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Lesson 4 ECG 2 Electrocardiography

Lesson Revision 1.20.2006

BIOPAC Systems, Inc. 42 Aero Camino, Goleta, CA 93117 (805) 685-0066, Fax (805) 685-0067

[email protected]

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Lesson 4 ELECTROCARDIOGPAHY 2

I.

The body contains fluids with ions that allow for electric conduction. This makes it possible to use electrodes on the surface of the skin to detect electrical activity in and around the heart and use an electrocardiograph to record the activity. Conveniently, the legs and arms act as simple extensions of points in the torso, allowing the recording and ground electrodes to be placed on the wrists and ankles (Fig. 4.1).

SCIENTIFIC PRINCIPLES

Recall from Lesson 3, ECG 1, that each cardiac cycle, or heartbeat, begins with a bioelectric impulse generated by the sinoatrial (SA) node, the pacemaker of the heart. The bioelectric impulse is conducted through the heart by the pacemaker system and travels as a wave of depolarization followed in a few thousandths of a second by a wave of repolarization. Ventricular muscle depolarization and repolarization are represented in the electrocardiogram (ECG) as the QRS complex and the T wave, respectively.

Lead I +-

Lead III

Lead II

-

+

+

+

-

left leg

left armright arm

right leg(ground)

Fig. 4.1 Electrode placement options

Fig. 4.2 Einthoven’s triangle and vectorgraph

The ECG is a record of the overall spread of electric current through the heart as a function of time in the cardiac cycle. The direction of polarity (+ or -) of the recorded waveforms depends upon the location of the recording electrodes on the surface of the body and whether the electrical activity during the cardiac cycle is coming toward or going away from the surface electrode. In general, as a wave of depolarization approaches a positive electrode, a positive voltage is seen by that electrode. If the wave of depolarization is traveling toward a negative electrode, a negative voltage will be seen. The term “lead” is defined as a spatial arrangement of two recording electrodes on the body. One lead is labeled positive (+) and the other negative (-). The electrode placements define the recording direction of the lead, which is called the lead axis or angle. The axis is determined by the direction when going from the negative to positive electrode. The electrocardiograph computes the voltage difference (magnitude) between the positive and negative electrodes and displays the changes in voltage difference with time. The standard bipolar limb leads are:

Lead I = right arm (-), left arm (+)

Lead II = right arm (-), left leg (+)

Lead III = left arm (-), left leg (+)

The three standard bipolar limb leads may be used to construct an equilateral triangle, called Einthoven’s triangle, at the center of which lies the heart (Fig. 4.2, left).

Each side of the triangle represents one of the bipolar limb leads, and each lead forms a 60° angle with the two opposite leads. By placing Lead I through the center of the heart and moving the axes of Lead II and Lead III horizontally until they intersect Lead I at the center of the heart, a vectorgraph can be constructed (Fig. 4.2, right). The vectorgraph is used to plot vectors and aid in visualizing the electrical axes of the heart.

At any given moment during the cardiac cycle (during the QRS complex, for example), the net electrical activity seen by a lead may be represented by a vector. A vector is an entity that can be represented by an arrow of specific length pointing in a specific direction. A vector has both magnitude and direction. The length of the arrow, divided equally into 0.1 mV units, represents the vector’s magnitude, and is expressed as a mean potential. The direction of the vector, in degrees, is represented by its position relative to the horizontal 0º to 180º line (Lead I) on the vectorgraph.

The dominant waveform in all three bipolar limb leads is the QRS complex signifying ventricular depolarization. The mean axis of ventricular depolarization is called the mean QRS axis. The mean QRS axis is diagnostically important

Lesson 4: ECG 2

because it represents the majority of the heart’s electrical activity occurring during a cardiac cycle. When the bipolar limb leads are used to obtain data for plotting the vector, the mean QRS axis lies in the frontal plane because the electrical “picture” seen by the recording electrodes placed on the extremities lies in the frontal plane. In normal persons, the mean QRS axis is between -30° and +90° (Fig. 4.2). In the frontal plane, the mean QRS axis is often between 0° and +90°.

A mean QRS axis between +90° and +180° indicates right axis deviation (RAD). RAD occurs, for example, in persons with right ventricular hypertrophy (over-sized right ventricle). A mean QRS axis between -30° and -90° indicates left axis deviation (LAD). LAD is found in patients with left anterior blockage in the conduction system of the ventricle.

