Introduction to Electrocardiography
Information contained in the ECG
Disturbances of rhythm and conduction.
Ischemic damage to the myocardium.
The anatomical orientation of the heart.
Relative size of the heart chambers.
The influence of some drugs and electrolyte
disturbances (for example, hypokalemia).
Interpretation of electrical activity of the heart
Precise interpretation of electrical activity of the heart as recorded with electrodes on
the skin is difficult because:
The distribution of charge within the heart is multidirectional and constantly changing.
The configuration of cardiac action potentials varies from region to region.
The position of the heart changes continually throughout the cardiac cycle.
Conductivity through the extracellular fluid is not uniform.
The surface of the body is not simple geometrically.
Cell model to show origin of
electrocardiogram record: resting cell
positive electrode
ECG record
Negative
electrode
Cardiac myocyte
isoelectric
In the resting cell the surface is uniformly positive compared to the cell interior.
There is no potential difference between different points on the cell surface.
The ECG records a baseline (the isoelectric line).
Cell model to show origin of electrocardiogram
record: partial depolarization creates a potential
difference between two points on the cell surface
Current flows
from negative to
positive regions
positive electrode
ECG record
depolarization
Negative
electrode
Depolarization of part of the cell membrane creates a potential difference
between the resting and depolarized regions on the surface.
By convention, current flows from negative to positive and from the
depolarized region to the “resting” region.
If the wave of depolarization is directed towards the positive electrode,
the ECG records a positive (upward) deflection.
Cell model to show origin of electrocardiogram record:
complete depolarization returns the ECG signal to baseline
positive electrode
ECG record
Negative
electrode
When the cell is completely depolarized, once again there is no potential
difference between any two points on the outside of the cell membrane.
The recorded potential returns to the baseline level.
Cell model to show origin of electrocardiogram record:
repolarization produces an ECG signal below baseline
positive electrode
ECG record
Negative
electrode
repolarization
As repolarization occurs, current flows in the opposite direction than
during depolarization.
Consequently the recorded potential is opposite in direction to the
potential during depolarization.
Cell model to show origin of electrocardiogram record:
repolarization completed: ECG signal returns to baseline
positive electrode
ECG record
Negative
electrode
Repolarization is slower than depolarization so the peak value is less than
during depolarization and the duration of the wave is longer.
With the return to the resting state the recorded potential returns to baseline.
Standard labeling of an ECG waveform in lead 1
R
Atrial
depolarization
T
S
P
Q
ST
PR
Atrial depolarization
& conduction
through AV node
QRS
QT
Ventricular depolarization
& atrial repolarization
Ventricular
repolarization
Factors affecting the amplitude of the ECG signal
Obese subject with
hypothyroidism
Hypertensive subject with
cardiac hypertrophy
(Signals are drawn to scale)
The amplitude of the ECG signal is affected by
the amount of myocardial tissue
the orientation of the heart in the chest
the thickness and type of tissue between the heart and the electrode.
For example, pericardial fluid, emphysematous lung tissue or obesity
increase electrical resistance between the heart and the electrodes,
reducing the amplitude of the ECG signal.
Correspondence between a ventricular action potential & the ECG
P
QRS
1
0
T
2
3
4
The QRS complex is produced by the summed upstrokes (phase 0) of the ventricular
myocyte action potentials.
The S-T segment corresponds to the plateaus of the action potentials.
The T wave is produced by ventricular repolarization.
Normal ECG intervals: calculating the intervals in
an ECG helps determine if the recording is
normal or what kinds of abnormalities may exist.
ECG intervals
(seconds)
average
range
Events in heart
0.18**
0.12 to 0.20
Atrial depolarization & conduction
through AV node
QRS duration
0.08
0.06 to 0.10
Ventricular depolarization
QT interval
0.40
Varies with HR
Ventricular depolarization &
repolarization
ST interval
0.32
---
PR interval*
Ventricles depolarized; ST interval
normally lies on isoelectric line
* Measured from the beginning of the P wave to the beginning of the QRS complex
** Shortens as heart rate increases from average of 0.18 at a HR of 70
to 0.14 a t a HR of 130.
Path of excitation in the heart
Sino-atrial node originates action potentials
Conduction velocity for action
potentials is greater in the Purkinje
fibers than in myocytes so
excitation spreads rapidly through
the ventricles allowing coordinated
contraction of the ventricular
muscle.
