Download Physio Lab 7 Using Burdick for Basic EKG and mean electric axis

Lab 7
Part One: Using Burdick for Basic EKGs
Part Two: Mean Electric Axis
If the vector arrows are going the same direction, the pen on the EKG machine goes up.
If the cardiac and lead vectors are in opposite directions (anti-parallel), the EKG pen points down.
If the vector lines intersect, the pen will go to baseline, because there is no electrical current.
After you get your EKG paper printed, look at leads one and three.
Find lead 1 (within the first group of peaks on your EKG readout).
Take your ruler and find your best baseline. Try to find a segment to the left and right of a good QRS
segment, place the ruler on the plateaus and draw a line.
Take your ruler or count the boxes. Every small box is 1mm. Measure from baseline to the top of the R
peak. If there is no R, it is a zero deflection. Then take baseline and measure down to your S deflection. If
you don’t have one, it is zero.
Say the first measurement was 7 and the second was -1; add them together, so the total is now 6. Now
look at the bottom of any of the four the compasses (they are all the same) on the last two pages of this
document. In the center is zero, to the right is positive values, to the left is negative. Find your number on
the circular grid. Take a ruler and use the perpendicular guidelines to draw a line to the top. Do the same
for lead 3. The plot on the circular grid will be to the left. The line for lead 3 should intersect with the line
for lead 1. Now draw a big dot on the intersection point. Then take your ruler and line it so you can draw a
line from the center circle of the compass, through your intersection dot, on out the right side of the
compass circle.
Normal is from 100 to -20.
From -20 to -30 is a left axial shift; more than that is a left axial deviation.
100-110 is right axial shift, more is right axial deviation.
The computerized EKG analyses this and gives you the analysis.
AN EXERCISE IN BASIC ELECTROCARDIOGRAPHY (EKG)
INTRODUCTION
Each time the normal heart beats, a characteristic electrical phenomenon initiates at the S-A node
(pacemaker) and sweeps across the heart to cause the chambers to contract. This contraction (systole) is designed
to force blood from the chambers. The recording of this electrical phenomenon is called electrocardiography (EKG
or ECG). The EKG is a trace representing both the depolarization and repolarization parts of the electrical
phenomenon. It is these electrical events that cause the cardiac muscles to undergo systole and pump the blood.
Because the human body contains about 40 liters of conductive body fluid, the electrical phenomenon, which
occurs on the heart, is conducted throughout the body to the surface of the skin. Here, surface electrodes
(transducers) may pick-up the signals, convey them to an amplifier and produce a trace on the physiograph. The
recorded trace is the EKG trace, or physiogram. This trace can be very diagnostic and useful in determining
abnormalities in the heart's conduction system.
W. Einthoven, a Dutch physiologist, invented the first EKG instrument around the turn of the 19th century.
He consequently developed the Einthoven Triangle to define the different Limb Leads used in EKG work. This
exercise will attempt to define the basic principles of EKG with special reference to the normal Lead II pattern.
Considerable lecture material will be presented to support the principles outlined in this Exercise.
MATERIALS NEEDED
A Burdick 10 EKG instrument with bipolar and unipolar cable electrodes (instructor will demonstrate the automatic
recording of the Burdick 10 EKG).
PROCEDURES
1. Burdick 10 EKG: The EKG instrument should already be set up with electrode cables in place. Attach the
disposable electrodes to the subject (L/R wrist and L/R ankle). Then attach the leads (note the electrode plastic clip
designation: RA, LA, LL AND RL) to the subject. Touch the “Auto” switch and the EKG trace for the bipolar (Leads I, II,
