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
Advantages and disadvantages of mass spectrometry
Versatile, can measure multiple components, rapid response time
Complex, expensive, water vapour interferes with measurement and prolongs rise time
What will infra red absorption detect
Where is it commonly used
A molecule comprised of 2 or more dissimilar atoms
Capnographs
How does ir absorption work in gas sensor eg a capnograph
IR shined through the sample
Absorption causes vibration and bending of covalent bonds increasing molecules rotational speed
Different gases absorb specific wavelengths of light
Detecting absorption at different frequencies can identify gas type and concentration
What two main classifications of ir gas analysers are there?
Dispersive - multiple gas analysers where ir light split and delivered sequentially
Non dispersive - single wavelength used to detect specific gas eg co2
What is the most commonly used ir gas analyser
Ir spectrophotometer
Advantages and disadvantages of ir absorption gas analysis
Fast to allow breath to breath monitoring
Used on co2 N2O and volatiles
Response time increased with molecular size thus decreased accuracy for large molecules at rapid resp rates
What does uv absorption gas monitoring detect
Gases composed of similar atoms eg o2, h2, N2
Halothane
Advantages and disadvantages of uv absorption for gas monitoring
Accurate
The absorbed energy is enough to disrupt the molecules forming toxins so must be vented via soda lime
Response time is slow
How does a paramagnetic analyser work
Molecules can be paramagnetic (attracted to magnetic field) or diamagnetic (repelled)
Paramagnetic gas distorts magnetic field and distorts a dumbell displacing a mirror proportionally to concentration. A beam of light reflected off the mirror can be used to assess displacement and thus concentration.
What is the analyser called on a paramagnetic analyser for o2
Pauling analyser
Why is oxygen paramagnetic
Pair of electons spinning in same direction in outer shell
Advantages and disadvantages of paramagnetic analyser
Compact, cheap, unaffected by other common gases
Slow response time (5-20s) due to large chamber needing filling
How does thermal conductivity gas analysis work
What is the analyser called
A katherometer
Differing gasses have differing thermal conductivities
Gas passed over heated wire, wire is cooled depending on gas thermal conductivity, temp and flow rate. This changes resistance and thus produces signal related to consecration. Read via a Wheatstone bridge.
What are thermal conductivity gas sensors used for.
Mainly co2 and helium
Often used as part of a gas chromotography apparatus
Advantages and disadvantages of gas chromotography
Cheap, simple
Slow
How does electrochemical gas analysis work?
Reaction occluding between two electrodes in an electrolyte solution creating an electrolytic current, magnitude dependant on pp of dissolved gas.
How can solubility be used to measure gas concentration
Drager use it (narkotest)
Measures effect of different concentrations of voletiles by having them dissolve on bands of silicon rubber. Linear relationship between concentration and length of band.
How can density be used to measure gas concentration
Sealed glass bulb with air compared against gas mix in another bulb, balance changes dependant on density
Sources of error in gas measurement
Contamination - secretions, debris, water vapour
Poor gas mixing - concentration gradient in circuit
Altered vapour pressure - temp gradients between inspired and expired gasses altering pressure
Altered patterns of flow - changes at sampling point can cause local concentration changes
Varied absolute pressure - may alter in circuit and give odd result if different at pressure vs gas sensor
Instrument error - ram gas effect - flow velocity of gas as it enters sampling port can alter sample composition
Patient error - non homogeneity in alveolar time constants over the lungs so variation in gas concentration over expiration, esp I chronic lung disease
How can oxygen levels in blood samples be measured
Electrochemical methods - polarographic electrode and galvanic fuel cell.
What’s the general principle behind electrochemical methods of assessing oxygen content in blood
Electrochemical reduction of oxygen at the cathode using electrons generated at the anode resulting in 4 electrons moving anode to cathode for each molecule of oxygen creating a circuit - the current of which is dependant on oxygen concentration.
What law is used to convert concentration of oxygen in the solution detected by an electrochemical method into a partial pressure
Henry’s law
Number of molecules in solution is proportional to pp.
What sort of polarographic electorode is used to measure oxygen
What does it consist of?
A Clark electrode
Negative platinum cathode
Positive silver/silver chloride anode
Immersed in KCl solution
How does a Clark electrode work
Oxygen filters in from sample into the kcl solution via a semi permiable membrane
Reacts with electrons at cathode creating OH- ions which migrate to the anode
At the anode reaction produces electrons which flow through the circuit back to the cathode creating a small current proportional to the oxygen concentration. This current needs to be driven by a power supply due to the redox potentials of the platinum and silver - current increases with voltage applied until oxygen reduction the limiting factor and then it plateaus at a voltage proportional to the oxygen concentration
What measures current in a Clark electrode
A galvinometer
Why does the Clark electrode need a power supply
Redox potential opposes flow of electrons from anode to cathode in external circuit so small voltage needed to overcome this. Generated current increases with increased power supply until plateau reached, the plateau value is where oxygen reduction is the limiting factor proportional to concentration.
