The EKGBiomedical Engineering

The University of Connecticut

BME Program Director: Dr. John D. Enderle

Instructors:April DixonPete FlosdorfChris Liebler Laura RomonoskyBeth Showers

UConn Biomedical EngineeringWelcome to the University of Connecticut Biomedical Engineering lab. In this session, you will learn about an electrocardiogram machine (EKG). You will begin by learning about some background information on EKGs. From there, we will cover some basic electronics, such as resistors and capacitors and how they function. Then, we will show you how to build your own EKG, and how to test it out.

BACKGROUNDElectrocardiography

Electrocardiography is a method of monitoring and recording the electric currents generated during the alternating contractions of the atria and ventricles of the heart. The device used to monitor and record these signals is an electrocardiogram, or ECG for short. An ECG is commonly called an EKG, which is what we will refer to an electrocardiogram as from this point on. When using an EKG, electrodes are applied to the skin in places where the heart’s signals can be measured easily. Usually, these spots are between muscles on the upper arms and lower legs. Cables connect the electrodes to the EKG, where the electrical signal is turned into a waveform on a computer or a paper plot. The results produced from this machine allow doctors to observe the performance and condition of the heart as well as diagnose any problems they may find in the signal. A

normal EKG signal is shown in Figure 1.

The heart’s electrical system is quite complex. Electrical rhythms begin as impulses emitted from the sinoatrial (SA) node, also known as the heart’s “natural pacemaker.” The impulse then travels across a specific route, or pathway, moving through the atrioventricular (AV) node and into the ventricles. Once the impulse reaches the ventricles, it serves as a set of instructions, causing the heart’s chambers to contract in a routine and consistent manner. The path of this electrical signal, called the PQRST waveform (Fig. 2), may be followed through the heart in Figures 2 and 3. This path constitutes a single heartbeat. The EKG breaks down each

Figures 1 and 2: A normal EKG signal and the different segments of a single waveform respectively.

heartbeat into a set of three distinct waves: the P wave, the QRS complex and the T wave. These waves indicate behavior of the impulse at each location along its pathway. The P wave is associated with the spread of the impulse through the heart’s upper chambers (atria). The QRS complex and the T wave reflect the contraction and relaxation of the ventricles respectively.

What is an EKG used for?

If this set of rhythms is interrupted, delayed or sent down the wrong path, the heartbeat may become irregular, moving too fast or slow. These abnormal rhythms are produced if a patient has suffered a heart attack or heart disease. An EKG is used to detect these changes. EKGs may also be used if patients experience any of the following symptoms:

Angina (chest pain resulting from the heart not getting enough oxygen)

Palpitations (strong, fast or otherwise irregular heartbeat)

Arrhythmias (irregular, fast or slow heart rhythms)

Dyspnea (shortness of breath)

Syncope (lightheadedness or loss of consciousness)

Figure 3: A cross-section of the anatomical structure of the heart. Blue indicates passages bringing blood into the heart (oxygen poor); red indicates passages through which blood exits (oxygen rich).

Pericarditis (inflammation of the pericardium - a thin, fluid-filled sac surrounding the heart)

Long Q-T syndrome (a disorder that could lead to fainting (syncope) or sudden cardiac death)

Myocarditis (Inflammation of the heart muscle due to viral infection)

Certain congenital heart defects

Many people with coronary artery disease, heart valve disease or heart muscle disease will eventually have abnormal EKG readings. Because many EKGs are done while the patient is at rest, certain abnormalities that occur during periods of stress may not appear even in patients with significant disease. In fact, it has been estimated that the resting EKG is accurate only about 50 percent of the time. Because it is very common to see this false-negative result (i.e., the EKG doesn’t find the damage or abnormality that is really present), a normal EKG is not enough to rule out suspected heart disease.

You will have the opportunity to create a plot of your own EKG and analyze your heart rate using a real electrocardiogram machine, called the Siemens Burdick EK10. Instructions on how to operate the EKG are located at the end of this booklet in APPENDIX A.

After creating your own EKG in this project you will apply the electrodes to your arms and legs and observe your heart’s own signal on a computer screen.

