Overview. Signal Averaged ECG

Transcription

1 Updated : Signal Averaged ECG Overview Signal Averaged ECG The Biopac Student Lab System can be used to amplify and enhance the ECG signal using a clinical diagnosis tool referred to as the Signal Averaged ECG. The technique allows you to identify small electrical currents, called ventricular (ven-trik'u-ler) late potentials. This test is used to help identify people at risk of a dangerous rhythm in the heart's lower (pumping) chambers. This is called ventricular arrhythmia (ah-rith'me-ah), and it can lead to sudden cardiac death. For years, cardiologists examined paper charts of ECG signals to create an averaged ECG complex. They took each individual cardiac cycle, measured each part of the complex with a ruler, wrote down each value, and calculated the means to derive an averaged ECG complex. As you can imagine (or have experienced), this task took a great deal of time and patience. When the need arose to analyze two or more different scenarios (e.g., baseline response), the task had to be performed even more carefully for accurate comparison of the ECGs. The BSL PRO software has a powerful averaging feature that automatically derives an averaged signal from a specific number of cycles (averaging is found in the Find Peak dialogue). And, you can easily compare two, three, or more averaged ECG signals with just a few mouse clicks. NOTE: This lesson uses L05 Electrocardiography I as source data. If you have already recorded the ECG data you want to average, click here to jump to the Analysis section. Page 1 of 7

2 Objectives 1. Optional: Record multiple segments of ECG data under different conditions. 2. Provide a Signal Averaged ECG for each of the recording segments. 3. Compare the Signal Averaged ECG data for different conditions. Equipment BIOPAC electrode set (SS2L) BIOPAC disposable vinyl electrodes (EL503) three electrodes per subject BIOPAC electrode gel (GEL1) BIOPAC abrasive pad (ELPAD) or skin cleanser or alcohol prep Cot or lab table and pillow Computer running Windows 7/VistaXP or Mac OS X Biopac Student Lab PRO software BIOPAC Data Acquisition Unit (MP36/35/30/45) Setup Hardware 1. Make sure that the MP3X unit is off. 2. Turn on the computer. 3. Plug the electrode lead (SS2L) into CH 2. Subject 1. Remove all substantial metal jewelry from the Subject and make sure that the subject is not touching any metal (metal pipes, chairs etc.). 2. Place three electrodes on the subject as the table below indicates. Electrode Right arm Right leg Left leg Position Centered on the anterior wrist (same side as the palm of the hand), about " down from the palm Medial, just above the ankle, flat on skin (not over bone) Medial, just above the ankle, flat on skin (not over bone). Lead Color White Black Red 3. Attach the electrode leads by color as shown in the table above. The pinch connectors work like small clothes pins; however, they will attach to the electrode only on one side. You may have to rotate the pin to make sure the metal on the inside of the clip is connected, touching and clamped onto the electrode at the base of the nipple. Clip connector cables to subject's clothing, or place so that there is no strain on the electrode clips or the cable wires at any point in the setup. 4. Turn the MP3X unit on (assuming the AC100A power adapter has already been connected). 5. Wait 5 minutes after the electrodes have been attached to the skin to begin recording to give the gel time to settle and maximize conductivity. Page 2 of 7

3 Software 1. Launch the BSL Lessons software on the host computer. If the BSL Lessons software is not available, click here to set up BSL PRO. 2. Choose Lesson 5: L05-ECG-1 3. Save as the desired file name when prompted. Calibration Automatic in BSL Lesson 5; not required if using BSL PRO. Recording Follow BSL Lesson 5 instructions to record the following segments and label markers as indicated: Segment Duration Marker Label Condition 1 60 sec Supine Subject should be lying down, and not moving or talking at any time while recording is taking place sec Seated Subject should sit up quickly, then start recording. Do not record while subject is sitting up or making any type of movements sec Deep breathing sec After exercise Subject should take about 5 deep breaths over the recording period. Subject should exercise for a few minutes, such as pushups, sit-ups, jumping jacks, etc., to dramatically increase heart rate. Connectors can be removed and reconnected to subject, however the electrodes should not be taken off. Subject should then relax and record the 60-second period after exercise has stopped, but while the heart rate is still increased. Note: Any movement of the body while recording will cause small electrical impulses that will be detected by the electrodes, and thus recorded as inaccurate data. It is very important that the subject not move at anytime during recording. Page 3 of 7

