Vital Signs Monitor UConn BME 4900

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.

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

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)

ECG Trace

Cardiac Electrical Activity

The ECG Waveform

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.

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

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 Feedback Amplifier Buffer 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

1 st 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

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

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

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

Respiration Non-inverting amplifier Gain = (1 + R 2 / R 1 ) Multiplies thermocouple voltage by gain R2 R1

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

Body Temperature Wheatstone Bridge R x = Thermistor R 1, R 2, R 3 = 4.7k Ohm Differential amplifier

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

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 2 8 = 256 counts over 5V range ~19.5 mV per count

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

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

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

Labview Software

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