BME 310 Lab 6 Electrocardiogram, John G. Webster 2/28/00

Introduction: Each time the heart beats, the contracting muscle generates potentials. The 1mV resulting signal can be picked up on the skin on the chest or limbs, using electrodes. The signal is known as the electrocardiogram (ECG). Continuously monitoring a single lead II, the physician can assess the rhythm and condition of the heart. Bradycardia is slow heart rate. Tachycardia is fast heart rate. Ventricular fibrillation indicates that the heart has stopped pumping and defibrillation is required to restart the heart. S–T segment depression indicates that the heart muscle is damaged (recent heart attack) or lacks oxygen (during a treadmill stress test). Using a few seconds of twelve different vector views of the electrical activity of the heart muscle, the cardiologist obtains a diagnostic ECG to more specifically diagnose heart disease.

Before the lab: Read material about ECG amplifiers from J. G. Webster (ed.), Bioinstrumentation, Section 2.2 at the coursepage http://www.engr.wisc.edu/cgi/courses/list/bme/310/webster/

Laboratory Equipment

1. ECG amplifier.

2. Signal generator.

3. Power supply.

4. Disposable electrodes.

5. ADC converter and PC.

6. Oscilloscope.

7. Coil that generates a 60 Hz magnetic field.

8. Burdick Eclipse 800 Electrocardiograph and manuals

At the Lab

1. An ECG amplifier is provided. A schematic diagram is provided but you are not required to understand the details of the circuit.

2. Turn on the power supply and adjust the voltage output to ±12 V, which is required by the circuit. Then turn it off. To prevent damage to the components, do not connect the wires while the power is on.

3. Connect the power supply to the amplifier properly. + output of power supply is connected to + 12 V on amplifier. – output to – 12 V. Connect the common port on the power supply to common on the amplifier.

4. Determine amplifier common mode gain Gc at 60 Hz by connecting the two inputs IN1 and IN2 together and driving them with common mode voltage vc from the signal generator low output. Connect signal generator ground to amplifier common. If you use vc = 2 Vp-p you may be overdriving the system. vc = 0.2 Vp-p may be better. By adjusting the CMRR pot, determine the maximal and minimal common mode gain Gc possible. Set the pot for minimal common mode gain and leave it at this setting for the remainder of the experiment. This step minimizes 60Hz interference from the power lines from appearing at the output.

5. Find the amplifier differential gain Gd at 60 Hz by connecting one of the two inputs to amplifier common. It is difficult to obtain a 1mV signal from a typical signal generator. Therefore we use a 100:1 attenuator. Feed the sinusoidal signal from the signal generator low output into the port of 100:1 IN and connect the output of 100:1 OUT to another input. Observe the amplifier output on an ac-coupled scope. Adjust the input amplitude to yield a sine wave at the output. Determine the differential gain of the amplifier at 60 Hz.

6. Determine the amplifier frequency response by varying the frequency from midband to high and low frequencies. When the output observed on the scope is reduced to 0.707 of its value at 60 Hz, write down these high and low corner (cut-off, break) frequencies.

7. Attach electrodes to each wrist and ankle to obtain ECG lead II. Determine the connection of electrodes and amplifier inputs by yourself. Show the TA your connection.

8. Connect the output of the amplifier to one channel of the ADC converter. Connect the ground of the ADC converter to common on the amplifier. Run the program of “Biobench” and set up the proper channel.

9. Monitor your signal on the screen and try to record a satisfactory ECG signal. To obtain a satisfactory ECG signal, you may find it helpful to relax, sit still and stop breathing. Print data.

10. Clench your right fist and hold for several seconds. What happens? Why? Print data.

11. Tap and push on one electrode. What happens? Why? Print data.

12. Breathe deeply. What happens. Why? Print data.

13. Bring a coil that generates a 60 Hz magnetic field near the input leads. What happens? Why? Print data. There is a 60 Hz cancellation filter in BioBench. Just click on the filter and watch the effect of the filter.

14. Read the manual of the Burdick ECG machine.

15. Turn the unit from standby to on. Connect the leads LA, RA, LL, RL.

16. Learn how to use the keyboard (page 3-2). Learn the use of left/right, up/down keys and underlined letters. Enter subject name and other information.

17. Learn the program setup features (page 4-4, 4-11). Understand the corresponding LCD displays. Use 10 mm/s to conserve paper. Start with 5 mm/mV and 150 Hz.

18. Acquire an AUTO ECG (page 6-2, 6-10). The filter menu must be set at 60 Hz to remove the interference from the power-line. Adjust the gain so that there is no distortion in the waveforms.

19. Acquire leads I, II, III, aVF, aVR, aVL.

20. Repeat procedure 10 to 13 with the Burdick ECG machine.

Results.

1. From 4, give the maximal and minimal common mode gain Gc.

2 From 5, give the differential gain Gd.

3. Calculate maximal and minimal amplifier CMRR (Common Mode Rejection Ratio) = Gd/Gc. Show your work.

4. From 6 indicate frequency response.

5. From 9 provide the recorded data of the ECG signal of lead II. Label P, Q, R, S, and T waves.

6. From 10 provide the recorded data of the ECG signal of lead II and explain results.

7. From 11 provide the recorded data of the ECG signal of lead II and explain results.

8. From 12 provide the recorded data of the ECG signal of lead II and explain results.

9. From 13 provide the recorded data of the ECG signal of lead II and explain results.

9. From 13, describe the effect of the filter.

10. In 20, do you get the same results as in 10 to 13? Explain any similarities or differences.