EKG
Mike Clark, M.D.
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Topics
What is an EKG
What is the Lead System
The EKG paper
EKG mounting
Standard EKG waves and segments
Interpretation of EKG
Cardiac Stress Test
Thallium scan versus Technetium scan
Myocardial Infarction (Heart Attack)
EKG Cardiac Monitoring
An EKG only measures the Charge Activity
of the Heart
The EKG monitors and measures the charge
activity of the heart. Muscle has the
properties of charge movement and
contraction- the EKG only evaluates the charge
movement activities of heart muscle and
conduction system cells. . The actual
contraction is monitored using other
diagnostic devices.
• Sensors are placed on the skin to monitor the charge
activities of the heart. The sensors are electrodes that
measure the charge movements (action potentials).
• The electrodes form the Lead system used in
electrocardiography.
• Since the Leads are placed on the chest wall and not
directly in the heart muscles or conduction system
cells- evaluation of the heart activity is at a distance.
• Whenever an activity is observed from a distance complete evaluation requires observations from many
different viewing locations. This is the case of the
EKG- numerous leads (sensors) must be placed on the
skin to observe the multitude of charges coursing
through the millions of muscle cells and conduction
cells all around and through the heart. The standard
evaluative EKG has 12 Leads.
Lead System
• The standard EKG for diagnostic interpretation has 12
leads termed I, II, III, AVR, AVL, AVF, V1, V2, V3, V4, V5 and
V6. Monitoring EKGs generally only use 3 leads.
• Leads I, II, and III are considered bipolar leads in that they
detect a change in electric potential between two points.
• Leads AVR, AVL, AVF, V1, V2, V3, V4, V5 and V6 are
considered unipolar leads in that they measure the
electric potential at one point with respect to a null point
(one which doesn’t register any significant variation in
electric potential).
• The right leg electrode always acts as a ground electrode.
The acts in protection and to eliminate some background
electrical interference.
• Lead I is between the right arm and left arm electrodes,
the left arm being positive.
• Lead II is between the right arm and left leg electrodes,
the left leg being positive.
• Lead III is between the left arm and left leg electrodes,
the left leg again being positive.
Remember the right leg
electrode always acts as a
ground electrode. It
acts as a backup to keep
the patient from getting an
electrical shock if the machine
malfunctions.
EKG Augmented Limb Leads
The same three leads
that form the standard
leads also form the
three unipolar leads
known as the augmented leads.
These three leads are referred
to as aVR (right arm), aVL (left
arm) and aVF (left leg) and also
record a change in electric
potential in the frontal plane.
Augmented Leads
• The Augmented Leads are formed by sending
the non-selected arm or leg through the Central
Terminal of Wilson ( 5,000 Ohm resistor). The
Central Terminal of Wilson acts as the null point.
This null point is obtained for each lead by
adding the potential from the other two leads.
For example, in lead aVR, the electric potential
of the right arm is compared to a null point
which is obtained by adding together the
potential of lead aVL and lead aVF.
Precordial Leads
• These six unipolar leads, each in a different
position on the chest, record the electric
potential changes in the heart in a cross
sectional plane. Each lead records the
electrical variations that occur directly under
the electrode.
x
To position chest leads – place V1 at in the 4th intercostal
space at the right sternal border- place V2 at the 4th
intercostals space at the left sternal border- place V4 next at
the 5th intercostals space at the midclavicular line- now place
V3 halfway between V2 and V4- place V5 in at the anterior
axillary line in the 5th intercostals space- place V6 at the mid
axillary line in the 5th intercostals space.
V3
V1
V2
V4
Intercostal spaces
The electrode position provided is the standard placement.
However, there are numerous variations of this placement
depending on certain conditions.
Figure 7.22a
The EKG Paper
• Consider EKG paper as a graph with an X and Y axis. The X
axis measures time in milliseconds. The Y axis measures
voltage in millivolts. What is being measured are action
potentials in the heart muscle cells and conduction system
cells. Since, the electrodes are placed on the skin and not in
a muscle cell or conduction system cell- a multitude of
action potentials from thousands of cells is being monitored
at the same time.
