THE PHYSIOLOGICAL
BASIS OF THE EKG
Dr. Guido E. Santacana
Professor
Dept. of Physiology
After this section you should be able to:
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Differentiate between an intracellular action potential and the electrocardiogram
as an extracellular recording.
Recognize the concept of the dipole and electrical potential vector and how it
applies to the heart and EKG recording.
Recognize that the cumulative electrical activity of the heart forms dipoles or
electrical potential vectors in different directions as the activation of the heart
progresses.
Recognize that the multiple dipoles or electrical potential vectors generated by
the heart produce the EKG recording.
Learn the 12 EKG Leads and their projection of the lead vectors in three
orthogonal planes.
Understand how each EKG wave is generated using Lead I as an example.
Perform a simple sequential analysis of the EKG.
Learn the standards of the EKG recording paper.
Understand the origin of the Mean QRS Axis concept.
Using the Electrical Axis Circle of the heart learn to estimate Mean Electrical
Axis.
Calculate the Mean QRS Axis by vector analysis using three or two standard
leads.
Recognize the effect of left or right hypertrophy on the Mean QRS Axis.
Recognize the effect of left or right Bundle Branch Block on the Mean Electrical
Axis.
Understand the concept of current of injury and its clinical implications.
Learn to estimate the site of an ischemic injury using the concept of the J Point.
The Conduction System of the
Heart
Action Potentials = Change in membrane
potential occurring in nerve, muscle, heart and
other cells
The ECG is not an action potential but
reflects their cumulative effect at the
level of the skin where the recording
electrodes are located.
THE DIPOLE CONCEPT
METER
C
0
+2
+
JAR
-2
B -
-
+ A
D
POS
NEG
BATTERY
EINTHOVEN’S TRIANGLE
AND LIMB LEADS
B
B
A
A
Einthoven’s Original EKG Recorder
String Galvanometer Based
EKG Recorder. Patient with
hands submerged in strong
salt solution.
String Galvanometer
Schematic.
What are we looking at in the EKG
waves?
VOLTAGE!!
As amplitude in
Millivolts.
Baseline at 0mv
TIME!!
Duration in fractions of a second
Atrial Depolarization
P wave (Lead I)
A
B
Zero
potential
-
P
+
+
0
Peak
Potential
Ventricular Conducting System
AV Node
Bundle of His
Left Bundle Branch
Right Bundle
Branch
Left Posterior Fascicle
P
Septal fascicle
P
Left Anterior
Fascicle
P= Purkinje Fibers
Ventricular Septal
Depolarization- the Q Wave
A
B
+
+
-
0
Q
-
Ventricular Depolarization-the R Wave
B
A
-
R
3
4
+
0
+
-
Ventricular Depolarization-The S Wave
B.
A
B.
+
+
0
A
S
QRS Configurations
RSR’
QRS
RS
QR
QS
Ventricular Repolarization- the T Wave
+
B
A
-
T
+
+
0
-
S
Review of the Sequence in the
Formation of the EKG
Intervals and Segments of the
Normal EKG
INTERVALS AND SEGMENTS !
PR Interval- Onset of P wave to
onset of QRS. (.12-.20sec or 35 small squares)
 QRS Interval-Beginning and end
of QRS wave.(<.12sec duration
or 3 small squares)
 QT interval- Beginning of QRS
to end of T wave.( Calculated as
corrected QT = .42 sec)
 ST segment ( no elevation or
depression)

B
A
0°
Limb Leads=Frontal Plane
B
60°
B
120°
A
A
+
A
+A
-150°
B -
-B
B 90°
-30°
A
Chest Leads = Horizontal Plane
B
A
+
The Chest or Precordial Leads
V6
V5
V1
V2
Over right
ventricle
V3
Over the
Left Ventricle
V4
Over Interventricular
Septum
Projection of the 12 Lead EKG Vectors
in Three Orthogonal Planes
Review of what each EKG Lead
looks at.