BASIC ELECTRONICS

Before discussing the elements used to create a circuit, the nature of electricity should first be discussed. Current is known as the flow of electricity through is circuit. Resistance is the opposition to the flow current. Voltage refers to the amount of electrical force that must be used to move current through the circuit.

In the case of a circuit, electricity acts much like water in a pipe. In this analogy voltage is the pressure in the pipe, current is how fast the water flows through the pipe, and resistance acts like a valve that, only allows a certain amount of water to pass through the circuit. The circuit acts as the different pathways the water can take. Each of the different circuit elements acts to manipulate the “water” in different ways. To understand what is happening in the circuit, keep this analogy of water in mind as you read the following section.

Printed Circuit Board

A Printed Circuit Board, or PCB, is what connects all of the electrical components together to form a circuit. A normal PCB is constructed with a

thin sheet of a fiberglass substrate, which is an insulator. An insulator keeps the electricity from traveling down paths that it is not supposed to. The fiberglass substrate has solder covered copper lines called traces that conduct electricity between components. These traces can be on one or both sides of the fiberglass substrate. Components are always mounted on the top layer of the board and soldered on the bottom layer of the board. Some special PCB’s can contain layers of traces embedded in between the top and bottom layers, these are called Multilayer PCB’s and are usually found in electronic devices where space and weight are a large concern like cell phones, laptop computers, airplanes, and satellites. PCB’s are used in every electronic device.

Resistors

A resistor acts exactly like its name. It resists the flow of current through the circuit. As the strength of a resistor increases, it becomes more difficult for current to flow in the circuit. A color-coded band indicates the strength of each resistor. The unit of resistance is the Ohm (Ω). Our EKG will consist of six resistors with resistance values of 1K (1,000) ohm and 1M (1,000,000) ohm. The schematic symbol of a resistor is shown below in Figure 4.

Reading the Color Code

Color Code Chart:

Black Brown Red Orange Yellow Green Blue Violet Gray White

0 1 2 3 4 5 6 7 8 9

First find the tolerance band; it will be gold. Starting from the other end, identify the first band. Write down the number associated with that color from the color code chart on the previous page. In the example here, the blue band is 6. Now read the next color, here it is red so write down a '2' next to the six. Now read the third or 'multiplier' band and write down that number of zeros. In this example it is two, so we get '6200' or '6,200'. If the 'multiplier' band is Black (for zero) don't write any zeros down. This is the strength of your resistor!

Figure 4: The schematic symbol for a resistor

Capacitors

A capacitor is a circuit element that is used for storage. The capacitor itself is made of two conductor plates that are separated by an insulator (dielectric) such as air, glass or water. A capacitor is “charged” when one plate of the capacitor has more electrons than the other. The unit of capacitance is the Farad (F). Your project will include two capacitors, a 3.2 micro (0.0032) farad and a 1 pico (0.000000001) farad. A diagram of a simple capacitor and its schematic symbol are shown below in Figure 5.

Integrated Circuit Chips

Integrated Circuit Chips are usually called IC chips. An IC chip is a specific circuit that has been miniaturized to fit into a small package. There are thousands of different types of IC chips, each performing a different function in a circuit. One type of IC chip is an Operational Amplifier, which is usually referred to as an op-amp. In this project, the op-amp acts similarly to the volume control on your TV. It enlarges the power, current, or voltage of the circuit without physically changing the signal. In the case of the EKG, the “volume” of the heart’s impulse is being turned up so we can see it in the tracing. In Figure 6 below is a picture of what some typical IC chips look like. There is also a diagram, called a pinout, of an op-amp. In the pinout diagram each of the legs of the chip is labeled to indicate a specific connection it has with the rest of the circuit. Some of these connections are input, output, voltage source (Vcc+), and ground (Vcc-).

Figure 5: A model of a simple capacitor and the schematic symbol for a capacitor

Soldering Techniques

Soldering is the way that the components are connected to the traces on the PCB. This process can be tricky because bad connections will result in problems with your circuit. Therefore, soldering requires some concentration and patience. Here are some tips for better soldering.

