Download Electrical Activity of the Heart / The ECG

Electrical Activity of the Heart / The ECG
Overview
Today you will be introduced to the common methods of monitoring and recording the electrical activity of the
heart during the cardiac cycle. Utilizing Bio Pac, you will record an electrocardiogram (ECG or EKG) and
analyze the results by measuring and calculating the amplitudes of the P wave, QRS complex, and T wave, as
well as the time intervals of these deflections. The ECG intervals, segments, and heart rate will also be
measured and calculated.
Introductory Notes
Intrinsic Conduction System
• The heart contains two specialized types of cardiac muscle cells:
- Contractile myocardial cells are responsible for the mechanical pumping of the heart.
- Autorhythmic cells of the conduction system are responsible for electrical stimulation of the
myocardial cells. These cells spontaneously depolarize generating an action potential (AP). The AP
travels throughout the conduction system, triggering contraction of the surrounding myocardial tissue.
• Different parts of the heart’s conduction system have different “autorhythmicity” rates:
- SA node= 70 beats/minute
- AV node= 50 beats/minute
- AV Bundle= 35 beats/minute
- Bundle branches= 35 beats/minute
- Purkinje Fibers= 30 beats/minute
Electrocardiogram (ECG) is a recording of the electrical changes that accompany the cardiac cycle. The
baseline (isoelectric line) is broken by wave deflections (P, QRS, and T). In addition to the waves, there are
interval and segment components to the ECG seen below.
• Isoelectric line: indicates periods when ECG electrodes did not detect any electrical activity. Serves as a
point of departure of the electrical activity (depolarization and repolarization) during the cardiac cycle.
• Deflections of the ECG:
- P wave: depolarization of the atria as a wave spreads from the SA node toward the ventricles
- QRS complex: depolarization of ventricular muscle. Atrial repolarization also occurs during this
phase although it is masked by the large QRS deflection
- T wave: repolarization of the ventricles
• Time Intervals and Segments of the ECG:
- P-R interval: interval between the activation of the SA node and the AV node
- P-R segment: interval between end of atrial depolarization and start of ventricular depolarization
- S-T segment: interval between the end of the S wave and the beginning of the T wave. Ventricles are
uniformly excited.
- Q-T interval: time from start of ventricular depolarization to the end of ventricular repolarization
- T-P segment: time from the end of ventricular repolarization to the onset of atrial depolarization
- R-R interval: interval of time between successive cardiac cycles
Electrode Placement
• A lead is an arrangement of electrodes used in recording the electrical changes in the heart during a
cardiac cycle. This allows us to view the changing pattern of the heart’s electrical activity from different
perspectives. Certain abnormalities are best seen with particular leads.
• Common Lead Arrangements for a 12 lead ECG
- Bipolar Limb Leads: records changes in electrical potential between two electrodes placed on two
limbs (arm/ leg).
> Lead I: Right arm (-) and Left arm (+)
> Lead II: Right arm (-) and Left leg (+)
> Lead III: Left arm (-) and Left leg (+)
- Unipolar Limb Leads: records changes in electrical potential in reference to a single exploratory (+)
electrode placed on the limbs.
> aVF: Left leg(+) and Left arm/Right arm(-)
> aVR: Right arm(+) and Left arm/Left leg(-)
> aVL: Left arm(+) and Right arm/Left leg(-)
- Unipolar Chest Leads: records changes in electrical potential in reference to a single exploratory (+)
electrode placed on the chest.
> V1 - V6 are placed on various intercostal spaces
Calculation for Heart Rate:
Given paper speed: 2.5cm
1 sec
Measured R-R: 1.5cm
2.5cm = 1.5cm
1 sec
x sec
2.5x = 1.5
2.5x = 1.5
2.5
2.5
1 beat x
0.6 sec
x= 0.6 sec
60 sec
1 min
=
60beats
0.6 min
=
100beats/ min
Experimental Procedures
1. Each lab group will need a 3-lead cable, 3 electrode pads, and an alcohol swab.
2. Determine the role each member of your lab group will play.
3. Plug the 3-lead cable into Channel 1 on the front of the Bio Pac acquisition box. Turn the box on.
4. Prepare your subject as follows:
a. Clean the anterior surface of the right wrist and the medial aspect of both ankles, with the alcohol swab.
b. After allowing the areas to dry, apply electrode pads. Make sure that the pads tightly adhere to the skin.
c. Use the information in your introductory notes to determine the electrode lead set-up to run a Lead II.
