Chest Lead EKG

Experiment HH-7: Electrocardiograms from the Six Chest Leads Background Discrete areas of electrical activity in the heart can be defined by an electrocardiogram recorded simulta- neously from different points of view around the heart. Each point of view is commonly called a lead. The multiple lead electrocardiogram that is most universally accepted as a clinical tool is known as the 12-Lead ECG. This electrocardiogram uses one electrode on each limb of the subject, and an electrode on each of six different positions on the chest. Many 12-Lead ECG monitors have inputs and wiring for each of the six chest electrodes. However, if the monitor has only one input and cable for recording from the chest, the cable must be moved from one chest electrode to another in order to complete the recording from all six positions on the chest. Unipolar and Bipolar Leads To record an electrocardiogram from twelve different points of view, the outputs of limb and chest electrodes need to be combined in nine of the twelve leads. On many ECG monitors, the combinations are created by rotating a simple switch called the ECG lead selector. Each of the standard limb leads (Leads I, II, and III) record electrocardiograms between two of the three limb electrodes. These leads are classified as bipolar because two electrodes are involved in the recording. The voltage at the active (recording) electrode is refer- enced against the voltage at the indifferent electrode. For example, the voltage at the electrode on the left leg is compared to the voltage on the right arm or shoulder in Lead II recordings. Each of the augmented limb leads (aVF, aVL, aVR) record electrocardiograms between an active limb electrode and an indifferent point which formed by the joining of the two remaining limb electrodes through the ECG lead selector. The augmented leads are classified as unipolar because the voltage at the active electrode is referenced against a point that is equidistant between the two non-active electrodes. The chest leads, known as V1 through V6, record electrocardiograms between an active chest electrode and another indifferent point formed by the joining of the limb electrodes through the ECG lead selector. The chest leads are also unipolar because the voltage at the active electrode on the chest is referenced against an indifferent point in the center of a triangle formed by the limb leads. Plane of the Limb Leads In the standard 12-Lead ECG, the six limb leads look at the heart along six directions in a vertical plane that is best described as being parallel to the floor when the subject is reclining. The information from these leads can determine the electrical axis of the atria and ventricles during depolarization (see Experiment 28: The Six-Lead ECG, iWorx Physiology Lab Manual). A variation in the angle of this electrical axis can be caused by a variety of physiological or pathological conditions. For example, right-axis shifts are common in extremely thin and healthy persons; the causes of the shifts in these cases are physiological. Right-axis shifts also appear in persons with right bundle branch blocks or right ventricular hypertrophy; however, in these cases, the deviations are caused by pathological problems. All of the deviations detected by the limb leads are around a horizontal axis that is perpendicular to the vertical plane of these leads. Plane of the Chest Leads The six chest leads of the 12-Lead ECG are often called the precordial leads. The chest leads are in an almost horizontal plane that is best described as being parallel to the floor when the subject is standing. The electrocardiograms from these leads can be used for diagnosing damage to cardiac tissue caused by coronary occlusions, or ventricular hypertrophy. These types of determinations are possible because chest leads are unipolar and placed in an array. The active electrodes in these leads respond to electrical activity directly beneath them whether the activity is moving towards or away from the electrode, or from side to side. The depolarizations that move toward an electrode appear as positive deflections; if the depolar- izations are moving away from the electrode, the deflection is negative. Recordings from a normal, healthy heart usually show a large S wave from the V1 and V2 leads. The V3 and V4 leads provide QRS complexes that are biphasic, R and S waves of equal magnitude in opposite directions. The lead with a biphasic QRS complex overlies the anterior edge of the interven- tricular septum, also known as the transition point. A shift in the transition point results from an abnormal orientation of the heart around a vertical axis that is perpendicular to the horizontal plane of the chest leads. Chapter 6: 1 In this lab, you will record electrocardiograms from Lead I and the six chest leads (Figure 6-1 on page 2). Lead I will be used as a reference. The amplitudes and shapes from each chest lead will be compared, and the transition point of each subject will be determined. Figure 6-1: Electrocardiograms recorded from Lead I (upper) and V1 (lower) positions at the same time. Equipment Required PC Computer iWorx unit, and USB or serial cable AAMI cable and ECG lead cables 3-way indifferent ECG lead cable Alcohol swabs Disposable ECG electrodes Equipment Setup 1 Connect the iWorx unit to the computer (described in Chapter 1). 