Clinical Measurement Flashcards
Factors required in a measurement system and examples
Transducer - Thermistor, piezoelectric crystal
Transmission path - electrical or optical cable, length of tubing, infrared link
Signal conditioning unit
Display unit - analogue (oscilloscope, moving coil), digital (digital voltmeter, led display)
Storage unit - analogue (magnetic tape) digital (hard disc, cd, usb)
Static characteristics of a measuring system
Accuracy - closeness between obtained and true value
Sensitivity - relationship between changes in output reading and changes in measured quantity
Linearity - how in proportion changes in output readings are to changes in measured quantity
Hysteresis - property that produces error depending on whether measured quantity is increasing or decreasing
Drift - variation in reading not related to change in measured quantity
Example of measuring systems that exhibit non linear measuring
Rotameter
Examples of why measuring systems may exhibit hysteresis
Frictional losses
Slack
Elastic energy storage
Examples of why a measuring system may exhibit drift
Changes I; temperature
Unstable components
Dynamic characteristics of measuring systems
Step response - response to rapid change in measured variable, ideally instantaneous but practically has a lag time
Dampening - following a step input system may vary in response with variable patterns
Frequency response - what frequency range the system will respond to, tails of at either end with inadequate gain below the lower and above the upper cut off.
What are the damping responses to a step input
underdamped - ovevershoots and corrects resulting in oscillating around ideal measurement, overdamped - response does not reach ideal value in time frame permitted,
critically damped - optimum compromise reaching ideal value without oscillation
How is damping created in a system
Unintentionally or intentionally to control oscillation
Frictional effect on movement
Fluid - viscosity opposing motion
Electrical - electrical resistance opposing current passage
What is bandwidth
The frequency range between the high and low cut off frequencies
What causes distortion due to frequency response
any input is made up of component parts with separate frequencies. If the frequency response of the measuring system doesn’t cover all the component frequencies distortion occurs.
Or
A system is more sensitive to certain frequencies thus overemphasises certain components of the input
What would be the disadvantage of a measuring system that was equally sensitive to all frequencies from 0 to infinity
Would allow noise causing error and distortion
What determines the frequency response to a system
It’s inertia and compliance elements
What are natural frequencies or resonances
How to deal
Frequencies of a measuring system that appear as peaks in frequency response, cause signal distortion
Design system so natural frequency outside of measurement range or apply dampening appropriately
What are the voltage ranges of an eeg and an ecg
EEG 1-500microvolts
ECG - 0.1-50mV
Frequency range of eeg and ecg
Eeg 0-60hz
Ecg 0-100hz
How can electrical signals be described
As a voltage or current varying in time - amplitude of signal being range of variation
Periodic or non periodic - repeating pattern (eg sine wave or ecg) or not (eeg)
Analogue or digital - continuous in time or discrete increments
Frequency components - description by frequency spectrum
How can electrical signals be described by their frequency spectrum
Any continuously varying frequency can’t broke down down into a collection of sine and cosine waves (paired sine cosine at same frequency)
What is analysis of a frequency spectrums component parts called
Spectral or fourier analysis
How ca electrical signals be processed?
Amplification
Filtering
Spectral analysis
Analogue to digital conversion
Averaging
Why is amplification; necessary for signal processing
Low amplitude signals could be adversely effected by noise in transmission (low signal to noise ratio obscuring the signal)
How can electrical signal filtering work
Blockage of unwanted signals (noise) that is at a different frequency to the desired signal
Either:
Low pass filter - rejects signals above given threshold
High pass filter - reverse
Notch filter - rejects a specific filter - eg 50Hz produced by electrical mains
Other than filtering, how else can noise be removed from a signal
Averaging
If wanted signal is repetitive and noise is random it can be averaged out
What may influence differences between measured pressures and actual values
Transmission path (transducers rarely at the sample site)
Sampling site may not be equal to desired site (eg lung pressures may be different in lungs vs at end of ett)
Rapidly fluctuating pressure - exacerbates errors born from the transmission path
Types of pressure measuring device for gases or liquids
Aneroid gauge
Manometer
Piezoresistive strain gauge
How does an aneroid gauge work?