APs are propagated between
myocytes via gap junctions.
Atrial myocytes
Atrioventricular node
Bundle of His
Right & left bundle branches
Purkinje fibers
Ventricular myocytes
Path of repolarization
Ventricular endocardium is depolarized before epicardium.
The duration of action potentials in the endocardium is greater than the duration of
APs in the epicardium.
Myocytes in the epicardium begin to repolarize before myocytes in the endocardium.
Therefore repolarization spreads through the ventricle in the opposite
direction to depolarization, so in the ECG depolarization (QRS) and
repolarization (T wave) are both positive deflections.
Right & left bundle branches
Purkinje fibers
endocardium
QRS
epicardium
P
T
Einthoven’s triangle: placement of electrodes
RA
+
-
-
+
RL (ground)
+
LL
LA
Einthoven’s triangle is a system of
placing electrodes to measure the
electrical activity of the heart in
the frontal plane.
Electrodes (red circles) are placed on
the right arm (RA), left arm (LA) and
left leg (LL).
The electrodes are grounded to the
right leg (RL).
ECG Leads measure potential differences (voltages) between electrodes.
Definition of leads in Einthoven’s triangle (frontal plane):
Lead I: between RA and LA
Lead II: between RA and LL
Lead III: between LA and LL
RA
-
Lead I
+
-
Lead II
LA
Lead III
+
+
LL
Leads I, II & III are bipolar: they measure the potential difference (PD) between 2 points.
Size & polarity of the potentials are related to the
path of depolarization or repolarization
RA
-
Lead I
+
-
LA
Normal range for the
mean electrical axis is
– 30 to + 90 degrees.
- 30
Lead III
Lead II
Lead I (zero)
+
+
+ 90
LL
The shaded arrow is a vector (has magnitude & direction).
This vector represents depolarization during the QRS complex & is called the mean
electrical axis of the heart in the frontal plane.
The mean electrical axis is normally is 60 degrees to the horizontal.
Example of a shift in the mean electrical axis
Lead I
RA
+
-
-
LA
Lead III
Lead II
+
+
LL
Example: a shift to the right to 120 degrees:
Lead I polarity is reversed because projection
of the vector on lead I is from + to  pole.
Lead II magnitude is decreased, lead III
magnitude is increased.
A shift in the electrical axis changes the magnitude & direction of the ECG
signals. When evaluating an ECG the axis may be determined from the
magnitude and direction of the signals in the six frontal plane leads.
Einthoven’s triangle and the axial reference system for the ECG
An axial reference system is used to compare
tracings from all 6 leads in the frontal plane.
The 6 leads include Leads I, II and III and the
unipolar limb leads AVR, AVL, & AVF (next slide)
RA
--
Lead I
+-
LA
I
III
Lead III
Lead II
+
- 90°
+
LL
Leads I, II & III of Einthoven’s triangle
+ 120 °
0°
II
+ 90 °
Leads I, II & III in axial reference system
The unipolar limb leads AVR, AVL, & AVF are measured as the PD
between one limb and the electrical average of the other two limbs.
+
+
aVR
aVL
+
aVR (augmented right arm)
aVF
aVF (augmented left foot)
aVL (augmented left arm)
Axial reference system: six leads measuring the ECG in the frontal plane
- 90°
For each lead, the arrow points
toward the positive pole.
- 150 °
An axis beyond - 30 °
is called left axis
deviation.
aVL
aVR
I
III
0°
II
aVF
An axis beyond + 90 °
is called right axis
deviation.
- 30 °
+ 120 °
+ 90 °
Normal range for the
mean electrical axis is
– 30 to + 90 degrees
The mean electrical axis is determined from the magnitude and
direction of the signals in the six frontal plane leads.
V6
V5
LV
Six standard precordial leads
V4
V1
V1
V2
V3
V2
V3
V6
V5
V4
The 6 precordial leads (V1 to V6) measure potentials in a transverse plane
around the apex of the heart at right angles to the frontal plane.
They measure the PD between the electrode and ground so they are unipolar leads.
There are 12 leads in a standard ECG
Six leads meaure potentials in the frontal plane
3 Leads (I, II & III) for Einthoven’s Triangle
3 Calculated leads:
aVR (augmented right arm)
aVL (augmented left arm)
aVF (augmented left foot)
Six leads measure potentials in a transverse plane across the heart.