2 The subject should remove all jewelry from his or her neck, wrists, and ankles. 3 Use an alcohol swab to clean and scrub regions on each wrist and ankle, under the lateral end of each clavicle and, on the chest (Figure 6-2 on page 2). Let the areas dry. 4 Obtain 12 disposable electrodes. Remove each electrode from its protective plastic sheet and apply it to one of the following scrubbed areas on the subject?s body: ? under the lateral ends of each clavicle; for use as the positive and negative electrodes of Lead I. ? on each wrist and each ankle; for use as the ground and the three electrodes that form the indifferent point for the chest leads. ? over the right border of the sternum at the 4th intercostal space; for use as the active electrode of the V1 chest lead. ? over the left border of the sternum at the 4th intercostal space; for use as the active electrode of the V2 chest lead. ? on the mid-clavicular line at the 5th intercostal space; for use as the active electrode of the V4 chest lead. ? halfway between V2 and V4; for use as the active electrode of the V3 chest lead. ? on the anterior axillary line at the same horizontal level as V4; for use as the active electrode of the V5 chest lead. ? on the mid-axillary line at the 5th intercostal space; for use as the active electrode of the V6 chest lead. Figure 6-2: A.Frontal view of the electrode positions for recording Lead I and the six chest leads. B. Top view of the electrode positions for the six chest leads Figure 6-3: Connections needed to record Lead I and chest lead electrocar- diograms. 5 Attach the AAMI connector on the end of the gray patient cable to the isolated Channel 1 and 2 inputs on the iWorx Chapter 6: 2 unit (Figure 6-3 on page 2). 6 Attach four color-coded electrode lead wires and the 3-way indifferent lead cable to the sockets on the lead pedestal of the gray patient cable. The 3-way indifferent lead cable is connected to the white ?+2? input. Snap the other ends of the lead cables to the electrodes on the subject, so that: ? the black ?-1? lead wire is connected to the electrode below the left clavicle, ? the red ?+1? lead wire is connected to the electrode below the right clavicle, ? the green ?C? lead wire is connected to the electrode on the right ankle. ? the brown ?-2? lead wire is connected to the electrode on the V1 chest electrode, ? each snap of the 3-way indifferent lead cable is connected to a different limb electrode. One is attached to the left arm, a second to the right arm, and the last one to the left leg. 7 The volunteer should sit quietly with their hands in their lap. Start the Software 1 Click the Windows Start menu, move the cursor to Programs and then to the iWorx folder and select LabScribe; or click on the LabScribe icon on the Desktop 2 When the program opens, select Load Group from the Settings menu. 3 When the dialog box appears, select AddedLabs.iws, and then click Load. 4 Click on the Settings menu again and select the ChestLe- adECG settings file. 5 After a short time, LabScribe will appear on the computer screen as configured by the ChestLeadECG settings. Exercise 1: ECG from the Chest Leads of a Resting Subject Aim: To record electrocardiograms from the six chest leads and determine the transition point. Procedure 1 The recording of electrocardiograms from Lead I and the six chest leads (V1 through V6) will be made with the Scope application of the LabScribe software. The settings file, ChestLeadECG, has configured the software to record a ten second sweep of data each time the Start button is clicked. Make sure the lead wires are attached correctly for recording from Lead I on Channel 1 and chest lead V1 on Channel 2. Note: While data is being recorded using the Scope, a progress bar is displayed on the LabScribe Main window; data will not appear in the window until the recording ends. 2 After the subject has been sitting quietly for at least one minute, click the Start button on the LabScribe Main electrocardiograms will appear after the recording ends. Click the AutoScale buttons on both channels to display the electrocardiograms in a suitable manner (Figure 6-1 on page 2). 3 After the electrocardiogram has been successfully recorded from chest lead V1, move the brown ?-2? lead wire from the active electrode for the V1 lead to the active electrode for the V2 lead. Have someone other than the subject move the lead wire because the subject must remain in the same position for each segment of the recording. Note: Before recording an ECG from the next lead position, it is important to let the electrodes equilibrate. Wait at least thirty seconds after the lead wire has been moved before recording. 