and III) and the unipolar (aVL, aVR, and aVF) should trace automatically. Press the “STOP” button following the last
unipolar trace to conserve paper since you will not be recording the precordial lead traces (V1 to V6.)
Analysis:
1. Using Einthoven's Triangle (as presented in lecture and the picture below), define each of the six Limb Leads (see
diagram) here. Which of the Leads are Bipolar and which of the Leads are Unipolar?
aVL
aVR
2. There are 6 unipolar leads called the PRECORDIAL LEADS that are usually included in an EKG study, termed V1
through V6. The placement of the movable electrode is shown on the diagram below. To take the Precordial Leads,
one must set the ELECTROCARDIOGRAPH LEAD SELECTOR SWITCH to the "V" setting and move the surface
electrode along the six different chest positions. Taking the Precordial Leads is normally not done in the Human
Physiology laboratory but it is important to note their character.
Looking at the different V1 through V6 traces (see diagram below), can you explain why the V1 through V3 show
negative QRS patterns and the V4 through V6 show more positive QRS patterns?
3. Now examine the Lead II EKG trace you obtained. What was your paper speed? What does each box along the
“X” axis represent in time at your paper speed? What does each box along the “Y” axis represent in mV? To help
you, see the picture on the previous page.
Heart rate (BPM)
60
70
80
90
100
120
P-R interval (sec)
0.18-0.20
QRS interval (sec)
0.07-0.1
Q-T interval (sec)
0.33-0.43
0.31-0.41
0.29-0.38
0.28-0.36
0.27-0.35
0.25-0.32
S-T segment (sec)
0.14-0.16
0.13-0.15
0.12-0.14
0.11-0.13
0.10-0.11
0.06-0.07
P wave = 0.1 sec and 0.2 mV
T wave = 0.16-0.27 sec and 0.2-0.3 mV
P-R segment = 0.03-0.06 sec (NO mV)
4. Tape your tracing here:
5. On your tracing above, find one of the Lead II EKG traces taken at PAPER SPEED of _________ cm/sec, which is
the same as __________mm/ sec, indicate and calculate the second and mV readings for the following (no
amplitudes for segments or intervals):
How to solve for duration of an episode: Paper speed reflects the velocity of the paper. Change in distance
reflects the length the event lasted on your EKG tracing (mark on your paper where a wave or interval or segment
begins and ends and then you measure the distance from beginning to end in mm) and solve for time using the
velocity equation V= D / T. To measure amplitude, simply measure the distance from baseline to the peak of
the wave and convert to amplitude (every 1mm equals 0.1mV).
Duration in time (seconds)
P wave
QRS complex
T wave
P-R interval
Q-T interval
S-T segment
Amplitude (mV)
T-P segment
6. Now define the following:
P Wave
P-R segment
P-R Interval
QRS Complex
S-T segment
Q-T Interval
T-P segment (how does it change with increased heart rate ?)
T-wave
5. An Accurate Determination of Heart Rate: Determine your Heart rate!
Mark any two identical waves on an EKG strip, over any number of beats. (ex. R to R.) Measure
the horizontal distance between these points in millimeters. Determine how many beats occur in this
interval (remember, from one R wave to the very next R wave represents one beat.) Calculate as follows:
Number of beats measured
Number of mm over interval
X
paper speed in mm X 60 sec = number of beats
sec
min
min
Your calculation of your BPM here:
Review Questions:
 Why is knowing your paper speed important? (your discussion should include how the values of
time for each of the boxes and thus your duration of events would change).
 What is a lead?
 Which are bipolar or unipolar? What are the precordial leads?
 What is vector analysis? What is a vector? How do we use vectors to represent direction and
magnitude of electrical current?
 Do we have to read from lead 2? Can we read from other leads? Would the tracing be slightly
different if we do so? Why?
 Know how to determine heart rate if given a sample tracing and paper speed.
 What is tachycardia? How would the R-R or T-P segment change?


