Cardio Lecture
ECG Part One
Jan 28th, 10:00 PM
Dr. Downey
Matt Messa for R. Kagan
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ECG Part One
Introduction
 This lecture was meant to be the foundation for understanding the physics behind
ECG’s and the waveforms on the ECG
 The waveforms of the ECG result from a sequence of electrical excitation within
the cardiac muscle
 Action potentials that occur at different times and in different parts of the
myocardium in sequence, cause there to be differences in voltage on the surface
of the body that we can amplify and display as the ECG’s
 As the cardiac mass depolarizes, there will be time varying differences in
electrical potential on the body surface
 The reason we are able to measure voltage on the chest wall when the heart is
inside the body is because the extracellular space conducts the voltage just as a
battery suspended in salt water would do
 There is a voltage loss though when measuring voltage across the chest wall
because the leads are not actually touching the heart itself. This is the same reason
that there would be a voltage loss when measuring a 1.5 volt battery in salt water
solution, when the electrodes are not actually touching the terminals themselves
 As the battery in this salt water solution changes orientation in relation to the
electrodes, the voltmeter will still read a positive reading but it will be different
(less)than when the electrodes were close to the terminals
 If the battery was placed exactly perpindicular to the electrode’s field of
measurment (turned 90 degrees), the voltmeter would read zero. In comparing this
situation to the ECG, the same would hold true if a particular two electrodes (lead
I for instance) measured a direction of depolarization perpendicular to lead I.
Thus, a very flat wave would appear in this lead in this situation.
 When we use ECG’s to evaluate the heart, we are actually measuring potential
differences at various locations on the chest and body wall, with respect to the
heart.
Depolarization of a muscle strip of myocardium
 At baseline, these cells are negative on the inside with respect to the outside
(normal polarization)
 As this strip depolarizes from left to right, or from A to B, the series of action
potentials are conducted in a wave-like propagation.
 In the strip shown in the figure, about two thirds of the membrane has been
depolarized. The depolarized area is now more negative on the outside relative to
the inside, and since the correlating ECG deflection shows an upward spike, we
know that the membrane must be depolarizing toward the positive electrode of the
lead being evaluated.
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ECG Part One
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 The reason that the ECG tracing is shown to be coming back downward is
because once the membrane passes the halfway point between the two measuring
electrodes, we have surpassed the greatest degree of charge separation.
 So once depolarization of the entire membrane is complete, the tracing of the
ECG should be back at baseline.
 KNOW that the ions on the outside of the cell membrane are the most important
conductors of potential, while the charge of the inside of the membrane has little
to do with potential measurement.
ECG Hints
 Even though millions of myocardial cells depolarize in various different
directions, the ECG uses a vectorial summation to show the primary direction of
depolarization of the heart as measured by a particular lead.
 The left ventricle in general overwhelms the portion of the depolarization of the
right ventricle because of its bulk, so the vectorial summation of the depolarizing
event is normally more toward the left side of the heart.
 Pathologic processes can shift this vector though; Imagine a person with an
enlarged right ventricle due to pulmonary hypertension, well this person’s vector
sum might show the depolarization to occur more toward the right than usual.
 Thus, the ECG is best used to determine the spatial direction of the depolarization
event in the heart.
 The ECG is also very useful for determining the time of atrial related to
ventricular depolarizations, and for determining the time it takes depolarization to
go from the atria through the AV node to the ventricles.
ECG Rules
 A depolarization coming toward a positive electrode produces a positive spike on
the ECG. What does this really mean? Well think of it in these terms. Lead I
measures the potential between basically the right arm and the left arm, and the
heart just happens to sit right between both arms.
 So, to place lead one on a person, you place a negative electrode on the right arm
and a positive electrode on the left arm. And since the heart sits at an angle in the
chest, you obviously expect the wave of depolarization to run down the atria and
ventricles from about the 10:00 oclock position toward the 4:00 oclock position,
or better thought of as Northwest to Southeast (downward and leftward)
 So some portion of the vector is obviously going to be traveling toward the
positive lead (left arm), and none would be traveling toward the negative
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ECG Part One
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electrode (right arm). In this situation (which is almost always the case in
humans), lead I should show on the paper ECG to be deflected more up than
down. Not to get ahead here, but if the sum of the depolarization vector would
have been traveling straight down toward the feet instead of what is expected in a
normal heart, the deflection in lead I should be very flat because the potential read
here would be zero (because this is just like the battery placed at 90 degrees from
the field of measurement)
 A depolarization going toward the negative lead would instead produce a negative
deflection on the ECG. So for instance, the only way you will get a primarily
negative deflection in lead I is if the summation of the depolarization vector is
somehow moving toward the negative lead on the right arm. (This can only be in
a pathologic state usually)
Repolarization
 A wave of repolarization approaching a positive lead produces a negative
deflection on the ECG, whereas a wave of repolarization approaching a negative
lead produces a positive deflection.