Y
Z
X
Anterior Leads
V1,V2,V3,V4
Inferior Leads
II,III,AVF
Left lateral
Leads
I, AVL,V5
V6
MEAN QRS AXIS BASICS
WHAT IS THE MEAN QRS AXIS?
IT REPRESENTS THE AVERAGE DIRECTION OF THE
INSTANTANEOUS FORCES GENERATED DURING
THE SEQUENCE OF VENTRICULAR DEPOLARIZATION.
NORMAL RANGE= -30 TO +90 DEGREES
NORMAL VALUE= 59 DEGREES
MORE - THAN -30 = LEFT AXIS DEVIATION
MORE + THAN +90 = RIGHT AXIS DEVIATION
Instantaneous and Mean Vectors of
Ventricular Depolarization
-90°
180°
G
F
E
A B
C D Mean Vector
+90°
0°
The Electrical Axis Circle
Where does it come from?
Lead I
Lead I 0°
Lead
II
Lead III
Lead III
Lead
120°
II
REMEMBER EINTHOVEN
60°
The three Leads with a Common
Center
Lead I 0°
Lead III
120°
Lead
II
60°
THE ELECTRICAL AXIS CIRCLE!
-120°
-90°
-60°
aVR- -150°
aVL - -30°
+180°
I- 0°
Normal
+150°
Range
III- +120°
+30°
II - +60°
aVF- + 90°
Using the Circle to Estimate MEA
TO ESTIMATE QRS AXIS
-90°
Lead II QRS UP
-30°
I-0°
+180°
Lead I
QRS UP
+150°
II - +60°
+90°
-30°
NORMAL AXIS
+90°
The Isoelectric QRS and its use!
aVL
-30°
+60
Lead perpendicular to the
isoelectric QRS
II
Why is a Wave Biphasic?
Cardiac Muscle
-----------------++++++++++
Lead
-
0
C
B
A
+
Meter
Cardiac Muscle
++++ ---------------- +++++
Lead
-
0
+
Meter
Cardiac Muscle
---------+++++
-
0
++++
------Lead
+
Meter
Electrode Perpendicular to Direction of Depolarization
Why is Lead aVL Biphasic?
+
Lead aVL
LA
RA
-
+
LV
RV
Lead I
+
+
Lead II
Quick Estimation of Axis Deviation
I
I
AVF
Extreme
Right axis
deviation
AVF
Left axis
deviation
0° Lead I
Right axis
deviation.
Normal axis
I
I
AVF
AVF
+90° Lead AVF
Ventricular Hypertrophy 1
Limb Leads
Precordial Leads
Ventricular Hypertrophy 2
Precordials
S wave
R exceeding 18mm
R exceeding 26mm
Review of Vectors and
Vectorial Analysis of the
EKG
Guido E. Santacana Ph.D.
Professor
Department of Physiology
Basis for Vectorial Analysis
The Boat example!!!
Actual Direction
(Resultant Vector)
Graphical Representation
y
Wind 10 knots
x
20 knots
Vectorial Analysis of the Mean
Electrical Axis
EKG (LEAD I): Projected Vectors for
different Mean Electrical Axes
Tilted Mean Vectors
Partial Voltage Reading
-
+
Parallel Mean Vectors
LEAD I
Higher Voltage Reading
-
Perpendicular
Mean Vectors
-
No Voltage Reading
+
LEAD I
NO PROJECTED VECTOR!!!!!
LEAD I
+
The Concept of the Projected
Vector
A=Mean Vector
B=Projected Vector
Figure A
Figure B
Projected Vectors for theThree
Standard Leads
Ventricular Depolarization Analysis
Using the Projected Vectors
.01 sec
.02 sec
.05 sec
.035 sec
.06 sec
Ventricular Repolarization Analysis
Using the Projected Vectors
What is the Vectorcardiogram?
It is simply the path marked by the positive ends of
The depolarization vectors.