What is Bradycardia? How would the R-R or T-P segment change?
What is 1st, 2nd, 3rd degree AV block?

What is hypokalemia?

What is hyperkalemia?

What is hyponatriemia?

What is Hypernatriemia?

What is Hypocalcemia?

What is Hypercalcemia?

How can these electrolyte problems affect the electrical conductance through the heart?
 What is Einthoven’s triangle? Why are the leads placed in a triangle—what is this supposed to
represent?
Each of the important phases of the typical EKG cardiac cycle is described below but you should also add
to this any information your instructor gives you about these events during class:
P Wave
The P wave represents the depolarization of the atrial musculature which spreads from the S-A
node to the A-V node. Enlargement of the right atrium (right atrial hypertrophy) will increase the voltage
and therefore produce an abnormally tall P wave.
P-R Segment
The P-R segment represents the delay in transmission of the impulse through the A-V node (A-V
delay). Part of this pattern is made up of the atrial repolarization.
P-R Interval
The P-R interval represents the time required to depolarize the atrial musculature plus the A-V
delay. A P-R interval greater than 0.2 sec may indicate a First Degree Block.
Q Wave
The Q wave is usually negative with Lead II and represents the left to right ventricular
septal depolarization. This left to right impulse movement is caused by the higher location of the Left
Bundle Branch (LBB) along the ventricular septum.
QRS Complex
The QRS complex represents the depolarization of the ventricles. It also includes some atrial
repolarization. A duration of more than 0.12 sec indicates Bundle Branch Block (BBB).
S-T Segment
The S-T segment represents the depolarized state of the ventricles or the time between
ventricular depolarization and repolarization. A condition of insufficient blood flow (ischemia) may occur
because of a blocked coronary artery. If an EKG is taken 2 minutes and 6 minutes after exercise, one
evidence of myocardial ischemia is an "ischemic depression" of the S-T segment -see below:
Q-T Interval
The Q-T interval represents the entire time required for depolarization and repolarization of the
ventricles.
T-Wave
The T wave represents the repolarization of the ventricles. This repolarization wave is moving
against the vector (Lead II) so it is usually positive. It may "tent" during hyperkalemia. A U-wave may
result from hypokalemia.
AN EXERCISE IN THE DETERMINATION OF THE MEAN ELECTRIC AXIS
INTRODUCTION
The depolarization wave (QRS) spreads through the ventricles in different directions from
instant to instant. If you could draw a single vector line to represent the "common vector" of all of the
different QRS directions, this would be the "mean QRS axis" sometimes called the "mean electric axis".
The measurement of this "mean electric axis" can be most useful as a diagnostic tool. Certain
myopathologies exist that may alter the "mean electric axis" from its norm.
In calculating the electric axis of the heart, the Einthoven Triangle is converted into a type of
protractor with angles (0 to 180 degrees). The mean QRS axis is read as a vector (arrow) through one of
these angles. This will be covered during lecture but remember, the normal mean electric axis angle is
about +59 degrees.
In order to use the Einthoven Triangle as a protractor, the Triangle must be converted to a
triaxial (3 axes) system. This is easily done by shifting the three bipolar leads (I, II, & III) so that they all
intersect at a common point (see diagram below).
If the three unipolar leads are added (aVR, aVL & aVF) to the triaxial diagram, then a hexaxial (6
axes) figure is produced. If Lead I is 0 degree, then the hexaxial figure is turned into a protractor (see
diagram below):
A right axis shift from +100 degrees to +110 degrees is strongly suggestive of cardiac disease.
Any shift greater than +110 degrees is considered a RAD (right axis deviation). Occasionally, in tall thin
adults, a normal right shift will be seen because there is more space between the heart and the
diaphragm, therefore, the heart may hang in a more vertical position. Some causes of RAD would
include: right ventricular hypertrophy, RBBB, mitral stenosis (why?), pulmonary valve stenosis, tetralogy
of Fallot and right ventricular hypertrophy failure. Be sure you know the basis for each of these RAD
causes (lecture).
A left axis shift from -20 to -30 degrees is suggestive of cardiac disease, with a shift greater
than -30 degrees to the left considered an LAD (left axis deviation). Cases of obesity as well as full term
pregnant women can show a natural shift to the left because of upward positioning of the diaphragm.
Some causes of LAD would include: left ventricular hypertrophy, LBBB, and aortic stenosis. Be sure you
can explain the causes of LAD
It should be noted that when students run EKG's in the Human Physiology laboratory that they
are doing so to learn the basic techniques on electrocardiography. Do not attempt any clinical diagnosis
based on your strips as there are many factors to be considered.
MATERIALS NEEDED
You will need the same set up as described in the previous Exercise for the burdick EKG. Have a person
in your lab group, who didn't set up the previous Exercise an opportunity to set up the equipment for
this lab. This will give more students a chance to work with the equipment.
PROCEDURES
1. Obtain a calibrated Lead I, Lead II and Lead III trace from a reclining subject. By visual inspection,
see if Lead I and Lead III appear to equal Lead II (Einthoven's Law). If they are visibly different,
perhaps the electrodes were placed incorrectly on the subject.
2. While the subject is still connected, take aVR, aVL and aVF unipolar lead traces. According to
Kirchoff's Law: the algebraic sum of all the potential differences in a closed circuit must equal
zero. Therefore, if you take the algebraic sum of the unipolar leads (as you did with the bipolar
leads) you would have:
aVR + aVL + aVF = 0
Also, the LA is positive in Lead I and negative in Lead III and therefore cancels out when the two leads
are added:
Lead I = LA-RA
+Lead III = LL-LA
-----------------Lead II = LL-RA