Mechanical
Measured pressure operates a mechanism connected to a pointer
Type of aneroid gauge
Specific Workings
Bourdon gauge
Measured pressure causes spinal tube to uncoil driving a pointer
Advantages and disadvantages of aneroid gauge
Advantages - simple, robust, convenient, no power supply, high or low pressures
Disadvantages - not suitable for very low pressures (<5cmH2O) hard to calibrate
How does a manometer work
Unknown pressure measured by comparing against pressure from column of liquid (usually water or mercury)
When is water used in a manometer
When is mercury used
Water for lower pressures
Mercury for higher pressure s
Source of error in a manometer
Effect
Surface tension between liquid and manometer tubing (water reads too high, mercury to low)
Advantages and disadvantages of a manometer
Advantages, simple, no need for calibration as measured directly ,,
Disadvantages - bulky
How does a piezoresistive strain gauge work
A semiconductor that varies electrical resistance when subjected to mechanical strain
A diaphragm is distorted by the pressure straining the semiconductor altering resistance.
Wheatstone bridge then used to calculate change
Advantages and disadvantages of piezoresistive strain gauge
Versatile, large range, electronic display and recording,
Needs power supply and processing unit, susceptible to electrical interference
A 10 cm change in arterial line tranducer results in what change in Bp
7.5mmHg
Indirect methods of measuring blood pressure
Manual occlusive cuff
Flush method
Automated occlusive cuff
Penaz technique
Continuous arterial wall tonometry
Doppler ultrasound
How does manual occlusive cuff Bp work
Inflate a cuff above sbp then deflate listening to sounds as pressure falls.
What are the korotkov sounds
1 tapping synchronous with pulse
2 softer tapping
3 more intense as diastolic point approached
4 muffled sounds just before DBP
5 loss of sounds
Advantages and disadvantages of manual occlusive cuff Bp monitoring
Advantages, simple, no power, patient contact
Disadvantages, operator dependant, time consuming
What is flush method of bp measurement
Usually neonates
Milk blood out of arm, inflate cuff, deflate until arm flushes, this is the sbp (though probs closer to map)
How do automated occlusive bp cuffs work
What is the bit of kit called
Use a Von Recklinhausen oscillotonometer
A cuff inflates then autodeflates slowly (2-3mmhg/s)
Vibrations in arterial wall produce pressure changes senses by an electrical transducer and analysed by a microprocessor calculating sbp, map and DBP
Vibrations are at maximum at map, 25-50% at sbp and 80% at DBP
Advantages and disadvantages of automated occlusive cuff bp measuring
Advantages - simple, hands free, non invasive, allow sampling when access difficult, auto calculates map, can have alarms built in, can data transfer
Disadvantages - less accurate at extremes (over reads low pressures, under reads high), inconsistent if pulse not regular, can cause nerve or tissue damage when frequency high
How does the penaz technique work for measuring bp
Uses an infrared plethysmograph to monitor artery diameter
Signal triggers inflation or deflation of attached cuff corresponding to map - as artery dilates in systole cuff pressure increases to keep diameter the same, and vica versa in diastole - cuff pressure is then displayed on screen as an arterial waveform
Advantages and disadvantages of penaz technique
Advantages - continuous, in normal vasculature corrolates well with invasive methods
Disadvantages - unreliable in pvd, needs regular calibration as tissue fluids relocated due to pressure, can cause discomfort or vascular damage if prolonged use
How does continuous arterial wall tonometry work
Issue?
