Download ECG INTRODUCTION (Lecture 1)

ECG INTRODUCTION
(Lecture 1)
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Associate Professor
Dr. Alexey Podcheko
Spring 2015
Intended Learning Objectives:
BRIEF REVIEW IN ANATOMY & PHYSIOLOGY OF THE
HEART
CARDIAC ELECTRICAL ACTIVITY
ECG READOUT GENERATION
ECG NOMENCLATURE
POSITION OF THE 12 ECG LEADS RELATIVE TO THE
HEART
TIME & THE ECG
THE PREDICTED NORMAL ECG & SOME REFINEMENTS
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“CARDIAC ANATOMY &PHYSIOLOGY”
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Conductive system of the heart
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“CARDIAC ELECTRICAL ACTIVITY”
 “During each cardiac cycle” the atria contract in
diastole to fill the ventricles which then contract
during systole to supply blood to the lungs and the
systemic circulation.
 Contraction of the atria and ventricles is tightly
coordinated by a wave of Depolarization spreading
through the muscular walls of these chambers.
 The Depolarization wave reflects movement of charge
across myocyte membranes and is in effect an
electrical current spreading through the heart.
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………..d.d…..
- Direction of electromagnetic wave
 Following contraction, cardiac muscle returns to a resting
state and this is associated with reversal of the movement
of charge across the myocyte membranes, this second
wave of electrical activity is termed cardiac repolarization.
 Cardiac depolarization is triggered by an electrical pulse
generated in the sinoatrial node.
 The atria and ventricles, however, are separated by a Nonconducting fibrous septum.
 The depolarization wave cannot penetrate this barrier and
in order to activate ventricular contraction the wave must
be transmitted into the ventricles by the specialized cardiac
conducting system. In a normal heart, the only route by
which the depolarizing wave can enter the ventricular
12 conducting system is through the AV node.
 In order to allow the ventricles to fill with blood following
atrial contraction, the AV node initially delays the spread of
the depolarization wave. After this short delay, the
depolarizing signal is transmitted into the ventricles via the
bundle of His.
 The bundle of His lies in the interatrial septum and divides
into right and left bundle branches. The right and left
bundle branches transmit the depolarizing signal into the
muscle mass of the right and left ventricles respectively.
 The interventricular septum is the first part of the
ventricular muscle mass to depolarize and it does so by
movement of depolarization across the septum from the
left towards the right bundle branch……….
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 In the walls of the ventricles, Depolarization spreads from
the terminal fibers of the conducting system Outwards
from the Endocardium towards the Epicardial surface of
the heart and also back along the ventricular wall to the
atrioventricular groove.
 Repolarization spreads through the ventricles in the
“opposite” direction to the depolarization wave moving
from the Epicardial to the Endocardial surface of the
chambers.
 Within the ventricular wall there is a gradient in the rate of
cellular repolarization, cells in the epicardial region have
the fastest rate of repolarization and repolarize first
following ventricular contraction. The rate of cellular
repolarization is then progressively slower as we move from
the epicardium towards the endocardium.
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“ECG READOUT GENERATION”
 The leads of the ECG machine detect the movement of the
cardiac depolarization and repolarization waves as they
spread through the atria and ventricles.
 In any ECG lead, the flat line recorded on the readout when
No Net current is flowing is termed the Isoelectric line.
 It is very important to realize that all of the leads on the
ECG machine are set up in such a way that Depolarizing
current moving Towards a lead produces a deflection on
the ECG paper above the isoelectric line, a Positive
deflection
 while Depolarizing current moving Away from the lead
produces a deflection below the isoelectric line, a Negative
deflection.
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No Net current (E.g. No
signal or wave of
depolarization moves at
right angle
Depolarizing current
moving Towards a lead
Depolarizing current
moving Away from the
lead
Very important to remember
•A wave of depolarization moving toward an
electrode will record a positive deflection on an
ECG
•A wave of depolarization traveling away from
an electrode will inscribe a negative deflection
on an ECG.
•A wave of depolarization moving at right
angles to an electrode will cause either no
deflection or a very small deflection on an
ECG.
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 Therefore, Repolarizing current moving Towards a lead
produces a Negative deflection on the paper
 while Repolarizing current moving Away from the lead
produces a Positive deflection.
 the SA node is situated towards the back of the right
atrium so the atrial depolarization wave not only
spreads downwards and to the left but also outwards
towards the front of chest towards the chest leads.
 As this depolarizing current is moving towards the
leads it produces a positive deflection on the ECG
paper, this is the P wave of atrial depolarization.
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 The Magnitude of the electrical signal generated by
depolarizing muscle is directly proportional to the
Mass of Muscle generating it, what this means is, that
the more muscle present the more electrical signal
generated and the more signal the ECG machine
detects.
 The left ventricle has a much greater muscle mass than
the right and so dominates the electrical signal of
ventricular depolarization in all leads.
