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Student Lectures 2008. Patrick Carena. Introduction . Patient Monitoring – are the numbers sensible? Sensors and devices used in patient monitoring. Very brief look at incidents. Typical physiological measurements : Temperature Pulse rate Pressure Weight Fluids Gases.
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Student Lectures 2008 Patrick Carena
Introduction • Patient Monitoring – are the numbers sensible? • Sensors and devices used in patient monitoring. • Very brief look at incidents
Typical physiological measurements : • Temperature • Pulse rate • Pressure • Weight • Fluids • Gases
Sensor/Equipment Essentials • Accurate • Repeatable • Standardised • Meaningful • Calibration
Accurate • Is the information correct and reliable. • Is there enough information to enable a judgment to be made.
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.
Standardised • Units of measurements • Use a known output language/graphics • Equipment layout • Patient connections
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
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
Translation to standard values. • 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.
Simple sensors • Many materials physical size varies with temperature. • Metals have a large temperature coefficient of expansion - in particular mercury.
Problems: It has got mercury in it.It is slowIt does not automatically record • Plus points:It is accurateIt is fairly simple to makeIt is very cheap to produce.
Substitute the mercury • Galinstan mixture of Gallium, Indium and Tin • It is liquid between -15ºC and +1300ºC.
Resistance wire • Metals vary there dimensions with varying temperature. • Basic formula is T = (Rt/Ro – 1)/ , where Ro is resistance at 0C 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.
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
Thermocouples • Discovered by Thomas Johann Seebeck about 1821.
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.
RET-1Rectal 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. • OT-1For 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.
Thermistors • 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
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.
This one uses mirrors • This one uses phosphorescent and a mirror as the sensor.
IR Sensors • Tympanic thermometers. Thermopile detector to view IR from the tympanic membrane.
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.
Measuring Pressure • Consider the following simple transducer made up of a cylinder with a flexible diaphragm D at one end. • Transducer – a device which translates from one physical quantity to another.
How to measure an applied pressure P – add a pressure scale. • Can this into an electrical transducer – resistance change.
Or capacitance change • Or inductance, optical, etc.
Stain gauges Tomlinson 1876-77 • Resistance of metal give by R = rL/A in ohm • r is r = conductor’sresistivity, 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.
Other transducers • Simple optical system
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
Elastic Resistance Strain Gauges • These are made by having elastic tubes filled with conducting fluid - mercury!!, electrolyte or conductive paste.
Non electrical devices • 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!!
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.
NIBP • The Oscillometric method used in automated non invasive blood pressure monitoring.
Defibs • Ancient history • ~AC simply a big switch, a timer and a step up transformer.
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.
Add some more to remove the peak and we now have a square wave output
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.
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 • Note the equipment does not have to be medical equipment.
