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Biology
Sensors:
Loggers:
ECG
An EASYSENSE logger capable of fast recording
Logging time: 10 seconds
46 ECG Graph demonstration
Read
An ECG (electrocardiogram) measures the electrical output of cardiac muscle during a heart beat
cycle. Heart muscle has a very small potential difference (voltage) across their cell membranes. The
cells of the heart can depolarize spontaneously, as the cells depolarize they contract. The group of
cells in the heart that depolarize the fastest form an area called the pacemaker or sinoatrial node.
The pacemaker is located in the right atrium.
The atria and the ventricles are isolated from each other electrically. A depolarization of atrial cells
does not directly affect the cells in the ventricles. A group of cells in the right atria, called the
atrioventricular or AV node, conduct the electrical signals from an atrial depolarization down a bundle
of conducting fibers, called the Bundle of His, to the ventricles. This process creates a short time
delay between the atria contracting and the ventricles contracting. Because the cells of cardiac muscle
are closely interconnected, the wave of depolarization, contraction and repolarization spreads rapidly
across all of the heart.
The changes in the polarity of the heart cells can be measured, amplified, and plotted over time. The
ECG is the graphical representation of the measured electrical currents, and indirectly of the muscle
contraction.
The Electrocardiogram
A typical ECG shows a pulse caused by the depolarization of the atria, this is called the P wave and it
lasts about 0.04 seconds. This wave is and is associated with the contraction of the atria.
There is a short delay after the P wave while the AV node depolarizes and sends a signal along the
Bundle of His to the Purkinje fibers in the ventricles.
After the AV node depolarizes there is a sharp downward pulse called the Q wave, followed rapidly by
an upward pulse of the line called the R wave. The R wave is followed by a strong downward pulse of
the line called the S wave. These three waves together are called the QRS complex and they form
the central portion of the ECG. This complex is caused by the depolarization of the ventricles and is
associated with the contraction of the ventricles.
The recovery of the cardiac cells back to their resting polarity takes place with the movement of ions
across the cell membranes. The movement of these ions generates a final wave called the T wave and
indicates repolarization of the ventricles.
The sequence from P wave to T wave represents one heart beat/cycle. The number of beats in a
minute is called the heart rate and is typically 70 to 80 beats per minute at rest. The exact value will
depend on the age, gender and fitness of the individual.
L3 Biology
46 - 1(V2)
R
ECG / EKG
uV
QRS
complex
T wave
P wave
Q
S
Typical recording of 4 heart beats using ECG.
P-R interval ....120 - 200 milliseconds (0.120 to 0.200 seconds)
QRS interval ....under 100 milliseconds (under 0.100 seconds)
Q-T interval .....under 380 milliseconds (under 0.380 seconds)
This sensor is NOT intended or suitable for medical diagnosis. Any results or patterns that
differ from a published norm should not be seen as an indication of a medical condition.
What you need to do
Green clip
Red clip
Electrode
patch
Black clip
1.
EKG
Sensor
Use three electrode patches per subject. The electrodes should be kept in an air-tight, clean,
dry container for storage.
L3 Biology
46 - 2(V2)
2. Because the electrical signal produced by the heart is so small, it is very important that the
electrodes make good contact with the skin. Rub the areas of skin where the patches will be
attached with a paper towel soaked in alcohol or light abrasive material to remove dead skin
and oils.
3. Peel three electrode patches from the backing paper. Place one electrode on the right wrist.
The second electrode on the right elbow / upper arm, and the third electrode on the left elbow
/ upper arm. (This is one of several possible arrangements for EKG electrodes on the body).
4. Connect the crocodile clips on the sensor to the tabs on the edges of the electrode patches.
•
•
•
The black reference lead must go to the right wrist electrode patch.
The green negative lead goes to the right elbow electrode patch.
The red positive lead goes to the left elbow electrode patch.
This arrangement described is appropriate for the classroom; it may not produce traces of the
quality seen in the media.
Collecting the resting ECG
1.
2.
3.
4.
5.
6.
From the Home screen select Open Setup (or File, Open Setup). Open the file Data Harvest
Investigations (Edition 2) \ Setup file \ Biology L3 V2 \ 46 ECG graph.
Allow the individual who is having their ECG taken to relax and stop moving.
Click on the Start icon to begin recording data. The person whose ECG is being measured
should remain calm and relaxed during the course of the recording; ideally they should not be
able to see the trace being recorded. Encourage the person to breath normally.
Data recording stops automatically at 5 seconds, there will be brief pause as the data is
retrieved and displayed.
If the ECG looks like the sample trace shown above, label and save the data. If the base line of
the trace is not stable it is possible the recording was taken before the ECG sensor had
stabilised. Wait for a few seconds and then click on Start to take a second reading.
If the trace shows a flat line, it is likely that one or more of the electrodes / crocodile
connector is not making good contact. Smooth the electrodes against the skin and reconnect
the crocodile clips. Take another reading.
Problems with recording?
• Check the connectors are on the correct electrodes.
• Check the electrode patches are not being pulled away from the skin by the weight of the crocodile clips.
• Make sure the subject is not moving, especially the limbs that the electrodes are connected to. All muscle
movement generates electrical energy, which will swamp the delicate signal from the heart.
• Check you are using the correct recording times; the heart beat is of a very small duration if the
recording speed is set to low or too high the electrical event of the heart beat can be missed.
Analysing the data
1.
Use Zoom to draw a rectangle around a region of the data that includes four or five complete
heart cycles. The graph will rescale to show the region you’ve selected.
2. Use Values find the peak of a P wave. Record the time indicated in the lower panel.
3. Move the line to the point that corresponds to the peak of the Q wave. Record the time.
4. Move the line to the point that corresponds to the peak of the R wave. Record the time.
5. Calculate the P-R interval and record the time.
6. Move the values line to the point that corresponds to the peak of the S wave. Record the time.
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46 - 3(V2)
7. Calculate the Q-R-S interval and record the time.
8. Move the values to the point that corresponds to the peak of the T wave. Record the time.
9. Calculate the overall Q-T interval and record the time.
Results table: Interval analysis
Item
Time (s)
P wave begins
Q wave begins
R wave begins
S wave begins
T wave begins
Item
Time (s)
Typical Time
P-R interval
0.120 to 0.200 s
QRS interval
under 0.100 s
Q-T interval
under 0.380 s
Extension work
•
Use the same individual and get them to do some light exercise for 3 - 5 minutes and then
record the ECG, compare the traces. Leave the patches on the individual but remove the leads.
Walking around a room for several minutes will be enough to show a difference.
•
Drink a caffeine containing drink and record the ECG at 5 minute intervals to see if the
caffeine has any effect over heart rate.
•
Take a resting ECG and then show the subject a short clip of an action scene from a film / TV
programme to see how the heart rate has responded.
Questions
1.
?
Compare your values for the P-R, Q-R-S, and Q-T intervals to the ones given earlier. What
could explain the differences?
2. How does the heart rate measured by the ECG compare to your heart rate measured with the
Heart Rate sensor or by the direct measurement of the pulse at the wrist or neck? What could
explain the difference?
L3 Biology
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