http://www.cvphysiology.com/Arrhythmias/A0
09.htm
Electrocardiography-ECG/EKG
Is a transthoracic interpretation of the electrical activity of the
heart over time captured and externally recorded by skin
electrodes. It is a noninvasive recording produced by an
electrocardiographic device
Electrocardiography: Introduction
• Body fluids are good conductors (the body is a volume conductor)
Fluctuations in potential (action potentials of myocardial fibers) can
be recorded extracellularly with surface electrodes
placed on the skin
• The record of these potential fluctuations during the cardiac cycle
is the electrocardiogram (ECG).
• The ECG provides information on: - Heart rate and rhythm
- The pattern of electrical activation of the atria and ventricles
- The approximate mass of tissue being activated
- Possible damage of the heart muscle
- Possible changes in the body’s electrolyte composition
Electrocardiography:
-ECG is a complex recording representing the overall
spread of activity throughout the heart during
depolarization and repolarization.
- The recording represents comparisons in voltage detected
by electrodes at two different points on body surface, not
the actual potential.
•
•
•
•
•
•
ECG graph paper
Paper moves at a speed of 25mm/second
At this speed
Each horizontal small cube represents 0.04 seconds
Each vertical small- 0.1 mv
Large cube- horizontal- 0.2 seconds
Large cube- vertically- 0.5 mv
•Important features of the ECG are the P wave, the QRS
complex and T wave.
•Relevant intervals and segments are the PR interval, the RR
interval, the QT segment and the ST segment.
P wave: Atrial depolarization as recorded from the surface of the
body
P – R interval: Time taken for the wave of depolarization to move
through
the atria, AV node, bundle of His, Purkinje fibres to the
ventricular myocardium.
QRS complex: Depolarization of the ventricles.
ST segment: Marks the end of the QRS complex and the beginning
of
the T wave. It occurs when the ventricular cells are in the
plateau phase of the action potential (i.e. there is no change
in potential occurring and so the ECG baseline is at zero
potential)
T wave: Repolarization of the ventricles (due to potential changes
occurring during phase 3 of the cardiac action potential)
Q – T interval: Period during which ventricular systole occurs
R – R interval: This time is usually used to calculate the heart rate.
Waves and normal values
• P wave- Atrial depolarization
•
•
0.1 seconds
0.25 milli volts
PR interval- AV nodal delay
0.12 seconds- 0.2 seconds
QRS complex- ventricular depolarization
0.1-0.11 seconds
ECG intervals
Intervals
Normal Duration(s)
Average
Range
Events on the heart
during intervals
PR interval1
0.182
0.12-0.20
Atrial depolarization
and conduction through
AV node
QRS duration
0.08
to 0.10
Ventricular
depolarization and
atrial repolarization
QT interval
0.40
to 0.43
Ventricular
depolarization plus
ventricular
repolarization
…
Ventricular
repolarization
ST interval (QT- 0.32
QRS)
1Measured
from the beginning of the P wave to the beginning of the QRS complex
2Shortens as heart rate increases from average of 0.18 at a rate of 70 beats/min to
0.14 at a rate of 130 beats/min
Fig. 2. Principles of the bipolar recording of an action potential
Recording the Electrocardiogram. Basic concept
When the wave of depolarization moves toward
the positive electrode, an upward deflection is
recorded, whereas depolarization moving in the
opposite direction produces a negative deflection
EKG RULES:
1) A wave of depolarization traveling toward a positive electrode
results in a positive deflection in the ECG trace.
2) A wave of depolarization traveling away from a positive
electrode results in a negative deflection.
3) A wave of repolarization traveling toward a positive electrode
results in a negative deflection.
4) A wave of repolarization traveling away from a positive
electrode results in a positive deflection.
EKG RULES: continued
5)A wave of depolarization or repolarization traveling
perpendicular to an electrode axis results in a biphasic deflection
of equal positive and negative voltages (i.e., no net deflection).
6) The instantaneous amplitude of the measured potentials depends
upon the orientation of the positive electrode relative to the Mean
QRS vector.
7)The voltage amplitude is directly related to the mass of tissue
undergoing depolarization or repolarization.
Electrical Vectors
http://www.cvphysi
ology.com/Arrhyth
mias/QRS%20vecto
rs%20animation.gif
Fig. 11. The basic direction of electrical conduction through the heart
How the polarity of the waveform depends on the position of
the recording electrodes relative to the heart.
