Transcript Document

Student Lectures 2008
Patrick Carena
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Introduction
Patient Monitoring – are the numbers sensible?
 Sensors and devices used in patient monitoring.
 Very brief look at incidents
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Typical physiological measurements :
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Temperature
Pulse rate
Pressure
Weight
Fluids
Gases
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Sensor/Equipment Essentials
Accurate
 Repeatable
 Standardised
 Meaningful
 Calibration
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Accurate
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Is the information correct and reliable.
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Is there enough information to enable a
judgment to be made.
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Repeatable
How well does the system respond to
environmental changes.
 The system should be tolerant to changing inputs
– high to low/low to high.
 Does the system age.
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Standardised
Units of measurements
 Use a known output language/graphics
 Equipment layout
 Patient connections
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Meaningful
Is the output from the device “3rd” party verified
does it have to be??
 Algorithms or hardware used to interpret the
measurement
 Frequency of update
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Calibration
Ensuring the system is interpreting the input
correctly and faithfully.
 Can the system be checked against a known
standard
 When was it last checked
 Has it ever been checked
 In a lot of cases calibration normally means
output check
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Translation to standard values.
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First why translate to standard values.
Example patients temperature
Use a hand – whose hand.
What terms to use – hand hot, patient is burning up,
patient is cold, seems OK to me.
Whose hand will be the standard for a definition check
What to write in the patient notes.
Can we prove that the patient’s temperature was OK.
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Simple sensors
Many materials physical size varies with
temperature.
 Metals have a large temperature coefficient of
expansion - in particular mercury.
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 Problems:
It has got mercury in it.
It is slow
It does not automatically record
 Plus points:
It is accurate
It is fairly simple to make
It is very cheap to produce.
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 Substitute the mercury
 Galinstan mixture of Gallium, Indium
and Tin
 It is liquid between -15ºC and
+1300ºC.
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Resistance wire
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Metals vary there dimensions with varying
temperature.
Basic formula is T = (Rt/Ro – 1)/ , where Ro is
resistance at 0C and  is the temperature coefficient
of the wire and T is the temperature and Rt is the
resistance at temp T.
If we have a thin long length and remembering that
resistance is proportional to the dimension of the
metal, and as we heat the metal up its dimension
change then we will get a change in resistance and
hence a measure of the temperature.
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Element
Metal
Temperature
Range
Benefits
Base
Resistance
TCR(Ω//°C)
Platinum
-260 to 850°C
Best stability, good
linearity
100 Ω at 0°C
0.00385
Copper
-100 to 260°C
Best linearity
10 Ω at 0°C
0.00427
Nickel
-100 to 260°C
Low cost, High
Sensitivity
120 Ω at 0°C
0.00672
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Thermocouples
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Discovered by Thomas Johann Seebeck about
1821.
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Output varies between 1µV/ºC and 20µV/ºC. Very sensitive and
stable electronics required.
Reference if used must be held at a stable and known.
Must ensure that the thermocouple effect being measured is the
correct one.
very stable 0.05ºC/ºC over the range 0ºC to 100ºC
Response times depend on size. For very small thermocouples
response times in milliseconds are possible.
Sizes of a thermocouple element can be as small as 5µm. Measure
the temperature rise in the eye due to laser treatment.
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RET-1
Rectal probe for humans, Flexible, vinyl covered, soft tipped.
Does not cause discomfort. Max Temp. 90°C (194°F). Time
constant 5.0 secs. 5 ft. lead. Isolated.
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OT-1
For fast reading oral use. Ball-tipped stainless steel shaft,
stainless handle. 5 ft. lead. Max Temp. 125°C (257°F). Time
constant 0.8 secs. Not isolated.
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Thermistors
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Made from oxides of various materials manganese,
cobalt, etc.
Thermistor is an acronym from Thermally Sensitive
Resistor
Two type Negative Temperature Coefficient, NTC,
and Positive Temperature Coefficient, PTC.
High sensitivity to temperature change
Only linear over a small temperature range
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Glass Bead Fast Time Response Probe
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Optical Temperature Sensor
Use special fibre optics
 Use fibre optics coupled with a prism
whose reflective index or shape
changes with temperature.
 Do it with mirrors and fibre optics.
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This one uses mirrors
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This one uses phosphorescent and a mirror
as the sensor.
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IR Sensors
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Tympanic thermometers.
Thermopile detector to view IR from the
tympanic membrane.
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Thermopile??
Quite simply it is a large number of
thermocouples placed in series on the
surface of a “black body” (heat
absorber).
 The trick with tympanic sensors is to
block external IR from the ear and only
look at certain IR frequencies.
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Measuring Pressure
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Consider the following simple transducer made up of
a cylinder with a flexible diaphragm D at one end.
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Transducer – a device which translates from one physical quantity to
another.
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How to measure an applied pressure P – add a
pressure scale.
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Can this into an electrical transducer – resistance
change.
P
R
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Or capacitance change
C
P
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Or inductance, optical, etc.
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Stain gauges Tomlinson 1876-77
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Resistance of metal give by R = rL/A in ohm
r is r = conductor’s resistivity, L is conductors length
and A is the conductors cross section area.
Stretch a piece of metal – length increases and cross
section area decreases – resistance goes up.
We could use this change in resistance to produce a
transducer.
S1
P
S2
S3
S x = S tr a i n G a u g e s
S4
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Other transducers
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Simple optical system
L i g h t In p u t
P
Ligh t Sen s o r
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Piezo electric materials – electrical characteristics
vary as there shape changes. Normally fabricated
using semiconductor type processes hence these
devices can be very small.
Custom Pressure Catheter sensor
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Elastic Resistance Strain Gauges
 These are made by having elastic tubes filled with
conducting fluid - mercury!!, electrolyte or conductive
paste.
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Non electrical devices
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Sphygmomanometer - use a a mercury column to
show the applied pressure in the bladder.
 Gold standard
 Easy to use
 No need for electrical
power
 Ensure column vertical
 Not dirty
 No leaks
 Mercury!!
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C. 1896 an Italian Scipione
Riva-Rocci developed a cuff
with an air filled bladder. He
could determine the systolic
pressure
It was a Russian Korotkoff
(c. 1905) who found that by
listening over the brachial
artery he could here different
sounds depending on the
blood flow. He sorted the
diastolic pressure.
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NIBP
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The Oscillometric method used in automated non
invasive blood pressure monitoring.
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Defibs
Ancient history
 ~AC simply a big switch, a timer and a
step up transformer.
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Next came the simple DC defib
Simply a big capacitor, a transformer and a switch.
Large peak current
Short time delivery
Need to deliver a high current over a period of time 5
to 30 msec.
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Add some components to shape the current/voltage
output.
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Add some more to remove the peak and we
now have a square wave output
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trapezoidal waveform
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Impedance matching
Patient impedance varies.
 Need to ensure that the energy
delivered to the patient is constant over
each shock.
 Measure the patient impedance prior to
shock and vary either the voltage or
current to the patient.
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Equipment incidents
When a item of equipment is involved in
 Causes an injury to somebody
 Nearly an injury to somebody
 If left may cause an injury
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Note the equipment does not have to be
medical equipment.
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Some pictures
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Nebulisers
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Cold Light Sources
The name is with reference to old
technology
 The light is produced more efficiently –
less wasted heat produced
 A 300 Watt light source gives about 50
Watt light output
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How hot??
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A small 30 watt soldering iron will attain
350ºC
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Short video
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Walking Frames
Very simple device.
 Who uses it.
 What happens if one fails?
 Have had a number of failures.
 Risk Alert issued a couple of weeks ago
– again.
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