Electrocardiogram (ECG) – I |
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Electrocardiogram (ECG) – I |
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Table of Contents
Educational Objectives
Understand the origin of electrocardiogram (ECG).
1. Anatomy and function of the heart
The heart serves as a four-chambered pump for the body's blood circulatory system. The four chambers are names as: left atrium, left ventricle, right atrium and right ventricle. The following diagram shows the route of blood circulation (where the red color indicates the oxygen-rich arterial blood while the blue color indicates the oxygen-poor venous blood):
Figure 1 – Blood circulation.
The main pumping function is supplied by the ventricles, and the atria are merely antechambers to store blood during the time the ventricles are pumping. A complete heart cycle is divided into two phases: systole and diastole. Systole refers to the contractile or pumping phase; and diastole refers to the resting or filling phase.
The rhythmic contraction of the atria and ventricles has an underlying electrical precursor in the form of a well-coordinated series of electrical events that takes place within the heart. This series of electrical events originates in the sinoatrial (SA) node which is located at the junction of the superior vena cava and the right atrium. The SA node acts as a pulse generator. Each impulse generated by the SA node leads to a heart beat. Therefore, in a normal heart, the heart rate (beats per minute) is determined by the period of the pulses generated by the SA node. The impulse generated by the SA node first spreads all over the muscles of the two atria, causing the contraction of the atria. After a short delay, the impulse spreads over the muscles of the two ventricles and cause their contraction. The ECG waveform recorded on the body surface is produced by the electrical activities associated with the muscles contraction and relaxation of the atria and ventricles.
2. Normal ECG waveform
The following figure shows two complete cycles of a normal ECG waveform.
Figure 2
– Normal ECG waveform.
P-wave is produced by muscle contraction of atria. R-wave marks the ending of atrial contraction and the beginning of ventricular contraction. Finally, T-wave marks the ending of ventricular contraction. The magnitude of the R-wave normally ranges from 0.1 mV to 1.5 mV. A narrow and high R-wave indicates a physically strong heart.
The R-R interval measures the period of heart beat. Its inverse is the heart rate:
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(1) |
where HR is the heart rate measured in beat-per-minute (bpm), R-R is the R-R interval measured in millisecond (ms). For example, if R-R is 800 ms, the heart rate is 75 bpm. The R-R interval should be relatively constant from beat to beat. A changing R-R interval indicates irregular heart rate.
The P-R interval is a measure of the time from the onset of atrial contraction to the onset of ventricular contraction. It normally ranges from 0.12 to 0.20 second. An abnormally prolonged P-R interval often indicates a special heart disease called "First Degree Heart Block". The R-T interval represents the ventricular systole (muscle contraction) and the T-R interval represents the ventricular diastole (muscle relaxation).
3. Standard limb leads
In clinical ECG measurements, four electrodes are attached to the four limbs: left arm (LA), left leg (LL), right arm (RA) and right leg (RL). The electrode on RL is usually grounded while the voltage drop between any two of other three electrodes are measured. In Lead I configuration, ECG is measured as the voltage drop from LA to RA. In other words, LA is connected to the positive input of the amplifier and RA is connected to the negative input of the amplifier. In Lead II configuration, ECG is measured as the voltage drop from LL to RA, and in Lead III configuration, ECG is measured as the voltage drop from LL to LA.
In this lab, you will use the following setup/circuit to measure the ECG:
Figure 3
– Experimental setup for measuring ECG.
AD620 instrumentation amplifier is an 8 pin Dual In-Line IC chip. The pins are labeled from 1 to 8 with pin 1 to the left of the small notch as shown on the right (top view). |
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1. Build the circuit
The circuit is built around a single-chip instrumentation amplifier AD620, manufactured by Analog Device. The AD620 is a low cost, high accuracy amplifier which requires only one external resistor to set gain of the amplifier. The gain of the amplifier is determined by the following formula:
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(2) |
Carefully insert the AD620 chip across the long groove that separates the holes on the board into two groups, so that the four pins on the left side of the chip (pin 1 to pin 4) are inserted into the four holes on the left side of the groove and the other four pins (pin 5 to pin 8) are inserted into the four holes on the right side of the groove, as shown in the following figure.