Arm cuff inflated to 30
Changes I;pressure from arterial dilatation sensed by cuff and interpreted into bp
Not very reliable
How can Doppler be used for bp monitoring
Doppler over artery
Detects degree of movement in vessel wall
Size of movement is a measure of bp
Advantages and disadvantages for Doppler BP measurement
Advantages - non invasive, all ages
Disadvantages - needs accurate positioning, prone to movement artefact and distortion
Sources of error in indirect bp measurement
All tend to slightly under-read vs invasive
Korotkov sounds often beneath audible range and dependant on oral acuity of observer
Korotkov sounds flow dependant thus can be effected by high flow states etc
Cuff size WIDTH - too narrow overestimates, too wide underestimates
Pneumatic leaks
Calibration errors
Deflates to fast missing signals
How big should a bp cuff be?
In adults width 12-14cm (4-8 year old 9cm, 1-4 year old 6cm, neonate 3cm)
Should cover around 2/3 of length of upper arm
Rate of saline into an art line
1-4ml/hr
Features of art line
Short, paralel sided cannula
Saline flow to reduce clots
Short narrow-bore non-compliant plastic tubing
Piezoresistive strain gauge
What is the upper frequency cut off of an arterial line
20Hz
Why should art line tubing be narrow
Shifts natural frequencie/resonance in the saline above the cut off frequency
What sort of damping do art lines aim for?
Critical damping
How would an overdamped art line trace appear
Under reads sbp and over reads DPb, appears flattened and no dicrotic notch
How would an under damped arterial line trace appear?
Overreads SBP and underreads dpb
Extra artifactual notches on downslope
Advantages and disadvantages of art line
Advantages - continuous, gold standard
Disadvantages - difficult, infection haemorrhage, vascular damage
Sources of error in art line
Air bubbles - decreases resonant frequency increasing the damping
Cather over compliance - energy goes into distending catheter increasing damping
Blood clots - increases flow resistance increasing damping
Drift from zero point
When do we need to measure gas flows
To monitor gas input
To monitor tidal volumes and resp flow
To test patients pulmonary function
Examples of gas flow meters used in pulmonary function testing
Benedict Roth spirometer
Vitalograph
Wright respirometer
Dry gas meter
Electronic volume meter
Peak flow meter
What is a Benedict Roth spirometer
A bell which traps a closed volume of air over water. As air is either removed or added (inspiration or expiration) the bell rises or falls. This is sensed and can be transcribed with a pen giving a waveform
How does a vitalograph work
Collects expired gas into bellows, these expand moving a recording pen.
How does a wright respirometer work
Gas flow drives a spinning vane coupled to gears and display dials
Disadvantages of a wright respirometer
Only records unidirectional flow
Accuracy and reliability depends on mechanical quality
What volume can a wrights respirometer record up to
How can flow rate be calculated
1000L
Derived not measured by total volume over time.
How does a dry gas meter work
Advantages and disadvantages
Directs gas in and out of two sets of bellows. Has a clockwork counter. Once a set is filled gas is vented and so on.
Can measure huge volumes 10^6 L but can’t measure flow.
How does an electronic volume meter work
Advantages and disadvantages
Similar to a wright respirometer with a spinning vain adapted to give electicial reading
Eg measure how often a light beam is interrupted by the vain
Advantages - less mechanical parts, can be bi-directional,
Disadvantages - needs power and processing / display unit
How does a peak flow meter work
Expired gas operates a shuttle controlling a variable orifice through which the gas escapes, the greater the flow the larger the orifice opened. The sitter is non returnable so can be measured (with a pointer) to record max flow
How can gas flow be measured in an anaesthetic machine
Rotameter
How does a Rotameter work
Gas flow passed upwards through a vertical tube with an increasing orifice size via a needle valve. A bobbin sits in the tube and moves up and down with changes in the gas flow.
Features of the bobbin in a Rotameter
Smaller diameter than the tube
Spiral grooves causing it to spin and not stick
Flat top to read value
Where does a bobbin settle in a Rotameter
Where force of gas flow equals bobbin weight
Why do Rotameter have to be calibrated to specific gas types?