Connect the electrode wires as follows:
Black
Ground
Red
Positive (+)
White
Negative (-)
5. Start the software by double-clicking on the “ECG” file, located on the desktop. This will launch the
Biopac Student Lab PRO software.
6. Record a resting ECG for approximately 15 seconds with your subject lying in a supine position. Click
START (bottom right of screen) to begin recording, and STOP to end it.
7. Observe the ECG’s. Note the P, QRS and T waves.
8. Choose a representative cycle and magnify it using the Zoom tool. The Zoom tool looks like a small
magnifying glass, and is located in the lower right of the screen. (NOTE: if you zoom too far, hold down the
option or alt key, click and you will reverse your zoom). After using the Zoom tool, be sure to replace it
with the normal cursor, also located in the lower right of the screen.
9. To change your horizontal or vertical scales, click on the outer axes to modify the scale values.
10. In order to measure and collect data, select the I-beam tool in the lower right of the screen. Measure
duration values by clicking and dragging to highlight your selected wave, segment, or interval. Record the
time values (Delta T) in the first column (“Resting”) of the table on your data sheet. Continue until each
duration value is measured and recorded.
11. To measure the amplitude changes (mV) for the P wave, QRS wave, and T wave highlight the vertical
deflection of the selected wave and record your mV values (Delta or p-p) in the data table on your data
sheet.
a. p-p = “peak to peak” finds the minimum amplitude value and subtracts it from the maximum amplitude
value in the selected area.
b. delta = calculates the difference in amplitude between the last point and the first point in the selected area.
This is particularly useful for taking ECG measurements because the baseline does not have to be at zero
to obtain accurate measurements.
12. Through discussions with your lab group, decide on an activity that you think will alter the ECG, and
describe the activity in the space below. Predict the specific changes you would expect to see in the ECG.
Note that you cannot record an ECG while your subject is moving – it will create too much interference
(electrical “noise”) from skeletal muscle activity. You can, however, collect the data immediately after your
subject stops moving.
13. Now, test your hypothesis by having your subject perform the described activity. Record an ECG, measure
as above, and put your data in the column labeled “Activity” in the data table.
14. After you compare the Resting and Activity ECG, run a Lead I and a Lead III ECG. Use the information in
your introductory lab notes to determine the electrode limb set up for each of theses leads. You will just
need to run each lead long enough to observe differences in the ECG deflections for the different lead setups.
15. Once you have collected all of your data and completed your observations, quit the Bio Pac program.
Carefully unplug your 3-lead cable from the Bio Pac acquisition box. Remove used electrodes from the skin
and throw them away.
NAME_____________________________
Data Collection and Follow Up Questions
ECG Component
Normative Values Resting
Activity - describe:
*based on 75 BPM
Duration measurements (secs)
CH 1 Delta T
P Wave duration
.07 - .18 sec
QRS Complex duration
.06 - .12 sec
T Wave duration
.10 - .25 sec
P-R Interval
.12 - .20 sec
P-R Segment
.02 - .10 sec
Q-T Interval
.32 - .36 sec
S-T Segment
< .20 sec
T-P Segment
0 - .4 sec
R-R Interval (one cardiac cycle)
.8
Heart Rate (BPM)
60 – 80 BPM
Amplitude measurements (mV)
CH 1 (p-p or Delta)
P Wave amplitude
< .2 mV
QRS amplitude
.10 – 1.5 mV
T Wave amplitude
< .5 mV
1. What does an ECG measure?
2. Compare the Activity ECG to the Resting ECG and answer the following question;
a. Are there any differences? Explain each difference noted and why do you think it occurs. Think about
how these changes affect heart function.
4. Compare the ECG deflections for your three bipolar lead set-ups, Lead I, Lead II, and Lead III. Describe the
differences you observe in these three recordings. What accounts for the differences you see in their
deflections?
5. Clearly explain all the electrical and mechanical events that occur during each measured wave, segment and
interval and relate it to the cardiac cycle. An electrical event would be the ECG wave; mechanical events
include blood flow, open or closed valves, heart sounds, contraction (systole), relaxation (diastole), pressure
changes, etc. An example of a complete answer for the P wave is give below. Refer to the cardiac cycle
graphs in your textbook and your lecture notes. (May need to complete after we get through the Cardiac
Lecture)
P Wave – During the P wave, depolarization is sweeping from the SA node throughout both atrial
(electrical event). This leads to the beginning of atrial contraction (mechanical event).
P-R Segment –
QRS Complex –
Q-T Interval –
S-T Segment –
T Wave –
T-P Segment –
R-R Interval –