Keep parts clean: grease, fingerprints, and dirt will keep solder from sticking properly.

Keep the soldering iron clean: clean the soldering iron by wiping it on a wet sponge. Make sure the iron is not blobbed with solder.

Keep your hands clean: solder contains lead, so it’s a good idea to wash your hands when you are done.

Heat parts: use the soldering iron to heat the parts. Touch the solder to the parts, not the iron. The hot parts melt the solder. Don't melt the solder with the iron directly because a blob of molten solder will not stick to your cold parts.

Amount of Solder: too little solder will not attach the parts, too much gets in the way and may touch other components.

Amount of Heat: you need to heat things up enough to melt the solder, but don't overheat components - most electrical components can only take a couple of seconds of heat.

Keep hands cool: remember, heat conducts along parts and wires...don't hold them in your hand. Use pliers, clamps, etc.

Cooling: things don't cool instantly. You need to hold the parts together a few seconds after removing the iron before you let go.

Wire-to-component: For things such as switches, there is often a little tab (often with a hole in it) to solder to. It is tempting to twist up the wire in and around the tab-hole and then heat and solder the

Figure 6: Some typical IC chips and op-amp pin out

whole mess. This usually produces a big messy blob that often doesn't conduct properly as it is hard to heat all that metal at once. It's better to pre-tin the wire and the tab (even if you fill the hole). Then heat the tab, stick the wire a short way into the hole (the solder plugging the hole will be molten) and heat the wire as well. A tiny bit more solder will fuse it all together. Keep in mind that large components take a long time to cool.

PCB: printed circuit boards are the easiest to solder. Push component/tab/wire through the hole. Lay the iron against both the wire and the pad for a second or two on one side and then touch the solder to the other side of the wire/tab and pad. The solder should melt and flow all around the wire/tab and pad. Be sure to hold components such as sockets firmly down to the board. To solder an IC socket down, do the two opposite corners first.

Instructions for Soldering

1. Solder the leads in place.

2. Obtain cone-shaped soldered joints.

3. Do not apply round solder joints. This will result in a bad connection.

4. Trim the excess wires up to the level of the solder.

EquipmentOscilloscope

An oscilloscope (right) is a machine that draws a graph of an electric signal. In most applications the graph shows how signals change over time, or time dependency. The vertical (Y) axis represents voltage and the horizontal (X) axis represents time. The Z axis in this case would represent the intensity of the signal. Figure 7 shows a sample-readout. This simple graph can tell you many things about a signal:

Specific voltage values per time.

Calculate the frequency of a signal.

Determine what portions of a signal are direct current (DC) and alternating current (AC).

Determine what portion of the signal is noise and whether the noise is time dependant.

For more information refer to APPENDIX B: Oscilloscope.

Figure 7: X, Y, and Z Components of a Displayed Waveform

Software

When viewing the EKG signal, software must be used to convert the electrical impulse into a visual representation that we can see and understand. There are different software packages for different applications.

LabView

LabView (Laboratory Virtual Instrument Engineering Workbench) is a software package developed to build programs with symbols (icons) rather than writing out lines and lines of text. LabView uses symbols, terminology and formats that are familiar to technicians, scientists, and engineers. LabView is programmed to act as an interface, helping pieces of hardware “communicate” with each other. LabView also has built-in libraries that allow the user to work over the internet and use different programming formats and systems.

LabView Applications

The applications to LabView are endless. In the past, multiple instruments were necessary to obtain the data a researcher wanted. In the case of the EKG, several oscilloscopes would be needed to view more than one heartbeat simultaneously. Instead, LabView has virtual instruments (VIs). These are programs that are built into LabView and perform the same function as another piece of equipment such as an oscilloscope. In our case we can view as many heartbeats as we want. We just need to specify how many channels we will need rather than use multiple oscilloscopes to do the same thing.

There are many advantages to LabView. Virtual instruments may be modified for each specific application. This means the user can manually add functions to their VI at any time. Furthermore, LabView can be “plugged” into the internet, so progress can be controlled from remote locations. In other words, someone in the U.S. could monitor and control the heat in their house in Russia!