•
• EKG paper has big boxes and small boxes. Each small box is 1
mm wide and tall. Each large box contains 25 small boxes.
•
• Usually the EKG runs at a paper speed of 25 mm per secondif set at this speed each small box has a .04 second time
duration. When the EKG is set at the standard setting for
voltage- each small box is .1 mv.
EKG paper has big boxes and small boxes. Each small box
is 1 mm wide and tall. Each large box contains 25 small
boxes. Usually the EKG runs at a paper speed of 25 mm
per second- if set at this speed each small box has a .04
second (large box 0.2 sec) time duration. When the EKG is
set at the standard setting for voltage- each small box is .1
mv.
EKG Mounting
• When a 12-lead EKG is taken – it is properly
mounted in the usual manner below.
I
AVR
V1
V4
II
AVL
V2
V5
III
AVF
V3
V6
Standard EKG Waves & Segments
• The mock tracing above delineates the standard EKG
waves. These waves are recorded in a lead when
action potentials move in various angulations to that
particular lead. If action potentials are moving
towards the positive pole of a lead the pin will defect
upward- if moving away the pen will deflect
downward. If no action potential actions are
perceived in a said lead at a said time- the pen will
stay on the baseline. If action potentials move a
perfect perpendicular angle to a lead the pin will
deflect as much up as it does down.
• The standard EKG waves are the P, QRS and T waves. The
P wave represents atrial depolarization, the QRS complex
reflects ventricular depolarization and the T wave
represents ventricular repolarization. No atrial
depolarization is observed in that it occurs at the same
time as ventricular depolarization- since this action is
much larger charge event the atrial repolarization event is
obscured by the ventricular action.
Figure 18.17
• The P wave is caused by atrial depolarization. The P
wave is usually smooth and positive. The P wave
duration is normally less than 0.12 sec.
• The PR interval is the distance from the start of the P
wave till the start of the R wave. The PR interval
represents the flow of action potentials from the SA
node to the AV node. This time period should be .12
sec till .20 sec.
• The PR segment is the portion on the EKG wave from
the end of the P wave to the beginning of the QRS
complex. The PR segment corresponds to the time
between the end of atrial depolarization to the onset
of ventricular depolarization. It is an isoelectric
segment, during which the impulse travels from the
AV node through the conducting tissue (bundle
branches, and Purkinje fibers) towards the ventricles
• The QRS complex represents the time it takes for
depolarization of the ventricles. - due to
ventricular depolarization. The normal QRS
interval range is from 0.04 sec - 0.12 sec
measured from the first deflection to the end of
the QRS complex.
• The ST segment represents the start of
repolarization of the ventricles. The ST
segment begins with the completion of the S
wave and ends with the beginning of the T
wave. This should be flat to the base line. If
upward or downward deflections are
observed- this could suggest certain
pathology.
• The T wave due to ventricular repolarization
The wave is normally rounded and positive.
Interpretation of EKG
Look at
1. Rate (normal resting heart rate 60 – 100 BPM
2. Rhythm – beats should occur with relative
regularity
3. Axis – During one entire cardiac cycle – which
direction do most of the action potentials
travel
4. Waveform – are the waves normally timed
and shaped
EKG Heart Rate Determination
• Knowing the paper speed, it's easy to work out
heart rate. It's also very convenient to have a
quick way of eyeballing the rate, and one method
is as follows:
• Remember the sequence: 300, 150, 100, 75, 60,
50
• Identify an R wave that falls on the marker of a
`big block'
• Count the number of big blocks to the next R
wave.
• If the number of big blocks is 1, the rate is 300, if
it's two, then the rate is 150, and so on. Rates in
between these numbers are easy to `interpolate'.
R to R Count how many large boxes?
