that is, when it
records a negative voltage. The term is not used with the
waves of the QRS complex, since the polarity of each of
these is part of its definition. Compare depressed.
isoelectric Showing neither positive nor negative
potential and therefore no deviation from the baseline in the
ECG tracing.
J (for junction) point The point in an ECG tracing
where the QRS complex ends and the ST segment begins.
No connection with junctional rhythm.
muscle artifact Jaggedness or fuzziness of the ECG
baseline due to tremors or tension of the subject’s muscles.
negative Referring to an ECG wave that goes below
the baseline, indicating movement of a wave of electrical
activity away from the recording electrode.
P mitrale (“my-TRAY-lee”) A large, notched P wave
seen in left atrial hypertrophy due to mitral stenosis.
P pulmonale (“pul-mo-NAY-lee”) A large P wave
due to right atrial enlargement, which occurs in cor pulmonale (right heart disease due to pulmonary disease, usually emphysema).
pacemaker artifact A narrow spike in the ECG tracing indicating an electrical impulse from an artificial pacemaker.
pacer spike Same as the preceding.
parasystole A complex arrhythmia in which two pacemakers, both of which may be ectopic, function simultaneously but independently. This is unusual because ordinarily
the pacemaker with the fastest rate remains dominant, canceling out any slower foci. In parasystole, an impulse from
either one of the two functioning pacemakers can activate
the ventricles if it reaches them outside the refractory
period following their activation by the other pacemaker.
Arrival of impulses from both pacemakers at the same
instant may result in a fusion or summation beat.
pseudoinfarct pattern An ECG pattern suggesting
myocardial infarction but actually due to some other abnormality, usually a conduction defect.
racemaker (slang) A runaway artificial pacemaker firing at an inappropriately rapid rate.
reciprocal changes Changes in an ECG lead due to
abnormalities on the side of the heart opposite the recording electrode. “The electrocardiogram shows elevation of
ST segments in leads II, III, and aVF and reciprocal
depression of ST segments in leads I, aVR, and aVL and
all of the precordial leads.”
R on T The appearance of an R wave from a premature ventricular contraction immediately on top of the T
wave of the preceding contraction. An ominous finding in
a patient with premature ventricular contractions (PVCs),
as it indicates a tendency toward ventricular tachycardia or
ventricular fibrillation.
somatic tremor Same as muscle artifact.
silent myocardial infarction A myocardial infarction
causing atypical symptoms or no symptoms, which is either
not recognized at the time of its occurrence and only diagnosed later on the basis of ECG findings (Q waves), or is
diagnosed only accidentally on ECG during its acute phase.
Stokes-Adams attack A syncopal episode due to transitory ventricular asystole. Strictly speaking it occurs only
in complete (third-degree) AV block when the idioventricular rate is too slow to maintain adequate blood flow to the
brain. The term is also applied, however, to syncopal
episodes due to other cardiac arrhythmias, such as ventricular fibrillation.
summation beat Same as fusion beat; see parasystole.
torsade de pointes A form of ventricular tachycardia
in which cyclical variations in the height of the R waves
yield an undulating or scalloped contour.
John H. Dirckx, M.D., Electrocardiography, The SUM Program Cardiology Transcription Unit, 2nd ed.
©2011, Health Professions Institute, www.hpisum.com