Of course, LabView would be one component in an entire system. In our system, we have a circuit board, a power source (the batteries), a data acquisition board that collects the data, LabView, and a computer. These components can be categorized into one of the following groups that comprise the entire system.

Computer

The acquisition board

The supervised system itself

An example of another system is shown in the figure below.

Figure 8: A typical virtual system.

How does LabView look?

LabView has two different screens. There is a front screen is cosmetic. It actually looks like the front of an instrument. There are buttons, meters, gauges and whatever else the user needs to monitor the data. The second screen is known as the back screen. This is where all the icons are arranged and programmed. This screen is the control center for your LabView application. It is here where changes can be made to the function of the VI.

The back screen has two sub-panels known as palettes. These palettes are used to build the VI. The tools palette changes the function of the mouse. An arrow can be used to select different controls and place them on the screen. A finger might be used to press the buttons and turn the dials on the front screen. The cursor allows labels to be added to the program so it is better organized and clear to follow. The front panel has a control palette where different knobs, switches and monitors may be selected. The back panel has a functions palette that allows one to choose icons that make the VI work specifically. Figures 9 and 10 show the front and back screens respectively.

Computer with

Virtual instrument

System

Figure 10: The back screen with the tool and functions palettes.

Figure 9: The front screen with the tool and control palettes.

Figure 11 shows an example of a constructed front panel. This VI is a converter from Celsius to Fahrenheit temperature scales. The “Numeric” palette is one of the options within the control palette. To construct the instrument, simply drag the components from the palettes to the desired location.

In order to convert temperature from Celsius to Fahrenheit, we know that the following mathematical conversation must be used

DegF=(DegC)(1.8)+32

Therefore, this mathematical operation must be constructed in the back screen. Figure 12 shows the addition and product operations and constants taken from the numeric sub-palette and constructed on the back screen to build this conversion.

Figure 11: A sample front panel that monitors temperature conversion, the numeric palette where the thermometers and readouts are obtained.

This is only a small example of the capability of LabView. A more complex application will be used for the visualization of your EKG project. The application involves composite levels of programming such as for loops, do loops, and charts for the drawing of the signal on the computer screen.

Assembly InstructionsChecklist

When you receive your EKG kit, go through the parts list below and make sure your kit contains everything listed to complete your project. Record the contents of your kit by checking the box next to each part name. If you have trouble identifying a part, or a part is missing from your kit, let someone know, and they will help you.

QTY ITEMCheck

1 Black Project Box 1 Printed Circuit Board (PCB) 2 9 Volt Batteries 2 9 Volt Battery Holders

14 Wire Bundle - Black Brown White Red

4Single Wires: 2 Orange, 1 Black, 1 Green

2 IC Chips

Figure 12: Construction of the back panel using different mathematical operations and constants.

2IC Sockets

21K Ohm Resistors – Brown Black Red

41M Ohm Resistors – Brown Black Green

2 Capacitors: 1Green, 1Brown 4 Screws

4Banana Jacks: 2 Orange, 1 Black, 1 Green

1 Toggle Switch

Procedure

1. The circuit board will be assembled from left to right. First place the 3.2 uf (microfarad) capacitor into the C1 slot.

2. Bend both ends of the wire to the back, solder, and clip. Refer to the instructions in the Soldering section.

3. Place the 1 pf (picofarad) capacitor into the C2 slot. Solder and cut.

4. Insert the 1M ohm resistor into slot R1. To make insertion easier, bend the wire straight down on both sides of the resistor, and place in the holes. Solder and clip. Put the second 1M ohm resistor into R2, solder and clip.

5. Strip both ends of the black wire with the strippers. Insert into the hole labeled BLK, solder and clip.

6. Repeat Step 5 for both orange wires and the green wire. These holes are labeled OR and GR, respectively.

7. Insert sockets into the circuit board at slots IC1 and IC2. Solder two opposite corners first. To avoid damaging the chips do not insert them into the socket now. The heat from the soldering iron could ruin the chip.