The effect of changing electrode position on the wave
form recorded.
EKG Leads
Leads are electrodes which measure the
difference in electrical potential between either:
1. Two different points on the body (bipolar leads)
2. One point on the body and a virtual reference point with
zero electrical potential, located in the center of the heart
(unipolar leads)
EKG Leads
The standard EKG has 12 leads:
3 Standard Bipolar Limb Leads
3 Augmented Unipolar Limb
Leads
6 Precordial Leads
The axis of a particular lead represents the viewpoint from which
it looks at the heart.
https://www.youtube.com/watch?v=u53UAr19
xcM
ECG recordings from Bipolar Limb Leads
Einthoven’s Triangle
•Einthoven’s triangle
hypothetical triangle created around the heart when electrodes are placed on both arms
and the left leg .The sides of the triangle are numbered to correspond with the three leads
("leeds"), or pairs of electrodes.
Lead I at the top of the triangle, is
orientated horizontally across the chest. This angle is taken as zero.
Lead II is angled at 60 degrees to Lead I, and Lead III at roughly 120 degrees to
Lead I.
Augmented Unipolar Limb Leads (aVR, aVL and aVF)
Three unipolar limb leads are also used for recording ECGs. Each
lead measures the potential difference between an exploring
electrode and an “indifferent” electrode (V) assumed to be at
zero potential. This indifferent electrode is constructed by
connecting the electrodes on the right arm (R), left arm (L) and
left leg or foot (F) together. This indifferent electrode is called V
and is assumed to be at zero potential (since the sum of the
potentials in all the leads cancel out).
Augmented limb leads
• Represented by aVR, aVF, aVR.
a- augmented
V-unipolar
Last letter represents the part of body
• aVR- between right arm and left arm+ left leg
• aVL- between left arm and rt arm+ left leg
• aVF- between left foot and rt arm+ lt arm
Precordial Leads
These are unipolar leads measuring the potential difference between
an electrode placed on the chest and an indifferent electrode, again
made up by connecting the RA, LA and LL electrodes (i.e. the V
electrode). There are 6 locations to place the chest electrode and
so there are 6 chest electrodes (V1 – V6).
With the chest leads, if the chest electrode is in an area of positivity,
which occurs if the wave of depolarization is approaching this
electrode, then an upward deflection is recorded.
Adapted from: www.numed.co.uk/electrodepl.html
Precordial Leads
Pre cordial leads
•
•
•
•
•
•
V1- 4th intercoastal space, rt side sternal boarder
V2- 4th intercoastal space lt side of sternal boarder
V3- between V2 and V4
V4- 5th intercoastal space in the mid clavicular space
V5- 5th intercoastal space in the anterior axillary line
V6- 5th intercoastal space in the mid axillary line.
Summary of Leads
Bipolar
Limb Leads
Precordial Leads
I, II, III
-
(standard limb leads)
Unipolar
aVR, aVL, aVF
(augmented limb leads)
V1-V6
Situs Inversus with dextrocardia
Arrangement of Leads on the EKG
Anatomic Groups
(Septum)
Anatomic Groups
(Anterior Wall)
Anatomic Groups
(Lateral Wall)
Anatomic Groups
(Inferior Wall)
Anatomic Groups
(Summary)
INTERPRETATION OF THE ELECTROCARDIOGRAM
What to inspect in an ECG
1. Heart Rate
2. Rhythm
3. Duration, segments and intervals.(P wave duration, PR interval,
QRS duration, QT interval)
4. Mean QRS Axis (mean electrical axis, mean QRS vector)
5. P wave abnormalities
Inspect the P waves in leads II and V1 for left atrial or right atrial
enlargement.
Left atrial hypertrophy would result in a taller P wave in Lead II
RA hypertrophy – taller P wave in V1.
6. QRS wave abnormalities
7. ST segment / T wave abnormalities
Determining the Heart Rate
Rule of 300
Take the number of “big boxes” between neighboring QRS
complexes( R – R interval), and divide this by 300. The result will
be approximately equal to the rate
Although fast, this method only works for regular rhythms.
(300 / 6) = 50 bpm
(1500/30) = 50 bpm
What is the heart rate?
www.uptodate.com
(300 / ~ 4) = ~ 75 bpm
(1500/20 ) = 75 bpm
What is the heart rate?