How to Plot the Mean Electrical
Axis Using Two EKG Leads
R wave only = 6mm
or .6mv
6mm
6mm
Mean Electrical Axis
RS waves
R= 8mm
S= -2mm
Total = 8-2=6mm
or .6mv
Abnormal Ventricular Conditions
That Cause Axis Deviation
Change in position of the heart in the
chest.
 Hypertrophy of one ventricle.
 Bundle Branch Block.
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Vectorial Analysis of Ventricular
Hypertrophy
-15°
Left ventricular hypertrophy in a hypertensive heart.
Reasons for deviation are LV mass and conduction time.
Vectorial Analysis of Right
Ventricular Hypertrophy
Notice also the
High voltage
EKG in Lead I
170°
170°
RV Hypertrophy caused by Pulmonary Valve Stenosis
Vectorial Analysis in Bundle Branch
Block
Prolongued
QRS due to
Slower
Conduction
Time Through
Block
-50° left deviation
Left axis deviation caused by a Left Bundle Branch Block
Vectorial Analysis in Bundle Branch
Block
Right Bundle Branch Block producing a right axis deviation.
Again observe the longer QRS interval. Longer QRS intervals
Can distinguish axis deviations due to BBBs vs. hypertrophies.
Low Voltage EKG
Normal voltage
between R wave
and S wave should
be from .5 to 2mv
If the sum of the
voltages in the QRS
of leads I,II,III is
greater than 4mv
the EKG is considered
as high voltage.
Low voltage EKG due to myocardial infarction.
Low voltage EKG is also caused by pericardial effusion, pleural effusion
and pulmonary emphysema.
The Current of Injury
Cardiac abnormalities specially those
that damage the heart muscle cause
part of the heart to remain partially
or totally depolarized all the time.
 The current that flows even between
heartbeats from the pathologically
depolarized area to the normal area
is the CURRENT OF INJURY.
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Causes of the Current of Injury
Mechanical Trauma.
 Infections
 Ischemia caused by coronary
occlusions. (Most common cause)

The Current of Injury
Current
of injury
remains after
the heart has
Repolarized.
EKG Generation in Normal vs.
Infarcted Heart
NORMAL
INFARCTED
The J Point and the Current of
Injury Vector Analysis
J Point is the
zero potential
line from which
the direction
of the injury
current is determined
Injury Potential in Anterior Wall
Infarction
Respective J Points
In Leads I and III
J point in V2
an anterior
Lead
Injury Potential in Posterior Wall
Apical Infarction
J Points of
Leads II &
III.
J point of
V2
Cardiac Arrhythmias
Result from disturbances of
IMPULSE
PROPAGATION
IMPULSE
INITIATION
Conduction Blocks
Reentry rhythms
SA Node
Ectopic Foci
Alteration of SA Rhythm
Autonomic nervous system usually
involved.
 P, QRS, T waves normal.
 Duration of Cardiac Cycle P-P interval
shortened or prolonged.
 Sinus Bradycardia- Slow Rhythm.
 Sinus Tachycardia- Fast Rhythm.
 Cardiac frequency changes gradually.
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Bradycardia
Normal Rhythm
Bradycardia
bradycardia occurs when the hearts rate is
slower than 60 beats per minute.
Tachycardia
Normal Rhythm
Tachycardia
Sinus tachycardia occurs when the sinus rhythm
is faster than 100 beats per minute
AV Transmission Blocks
Impulse transmission through
conduction tissue blocked.
 His Bundle Electrogram may be used
to localize block.
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His Bundle Electrogram
Prolongation of either
the A-H or H-V interval
indicates block above or
below the Bundle of His
A
Atrial Wave
H
His Bundle
Wave
V
Ventricular wave
Paroxysmal Tachycardia
Abrupt onset and termination.
 Origin is ectopic site.
 Reentry circus movements most
frequent cause.
 High frequency.
