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ECG_lab_S2010

Page history last edited by Julia Gouvea 12 years, 7 months ago

© Catherine H. Crouch, permission to distribute for educational purposes with proper attribution.

 

Laboratory 5:

ELECTROCARDIOGRAPHY

 

 

Purpose

To observe the potential differences on the surface of the human body generated by the human heart by measuring an electrocardiogram (ECG)

To gain further familiarity with measuring potential difference (“voltage”) and with potential differences in the field of an electric dipole.

 

Introduction

In this lab, you will measure your own (or your lab partner’s) electrocardiogram: the time-dependent potential difference that the heart generates between selected points on the surface of the body. This measurement allows determining the components of the time-dependent electric dipole moment of the heart, which results from the discharging of the cell membranes of the cells making up the heart muscle.

A leading interventional cardiologist says that the electrocardiogram is the “among the most valuable clinical tests available to medicine because it is quick, completely safe, painless, inexpensive, and rich in information.” Today you’ll learn a little bit about such measurements.

Why does the heart have a time-dependent electric dipole moment? The heart is made up of muscle cells. When resting (not contracting), these cells have oppositely charged layers of ions on either side of the cell membrane. When contracting, these cells have specialized proteins crossing their membranes called “ion channels” that open up pathways that allow positive ions to flow across the membrane inward, changing the potential difference across the membrane. This process in a capacitor would be called discharging; the physiological term is “depolarization”. The contraction travels from cell to cell, so the edge of the depolarized region likewise travels across the heart muscle.

The traveling edge of the depolarized region moves as the depolarization spreads across the heart’s muscle as it undergoes its regular cycles of contraction and pumping. The traveling edge is equivalent to a moving electric dipole.1It’s this moving electric dipole that is observed with the electrocardiogram. Any changes in heart function, such as diseased regions of the heart muscle that fail to contract properly, will change the moving dipole moment—hence its diagnostic utility.

Cardiologists and cardioelectrophysiolologists obtain a great deal of information from the electrocardiogram, much of which relies on detailed understanding of the heart’s normal function and physiology. In today’s lab we will focus on just one item of the standard ECG analysis: identifying the direction of the heart’s peak dipole moment within the “frontal plane” (plane parallel to the front of the torso).

 

Experimental Setup

Each ECG sensor box has three wires with clips on the end, red, black, and green. The sensor measures the potential difference between the green and the red wires, with the red lead taken as the positive side. In other words, if the red wire is at higher potential than the green, the reading will be positive; if the red is at lower potential than the green, the reading will be negative. The black lead serves as the reference point of potential. (This is equivalent to the “ground” terminal on the power supplies.) The black wire is used as a reference so that multiple sensors can be used simultaneously, all with the same zero.

Each sensor thus measures a single potential difference. In clinical electrocardiography, twelve potential differences are measured (if you’re curious, some sample clinical ECG data are available for you to look at). In this lab we measure just two, to determine the approximate dipole moment of the heart in the plane shown in Figure 1. For all measurements, the black reference wire is attached to the right ankle.

Clinically, each potential difference is referred to as a “Lead”. We will measure two potential differences called “Lead I” and “Lead II” by cardiologists, shown in Figure 1 (drawn as if you are the doctor looking at the patient). The + and – mark where the positive and negative sides of the potential difference should be measured, so + corresponds to where to attach the red wire of our sensors and – to the green.

Figure 1: Two electrocardiography measurement configurations.

Lead I measures the potential difference between two points on an approximately horizontal line. Consequently, as discussed in the “ECG Analysis” handout, the Lead I signal reflects the x-component of the dipole moment of the heart. Because of how the heart’s electric field spreads to the limbs, the line between the Lead II points is approximately 60 degrees to the horizontal. As described in the “ECG Analysis” handout, the nature of the dipole electric field allows you to combine Lead I and Lead II to determine the vertical (y-component) of the dipole moment of the heat.

 

Figure 2: Angular relationship between the two voltage measurement configurations.

The ECG setup measures the potential difference between the red and green leads as a function of time. A data point can be measured as often as every 5 ms. You will measure Lead I and Lead II potential differences vs. time simultaneously, and from these data determine the direction of the peak dipole moment of the heart. An example of clinical Lead II data are shown in Figure 3.