To define the mean QRS axis precisely, the cardiologist defines it in three dimensions (X, Y and Z planes). This is done in practice by using a standardized set of 12 leads. In this lesson, you will sequentially record two of the bipolar limb leads (Lead I and Lead II) and extract the data from the ECG necessary to determine the mean QRS axis in the frontal plane.

A simple way to approximate the mean QRS axis in the frontal plane is to plot the magnitude of the R wave as measured from Lead I and Lead II (Fig. 4.3).

--- -

+

++

+ +

I0,0

magnitude of R wave from leads

+

III

Mean Electrical

Axis

Mean Electrical

Axis

Fig. 4.3 Mean electrical axis

In this lesson, you will record data from Lead I and Lead III connections with the Subject lying down and sitting up.

II.

III.

EXPERIMENTAL OBJECTIVES

1) To record ECG from Leads I and III in the following conditions: lying down, sitting up, and breathing deeply while sitting.

2) To compare the direction of the QRS complex (+ or −) with the direction of the lead axis.

3) To estimate the mean ventricular potential.

4) To estimate the mean electrical axis of the QRS complex using two methods.

MATERIALS

Computer system (running Windows XP or Mac OS X) Biopac Science Lab system (MP40 and software) Electrode lead set (40EL lead set) Disposable vinyl electrodes (EL503), four electrodes per subject Cot or lab pillow Protractor Two different color pens or pencils


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IV. EXPERIMENTAL METHODS

A. Set Up

EQUIPMENT

Fig. 4.4

SUBJECT

Lead I +-

left forearmRED lead

right forearmWHITE lead

right legBLACK lead

(Ground) Fig. 4.5 Electrodes for Leads I & III/Lead I configuration

FAST TRACK Details

1. Turn the computer ON.

2. Set the MP40 dial to OFF.

3. Plug the equipment in as follows:

Electrode leads (40EL) MP40.

4. Attach four electrodes to the Subject as shown in Fig. 4.5.

Attach four electrodes to the Subject to allow Lead I and Lead III configurations, as shown in Fig. 4.5:

one electrode on the right leg, just above the ankle bone one electrode on the left leg, just above the ankle bone one electrode on the left wrist (same side of arm as the palm of hand) one electrode on the right wrist (same side of arm as the palm of hand). For optimal electrode response, place electrodes on the skin at least five minutes before starting the Check section.

5. Connect the electrode leads (40EL) for a Lead I configuration, matching lead color to electrode position as shown above.

IMPORTANT Clip each electrode lead color to its

specified electrode position.

Lead I will be recorded first, so connect the leads for a Lead I configuration: white: right wrist; red: left wrist; black: right ankle.

6. Start the Biopac Science Lab software.

7. Choose lesson L04-ECG-2 and click OK.

8. Type in a unique file name. No two people can have the same file name, so use a unique identifier, such as the Subject’s nickname or student ID#.

9. Click OK. This ends the Set Up procedure.

Lesson 4: ECG 2

B. Check

FAST TRACK Details

MP40 Check

1. Set the MP40 dial to ECG/EOG.

2. Press and hold the Check pad on the MP40.

3. Click when the light is flashing.

4. Wait for the MP40 Check to stop.

5. Let go of the Check pad.

6. Click Continue.

Continue to hold the pad down until prompted to let go.

The MP40 check procedure will last five seconds.

The light should stop flashing when you let go of the Check pad.

When the light stops flashing, click Continue. Signal Check

7. Click Check Signal.

8. Wait for the Signal Check to stop.

Lead I configuration is used for the signal check. Subject should be supine, relaxed, and still. The electrocardiograph is very sensitive to small changes in voltage caused by contraction of skeletal muscles, and the Subject’s arms and legs need to be relaxed so that the muscle (EMG) signal does not corrupt the ECG signal.

9. Review the data.

If correct, go to the Record section.

If incorrect, click .

The eight-second Signal Check recording should resemble Fig. 4.6.

Fig. 4.6

If the recording does not show a recognizable ECG waveform with R-wave peaks and no large baseline drift, click Redo Signal Check.