Figure 4
– A picture of the breadboard used to build the circuit.
As you have already learned, on each side of the groove, the five holes along the horizontal line are internally connected together. With the chip inserted as shown above, four holes are available for you to make connections to any particular pin of the chip. Also shown in Fig. 4 is a resistor which is connected to pin 1 and pin 8, and one end of a cable which connects pin 3 to the electrode on LL.
The input signal is connected to pin 2 and pin 3, and the amplified signal from pin 6 is first connected to a high-pass filter (C and R2), and then to Channel 1 of the oscilloscope. Make sure the input coupling of Channel 1 is set to DC. Do not use <Autoscale>. The Time/Div of the oscilloscope is set to 200 ms.
The +6V and -6V power supply will be obtained from the ±25V group of VHP E3631A Triple Output DC Power Supply.
Figure 5
– A picture of the power supply.
Turn on the power supply by pressing 'Power' button. While keeping output disabled (output 'Off'), turn the knob to adjust +25 V supply to +6 V and the current limit to 0.3 A, and adjust -25 V supply to -6V and the current limit to 0.3A. The terminal 'COM' should be connected to the ground of your circuit.
After completing the connections (except the wires to the electrodes), turn on the oscilloscope. Do not enable the output of the power supply yet.
2. Attach electrode
Have one student volunteer as the subject. Three electrodes will be attached to the following sites: the inner surface of right forearm near the wrist, the inner surfaces of left and right legs near the ankles. Prior to attaching the electrodes, the subject should swab the skin at each site of electrode contact with alcohol. Peer the electrode from the protective cap to expose the electrolyte gel. Place one edge of the electrode on the skin and press down; gently put the opposite edge of the electrode and press it down. To promote firm contact, smooth down the remaining adhesive area while pressing down firmly. Avoid pressing down on the center of the wet gelled area. Finally clamp the cable to the metal end of the electrode, as shown in the following figure.
Figure 6 – A picture of the electrode stuck on the arm with a cable connected.
Complete the connections between the three electrodes and the circuit (RA to pin 2, LL to pin 3, and RL to ground). The subject should remain calm and still during the measurement.
3. Monitor and measure ECG
Now, enable the output of the power supply. If everything is right, you should see the ECG waveform on the oscilloscope. You may need to adjust the time base (Time/Div knob) and gain (Volts/Div) of the oscilloscope to display 2 – 3 complete cycles of ECG waveform with adequate amplitude. Try to identify the P-wave, R-wave and T-wave. You may notice that the signal shown on the oscilloscope is much noisy than the one shown earlier in this lab manual. A major source of the noise is the 60 Hz power line interference that needs a special circuit to get rid off. In addition, if the subject moves his/her arm or body, the muscle activity will produce another kind of biopotential called electromyogram (EMG) which will show up in ECG waveform as a noise.
If you are satisfactory with the displayed waveform, store the waveform by pressing the storage key "stop". Then, measure the values of the following parameters:
a. The magnitude of the R-wave of the recorded ECG waveform
b. The R-R interval
c. The P-R interval
d. The R-T interval (the period of systole)
To improve the accuracy, measure the above parameters in each heart cycle, and then calculate the average values.
From the measured average values, calculate the values of the following parameters:
A. The magnitude of the original R-wave (divide the measured R-wave of the recorded ECG waveform by the gain of the amplifier).
B. The heart rate in bpm.
Pre-lab
Assignment
What is the "systole" and what is the
"diastole"?
Describe the heart events associated with the P-wave, R-wave, and T-wave.
If the heart rate is 80 bpm, what is the R-R interval in ms?
What is the Lead Configuration used in Fig. 3?
Using Eq. (2) to determine the gain of the amplifier shown in Fig. 3.
