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Analyzing the Heart with EKG
12
An electrocardiogram (ECG or EKG) is a graphical recording of the electrical events occurring
within the heart. In a healthy heart there is a natural pacemaker in the right atrium (the sinoatrial
node) which initiates an electrical sequence. This impulse then passes down natural conduction
pathways between the atria to the atrioventricular node and from there to both ventricles. The
natural conduction pathways facilitate orderly spread of the impulse and coordinated contraction
of first the atria and then the ventricles. The electrical journey creates unique deflections in the
EKG that tell a story about heart function and health (Figure 1). Even more information is
obtained by looking at the story from different angles, which is accomplished by placing
electrodes in various positions on the chest and extremities. A positive deflection in an EKG
tracing represents electrical activity moving toward the active lead (the green lead in this
experiment).
Although the ECG has great diagnostic value in terms of heart muscle arrthymias, it does not
directly show when the atria contract (atrial systole) or the ventricles contract (ventricular
systole). The ECG also does not show when the AV values close/open or the semilunar valves
close/open. The timing of heart chamber systole and diastole and the shutting or opening of
values are inferred rather than directly visualized in an ECG tracing. The “thunk” of the closing
of heart valves can be heart as a “lub dub” sounds (known as S1 and S2) by auscultation with a
stethoscope.
Five components of a single beat are
traditionally recognized and labeled P, Q, R,
S, and T. The P wave represents the start of
the electrical journey as the impulse spreads
from the sinoatrial node downward from the
atria through the atrioventricular node and
down the interventricular bundle to Purkinje’s
fibers in the inferior walls of the ventricles.
Actual atrial contraction (systole) is
completed between the P and the Q peaks.
Ventricular activation is represented by the
QRS complex. The Q-R interval represents
depolarization of the ventricles after the
impulse has arrived via the S/A node,
interventricular bundle, and Purkinje’s fibers.
At the R peak, the ventricles begin to contract,
with full contraction (systole) sometime
between the R peak and a bit after the S wave.
The T wave results from ventricular
repolarization, which is a recovery of the
Figure 1
ventricular muscle tissue to its resting state.
By looking at several beats you can also calculate the rate for each component.
Doctors and other trained personnel can look at an EKG tracing and see evidence for disorders of
the heart such as abnormal slowing, speeding, irregular rhythms, injury to muscle tissue
(angina), and death of muscle tissue (myocardial infarction). The length of an interval indicates
whether an impulse is following its normal pathway. A long interval reveals that an impulse has
Human Physiology with Vernier
12 - 1
been slowed or has taken a longer route. A short interval reflects an impulse which followed a
shorter route. If a complex is absent, the electrical impulse did not rise normally, or was blocked
at that part of the heart. Lack of normal depolarization of the atria leads to an absent P wave. An
absent QRS complex after a normal P wave indicates the electrical impulse was blocked before it
reached the ventricles. Abnormally shaped complexes result from abnormal spread of the
impulse through the muscle tissue, such as in myocardial infarction where the impulse cannot
follow its normal pathway because of tissue death or injury. Electrical patterns may also be
changed by metabolic abnormalities and by various medicines.
In this experiment, you will use the EKG sensor to make a five second graphical recording of
your heart’s electrical activity, and then switch the red and green leads to simulate the change in
electrical activity that can occur with a myocardial infarction (heart attack). You will identify the
different components of the waveforms and use them to determine your heart rate. You will also
determine the direction of electrical activity for the QRS complex.
OBJECTIVES
In this experiment, you will

Obtain graphical representation of the electrical activity of the heart over a period of time.
Learn to recognize the different wave forms seen in an EKG, and associate these wave
forms with activity of the heart.
 Determine the heart rate by determining the rate of individual wave forms in the EKG.
 Compare wave forms generated by alternate EKG lead placements.