Figure 3: Clinical Lead II ECG data

 

 

Exploration:Single lead ECG measurement:

The ECG measurements are made using LoggerPro. Connect two ECG sensors to the Ch1 and Ch2 inputs on the USB interface box. The sensor should be automatically recognized by the computer when you launch the LoggerPro software. Under Experiment -> Data Collection, set the sampling rate to 500 samples/second, so that a data point is measured every 0.002 s. (Ignore Logger Pro’s warning that this is faster than recommended.) Set the experiment duration to 3 seconds.

To measure potential differences on the surface of the body, we put sticky gel electrodes on the skin of the “patient,” and then clip the wires from the sensor onto the black tabs on the electrodes. The sensor therefore measures potential differences between the electrodes. Before attaching the electrodes, use alcohol wipes or soap and water to clean the skin where they are to be attached, since skin oils will degrade the electrical contact.

One member of the lab group (the “patient”) should attach electrodes in the four positions (right and left ankles and right and left shoulders/upper arms) and then connect the ECG sensors in the Lead I and Lead II configurations (Figure 1). Zero the sensors in Logger Pro and then measure an ECG (press the “play” button in the toolbar or choose Experiment -> Start Collection).

Your Lead I or Lead II data (or both) should look something like those shown in Figure 3. The repetition in the signal reflects the heart’s repeated contraction.

Figure 4 shows a schematic of a single cycle with the different features labeled using electrocardiography nomenclature. The peak labeled P corresponds to the spreading of the depolarization across the smaller chambers of the heart (the atria), and is typically very small because the atria include only a small fraction of the muscle mass of the heart (see heart image at end of lab). The interval from Q to S, called the “QRS complex,” comes from contraction of the large chambers (ventricles) of the heart; these chambers accomplish most of the pumping. On at least one of the leads, the QRS complex is the largest feature in the ECG. Finally, the T peak is associated with the repolarization, or recharging, of the ventricles in preparation for the next cycle. (Repolarization of the atria occurs during the contraction of the ventricles and is hidden by the QRS complex.) (If you would like to know more about the interpretation of the signal, your lab instructor can provide notes from a medical school lecture on electrocardiography.)

 

 

 

 

 

P QRS T

Figure 4. Schematic of a single cycle in an electrocardiogram.

 

The voltage level when the P, QRS and T features are not present is called the baseline. Zeroing the sensors should bring the baseline close to zero but it will not be exactly zero.

Q1: What happens to the ECG if you switch which electrode is attached to the green wire and which to red? (Try it — you’ll need to record a new set of data.) Explain your observations briefly. (Clinically this is referred to as “lead reversal.”)

 

Affix a printout of your two-lead ECG data (with the electrodes attached properly) in your lab book, and label the overall voltage amplitude and time duration of the QRS complex on each of the two measurements. The amplitude on at least one lead should be a few mV and the duration should be 60-80 ms; if yours are very different, check with your lab instructor to be sure you have the experiment configured correctly. (In LoggerPro you can measure voltages using the cursor; note that the values pointed to by the cursor show up at the bottom of the plot itself at right, notin the large colored Ch1 value displayed below the data spreadsheet at left.)

 

Q2: For which of the two lead configurations is the amplitude of the QRS complex greatest? How do the two signals differ or resemble each other?

 

 

Analysis:Calculating and plotting the time-dependent electric dipole moment of the heart

 

The following analysis will be done on your numerical ECG data. Before starting this analysis, make sure you have a good data set; retake the data if anything looks strange. Check that the baseline level of the Lead I and Lead II data (before start of P wave) is close to zero; if either has a nonzero baseline, either rezero the sensors and record new data, or create new columns of data and subtract the baseline value from the data.

The equivalent electric dipole moment of the heart, p, is a vector; it can be written as the sum of components, px, py, and pz. As discussed on the “Analysis” handout, pxand pyare proportional to the Lead I and Lead II - (Lead I)/2 measurements respectively. (We can’t easily measure the third component of the dipole today, since you’d have to take off your shirt to measure the voltage from front to back!).