 Therefore, as the wave of electrical activity reaches the
main muscle mass of the ventricles, the left ventricular
signal overwhelms all other signals and as it is moving
away from V1, the deflection produced on the ECG
recording from this lead becomes negative.
(Overall……Negative deflection)
 In contrast, however, this signal is moving towards lead
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 When ventricular depolarization is complete there is a brief
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period when No current is flowing and the recording returns to
the Isoelectric line.
The deflection produced by Ventricular Repolarization is termed
a T wave.
Cardiac Repolarization spreads relatively Slowly through the
muscle mass, Outside the conducting system. Hence, the T
wave is considerably Longer in duration and, therefore,
Broader on the ECG paper than the QRS complex.
In leads with an “overall” positive QRS complex that is the
positive deflection is larger than the negative deflection, the T
wave also tends to be positive above the isoelectric line, while
in leads with an overall negative QRS complex the T waves tend
also to be negative, inverted below the isoelectric line.
To use the jargon, in Non-diseased hearts the QRS complexes
and T waves tend to be concordant.
o atrial repolarization produces a relatively weak electrical
signal which is buried in the QRS complex and is generally
Not detectable on a standard 12 lead ECG.
Early left to right septal depolarization may produce small
physiological q waves in left sided leads…
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“ECG NOMENCLATURE”
 The deflection produced by atrial
depolarization is termed a P wave
 ventricular depolarization produces the QRS
complex.
 The diffuse deflection produced by ventricular
repolarization is termed a T wave.
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Within the QRS complex:
any positive deflection, that is a deflection above
the isoelectric line, is termed an R wave.
Any negative deflection which follows an R wave is
termed an S wave.
However, if the first deflection of the QRS complex
is negative this deflection is termed a q wave.
Remember, a q wave can only exist if, and only if, the
first deflection of the QRS complex is negative.
A negative deflection following a positive deflection
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(no matter how small that positive deflection may
be) is an S wave.
 The section of the ECG recording connecting the End
of the QRS complex and the Beginning of the T
wave is termed the ST segment.
 the junction between the ST segment and the end of
the QRS complex is termed the ‘J point’. As all the
ventricular muscle mass is depolarized and there is
no flow of depolarization through the heart at this
time, the J point and ST segment should lie on the
isoelectric line.
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“POSITION OF THE 12 ECG LEADS RELATIVE TO
THE HEART”
 The different leads of the ECG examine cardiac
electrical activity from different perspectives.
Learning the position of the leads of a standard ECG
relative to the heart is not as difficult as it seems
and as we’ll see later pays dividends in clinical
practice.
 The Chest leads examine the heart in the Horizontal
plane.
 The Frontal leads (Limb leads = Standard leads +
Augmented leads) examine the heart in the Vertical
plane.
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Augmented Limb Leads
 In addition to the three bipolar
limb leads described above,
there are three
augmented unipolar limb
leads. These are termed
unipolar leads because there is a
single positive electrode that is
referenced against a
combination of the other limb
electrodes. The positive
electrodes for these augmented
leads are located on the left arm
(aVL), the right arm (aVR), and
the left leg (aVF).
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“Chest Leads / Precordial Leads (Unipolar)”
 the organ is rotated toward the left, so that the right
ventricle lies anterior to the left immediately behind the
sternum. Therefore, V1 and V2 face the anterior surface
of the right ventricle,
 V3 and V4 look at the anterior surface of the left ventricle
 V5 and V6 look at the lateral surface of the left ventricle.
 V1 is applied to the chest in the 4th right intercostal space to the
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right of the sternum,
lead V2 in the fourth left intercostal space to the left of the
sternum
lead V4 is placed on the chest over the apex beat.
Lead V3 is placed down from V2 midway between V2 and V4.
V6 is placed horizontally and laterally from V4 in the midaxillary
line
V5 is placed at the midpoint between V4 and V6.
“The Frontal (Limb) Leads:
Standard Leads + Augmented Leads”
(Frontal or Vertical Plane) :
 the Standard leads (Bipolar leads): leads I, II
and III
 the Augmented leads (Unipolar leads): aVR,
aVL and aVF.
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“Lead & Blood Supply”
 For the experienced practitioner, looking at different areas on
the ECG readout is like looking at different anatomical regions
of the heart.
 Leads II, III and aVF examine the region of the heart supplied
by the right coronary artery (RCA) – Inferior leads
 Leads V1-V4 examine the region of the heart supplied by the
left anterior descending (LAD) branch of the left coronary
artery – Anterior leads
 Leads I, aVL, V5 and V6 examine the region of the heart
supplied by the left circumflex branch (LCA) of the left
coronary artery – Lateral leads
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“TIME & THE ECG”
 If you look at the bottom of the ECG readout you will see a
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long run of recording from lead II, this is called the rhythm
strip.
We use the rhythm strip to calculate the heart rate and to
diagnose abnormal cardiac rhythms (arrhythmias).