Therefore, in electrocardiography, the sum of 1 plus III equals II.
All of the above information may be combined to determine the mean QRS electric axis of the heart.
3. To measure the different bipolar Leads, on must measure the UPWARD (+) deflection of the "R"
wave in millimeters for each of the three Limb Leads (R1, R2 and R3). Then measure in
millimeters the DOWNWARD (-) deflection of the "S" wave (S1, S2 and S3) for all three Leads.
Take the algebraic sum between the R wave and the S wave for each lead. Below is an example
using lead 3.
2. Measure, in mm,
the upward
deflection of the R
wave (+10 mm)
3. measure (if any), in mm the
downward deflection of the S
wave (-2 mm)
4.) Now take the algebraic sum for lead three: 10 + (-2)= 8mm (overall positive deflection.
Now repeat this for lead one and lead two. See the example below:
1. Draw your
“best fit” line
across the
baseline
4. To verify Einthoven's Law, the algebraic sum of Lead II would have to equal a +15 (+7 plus +8
=+15). To obtain the mean electric axis of the heart, usually only Lead I and Lead III are used
because it is assumed that their sum will equal Lead II. In calculating the mean electric axis of
the heart, you are answering: in what general direction or toward which Lead is the QRS
predominantly oriented? As a general rule, the mean QRS axis will be directed midway between
two Leads that show tall R waves of equal height.
The direction of the normal mean electric axis is going to vary with the age of the subject. For
example, a child under the age of six months will usually have a shift to the RIGHT, as much as +130
degrees. For the child between the ages of one to five years, the axis shifts to the LEFT with an average
of perhaps +52 degrees. At the time of puberty, the axis will again shift to the RIGHT to about +67
degrees and then returns to the LEFT to the norm of about +59 degrees.
5. While still connected, have the subject take as deep a breath as possible and hold the breath
while you take a Lead I and Lead III trace. Let the subject breath normally for a minute and then
have the subject inhale normally but exhale as deeply as possible and hold the breath. Take a
quick Lead I and Lead III, and then let the subject relax for a minute or so. Save these breathing
traces as you will be using them later in the Exercise.
6. Disconnect the subject. Clean and return all materials to their proper locations.
7. Now it is time to actually plot the mean electric axis using your traces. You will use the grid chart
(as shown below) to determine the angle of the mean QRS electric axis vector. The grid chart is
derived from the hexaxial fiqure derived early in this Exercise. Compare the placement of the 0,
-90, -+180, and +90 degree points on the circular protractor around the grid with the same
angles as described for the hexaxial figure.
8. As an example of the use of the grid chart, suppose that a Lead I has an UPWARD deflection of
+7mm (R1 = 7mm) and a DOWNWARD deflection of -1mm (S1 = -1mm). Also, the Lead III for the
same subject, has an UPWARD of +14 (R3 = 14mm) and a DOWNWARD of -2mm (S3 = 2mm).
This means that the algebraic sum for Lead I would be +6 (+7-1) and the algebraic sum for Lead III
would be +12 (+14-2). Examine the grid chart below. Look along the Lead I axis to the +6 point. Notice
that there is a 90 degree vertical line drawn on the grid. Now look at the Lead III axis and observe that
there is a 90 degree line drawn from the +12 point. The straight lines from Lead I and Lead III converge
at a point in the grid. Observe the straight line drawn from the little circle in the center of the grid
through the point of Lead I and Lead III convergence. This line is continued to the described angles
around the grid. Where the line crosses the angles, this is the mean electric QRS axis. In this case, the
angle is +71 degrees.
9. Use the EKG traces obtained while the subject was breathing normally to plot the QRS mean
electric axis. Plot all of the mean axes using the grid charts in the Laboratory Manual. There are
extra grids in case you need them.
10. Plot the electric axis of the subject when the person was holding the breath after a deep
inspiration. During inspiration, the thoracic cavity must increase in volume so the diaphragm
moves downward towards the abdominal cavity. Considering that the movement of the
diaphragm affects the position of the heart (review your anatomy!), the heart should shift to the
RIGHT to a more vertical position when the diaphragm descends. This new plot should show a
slight right shift when compared with the same subject's normal breathing mean axis plot.
11. Now plot the mean axis when the subject had a deep expiration. Consider the position of the
diaphragm and determine in which direction the heart should shift. See if the plot will verify your
prediction.
Visually determining the Electric Axis by Examination of Lead I and aVF EKG Traces
12. Your instructor will give you a demonstration on how to take any two leads and visually estimate
which electric axis quadrant the mean electrical axis would be located. Another way to visually
determine MEA is to find a QRS complex on the EKG strip that has a 0 net voltage (that is, the
upward R is equal to the downward S in size), and identify a lead that is perpendicular to the 0
net voltage lead. The MEA will lie along this perpendicular lead.
Review Questions:
 What is the
meant by the
mean electrical
axis of the
heart?
 How is Einthoven’s Triangle turned into a protractor? Where are the positive vs. negative
degrees?
 Know the bipolar and unipolar leads and their directionality. If you weren’t given the hexaxial
(protractor) figure to plot your deviations on, could you recreate your own and VISUALLY
determine MEA? Practice drawing them here:
 What is LAD? LAS? RAS? RAD? What can cause these things?
 Why does LBBB cause a LAD? Why does a RBBB cause a RAD?

Why can someone who runs a lot exhibit LAS (you should be able to explain this with respect to
time needed for depolarization in the myocardium).
 What is Einthoven’s Law?
 How did you calculate your lead deviations?
 Why did holding your breath on an inhale or an exhale shift your MEA? In what direction did
these actions shift your MEA?