8. Repeat Step 2 for the other 4 resistors. Put a 1K ohm resistor into R3, a 1M ohm resistor into R4, a 1K ohm resistor into R5, and a 1M ohm resistor into R6.

9. Insert the red wire of one of the battery straps into the hole labeled V1+. Solder and cut. Do the same for the black (ground) wire of the same strap into the V1-. Repeat for the second battery strap, using slots V2+ and V2-.

10. Before inserting the switch into the board wires must be soldered to it. Then place one wire into one of the holes labeled SW. Place the other wire into the other SW hole. Solder and clip each. Do not be concerned with polarity of the switch.

11. Now insert the IC Chips in the sockets at slots IC1 and IC2. Each chip has a left and a right side. The left side has a small black dot on the bottom corner. This must go over the shiny notch on the circuit board.

12. The circuit board is now complete. Place a piece of double sided tape , which we will supply individually, on the dot in the center of the box. Place it in the bottom of the black box. Put the screw in the corner of the PCB

13. Attach the jacks to the box in the color order orange, orange, green, black.

14. Tape the battery straps to the top of the box with the double-sided tape

15. Close the box with the four screws.

16. After completion we will instruct you on how to connect the EKG to the computer.

Have Fun!

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Appendix A:Using the “SIEMENS Burdick EK10” EKG Machine

The Basics of an Electrocardiogram

The electrical activity of the heart can be recorded at the surface of the body using an electrocardiogram. The electrocardiogram (EKG) is simply a voltmeter that uses up to 12 different leads (electrodes) placed on designated areas of the body.

Why is an Electrocardiogram Done?

This test will help the doctor to evaluate the patient's cardiac condition related to:

If a heart attack has occurred

What part of the heart was damaged

If there are any irregular heart beats or rhythm

If there is a decreased supply of blood and oxygen to the heart

What do the Waves and Intervals mean?

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P-Wave QRS. T-Wave.

If it is longer than .20 seconds, it may indicate first-degree heart block.SA Node fires. Atrial Depolarization Sinus initiated rhythm.

Depolarization of the ventricles. Ventricular Contraction. Atrial Repolarization. (Can't always be seen on EKG)

Ventricle Repolarization.May be affected by changes in serum K levels.

The P wave is caused by atrial depolarization. The P wave is usually smooth and positive. The P wave duration is normally less than 0.12 sec.

The PR interval is the portion of the EKG wave from the beginning of the P wave (onset of atrial depolarization) to the beginning of the QRS complex (onset of ventricular depolarization). It is normally 0.12 - 0.20 seconds.

The PR segment is the portion on the EKG wave from the end of the P wave to the beginning of the QRS complex. The PR segment corresponds to the time between the ends of atrial depolarization to the onset of ventricular depolarization. It is an isoelectric segment, during which the impulse travels from the AV node through the conducting tissue (bundle branches, and Purkinje fibers) towards the ventricles.

The QRS complex represents the time it takes for depolarization of the ventricles. - Due to ventricular depolarization. The normal QRS interval range is from 0.04 sec - 0.12 sec measured from the first deflection to the end of the QRS complex.

The ST Segment represents the period of ventricular muscle contraction before repolarization. The ST segment is normally isoelectric (no electrical activity is recorded). However, the ventricles are contracting.

The QT interval begins at the onset of the QRS complex and to the end of the T wave. It represents the time of ventricular depolarization until ventricular repolarization.

The T wave due to ventricular repolarization. The wave is normally rounded and positive.

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EKG Procedure:

Remove one limb sensor, (sticky pad) and place it on your right bicep, between your bone and muscle.

Repeat step 1 for left arm.

Remove a limb sensor and place it on the inside of your right calf between your bone and muscle.

Repeat step 3 on left leg.

Attach lead LA to the tab of the limb sensor on your left arm.

Attach lead RA to the tab of the limb sensor on your left arm.

Attach lead RL to the tab of the limb sensor on your right leg.

Attach lead LL to the tab of the limb sensor on your left leg.

Press auto button.

Press 1 or 2, and use the EKG that looks most like the model EKG.

Press the stop button when satisfied with your reading.

Calculating Heart Rate

You can measure your heart rate from your EKG print out.