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C. Record

FAST TRACK Details

1. Prepare for the recording and have the Subjectlie down and relax.

Watch the Help menu videos to prepare for the recording. You will record two segments for each lead configuration, one with the Subject lying down, and another with the Subject sitting up. In order to work efficiently, read this entire section so you will know what to do before recording.

Check the last line of the journal and note the total amount of time available for the recording. Stop each recording segment as soon as possible so you don’t use an excessive amount of time (time is memory).

Tips for obtaining optimal data To minimize muscle (EMG) corruption of the ECG and baseline drift:

a) The Subject should not talk or laugh during any of the recording segments.

b) When lying down or sitting up, the Subject should be completely relaxed.

c) When sitting up, the Subject’s arms should be supported on an armrest (if available).

d) The recording should be suspended before the Subject prepares for the next recording segment.

e) The Subject should breathe normally during the recording to minimize EMG from the chest area.

f) Make sure electrodes do not peel up.

SEGMENT 1 — Supine, Lead I

2. Click . When you click Record, the recording will begin and an append marker labeled Supine, Lead I will automatically be inserted.

3. Record for 30 seconds. Subject should be supine, relaxed, and still (seconds 0-30).

4. Click Suspend. When you click Suspend, the recording will halt, giving you time to review the data and prepare for the next recording segment.

5. Review the data.

If correct, go to Step 6.

Your data should resemble Fig. 4.7.

Fig. 4.7 Segment 1 (Supine, Lead I)

If incorrect, click Redo.

Recording continues…

A little baseline drift when the Subject breathes in and out does not indicate incorrect data. The data is incorrect if:

a. The Suspend button was pressed prematurely b. An electrode peeled up, giving a large baseline drift c. The Subject has too much muscle (EMG) artifact

If the data is incorrect, click Redo and repeat Steps 1-5; the last data segment you recorded will be erased.

Lesson 4: ECG 2

SEGMENT 2 — Supine, Lead III

6. Disconnect the leads from the Lead I configuration and reconnect to establish a Lead III configuration (Fig. 4.8).

+

-

left forearmWHITE lead

left legRED lead

right legBLACK lead

(Ground)

Lead III

Fig. 4.8 Lead III Configuration

white: left wrist

red: left ankle

black: right ankle

7. Click Resume. When you click Resume, the recording will begin and an

append marker labeled Lying down, Lead III will automatically be inserted.

8. Record for 30 seconds. Subject should be supine, relaxed, and still (seconds 30-60).


10. Review the data on the screen.



Fig. 4.9 Segment 2 (Supine, Lead III)



A little baseline drift when the Subject breathes in and out is normal and does not indicate incorrect data. The data would be incorrect if:

a. The Suspend button was pressed prematurely b. An electrode peeled up, giving a large baseline drift c. The Subject has too much muscle (EMG) artifact

If data was incorrect, click Redo and repeat Steps 7-10; the last data segment you recorded will be erased.


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SEGMENT 3 — Seated, Lead III

11. Have the Subject quickly get up and sit in a chair, with arms relaxed.

Subject should sit with arms relaxed, preferably in a chair with armrests.

In order to capture the heart rate variation, it is important that you resume recording as quickly as possible after the Subject sits. However, it is also important that you do not click Resume while the Subject is in the process of sitting or you will capture motion artifact.

12. Click Resume as soon as possible after the Subject sits up.

When you click Resume, the recording will continue and an append marker labeled Seated, Lead III will be automatically inserted.

13. After 10 seconds of recording, the Subject should breathe in and out once such that the breath is audible

Note Do not insert markers.

After 10 seconds of recording, the Director should instruct the Subject to breathe in and out once such that the inhale and exhale are audible.

Note Do not insert markers in this segment because the automatic processing for this lesson splits the recorded segments into separate channels (after Done is clicked), which makes the time base for markers meaningless so they are not retained.

The recording should run for 30 seconds (seconds 60-90).


15. Review the data on the screen.



Fig. 4.10 Segment 3 (Seated, Lead III)




a) The Suspend button was pressed prematurely.

b) An electrode peeled up, giving a large baseline drift.

c) The Subject has too much muscle (EMG) artifact.