300, 150, 100, 75, 60, 50
This patient’s heart rate is a little greater than 60 BPM. The
R waves are separated by a little less than 5 large boxes –
thus not 300, not 150, not 100, not 75 but a littler greater
than 60 BPM.
Heart Rhythm on EKG
• On the mounted EKG – look at the very bottom tracing
strip. Take a ruler and measure between one R wave to
another one. See if the interval is the same.
• There will be slight variations in the beat to beat timing
and that is normal – particularly when inhaling and
exhaling.
• There is normally a slight degree of chaotic variation in
heart rate, called sinus arrhythmia. Sinus arrhythmia is
generally a good thing, and loss of this chaotic
variation is of ominous prognostic significance.
R to R Distances – are they the same?
This patient’s heart rate shows slight appropriate variation in
rhythm.
Measuring distances of R waves to see if the beats are rhythmic
Cardiac Axis as viewed on EKG
• The axis of the heart is what direction are the
greatest action potentials traveling during one
entire cardiac cycle.
• Considering the left ventricle – should be the
thickest cardiac chamber in the normal person –
the main action potential direction should be
down and towards the left of the patient.
• A shift from this direction could correspond to
heart pathology – i.e. hypertrophy of improper
chambers.
• It could also correspond to vertebral
abnormalities like scoliosis.
Figure 18.4e
Figure 18.1
How does one determine cardial axis?
• Remembering the fact that if action potentials
travel towards the positive end of a lead the
EKG pen is deflected upwards.
• Considering the fact – one wants to see the
action potentials traveling down and to the
left
• A novelist EKG reader looks at leads I and AVF
– paying attention to the R waves in these
leads
• To an expert EKG interpreter – one is looking
for the “Greatest Net Vector Force”
Quadrant I
Quadrant III
The R wave is down in AVF in this
drawing so it is abnormal.
Quadrant II
Quadrant IV
If the R wave is up in Lead I and up in Lead AVF – the axis is
in Quadrant IV that is the normal axis direction
R wave deflections
Quadrant I
Down in Lead I and down
In AVF
Quadrant III
Up in AVF but down in
Lead I
Quadrant II
Up in Lead I but down in
AVF
Quadrant IV
Up in both Lead I and AVF
Evaluation of EKG Waveforms
• The P wave is caused by atrial depolarization. The P
wave is usually smooth and positive. The P wave
duration is normally less than 0.12 sec.
• A tall P wave (3 blocks or more) signifies right atrial
enlargement, a widened bifid one, left atrial
enlargement
Right Atrial
Enlargement
Left Atrial Enlargement
The PR interval
• The PR interval is the distance from the start
of the P wave till the start of the R wave. The
PR interval represents the flow of action
potentials from the SA node to the AV node.
This time period should be .12 sec till .20 sec.
• It is widened in Heart Blocks – First degree,
second degree or third degree
First degree block
• This is manifest by a prolonged PR interval
Second degree block
• Conduction intermittently fails completely. This may
be in a constant ratio (more ominous, Type II second
degree block), or progressive (The Wenckebach
phenomenon, characterized by progressively
increasing PR interval culminating in a dropped beat -- this is otherwise known as Mobitz Type I second
degree heart block
Third degree block
There is complete dissociation of atria and
ventricles.
The QRS complex
• The QRS complex represents the time it takes for
depolarization of the ventricles. - due to
ventricular depolarization. The normal QRS
interval range is from 0.04 sec - 0.12 sec
measured from the first deflection to the end of
the QRS complex.