 This is the opposite of depolarization
Electrode Placement
 Standard lead placement systems have been established long before any of us
were on this earth, and it was done to create a consistent way to look at and
measure potential of the heart. What does this mean? It means that if you get an
ECG done at the Cleveland Clinic, and then one done at the “O” across the street,
it should look relatively similar.
 Standard ECG limb leads are best utilized to begin the understanding of ECG’s,
prior to adding all the other leads to confuse you!!!
 Standard limb leads utilize 3 electrode placements involving the right arm, left
arm, and left leg. In theory, this creates an equilateral triangle of sorts that
involves 360 degrees of possible measurement of potential
 You may have heard the term “3 lead ECG”. This refers to machines that only use
3 leads to measure the potential, like on many EMS ambulances.
 Now you can’t just say that lead I is the electrode on the right arm, because a lead
always involves potential difference between two separate electrodes.
 Lead I for instance measures the potential along a line parallel to the top (flat) part
of the equilateral triangle, with the right arm being the negative electrode and the
left arm being the positive electrode.
 Lead III measures potential difference from the line on the equilateral triangle
starting basically at the left shoulder and traveling toward the right leg. Now I
know what you are thinking; How can lead III measure potential along a line from
the left shoulder to the right leg when we don’t even place an electrode on the
right leg. Well, it is all an approximation of the potential along the imaginary line
from the left arm to the left leg, so in the anatomical position, the angle is pretty
close to what the angle is in the triangle.
 Lead II measures potential along the last side of the equilateral triangle, which
basically runs from the right arm to the left leg.
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 So what is special about the electrode on the left arm? When measuring lead I, the
electrode is the positive lead, but when measuring lead III, the left arm electrode
is the negative electrode. Work this out in your head!!
Specific ECG Waveforms
 Specific waveforms are used to determine standard measurements of the
depolarization and repolarization of the atria and ventricles
 The “P” wave on the ECG is the depolarization of the atria collectively
 After the P wave there is a return to baseline temporarily where there is no
deflection for a brief period. The period from the beginning of the P wave to the
beginning of the next deflection (beginning of the QRS) is termed the P-R
interval. This interval is an extremely important interval used in measuring
pathologic processes that cause a delay through the AV node.
 The textbook ventricular depolarization wave is a much sharper set of deflections
termed the “QRS” complex. The first downward wave is called a Q wave, and the
first upward deflection is termed the R wave. The last downward deflection is
called the S wave.
 You will notice that the ventricular depolarization occurs significantly faster than
atrial depolarization, which is something we learned from the last exam material.
(Recall purkinje fibers conduct at 1-4 meters/second while the atria are less than 1
m/s)
 Once the S wave of the QRS comes back to baseline, the ventricle is done
depolarizing.
 The segment from the end of the S wave to the beginning of the T wave (next
wave to be discussed) is termed the ST segment.
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The “T” wave is basically the repolarization of the ventricles! So what does this
tell you if the normal T wave is upright in most leads as is customary? It tells you
that your ventricle repolarizes backwards toward where the depolarization
initiated, because recall that a positive deflection for repolarization must mean
that it occurred toward a negative lead. So, the first part of the ventricle to
repolarize is actually the last part that depolarized. Its like driving your car down
a culdesac; you have to come back the way you came.
As you will notice, the repolarization wave (T wave) is not a mirror image of the
depolarization complex (QRS), thus this tells you that repolarization occurs at a
different velocity.
All intervals contain at least one waveform and one segment; a segment is just a
part of the ECG not contained in any wave, such as the portion between the end of
the P wave and the beginning of the QRS.
The last interval not discussed already is the QT interval, which contains the
QRS, the ST segment, and the T wave.
Intervals are fancy standards to help doctors determine if something is wrong or
not. For instance, in certain patients with abnormal calcium levels, the QT interval
may be altered out of normal range.
And just in case you didn’t hear Jeremy Johnson’s question in lecture, he asked
where the atrial repolarization is on the ECG. Well, it is buried inside the much
more prominent QRS complex and is not visible, because the atria are
repolarizing at the same time that the ventricles are depolarizing.
Tidbits of ECG Knowledge
 The paper speed of a normal ECG is 25mm/ second
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 Why do you need to know this? Because each interval and cardiac cycle has a
specific normal range of time outside of which it is considered pathologic, and
thus you must have a way to measure time.