Lead II is used as the rhythm strip as it is usually the
easiest lead in which to see P waves
For our present purposes we can consider that in all ECG
machines the recording needles run at a constant speed
over the ECG paper of 25 mm/sec.
At a recording rate of 25mm/sec, 5 large squares are
covered in one second. So, 300 large squares represents
one minute. Therefore, the number of R waves in 300 large
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squares is the heart rate in beats per minute…………..
 one large square corresponds to 0.2 seconds and one
small square to 0.04 seconds.
 In a normal heart, the time between the onset of atrial
depolarization (the beginning of the P wave) and the
onset of ventricular depolarization, (the beginning of the
QRS complex) varies between 0.12 and 0.2 seconds that is
between 3 and 5 small squares. This is the PR interval.
 The duration of the QRS complex represents the time
taken for ventricular depolarisation to be completed
 A normal QRS complex is less than three small squares in
width (<0.12s in duration).
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 The time between the onset of ventricular
depolarization and the end of ventricular
repolarization (that is the beginning of the QRS
complex and the end of the T wave on the ECG) is
termed the QT interval.
 “When the heart rate is 60 beats per minute”, the
upper limit of normal for the QT interval is 0.44
seconds or 11 small squares.
 It is important to realize, however, that the measured
QT interval varies with heart rate
 becoming shorter as the heart speeds up
 longer when the heart slows down.
 Therefore, particularly at higher heart rates it is
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 When faced with an ECG with a “heart rate other than”
60 beats per minute”, to calculate the true underlying
QT interval referred to as the corrected QT interval we
use the following formula: QTc = QT/√RR
 QTc is the corrected QT value, QT is the actual QT
interval measured on the ECG and the RR interval is
the distance between consecutive R waves on the ECG
measured in seconds.
To give you a rule of thumb, when looking at an ECG
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readout with a “heart rate other” than 60 beats per
minute”, if the QT interval is more than half the
distance between consecutive R waves, at least
consider the possibility of prolonged QT.
SUMMARY
The normal PR interval is 0.12 to
0.20 seconds (3 to 5 small squares).
The normal QRS duration is less
than 0.12 seconds (<3 small squares).
The QT interval; (approximately)
less than half the RR interval.
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“To Calculate the Heart Rate – with Regular Rhythm”
Rule of 300
 To calculate the heart rate in beats per minute from
an ECG with a regular rhythm, count the number of
Large squares between two consecutive R waves and
divide this number into 300.
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“To Calculate the Heart Rate”
Rule of 30:
 To calculate the heart rate in beats per minute from an
EKG with an irregular rhythm (also regular…) count the
number of R waves in 30 Large squares and multiply this
number by 10
Use “6-second EKG Strip” for doing this
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“PREDICTED NORMAL ECG & SOME
REFINEMENTS”
 The transition from dominant S waves to dominant R
waves in the chest leads occurs at or around lead V3 or V4.
 The transition at this point from an overall negative QRS to
a positive QRS complex in the chest leads is termed normal
R wave progression and reflects a normal healthy left
ventricle dominating QRS morphology with a normal
pattern of flow of depolarization around this chamber.
 It is quite normal for P waves to differ in morphology
between the 12 leads as each lead has a different
perspective on atrial depolarization. For example, as atrial
depolarization spreads away from lead aVR on the right, the
P wave in this lead is negative.
 P waves are usually most prominent in lead
II……………….(Overall Magnitude of the Depolarization
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Vector…………)
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 in a “healthy heart in leads with dominant R waves the
T wave tends to be less than half the amplitude of the
preceding R wave in height.
 Small physiological q waves in the left lateral leads
produced by septal depolarization are a normal finding
on the ECG.
 We’ve seen that the interventricular septum is the first
region of ventricular myocardium to depolarize and
that it does so by movement of depolarization from left
to right.
 This early left to right depolarization wave is the key to
understanding the potential existence of small
physiological q waves in any left lateral lead.
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 Depolarization of the septum does indeed spread from left to
right but this left to right movement also travels upwards and
somewhat backwards from the lower part of the left side of the
septum towards the upper part of the right.
 For this reason it is also perfectly acceptable to
see physiological q waves in the inferior leads.
 Physiological q waves may be seen in the Left Lateral and
Inferior leads.
 The square deflection at the end (or beginning) of each
recording strip on the ECG readout is the calibration box, an
internal standard in the ECG machine. It should be two large
squares in height indicating that 1mV of electricity yields a 10
mm vertical deflection on the ECG paper. This is the
standardized sensitivity of ECG machines.
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 The calibration box should also be one large
square in width this indicates that the machine is
recording at a needle speed of 25 mm/s.
 A standard calibration box of two squares
vertically and one box horizontally indicates that
the ECG machine is correctly calibrated.
 When you start to review patients ECGs in
clinical context it will rapidly become obvious
to you that there is tremendous variation in
normal ECG morphology in perfectly healthy
individuals.
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