Lesson 4: ECG 2

SEGMENT 4 — Seated, Lead I

16. Disconnect the leads from the Lead III configuration and reconnect to establish a Lead I configuration (Fig. 4.11) while Subject remains seated and relaxed.

Lead I +-

left forearmRED lead

right forearmWHITE lead

right legBLACK lead

(Ground) Fig. 4.11 Lead 1 configuration

white: right wrist

red: left wrist

black: right ankle

17. Click Resume. When you click Resume, the recording will continue and an

append marker labeled Seated, Lead I will be automatically inserted.

18. After 10 seconds of recording, the Subject should breathe in and out once such that the breath is audible.

Note Do not insert markers.

After about 10 seconds of recording, the Director should instruct the Subject to breathe in and out once such that the inhale and exhale are audible.

Note Do not insert markers in this segment because the automatic processing for this lesson splits the recorded segments into separate channels (after Done is clicked), which makes the time base for markers meaningless so they are not retained.

The recording should run for 30 seconds (seconds 90-120).

19. Click Suspend. When you click Suspend, the recording will halt, giving you time to review the data.

20. Review the data.



Fig. 4.12 Segment 4 (Seated, Lead I)




a) The Suspend button was pressed prematurely.

b) An electrode peeled up, giving a large baseline drift.

c) The Subject has too much muscle (EMG) artifact.



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21. Optional: Click Resume to record additional segments.

Optional: You can record additional segments by clicking Resume instead of Done. A time marker will be inserted at the start of each added segment.

22. Click Done.

23. Click Yes.

24. Choose an option and click OK.

A pop-up window with options will appear. Click Yes (or No if you want to redo the last segment).

When you click Yes, a dialog with options will be generated. Make your choice, and click OK.

If you choose Analyze current data file, go to the Analyze section for directions.

25. Remove the electrodes.

END OF RECORDING

Unclip the electrode lead connectors and peel off the electrodes. Throw out the electrodes.

Lesson 4: ECG 2

V. ANALYZE

In this section, you will examine ECG components of cardiac cycles and measure amplitudes (mV) and durations (msecs) of the ECG components.

Note: Interpreting an ECG is a skill that requires practice to distinguish between normal variation and those arising from medical conditions. Do not be alarmed if your ECG is different than the examples shown or from the tables and figures in Section I (Scientific Principles).

Because ECGs are widely used, basic elements have been standardized to simplify reading ECGs.

ECGs have standardized grids of lighter, smaller squares and, superimposed on the first grid, a second grid of darker and larger squares (Fig. 4.13). The smaller grid always has time units of 0.04 seconds on the x-axis, and the darker vertical lines are spaced 0.2 seconds apart. The horizontal lines represent amplitude in mV. The lighter horizontal lines are 0.1 mV apart, and the darker grid lines represent 0.5 mV.

Fig. 4.13 Standard ECG grid

FAST TRACK Details

1. Enter the Review Saved Data mode and choose the correct data file.

Note Channel Number (CH) designations:

Channel Displays

CH1 Supine, Lead I

CH2 Supine, Lead III

CH3 Seated, Lead III

CH4 Seated, Lead I

Important For this lesson analysis, segments that were recorded sequentially have been split into channels for relative comparison and do not share a time base.

To review saved data, choose Analyze current data file from the Done dialog after recording data, or choose Review Saved Data from the Lessons menu and browse to the required file.

Fig. 4.14 IMPORTANT: Channels do not share a time base

2. Set up measurement boxes for:

Channel Measurement

SC Max

SC Delta

SC Mean

The measurement boxes are above the marker region in the data window. Each measurement has three sections: channel number, measurement type, and result. The first two sections are pull-down menus that are activated when you click them.

Max: finds the maximum amplitude value within the selected area (including the endpoints) by the I-Beam tool.

Delta: computes the difference in amplitude between the last point and the first point of the selected area. It is particularly useful for taking ECG measurements because the baseline does not have to be at zero to obtain accurate, quick measurements.

Note The polarity of Delta is based on (first point-last point). First point is where you begin your selection; last point is where you release the mouse to end your selection.

3. For each of the leads, note whether the R wave is positive or negative.

A

This is just a visual check as to whether the R wave goes up (positive) or down (negative).