(300 / 1.5) = 200 bpm
Heart Rate < 60 beats / min  Bradycardia
Heart Rate > 100 beats / min Tachycardia
The Rule of 300
It may be easiest to memorize the following table:
# of big
boxes
Rate
1
300
2
150
3
100
4
75
5
60
6
50
2. Rhythm
Is the rhythm determined by the SA node pacemaker?
i.e. is it a “sinus rhythm”?
If normal, the following should be present:
· The P wave should be upright in leads I, II and III.
· Each QRS complex should follow a P wave
Einthoven’s Triangle
•Einthoven’s triangle
hypothetical triangle created around the heart when electrodes are placed on both arms
and the left leg .The sides of the triangle are numbered to correspond with the three leads
("leeds"), or pairs of electrodes.
Lead I at the top of the triangle, is
orientated horizontally across the chest. This angle is taken as zero.
Lead II is angled at 60 degrees to Lead I, and Lead III at roughly 120 degrees to
Lead I.
All Limb Leads
The QRS Axis
The QRS axis represents the net overall direction
of the heart’s electrical activity.
Abnormalities of axis can hint at:
Ventricular enlargement
Conduction blocks (i.e. hemiblocks)
The QRS Axis
By near-consensus, the normal
QRS axis is defined as ranging
from -30° to +90°.
-30° to -90° is referred to as a
left axis deviation (LAD)
+90° to +180° is referred to as a
right axis deviation (RAD)
Determining the Axis
• The Quadrant Approach
• The Geometric method.
Determining the Axis
Predominantly
Positive
Predominantly
Negative
Equiphasic
The Quadrant Approach
1. Examine the QRS complex in leads I and aVF to determine if they
are predominantly positive or predominantly negative. The
combination should place the axis into one of the 4 quadrants
below.
The Quadrant Approach
2. In the event that LAD is present, examine lead II to determine if
this deviation is pathologic. If the QRS in II is predominantly
positive, the LAD is non-pathologic (in other words, the axis is
normal). If it is predominantly negative, it is pathologic.
Quadrant Approach: Example 1
The Alan E. Lindsay ECG
Learning Center
http://medstat.med.utah.
edu/kw/ecg/
Negative in I, positive in aVF  RAD
Quadrant Approach: Example 2
The Alan E. Lindsay ECG
Learning Center
http://medstat.med.utah.
edu/kw/ecg/
Positive in I, negative in aVF 
Predominantly positive in II 
Normal Axis (non-pathologic LAD)
QRS Axis DeterminationUsing the Hexaxial
Diagram
First find the isoelectric lead if
there is one; i.e., the lead with
equal forces in the positive and
negative direction. Often this is
the lead with the smallest QRS.
The QRS axis is perpendicular
to that lead's orientation.
Since there are two
perpendiculars to each
isoelectric lead, chose the
perpendicular that best fits the
direction of the other ECG
leads.
Applied physiology
• Myocardial infarctionQ wave
elevated ST segment
• IschemiaST depression
• Heart block• First Degree AV Block
• There is a slowing of conduction through the
AV node. The P-R interval is unusually
long (> 0.20 s). However each P wave is followed
by a QRS complex.
Second Degree Block
As the PR interval increases to > 0.25s, sometimes conduction
through the AV node fails and a P wave does not result in a QRS
complex. This is intermittent conduction failure with a subsequent
loss of ventricular contraction and is typical of a second degree
block. There are 3 types of Second Degree Block:
Mobitz type I
Mobitz type II
Bundle Branch Block
Complete Conduction Block:- Third Degree Block
In this condition no impulse goes through the AV node. The atria
and the ventricles are now severed – electrically speaking – and
each beats under control of its own pacemakers. This is also called
AV dissociation. The atria have an inherent rhythm of 60
– 80 bpm and the P-P interval will be regular and consisten. The
only ventricular pacemaker that are available to initiate ventricular
contractions are the Purkinje fibres - their inherent rhythm is 20 –
40 bpm.
Arrhythmias caused by changes in Electrolyte Composition
Both Hypokalemia and Hyperkalemia can cause serious cardiac
arrhythmias. This is not surprising considering how dependent
the membrane potential is on extracellular K+ levels. To treat
arrhythmias due to hyperkalemia calcium gluconate is infused.
Ca++ has the opposite effects to K+ on the action potential.
Hypokalemia:
Flattened T wave
ST depression
More prominent U wave
Hyperkalemia:
Peaked T wave
Loss of P wave
Widened QRS