Flow at the base is laminar (as area round bobbing tube like - longer than it is wide)
Flow at top is turbulent (as area round bobbing aperture like - wider than it is long)
Laminar flow more dependant on viscosity, turbulent more on density
Thus gases with different viscosities and densities will behave differently in the Rotameter giving different readings at the same flow unless in specifically calibrated tube.
Which side of a Rotameter is the most distal
Which gas is added there, why?
Left
Oxygen, if there is a proximal breakage/leak the oxygen is still delivered. If oxygen was first it could reverse flow out of the leaking tube giving an hypoxic/anoxic mix
What safety features exist on Rotameter
Oxygen added last (even if dial first - it is channeled last)
Hypoxic guard, mechanically linking the oxygen control to others to ensure it is not possible to have <21%
How can gas flow be measured in breathing circuits
Penumotachograph
Types of pneumotachograph
Fixed resistance (screen or fleisch)
Hot wire
Pitot tube
How does a pneumotachograph work
Pneumotachograph head sits in breathing circuit
Patient breaths through it
Flow signal passed to conditioning unit where it is analysed and displayed.
Volumes can be calculated by measuring flow and duration
What do pneumotachograph have to compensate for during an anaesthetic to calculate volumes
Changes in gas mixture and thus changes in viscosity and density
How does a screen pneumotachograph head work
Permiable screen put in tubing
Pressure sensor either side
Gas passes through screen and is resisted thus slight pressure drop
Difference detected by the two sensors and flow calculated from this
Issue with screen pneumotachograph
Works well with laminar flow but high flow rates may cause turbulent flow thus innacuracy
How does a fleisch pneumotachograph work
Same two pressure sensors as a screen pneumotachograph but connected by a series of many ducts as opposed to a membrane. This ensures laminar flow but makes it bulky
How does a hot wire pneumotachograph work?
Two wires at right angles heated by an electric current
Gas flow produces cooling and a resistive change
How does a pitot tube pneumotachograph work
Two pressure sampling tubes in centre of gas flow path, one facing upstream, one downstream
Downstream measures static pressure
Upstream measures total pressure which is increased by flow of gas
Total - static pressure gives the dynamic pressure
What sort of flow is encountered in a pitot tube
Turbulent
Why do signals from gas analysers need processing
Small so need enhancing
Compensation for non linearity
What measures of lag are there in gas analysers
Delay time - time from change in concentration to detection of a 10% increase in sample chamber - time for gas to pass from sample orifice to measurement chamber
Rise time - time for display time to rise from 10-90% of the step change
Response time - time from gas reaching sample chamber to displaying 95% of final measurement
Types of gas analyser
Discrete analysers
- gas liquid chromotography
Continuous analysers
- mass spec
- ir absorption
- uv absorption
- paramagnetism
- thermal conductivity
- polarography
- galvanic fuel cell
How does gas liquid chromatography work?
Unknown mix injected into stream of carrier gas eg nitrogen
This flows through a column of liquid coated particles (the stationary phase)
Gases in the mix are slowed based on their solubility in the liquid Thus are separated at the end of the column.
Gases then analysed on exit by ir absorption or thermal conductivity giving peaks of component gases in mix
Type of gas given by time taken to exit, concentration by height of peak
Advantages and disadvantages of gas liquid chromatography
Very accurate
Expensive
How does mass spectrometry work
Sample drawn in continuously via sample tube
Ionised by a beam of electrons
Passed through a slit in the chamber and accelerated by a negatively charged plate
Ions then separated giving a spectrum according to mass and charge
Types of sensor in a mass spectrometer
Magentic sensor - ions deflected varying amounts based on mass (mainly) or charge (minor component as most have same charge) - can detect 4-6 different things
Quadrupole - 4 energised steel rods vary radio frequencies enabling selected ions to pass and be detected