Biopac Science Lab

Analysis continues…

4. Set up the display window for optimal viewing of one cardiac cycle in the supine channels.

Fig. 4.15

The following tools help you adjust the data window: Autoscale Horizontal Horizontal (Time) Scroll Bar Autoscale Waveforms Vertical (Amplitude) Scroll Bar Zoom Zoom Previous/Back

Your window should resemble Fig. 4.15.

5. Using the I-Beam tool, select and measure Max from a cycle in the center of each channel.

B

You’ll use the recorded measurements to construct the graphical estimate of the Mean QRS Potential in the Data Report.

6. Place an event marker to indicate where the QRS measurement was taken — you will add four markers.

Insert the event marker directly above the selected R wave.

To place an event marker after the data has been recorded, click the cursor in the event marker region under the marker label bar (above the top data channel). An inverted triangle should appear, which, when selected, will be yellow.

Type marker labels: Max Lead I down, Max Lead I up, Max Lead III down, and Max Lead III up.

7. Repeat Steps 5 and 6 for a cardiac cycle in the Seated channel.

B

8. Select the “breathe in” area of a cardiac cycle in the CH3 and repeat Steps 5 and 6.

B

“Breathe in” was recorded in the Seated, Lead I and Seated, Lead III segments, now displayed as CH3 and CH4. The breathe change should occur as an upward spike in the data.

9. Select the “breathe in” area of a cardiac cycle in the CH4 and repeat Steps 5 and 6.

B

10. Select the “breathe out” area of a cardiac cycle in the CH4 and repeat Steps 5 and 6.

B

“Breathe out” was recorded in the Seated, Lead I and Seated, Lead III segments, now displayed as CH3 and CH4. The breathe change should occur as a downward spike in the data.

11. Select the “breathe out” area of a cardiac cycle in the CH4 and repeat Steps 5 and 6.

B

12. Go back to the Max Lead I down marker created in Step 6.

This is the same QRS region you measured and marked in Steps 5 and 6. It represents a QRS cycle in the supine, relaxed

Lesson 4: ECG 2

state.

13. Measure and record the amplitudes of the Q, R, and S waves individually for both Lead I and Lead III for each segment.

C

You will be taking a total of six measurements.

You may wish to look at one channel at a time by hiding the other channel(s) using the channel boxes. To measure a peak, go from the baseline (isoelectric line) to the peak of the wave.

Fig. 4.16 Sample measurement of the R-wave peak

14. Save or print the data file. You may save the data, save notes that are in the journal, or print the data file.

15. Exit the program.

16. Set the MP40 dial to Off.

END OF LESSON 4

Complete the Lesson 4 Data Report that follows.


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The Data Report starts on the next page.

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Lesson 4 ELECTROCARDIOGRAPHY 2


DATA REPORT

Student’s Name:

Lab Section:

Date: I. Data and Calculations Subject Profile

Name Age Gender: Male / Female Height Weight

Direction of R-Waves for Different Leads

A. Put a check in the column to indicate whether the R-wave is positive (+) or negative (−) for each lead configuration:

Table 4.1 R-wave Lead + −

Lead I

Lead III

Mean QRS Potential and Axis — Graphical Estimate Using R-wave Amplitude

B. Use Table 4.2 to record measurements from the Data Analysis section: Table 4.2 QRS Potential and QRS Axis

Condition Potential QRS

Lead I Max

Lead III Max

Mean QRS Potential

Mean QRS Axis

Supine

Seated

Breathe In

Breathe Out

These are sample questions. You should amend, add, or delete questions to support your curriculum objectives.

Biopac Science Lab

A simple way to approximate the mean QRS axis in the frontal plane is to plot the magnitude of the R wave as measured from Lead I and Lead III (Fig. 4.3, repeated below from Section 1).

--- -

+

++

+ +

I0,0

magnitude of R wave from leads

+

IIIM

ean Electrical

Axis

Mean Electrical

Axis

Fig. 4.3

1) Determine the R-wave amplitude (mV) in Lead I and Lead III.

2) Plot the Lead I R-wave amplitude value on the Lead I axis. Draw an arrow from the center of the vectorgraph to the amplitude value.

3) Plot the Lead III R-wave amplitude value on the Lead III axis. Draw an arrow from the center of the vectorgraph to the amplitude value.

