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Vital Signs Monitor
UConn BME 4900
Vital Signs Monitor
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Purpose
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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
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What can it measure?
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Electrocardiogram
(ECG)
 Three leads (Right
arm, left arm, right
leg)
Respiration
 Thermistor
Body Temperature
 NTC Thermocouple
What is an ECG?
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Records a time waveform of heart electrical
activity.
Used to diagnose heart problems
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Arrhythmia
Myocardial infarction
Conduction blocks (bundle branch block)
ECG Trace
Cardiac Electrical Activity
The ECG Waveform
Measuring Electrical Activity
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3 leads
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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
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Circuit consists of:
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3 op amp differential amplifier
Low pass filter (Cutoff: 0.05 Hz)
High pass filter (Cutoff: 160 Hz)
Gain Amplifier
Differential Amplifier
Feedback
Amplifier
Buffer
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What does the buffer do?
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Provides a high input impedance
What is the purpose of C2 and C3?
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Remove DC offset
Why?
Offset would be amplified by 1000x
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10 mV at 1000x gain = ~10V
Differential
Amplifier
Differential Amplifier
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V1 & V2 are in the
microvolt range
We need it to be in the
milivolt range so Rf/R1 is
set to ~1,000
1st Order LPF
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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
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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
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Measured with thermocouple placed in nostril
During exhalation warm air passes through nose
During inhalation cooler air is drawn in
Respiration
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What is a thermocouple?
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Two different metals joined together
Temperature changes induce a voltage
Voltage can be linearized to temperature
Respiration
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Non-inverting amplifier
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Gain = (1 + R2 / R1)
Multiplies thermocouple
voltage by gain
R2
R1
Body Temperature
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Often indicates infection
“Normal” temperature – 98.6ºF (37ºC)
Measured with negative temperature coefficient
(NTC) thermistor
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Resistance decreases as temp. increases
Body Temperature
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Wheatstone Bridge
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Rx = Thermistor
R1, R2, R3 = 4.7k Ohm
Differential amplifier
Body Temperature Waveform
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Voltage decreases over
time
Time shown at right ~90
seconds
Voltage at steady-state
can be converted to a
temperature
measurement
Microprocessor
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Provides Analog to Digital (A/D) conversion
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Waveform y(t) sampled at a fixed rate Δt
Voltage read every Δt and converted to a number
If resolution is 8 bits
8
 Gives us 2 = 256 counts over 5V range
 ~19.5 mV per count
Digital Isolator
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Isolates the patient from the computer
Receives data from microprocessor
Uses Giant Magnetoresistance for isolation
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Digital pulse induces a magnetic field across an isolated
barrier
RS232 Converter
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Input from isolator
Converts 5 V UART signal from microprocessor
to RS-232 standard
RS-232 standard
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+10 V – 10 V digital signal
Switching Between Signals
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Three signals that can be measured
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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
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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