ELECTROCARDIOGRAPHY
(Adapted by Peter Love and Suzanne Amador Kane from Catherine Crouch’s Swarthmore lab on ECG’s 4/2010)
PURPOSE
• To observe the electric fields generated by the human heart by measuring an electrocardiogram (ECG)
• To gain further familiarity with measuring potential difference (voltage)
INTRODUCTION
In this lab exercise, you are measuring the electric dipole moment of the heart as a function of time. You
will take an electrocardiogram (ECG) by measuring the potential difference between selected points on
the surface of the body. A leading cardiologist says that the ECG is the “among the most valuable clinical
tests available to medicine because it is quick, completely safe, painless, inexpensive, and rich in
information.” Today you’ll learn how to take these measurements.
As you saw in the electric field mapping lab, if we have two
opposite electric charges, q and –q, separated by a distance
d, this is called an electric dipole. In the field mapping lab
you should have sketched some of the equipotential lines
shown in the figure on the left.
How are dipoles produced in the heart? The muscles in the heart
are stimulated to beat by waves of electrical charging and
discharging across cell membranes. Figure 2 shows a membrane
separating positive and negative electrical charges.
Since an ECG measures the voltage generated by the heart’s
electrical activity, we need to find how to relate electric
dipole moments to their associated electrical potential
difference. First let’s define the electric dipole moment
vector, p, as: p = q d, where d is a vector that points from
the negative to the positive charge and has a magnitude equal
to the distance d between the two charges
QuickTime™ and a
decompressor
are needed to see this picture.
Figure 2: Schematic diagram showing positive and negative charges separated by a cell membrane. For a cell at
rest (like a muscle cell in the heart), the membrane potential is usually negative inside the cell. When a cell is
excited, the potential inside the cell become more positive, and the potential outside the cell more negative. The
dipole moment of the heart arises because different cells are excited in different places at different times.
What is the electrical potential of a dipole? This is just equal to the sum of the potentials from each
charge. In the case that the potential is measured at a point which is far from the dipole we obtain:
Eq. 3
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Physics 102
ECG Lab
Using this formula we can compute the dipole moment of the heart if we measure several voltage
differences at a known distance and orientation. The important facts to remember are that the ECG
potential, V, depends upon:



The magnitude, p, of the heart’s dipole moment
The distance, r, from the measurement point (the electrode at which you measure voltage) to the
heart’s dipole moment
The angle, , between the heart’s dipole moment and the orientation of the electrode.
QuickTime™ and a
decompressor
are needed to see this picture.
Figure 7 (right) Schematic diagram of the chambers of the heart (shown by convention looking toward
the front of the body.) (bottom left) Schematic of an electrocardiogram. (top left) one cycle of an ECG
taken from Professor Love, Lead I, showing the PQRST features.
Your heart is a pump. Blood enters the atria, the smaller chambers of the heart, which then contract,
forcing the blood into the larger, lower chambers of the heart, the ventricles. Then these contract, forcing
the blood around your body. The ECG you will record has a characteristic shape, labeled by five letters
in cardiology to denote different phases of the heartbeat.
1. The peak labeled P corresponds to the initial electrical pulse triggering the heart’s contraction.
The interval from P to Q corresponds to the spreading of the depolarization across the smaller
chambers of the heart (the atria—singular atrium), and is typically very small because the atria
are only a small fraction of the heart
2. The QRS complex – the great big feature in any ECG trace – shows where the ventricles contract.
These are the largest, lower chambers of the heart, explaining why this feature is bigger than the
P feature for the atria
3. The T wave – this shows where the ventricles re-polarize in preparation for the next heartbeat.
This is where the muscles are relaxing after contraction.
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Physics 102
ECG Lab
EXPERIMENTAL SETUP
The ECG sensor has three electrodes, connected to wires colored red, black, and green. The sensor
measures potential difference between the red and green wires, with the red wire serving as the positive
wire and the green wire negative. The black wire serves as the reference for the zero of potential, so when
you zero the ECG sensors, it uses the reading of the black wire as zero. In our setup, for all three
measurements, the black lead is attached to the right ankle.