4) Draw a line perpendicular to the tip of each arrowhead

5) Determine the point of intersection of these two perpendicular lines.

6) Draw a new vector from point 0,0 at the center of the vectorgraph to the point where the perpendicular lines intersect with one another. The direction (in degrees) of this resulting vector approximates the mean QRS axis. The length (mV) of this vector approximates the mean QRS potential.

To determine Mean QRS Potential from the plotted vector, subdivide it into 0.1 mV increments exactly as the leads are in the vectorgraph. Alternatively, the length of the vector can be measured (with a compass, string, notepaper, etc.) and then transferred to a lead line to determine the mV value.

One way to approximate the mean electrical axis in the frontal plane is to plot the magnitude of the R wave from Lead I and Lead III, as shown in Fig 4.2 in Section I (Scientific Principles).

1. Draw a perpendicular line from the ends of the vectors (right angles to the axis of the Lead) using a protractor or right angle guide.

2. Determine the point of intersection of these two perpendicular lines. 3. Draw a new vector from point 0,0 to the point of intersection.

The direction of this resulting vector approximates the mean electrical axis of the heart. The length of this vector approximates the mean potential of the heart.

Lesson 4: ECG 2

Create two plots on each of the following graphs, using data from Table 4.2. Use a different color pencil or pen for each plot.

Graph 1: Lead I Lying Down and Sitting Up

10°

20°

30°

40°

50°

60°70°80°

0°

Lead II

Lead I.1 .2 .3 .4 .5 1 1.5 2-.1

2

mV.1-.1

Lead III

.5

1

1.5

2

mV

90°

100°

110°

120°

0,0

From the above graph, find the following values:

Condition Mean QRS Potential Mean QRS Axis Lying Down

Sitting Up Explain the difference (if any) in Mean QRS Potential and Axis under the two conditions:


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Graph 2: Breathing In /Breathing Out or Lead III Lying Down/Sitting Up

10°

20°

30°

40°50°

60°70°80°

0°

Lead II

Lead I.1 .2 .3 .4 .5 1 1.5 2-.1

2

mV.1-.1

Lead III

.5

1

1.5

2

mV

90°

100°

110°

120°

0,0

From the above graph, find the following values:

Condition Mean QRS Potential Mean QRS Axis Lying Down

Sitting Up Explain the difference in Mean QRS Potential and Axis for the Lying Down data in this plot (Graph 2) and the first plot (Graph 1).

C. Mean QRS Potential and Magnitude — More Accurate Approximation

Add the Q, R, and S potentials to obtain net potentials.

Lying down: Lead I Lead III

Q Q R R S S

QRS Net 1 QRS Net 2

Lesson 4: ECG 2


II. Data Summary and Questions

D. Define ECG.

E. Define Einthoven’s Triangle.

F. What factors affect the orientation of the mean electrical axis?

G. Refer to Table 4.2:

How did the amplitudes of Lead I and Lead III change between inhalation and exhalation? Did the heart’s axis and magnitude change?

H. What factors affect the amplitude of the R wave recorded on the different leads?

I. Compare the mean electrical axis and magnitude obtained when

i. using just the amplitude measurement of the R wave vs. net potentials

ii. lying down vs. sitting up

End of Biopac Science Lab Lesson 4 Data Report

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VI. ACTIVE LEARNING LAB

Design a new experiment to test or verify the scientific principle(s) you learned in the Biopac Science Lab recording and analysis segments.

For this lesson, you might examine how gender, duration of exercise, and changing body position influence the ECG and/or time to return to baseline.

Design Your Experiment

Use a separate sheet to detail your experiment design, and be sure to address these main points:

A. Hypothesis

Describe the scientific principle to be tested or verified.

B. Materials

List the materials will you use to complete your investigation.

C. Method

Describe the experimental procedure—be sure to number each step to make it easy to follow during recording.

See the Set Up section or Help > About Electrodes for electrode placement guidelines.

Run Your Experiment

D. Setup

Set up the equipment and prepare the subject for your experiment.

E. Recording

Use the Record, Resume, and Suspend buttons in the Biopac Science Lab program to record as many segments as necessary for your experiment.

Click Done when you have completed all of the segments required for your experiment.

Analyze Your Experiment

F. Set measurements relevant to your experiment and record the results in a Data Report.


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