4 Analysis Set up the Averaging Function To use the averaging function for ECG in BSL PRO, first determine the typical length of each ECG cycle. BSL PRO identifies the peak of each R-wave as a reference point. Because of this, set a window around the R- wave to look backward at the P and Q waves and forward to examine the S and T components. 1. Launch the BSL PRO software. 2. Open the ECG data file in BSL PRO. 3. Set a measurement channel to SC (selected channel) and the measurement to Delta T. 4. Zoom in on one representative ECG complex from the first segment. 5. With the cursor on the peak of the R-wave, click and drag backward past the P-wave to the midpoint of the P-wave and previous T-wave (see below). 6. Write down the delta T value (remember to note negative time); this time period represents the averaging period before the R-wave. 7. Click on the peak of the R-wave, and drag the cursor past the T wave. 8. Write down the delta T value (positive time); this time period represents the averaging period after the R-wave. 9. Use the markers to identify segments and highlight the area that you are averaging, as shown below. 10. Select Transform > Find Peak. Page 4 of 7

5 11. Check the Off-line Averaging box. 12. Set the cursors: o Enter the negative delta T measurement into the Set first cursor to box. o Enter the positive delta T measurement into the Set second cursor to box. 13. Check that the Threshold Level is not zero. o The value of the Threshold Level in the Peak Detector box determines how the software will read each R-wave. The value displayed in the Threshold Level box was determined by the selected area you chose. If the value is 0, cancel and reselect the area you want to view. 14. Click the Setup Averaging box. 15. Set the Averaging Range to Selected Area and click OK. 16. Click the Ave Start button. o The system will calculate the averaged ECG Complex. The signal will look similar to this: Page 5 of 7

6 Comparing Multiple Waveforms While this averaged ECG signal by itself provides an excellent diagnostic tool, the ability to compare other scenarios will be an even more valuable tool. The following graph shows the averaged ECG complex for the segments of data recorded when the subject is at rest, after sitting up, breathing deeply, and immediately after exercise. 1. Repeat above procedure for all segments, using the same peak parameters for each segment. IMPORTANT! Do not recalculate or change the time before and after the peak (the cursor settings in the Find Peak dialog) for each segment of data. You must use the same time frame for all segments to center all the averaged data around the R-wave so that when all the averaged graphs are compared they will align with respect to the R-wave. 2. View all analyses in one graph window. a. Select one of the averaged wave forms and choose Edit > Select All. b. Copy the averaged complex. c. Choose another R-waveform box (the one you wish to paste the copied waveform in to). d. Choose Edit>Insert Waveform. All your R-waveforms should be in one graph window. 3. Label each channel individually: On the first averaged waveform, click the Channel Label box (far left of the channel), and change the name of the channel. 4. Change the colors of the waveforms to provide more contrast in the final presentation: Right-click on the waveform, choose Color, select a color from the options provided. 5. Set all channels to the same Scale: Click on the Vertical Axis (right side of each channel) and click all three of the "Apply to All Channels" buttons in the Set Screen Vertical Axis dialog. NOTE: You must use the "Apply to All Channels" command. If you do not, the channels will not be uniform in Scope mode, and you will not get an accurate comparison. 6. Click the Scope Mode icon in the Tool Bar (top left corner) to change the graphs from Chart mode to Scope mode. Chart mode (the default) displays each individual averaged waveform, while Scope mode provides an overlapped comparison of each averaged ECG cycle. Chart Mode Page 6 of 7

7 Scope mode If Scope mode does not display all the waveforms in the window: Two very similar R-waves may be overlapping one another. All of the waveforms may not be set to the same scale. Make sure to select the "Apply to All Channels" commands in the Set Vertical Scale box. If Apply to All Channels is not selected, the channels will not be uniform in Scope mode and will not provide an accurate comparison. Recording with BSL PRO if BSL Lesson 5 is not available 1. Launch the BSL PRO software on the host computer. 2. From the File > Open menu, select Files of type: Graph Template (*.gtl) and open the H23 Signal Averaging ECG.gtl file. 3. Follow the Recording instructions. IMPORTANT! Remember to type your comments into the Marker field when commencing a new recording segment. This will allow you to correlate the body position with the data when performing the averaging process. Appendix GRAPH TEMPLATE SETTINGS Click here to open a PDF of the graph template file settings. The BSL PRO Graph Template file automatically establishes the settings shown in the table. Page 7 of 7