 Can cause lightheadedness or
syncope.
 Rapid contractions reduce ventricular
filling.
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Paroxysmal Supraventricular
Tachycardia
Originate in atria or AV tissue.
Usually from a reentry loop in atrial, AV tissue or both.
Paroxysmal Ventricular Tachycardia
From ectopic foci in the ventricles.
From considerable ischemic damage.
Bizarre QRS complexes
May be a precursor of Ventricular Fibrillation
Results from digitalis toxicity.
Fibrillation
Arrhythmia that is ineffectual in
pumping blood.
 Atria or Ventricles may be involved.
 Is due to fragmentation of reentry
loop into multiple irregular circuits.
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Atrial Fibrillation
Atria do not contract and relax sequentially.
No contribution to ventricular filling.
No P waves. Irregular fluctuations or f waves.
Normal QRS complexes but irregular rhythm.
Compatible with life and full physical activity.
20-30% reduction in ventricular pumping.
Ventricular Fibrillation
Irregular continuous twitching of the ventricular muscle.
No pumping of blood possible.
Loss of conciousness occurs rapidly
Irregular fluctuations in the EKG
Often initiated by a premature impulse arriving in the
vulnerable phase.
Vulnerable phase coincides with downslope of the
T wave.
Electric shock used to treat VF by leaving the
ventricles temporarily refractory and allowing
the SA node to take over again.
Mechanism of Ventricular
Fibrillation
Causes of reentry
Circus movements
Long Pathway= dilated hearts
Decreased velocity of conduction=blockade of Purkinje System
Greatly shortened refractory period= Epinephrine
60 Hz AC Induced VF
60Hz 120VAC
Applied here
End result
Atrial Flutter
F wave
Normal EKG
Wolf Parkinson-White Syndrome
Normal
Wolf Parkinson White
Alternate Conduction Pathway
Bundle of Kent
SEQUENTIAL APPROACH TO
THE EKG
Gain familiarity with the normal EKG.
 Evaluate the rhythm.
 Calculate rate.
 Evaluate each P wave, QRS, ST
segment and T wave in each lead.
 Mean QRS Axis
 Abnormalities of the P wave
 Abnormalities of the QRS
 ST and T wave abnormalities.
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The Normal EKG - 12 lead
Reading the EKG Paper
BASICS
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Each individual
horizontal and vertical
line is ruled in 1 mm
Each horizontal space
represent a time
interval of 0.04 sec
Each vertical space
represents a voltage
change of 0.1 mv
0.5
mv
0.2 sec.
HEART RHYTHM
Every P wave followed by a QRS.
 Every QRS preceded by a P wave
 P wave upright in leads I, II, III
 PR interval greater than .12 sec
 P wave rate 60-100BPM with < 10%
variation. < 60 - sinus bradycardia,
>100sinus tachycardia. Variation of
more than 10% = Sinus arrhythmia
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CALCULATE HEART RATE
STANDARD EKG PAPER SPEED OF 25MM SEC
HEART RATE = 25MM SEC X 60SEC/MIN
MM/BEAT
OR!!
= 1500
# of small boxes between 2 beats
Example
23
There are 23 mm between the
two QRS complexes, therefore:
Heart rate = 1500/23 = 65
beats/min
Analyzing The Normal EKG
Left Atrial Enlargement
Right Atrial Enlargement
Sample Abnormality of the
P wave
MITRAL
STENOSIS
P waves not visible
random rhythm
right ventricular hypertrophy
atrial fibrillation
AV BLOCK
Exercise Intolerance
Ventricular Escape Rythm
Hyperkalemia
Small or absent P waves
Atrial Fibrillation
Wide QRS
Shortened or absent ST segment
Wide and tall T waves
Ventricular fibrillation (sometimes)
Haemodialysis
Hypokalemia
Small or absent T waves
1st or 2nd degree heart AV block
slight depression of ST segment
Vomiting (prolonged)