Figure 3: Two electrocardiography measurement configurations. The distances r are the distances from
the heart to the electrode, so there is a distance rI for Lead I and rII for Lead II. The figure on the right
shows the angular relationship between the two “Lead” voltage measurement configurations.
The ECG measures the potential difference between the red and green leads as a function of time. A data
point can be measured as often as every 5 milliseconds. In medicine, each potential difference is referred
to as a “Lead” (short for a pair of wires). The two potential differences we will measure are called “Lead
I” and “Lead II” and are shown in Figure 3.


“Lead I” places the red (positive) lead on the left wrist and the green (negative) lead on the right
wrist. The black lead is attached to the right ankle. Lead I measures the potential difference
between two points on a horizontal line, and is generated only by the x-component of the heart
dipole moment. (Fig. 3 and 4)
“Lead II” places the red (positive) lead on the left ankle and the green (negative) lead on the
right wrist. The black lead is attached to the right ankle. The line between the Lead II points
approximately 60 degrees to the horizontal. (Fig. 3 and 4) Thus, Lead II will depend upon both x
and y-components of the heart’s dipole.
Vector analysis allows you to combine Lead I and Lead II to determine the vertical (y-component) of the
dipole moment of the heart.
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Physics 102
ECG Lab
The Experiments: Getting started
1. You will use two ECG devices to measure Lead I and Lead II AT THE SAME TIME. You will
record your data in Logger Pro. First connect your two ECG sensors to Ch1 and Ch2 inputs on
your LabPro computer interface, and make sure you know which is Lead I and which is Lead
II.
2. Under Experiment  Data Collection, set the sampling rate to 500 samples/second, so that a data
point is measured every 0.002 s (2 milliseconds). Take data for 10 seconds so you record a
reasonable number of heartbeats.
3. The leads have so-called “alligator clips” on their end which can be clipped onto the black tabs on
sticky gel electrode patches that attach to the skin of the “patient”. Before attaching the electrode
patches, use alcohol wipes and clean the skin where they are to be attached, since otherwise skin
oils will degrade the electrical contact.
4. One member of the lab group (the “patient”) should attach electrode patches in the four positions
(right and left ankles and right and left wrists) and then connect the ECG sensor in Lead I and
Lead II configurations.
5. Zero both sensors in Logger Pro, have the patient stand with her/his arms straight out at
her/his sides and then measure an ECG.
6.
Experiment 1. Double lead ECG measurements and a movie of your hearts dipole
To display your dipole moment data using the Vpython animation:
1. Once you have taken a 10s ECG, export your data from LoggerPro in .csv format.
2. Estimate and record the distances rI and rII , the average distance from the heart to the lead
electrodes in the I and II configurations, respectively.
3. The Vpython program provided (ECG.py) takes as input a .csv (comma separated values) file
with three columns: time, voltage corresponding to Lead-I, and voltage corresponding to Lead-II.
4. When you have your data file ready, open the Vpython program. Put the values for rI and rII and
the correct file name for your data file into the section labeled “student labeled parameters”.
5. Run the vpython program
Experiment 2: Lead reversal
What happens to the signal if you reverse which electrode is attached to green and which to red on one of
the leads? Explain briefly why this happens. (Clinically this is referred to as “lead reversal.”) Why might
this be confusing?
Record a second dual lead ECG with one of the leads reversed
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Physics 102
ECG Lab
ECG Lab Report form Physics 102
Your name: ___________________________Your lab partner: ________________________
Experiment I: Double Lead ECG’s and making a movie of the hearts dipole moment.
rI = _________(m)
rII = _________(m)
ATTACH YOUR PLOT OF YOUR TYPICAL ECG for the two leads
Email your .csv file and edited visual python program to plove@haverford.edu, the title of the email
should be (your name) and (your partners name) ECG data.
Experiment 2: Lead reversal
ATTACH YOUR PLOT OF YOUR ECG for the two leads with one lead reversed.
Explain:
a) What causes the difference between the lead reversed ECG and the normal ECG?
b) Why this might be confusing in a clinical setting?