Home Vital Signs Monitor for Daily Health Tracking

1. Vital Signs MonitorUConn BME 4900

2. Vital Signs Monitor • Purpose • As the population ages, many people are required by their doctors to take vital signs on a daily basis. • Developed for the average person to use at home. • Vital signs transmitted to a computer and ultimately to a doctor.

3. Measurements • What can it measure? • Electrocardiogram (ECG) • Three leads (Right arm, left arm, right leg) • Respiration • Thermistor • Body Temperature • NTC Thermocouple

4. What is an ECG? • Records a time waveform of heart electrical activity. • Used to diagnose heart problems • Arrhythmia • Myocardial infarction • Conduction blocks (bundle branch block)

5. ECG Trace

6. Cardiac Electrical Activity

7. The ECG Waveform

8. Measuring Electrical Activity • 3 leads • 2 arm • 1 leg • As cardiac muscle depolarizes it creates a potential on the skin • Potential measured as a vector • Difference between right and left electrode measured with a differential amplifier.

9. Measuring Electrical Activity • Circuit consists of: • 3 op amp differential amplifier • Low pass filter (Cutoff: 0.05 Hz) • High pass filter (Cutoff: 160 Hz) • Gain Amplifier

10. Differential Amplifier Feedback Amplifier Buffer Differential Amplifier • What does the buffer do? • Provides a high input impedance • What is the purpose of C2 and C3? • Remove DC offset • Why? • Offset would be amplified by 1000x • 10 mV at 1000x gain = ~10V

11. Differential Amplifier • V1 & V2 are in the microvolt range • We need it to be in the milivolt range so Rf/R1 is set to ~1,000

12. 1st Order LPF • The ECG is known to be a low frequency signal • A LPF can be used to remove the high frequency noise in the signal

13. High Pass Filter • ECG frequency range ~0.05 Hz – 150 Hz • Why do we need to eliminate frequencies <0.05 Hz? • Avoid distortion of the ST wave

14. Respiration • Measured with thermocouple placed in nostril • During exhalation warm air passes through nose • During inhalation cooler air is drawn in

15. Respiration • What is a thermocouple? • Two different metals joined together • Temperature changes induce a voltage • Voltage can be linearized to temperature

16. Respiration • Non-inverting amplifier • Gain = (1 + R2 / R1) • Multiplies thermocouple voltage by gain R2 R1

17. Body Temperature • Often indicates infection • “Normal” temperature – 98.6ºF (37ºC) • Measured with negative temperature coefficient (NTC) thermistor • Resistance decreases as temp. increases

18. Body Temperature • Wheatstone Bridge • Rx = Thermistor • R1, R2, R3 = 4.7k Ohm • Differential amplifier

19. Body Temperature Waveform • Voltage decreases over time • Time shown at right ~90 seconds • Voltage at steady-state can be converted to a temperature measurement

20. Microprocessor • Provides Analog to Digital (A/D) conversion • Waveform y(t) sampled at a fixed rate Δt • Voltage read every Δt and converted to a number • If resolution is 8 bits • Gives us 28 = 256 counts over 5V range • ~19.5 mV per count

21. Digital Isolator • Isolates the patient from the computer • Receives data from microprocessor • Uses Giant Magnetoresistance for isolation • Digital pulse induces a magnetic field across an isolated barrier

22. RS232 Converter • Input from isolator • Converts 5 V UART signal from microprocessor to RS-232 standard • RS-232 standard • +10 V – 10 V digital signal

23. Switching Between Signals • Three signals that can be measured • Only one is recorded at a time • A mode switch is used to select which signal is processed • When a mode is selected a flag is sent over the serial port • Flag indicates which mode • Labview reads flag and changes modes

24. Labview Software

25. Labview Requirements • Receive serial data from microprocessor • Read mode flag and switch to appropriate screen • Display EKG, and Respiration waveforms • Calculate respiration rate (number of peaks / time) • Read voltage from thermometer and convert to body temperature