• An initial downwards deflection is a Q (or q), any
negative deflection after this is an S. An upward
deflection is an R. Note that we refer to a second
deflection in the same direction by adding a
prime, so we have R', R'', S' and so on
Q wave versus Q point
• A Q wave is greater than 1/3 the height of the
next R a Q point is less than that height
• A Q wave means the person has had a heart
attack in the past
• Many people who have had a prior MI will have
an ECG that appears normal. There may however
be typical features of previous MI, and the most
conspicuous of these is Q waves. A simplistic
explanation of these prominent Q waves is that
an appropriately placed lead "sees through" the
dead tissue, and visualizes the normal
depolarization of the viable myocardial wall
directly opposite the infarcted area
Prominent Q wave
QRS Complex
• Bundle Branch Blocks
• Hemiblocks
• Hypertrophy of ventricles
Figure 18.14
Figure 18.17
The ST segment
• The ST segment represents the start of repolarization
of the ventricles. The ST segment begins with the
completion of the S wave and ends with the
beginning of the T wave. This should be flat to the
base line. If upward or downward deflections are
observed- this could suggest certain pathology.
The ST segment
• A tall elevated ST segment – signifies MI (heart attack
happening at the time)
• Depressed can mean ischemia, hypokalemia,
hypocalcemia and other non-specific findings
• Stress testing – if the ST segment drops significantly – 1
small box or greater – that is a positive sign for ischemia
Cardiac Stress Test
Evaluation for cardiac
ischemia
May inject a radiotracer
– like Technetium – 99 or
thallium -201 – then
termed nuclear stress
test
Thallium versus Technetium
• During the thallium scan, a small amount of thallium tracer is
injected into the vein. It travels through the bloodstream into
the coronary arteries and is picked up by the heart muscle cells.
Areas of the heart muscle that receive an adequate amount of
blood supply up the tracer immediately. Areas that do not
receive the adequate amount of blood pick up the tracer very
slowly, or not at all
• After the technetium is injected into a blood vessel in the arm, it
accumulates in heart tissue that has been damaged, leaving
"hot spots" that can be detected by the scintillation camera.
The technetium heart scan provides better image quality than
commonly used radioactive agents such as thallium, because it
has a shorter half-life and can thus be given in larger doses.
Generally used to diagnose an MI.
•
Diagnostic Value of Stress Test
• Treadmill test: sensitivity of 67%, specificity of 70% (of
advanced lumen narrowing) Nuclear test: sensitivity of
81%, specificity of 85-95% (of advanced lumen narrowing)
Heart Attack (Acute inferior MI)
Note the elevated ST segments signifying a heart
attack happening at the time – Q waves show up
later
The key finding of an acute transmural (all the way through the
heart muscle wall) MI is the steeply elevated ST segment. The
leads in which the elevated ST segments are found determine the
location of the MI.
Though a Q-wave is shown – it is not a main finding in an acute MI –
generally indicative of an old MI.
Leads to Look at for MI – now or in the past!
• Remember an MI happening now has elevated ST
segments – one in past shows Q-waves
• A Posterior wall MI – will show in leads V1 and V2 – it
involves occlusion of the Right Coronary Artery
• A Lateral wall MI – shows in lead I and AVL- it involves
occlusion of the Circumflex artery
• An Inferior wall MI is seen leads II, III, and AVF- it
involves occlusion of the Right or Left Coronary arteries
• An Anterior wall MI – will show in leads V1, V2, V3 and
V4 – it involves occlusion of the Anterior
Interventricular artery – clinically known as the
Anterior Descending
Anterior Wall MI – anterior interventricular affected
Left Lateral Wall MI – circumflex
affected
Posterior wall MI affected by The posterior interventricular
artery –which generally comes off of the Right Coronary Artery.
Figure 18.7
Monitoring EKGs
The 24 – hour Holter monitor is a device used to evaluate the
charge activity of the heart over a longer period of time
and while the patient performs his/her normal daily
activities. This is a 3 lead
EKG.
EKEKG
In the Medical Intensive Care Unit (MICU) or the Surgical
Intensive Care Unit (SICU) generally only three leads are
viewed. This is done to continuously monitor the status
-of the patient. Other parameters are also monitored like
blood pressure, blood gases using pulse oximetry and CVP.
The screen can be
seen in the patient’s
room and at the
monitoring station.