The x-component of the heart’s dipole moment, px, is simply proportional to the zeroed Lead I potential difference, or “horizontal” potential difference, as shown in the analysis writeup. The y-component is proportional to the “vertical” potential difference, Lead II – (Lead I)/2:

Horizontal:

Vertical:

 

Calculate DVvert= Lead II – (Lead I)/2 in a new column in Logger Pro (Data -> Calculate Column).

Make a new graph in Logger Pro displaying DVhorizand DVvertvs. time.

On the graph, select only a single cycle of the heartbeat as in Figure 4 (starting just before the P wave). This will select the corresponding data in the data table. Then put the cursor on the data table, copy the selected data, and paste them into a blank Excel spreadsheet. You should end up with a time column, the DVhorizdata, another time column, and then the DVvertdata, corresponding to the range you selected. Delete the second time column so that you have three columns, time, DVhoriz, and DVvert. Also delete the headers so you’re your sheet contains only data. Check that you have the right data, then save the Excel file in .csv format with a filename that has no spaces.

 

To calculate the peak dipole moment magnitude and direction:

You previously identified which of the Lead I and Lead II gives the biggest amplitude signal of the QRS complex. At the instant when this lead is at its peak, find DVhoriz, and DVvert. This instant roughly corresponds to the overall peak.2Now use these values to estimate the magnitude and direction of the equivalent electric dipole moment of the heart at this peak. Make a reasonable estimate for the distance rfrom the center of the chest cavity to the point where the potential is measured. The dielectric constant of water kwateris about 80 and e0= 8.85 x 10-12C2/(N•m2).

From your estimates for peak pxand py, determine the magnitude and direction of your heart’s peak equivalent electric dipole moment in the frontal (xy) plane. You should find a value on the order of 10-13Cm, but as this is just an estimate, yours might be somewhat different.

Add a vector showing the orientation of your peak dipole moment on the chest to the figure at the right, and tape this into your notebook. How does its direction compare with Figure 5 showing the path followed by the heart’s dipole moment? (Note: Among normal humans, this direction varies from 30° above the +x direction to the +y direction, as defined on the figure.)

 

 

To display your dipole moment data using the Mathematica animation:

The Mathematica notebook provided on the lab computers (called ecg.nb) takes as input a .csv (comma separated values) file with three columns: time, time, DVhoriz, and DVvert. When you have your data file ready, open the Mathematica notebook. Put the correct path and file name for your data file into the first line:

data = Import[]

by putting the cursor between the square brackets and then using the Insert -> File Path command to select your data file in the dialog box. Then choose Evaluation -> Evaluate Notebook, which may take a little while. Play the animation by pressing the “play” button. The animation shows an arrow representing the dipole moment and also keeps a trace of the position of the peak of the arrow, so that the path followed by the tip of the dipole moment is visible. It should look something like Figure 5!

 

Figure 5: Example of path followed by the tip of the cardiac dipole moment during a cardiac cycle. The vector is shown at the peak instant pointing to R. From Benedek & Villars, Physics with Illustrative Examples from Medicine and Biology, AIP Press-Springer, 2000.

To Include in your Lab Notebook

Printout of the ECG data from the two simultaneous measurements

Answers to exploration questions

Calculation of the magnitude and direction of heart’s dipole moment at the peak signal, sketch of the direction of the dipole vector on a chest, and how it discussion of how agrees with the figure below

Printout of finished animation showing path taken by dipole moment

 

 

Appendix: schematic of heart chambers and path of depolarization.

Atria are small upper chambers, ventricles are large lower chambers.

 

 

 

1 To be precise, usually “electric dipole” refers to a charge dipole, a pair of positive and negative charges of equal magnitude separated by a small distance. The traveling edge is actually a current dipole rather than a charge dipole, and the human torso is a good conductor. It turns out that the electric field produced by a current dipole in a conductor is exactly like the electric field produced by the familiar electric charge dipole in a vacuum or dielectric medium. Your instructor can discuss this further with you if you are interested. Practicing electrophysiologists typically describe the traveling edge as a charge dipole and therefore we will do the same for the purpose of this lab.

2 If you want to be more precise, you can calculate a column of DVhoriz2 + DVvert2 and look for the instant where the value is largest.

 

 

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