4 Click the Start button on the LabScribe Main window to record another ten second sweep of data. In this particular sweep, electrocardiograms from Lead I and chest lead V2 are recorded. 5 Repeat Steps 3 and 4, moving the brown ?-2? lead wire from the active electrode of one chest lead to the next (V2->V3, V3->V4...) until electrocardiograms have been recorded from all six chest leads 6 Select Save As in the File menu, type a name for the file. Choose a destination on the computer in which to save the file (e.g. the iWorx or class folder). Click the Save button to save the file (as an *.iwd file). Data Analysis 1 Click the ScopeView icon (Figure 6-4 on page 3) to display the data from each sweep in the ScopeView window. Figure 6-4: The LabScribe toolbar. 2 To display the data from the chest lead channel, V(1-6) Leads, click on its name in the Channel pull-down menu on the upper left side of the ScopeView window. Note: If you followed the procedure for this exercise, the V1 chest lead should be displayed on Sweep 1, the V2 chest lead on Sweep 2, and so on. 3 Click on Sweep 1 in the Display list on the left side of the ScopeView window to display the recording from the V1 chest lead. Additional sweeps from other chest leads can be displayed on the same window by clicking on the sweep numbers as the Control key is held down (Figure 6-5 on page 4). 4 To take measurements from Sweep 1, select its number from the Value from pull-down menu. Then, select Title and V2-V1 from the Table Functions list on the left side of the Chapter 6: 3 window. The recording will proceed for ten seconds. The ScopeView window (Figure 6-5 on page 4). Figure 6-5: Electrocardiograms from V1 through V6 leads displayed at the same time in the ScopeView window. 5 Use the mouse to click and drag the cursors across the Sweep 1 to measure: ? the amplitude of three adjacent R waves in the ECG from Sweep 1 (V1 chest lead). The amplitude is the value, V2-V1, from the Function Table at the top of the ScopeView window. The amplitude is measured from the onset of the Q wave to the peak of R wave. ? the amplitude of three adjacent P waves in the ECG from the same selected sweep. The amplitude is the value, V2-V1, from the Function Table at the top of the ScopeView window. The amplitude is measured from the baseline, between the T and P waves, to the peak of P wave. ? the amplitude of three adjacent T waves in the ECG from the same selected sweep. The amplitude is the value, V2-V1, from the Function Table at the top of the ScopeView window. The amplitude is measured from the baseline to the peak of T wave. 6 Measure and record the amplitudes of three P, R, and T waves from each chest lead and one sweep of Lead I. 7 Calculate the average amplitudes of the P, R, and T waves for each lead. Enter these values into the table (Table 6-1 on page 4). 8 Find the transition point of the subject by examining the recordings from the six chest leads. Find the lead where the R wave is equal in magnitude to the S wave but in the opposite direction. 9 Data can be entered into the Journal by either typing the titles and values directly or by using the right-click menu while in the ScopeView window. Place the cursors to take measurements; then, select Add Title to Journal or Add Data to Journal from the right-click menu to add the measurements to the Journal Questions 1 From which leads are inverted P and T waves recorded? From which leads are inverted R waves recorded? 2 From which lead is the amplitude of the R wave equal, and opposite in direction, to the S wave? 3 Which lead is over the anterior edge of the interventricular septum? Exercise 2: ECG from the Chest Leads of Other Students Aim: To record electrocardiograms from the six chest leads of other subjects and determine their transition points. Procedure 1 Disconnect the leads from the first volunteer?s electrodes and place them on the electrodes of a second student. 2 Follow the procedures in Exercise 1 to record electrocardio- grams from the six chest leads of the new subject. 3 Repeat the procedures until all subjects have recorded their chest lead electrocardiograms. Data Analysis Make the same measurements as done in Exercise 1. Questions 1 Do the R waves, from the same lead position, go in the same direction for all subjects? The P waves? The T waves? 2 What would cause the same type of wave from the same lead to go in the opposite direction in different subjects? 3 Do all subjects have the same transition point? Which leads were found to be the transition points in the subjects tested? Table 6-1: The average amplitudes (in millivolts) for each of the waves recorded from each lead. Average Amplitudes P Wave R Wave T Wave V1 V2 V3 V4 V5 V6 Lead I Chapter 6: 4 What could cause a difference in the transition points? Stephen Andre 29-ChestLead ECGNL.fm