Precordial leads V1 to V6
12 Lead ECG: Normal sinus rhythm at a rate of 71 beats/min,
a P wave axis of 45°, and a PR interval of 0.15 sec.
Cardinal features of sinus rhythm:
The P wave is upright in leads I and II
Each P wave is usually followed by a QRS complex
The normal adult resting heart rate is 6099 beats/min
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Example: Ventricular depolarization recorded by precordial leads V1 & V6
(1) At rest ECG leads show baseline PD
+
- -
-
LV
- -
-
+
-
-
+
V6
+
-
+
-
+
--
+
(2) Depolarization begins in the septum
V6
-
++
+
+
+
V1
V1
(3) Depolarization spreads to apex
(4) Depolarization spreads to LV & RV
V6
+
-
--
+ +
+
+
+
+
++ -
-
V1
+
-
--
+
- -
+ +
+
-
+
+ +
-
V6
+
+
+
++
V1
+
-
--
Depolarization is complete
-
+ +
+
-
--
+
+
-
+
+ +
-
-
+
++
-
+
-
+
--
V1
Note that step 4 is repeated
from previous slide
V6
+ +
V6
+
+
(5) Depolarization is complete;
trace returns to baseline.
+
--
(4) Depolarization spreads to LV &
RV; signal from LV dominates
+
-
+
+ +
+
+
+
++
V1
+
--
Calibration of the ECG
Paper speed = 25 mm/sec
1 mm = 1/25 sec or 0.04 sec
1 sec
0.2 sec
Calibration Signal (1 mV)
paper speed = 25 mm/sec
0.1 mV
0.04 sec
1 mm
Calculation of heart rate from the ECG
16 mm
Small box = 1 mm2
Paper speed = 25 mm/sec
1 mm = 0.04 sec = 1 small box
Heart rate = beats/min = [mm per minute]/[number of mm between beats],
Substituting numbers:
mm
beats min
Heart rate 

mm
min
beat
 25 mm  60 sec 



beats
sec  min 

HR 
 94
16 mm
min
beat
Since 1 small box = 0.04 sec, intervals can be calculated from trace also
Heart Block
Heart block is due to delay or blockade of excitation in the conducting system.
First degree heart block refers to a prolonged PR interval (> 0.2 sec) and is due to a
defect at the AV node & may be functional or structural.
Functional block occurs when the conducting impulse reaches a region that is still
refractory from the preceding depolarization. Causes include some anti
arrhythmic drugs, or AV nodal ischemia.
Structural block is due to irreversible damage in the conducting system, for
example due to infarction or degeneration with aging.
First degree block is usually benign.
Second degree heart block refers to intermittent complete blockade of the conducting
signal so that some P waves are not followed by a QRS complex. There are two types
QRS
and several causes.
P
P
2nd degree block
Third degree heart block is complete dissociation between the atrial rhythm and the
QRS complex. The ventricular rhythm is due to an ectopic pacemaker distal to the AV
node. Third degree block requires treatment with a pacemaker.
QRS
3rd degree block
P
P
QRS
P
P
Reentry and arrythmias
Errors in conduction can cause abnormal rhythms or arrhythmias
One common cause of potentially serious arrythmias is re-entry
Reentry occurs when an anatomical or pathological fault causes
action potentials to continuously circulate around an abnormal
path in the myocardium
Reentry may occur as multiple, irregular continuously moving
circuits leading to fibrillation or disorganized contraction of the
myocardium
Reentry can occur where there is a branching path of myocardial tissue
Normally APs in the connecting
branch extinguish each other.
Unilateral block with normal
conduction velocity
Cells in blocked area
are in refractory period
so AP cannot propagate
in a retrograde direction
Unilateral block with
slow conduction
Reentry: If conduction is slow, as APs travel in
a retrograde direction, they reach tissue which
has passed the refractory period and is
excitable, so the APs continues to cycle.
Conditions for reentry: A branching pathway of myocardium with unilateral
block plus decreased conduction velocity.
Conduction blocks may be caused by ischemia, inflammation, fibrosis or some drugs.
Reentry circuits may cause tachycardia, atrial or ventricular fibrillation.