MATERIALS
computer
Vernier computer interface (labpro)
Logger Pro software
Vernier EKG Sensor
electrode tabs
PROCEDURE
Part I: Standard limb lead EKG
1. Start computer and plug in Vernier labpro into outlet. Connect the EKG Sensor into CH1 (on
the front left side) on the labpro using the white square plug. Use the USB cable to connect
the labpro to the computer (The small, square connector plugs into the USB port on the right
side of the labpro. The wide connector end plugs into the USB port on the left side of the
laptop.)
2. Launch the Logger Pro software using the icon on the quick launch toolbar on the bottom of
the laptop screen. From the ‘File’ menu, click on ‘Open’ and select “12 Analyzing Heart
EKG” from the Human Physiology with Vernier folder.
Bio 242 Anatomy and Physiology II ECG Lab Using Computer-Assisted Sensors pg. 2
Analyzing the Heart with EKG
3. Attach three electrode tabs to your arms, as shown in Figure 2. Place a single patch on the
inside of the right wrist, on the inside of the right upper forearm (distal to the elbow), and on
the inside of the left upper forearm (distal to elbow).
4. Connect the EKG clips to the electrode tabs as shown
in Figure 2. Sit in a relaxed position in a chair, with
your forearms resting on your legs or on the arms of
the chair. The graph is automatically setup to collect
data for 3 seconds. To extend the collection time press
‘Ctrl+T.’ this will double the set run time for the
experiment. When you are properly positioned, have
your lab partner click
to begin data collection.
5. Once data collection is finished, click and drag to
highlight each interval listed in Table 1.
7. Use Figure 3 as your guide when determining these
intervals. Enter the x value of each highlighted area to
the nearest 0.01 s in Table 1. This value can be found
in the lower left corner of the graph.
8. Calculate the heart rate in beats/min using the EKG
data. Record the heart rate to the nearest whole number in Table 1.
Figure 2: How to hook up
the leads on your arms.
9. To save the data:
a. Go to the data table to the left of the graph. Hold down the left mouse button and select
all of the data in both columns. Click ‘Ctrl C’
b.
Open the Excel program and click ‘Ctrl V’.
c.
Click on the ‘Chart Wizard’ in the Excel toolbar to draw graph. Alternatively, cut
and paste the graph from Logger pro into Excel
Save as an excel document to memory stick or email the data to your email account.
Part II: Alternate limb lead EKG
10. Exchange the red and green EKG clips so that the green clip is now attached to the electrode
tab on the left arm and the red clip is on the right arm. Sit in a relaxed position in a chair,
with your forearms resting on your legs or on the arms of the chair. When you are properly
positioned, have your lab partner click
to begin data collection.
11. Save or sketch the tracing for alternate limb lead placement only (see instruction # 9 above).
12. When finished:
a) Shut down the computer and unplug the Vernier LabPro from power source.
b) Disconnect the EKG from the Vernier LabPro. Disconnect the USB cable from the laptop
computer and place it and the labpro device and power supply in the labpro container.
Place all other equipment into the ANP storage bins. Please use the Velcro power chord
tie wraps on all of the devices to roll up the chords before placing them in the storage
bins.
Human Physiology with Vernier
12 - 3

P-R interval:


QRS complex:
Q-T interval:
Figure 3
time from the beginning of P wave to the start of the QRS complex
OR from the peak of the P wave to the peak of the Q wave
time from Q deflection to S deflection
time from Q deflection to the end of the T OR from the bottom of the
Q wave to the peak of the T wave.
DATA
Table 1
Interval
Time (s)
P–R
QRS
Q–T
R–R
Heart Rate (bpm)
Table 2
Standard Resting Electrocardiogram Interval Times
P–R interval
0.12 to 0.20 s
Bio 242 Anatomy and Physiology II ECG Lab Using Computer-Assisted Sensors pg. 4
Analyzing the Heart with EKG
QRS interval
less than 0.12 s
Q–T interval
0.30 to 0.40 s
QUESTIONS
Describe what happens (or what is happening) in the cardiac cycle in the following situations.
1.
immediately before the P wave:
______________________________________________
2.
during the P wave:
_______________________________________________________
3.
immediately after the P wave (P-R segment):
___________________________________
4.
during the QRS wave:
_____________________________________________________
5.
immediately after the QRS wave (S-T interval) :
________________________________
6.
during the Q-T interval:
____________________________________________________
7.
during the T wave:
________________________________________________________
Define the following terms.
7.
Tachycardia-
8.
Bradycardia-
9.
Fibrillation-
10.
Which would be more serious, atrial or ventricular fibrillation?
_____________________ Why?
Human Physiology with Vernier
12 - 5
__________________________________________________________________
_______________________________________________________________________
11.
Abonormalities of the heart valves can be detected more accurately by auscultation than by
electrocardiography. Why is this so?
______________________________________________________________________
12. “Long QT Syndrome” is a genetic heart condition associated with fainting due to
tachycardia (rapid heartbeat), arrhythmias, and in certain instances atria or ventricular fibrillation
and sudden death. Some individuals with Long QT Syndrome have a mutation in voltage
sensitive K+ channels of the cardiac muscle cell membrane such that they open prematurely
during the action potential that stimulates those cells. The increase in the Q-T interval results
from the Q wave occurring earlier than normal. Knowing what occurs during the Q-T interval,
propose an explanation for mutations in K+ channels leading to these symptoms.
Bio 242 Anatomy and Physiology II ECG Lab Using Computer-Assisted Sensors pg. 6