Figure A-1: Model EKG format, showing P wave, QRS complex, and T wave.

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Take a ruler and measure in centimeters from the peak of a QRS complex to the peak of an adjacent QRS complex of to adjacent waves.

Convert centimeters to millimeters by multiplying this number by ten.

Divide that number by 50 mm/s.

Multiply this number by 60. That will give you your heart rate in beats per minute.

Fill out the back of the EKG paper with your general information. The number you calculated in part 4 can be placed as “Rhythm.”

On the reverse side of the given paper, peel off the stickers and place you r four best EKG readings on it.

EKG Paper

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APPENDIX B:

Oscilloscope

Electronic equipment can be divided into two types: analog and digital. Analog equipment works with continuously variable voltages, while digital equipment works with discrete binary numbers that may represent voltage samples. For example, a conventional phonograph turntable is an analog device; a compact disc player is a digital device. The oscilloscope we will be using is capable of both analog and digital signals.

The generic term for a pattern that repeats over time is a wave - sound waves, brain waves, ocean waves, and voltage waves are all repeating patterns. An oscilloscope measures voltage waves. One cycle of a wave is the portion of the wave that repeats. A waveform is a graphic

representation of a wave. A voltage waveform shows time on the horizontal axis and voltage on the vertical axis.

Figure B-1: Common Waveforms

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You can classify most waves into these types:

Sine waves

Square and rectangular waves

Triangle and sawtooth waves

Step and pulse shapes

For our purposes, we will be using sine waves and square waves. The sine wave is the fundamental wave shape for several reasons. It has harmonious mathematical properties - it is the same sine shape you may have studied in high school trigonometry class. Test signals produced by the oscillator circuit of a signal generator are often sine waves. The square wave is another common wave shape. Basically, a square wave is a voltage that turns on and off (or goes high and low) at regular intervals. It is a standard wave for testing amplifiers - good amplifiers increase the amplitude of a square wave with minimum distortion. Television, radio, and computer circuitry often use square waves for timing signals.

If a signal repeats, it has a frequency. The frequency is measured in Hertz (Hz) and equals the number of times the signal repeats itself in one second (the cycles per second). A repeating signal also has a period - this is the amount of time it takes the signal to complete one cycle. Period and frequency are reciprocals of each other, so that 1/period equals the frequency and 1/frequency equals the period. So, for example, the sine wave in Figure 7 has a frequency of 3 Hz and a period of 1/3 second.

Voltage is the amount of electric potential (a kind of signal strength) between two points in a circuit. Usually one of these points is ground (zero volts) but not always - you may want to measure the voltage from the maximum peak to the minimum peak of a waveform, referred to at the peak-to-peak voltage. The word amplitude commonly refers to the maximum voltage of a signal measured from ground or zero volts.

Figure B-2: Frequency and Period

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The volts per division (usually written volts/div) setting varies the size of the waveform on the screen. A good general purpose oscilloscope can accurately display signal levels from about 4 millivolts to 40 volts. The volts/div setting is a scale factor. For example, if the volts/div setting is 5 volts, then each of the eight vertical divisions represents 5 volts and the entire screen can show 40 volts from bottom to top (assuming a graticule with eight major divisions). If the setting is 0.5 volts/div, the screen can display 4 volts from bottom to top, and so on. The maximum voltage you can display on the screen is the volts/div setting times the number of vertical divisions.

**IMPORTANT--Setting the Oscilloscope Controls**

Some oscilloscopes have an AUTOSET or PRESET button that sets up the controls in one step to accommodate a signal. If your oscilloscope does not have this feature, it is helpful to set the controls to their standard positions before taking measurements.

Standard positions include the following:

Set the oscilloscope to display channel 1

Set the volts/division scale to a mid-range position

Turn off the variable volts/division

Turn off all magnification settings

Set the channel 1 input coupling to DC

Set the trigger mode to auto

Set the trigger source to channel 1

Turn trigger holdoff to minimum or off

Set the intensity control to a nominal viewing level

Adjust the focus control for a sharp display

Turn the three red knobs all the way to the right