What is the chemical reactions occurring in the Clark electrode
Cathode:
O2 + 2 H2O + 4e- to 4(OH)-
Solution
KCl + OH- to KOH and Cl-
Anode
Ag + Cl- to AgCl +e-
Why is a semipermeable membrane needed in a Clark electrode
Disadvantage
Stop protein deoposition on cathode which limits area for redox.
Creates a time lag in results
Advantages and disadvantages of Clark electrode
Robust, portable
Limited lifespan as silver/silver chloride electrode consumed
What does a galvanic fuel cell consist of?
Lead anode
Gold mesh cathode
KOH solution
What does a galvanic fuel cell consist of
Lead anode
Gold mesh cathode
KOH solution
How does a galvanic fuel cell work?
Oxygen reduced at cathode to OH-
OH migrates to anode combining with the lead forming lead oxide, water and electrons.
Electrons travel back to the cathode for further reduction via external circuit producing current measured by galvanometer
Advantages and disadvantages of galvanic fuel cell
Compact, power free, inexpensive
Slow response time (30s), can become contaminated by N2O, lifespan 6-12/12
Sources of error in electrochemical methods of measuring gases
Blood gas factor - values lower for blood likely because of slower diffusion and local depletion of oxygen
Stability - calibration drift due to lipid or proteins build up on electrodes
Interference - other substances eg N2O can be oxidised
Need mathematical correction for temp variation;
How can oxygen concentration be measured in vivo?
Intravascular oxygen electrode
Transcutaneous oxygen electrode
Conjunctival oxygen tension electrode
Mass spectrometer
Optodes
How does an intracascular oxygen electrode work
Clark electrode shoved in a vein
Can fit through a 18G cannula
Advantages and disadvantages of intravascular oxygen electrode
Continuous measurement
Temp sensitive, subject to drift, flow dependant, response times up to a minute, more rapid reduce accuracy
How does a transcutaneous oxygen electrode work?
Ring shaped anode and central cathode in electrolyte solution encased in semipermeable membrane
Held in direct contact with skin by tape
Local heating increases capillary flow
Oxygen diffuses out and is measured.
What temperature does a transcutaneous oxygen electrode start to cause tissue damage
44-46oC
When are transcutaneous oxygen electrodes useful
Neonatal monitoring
Advantages and disadvantages of transcutaneous oxygen electrode
Response times a function of distance thus faster in neonates and infants
Heating can cause oedema and burns, also increases skin metabolism causing odc right shift
Changes in skin pO2 lag behind PaO2
How does a conjunctival oxygen tension electrode work
Gold or platinum ring cathode
Silver/silver chloride anode
Thermistor for temp compensation
Covered in a memebrane of silicon or polyethylene
Placed under the eyelid in fornix
Works like a transcutaneous oxygen electrode
Advantages and disadvantages of conjunctival oxygen tension electrode
Shorter diffusion distance vs transcutaneous so faster and better correlation. No need for heating
Expensive, awake patients will need LA
How can a mass spec be used for in vivo oxygen measurement
Have mass spec in direct contact with patient via a metal catheter with a gas permiable membrane or a transcutatnious o2 electrode.
What is an optode, how does it work?
Intravascular probe with dye coated tip covered by oxygen permiable membrane and fibre optic cable
Illumination of fibre causes dye to fluoresce
Oxygen quenches the dye
Degree of fluorescence measured using a photomultiplier
Contains a thermocouple to allow temp compensation
Advantages and disadvantages of an optode
Independent of blood flow
Expensive, may deteriorate with time
How can co2 be measured in vitro?
The carbon dioxide electrode
How does the carbon dioxide electrode work?
Glass ph electrode
Silver / silver chloride electrode
Maintained at 37oC
Glass electrode in mesh bag containing sodium bicarb
All sat in in electrolyte solution
Co2 diffuses into electrolyte solution via semipermeable membrane
pH falls - proportional to co2 concentration
Advantages and disadvantages of carbon dioxide electrode
Accurate and stable
Needs monitoring and calibrating, can be slow
How can co2 levels be monitored in vivo?
Transcutaneous electrode
Intravascular probes
Optodes
Capenography
How does the transcutaneous electrode measure co2
Like the normal co2 electrode but held onto skin and thermistor heats to 42-44oC to dilate vessels.
How does transcutaneous co2 correspond to PaCO2
Generally higher (1.33xPaCo2+0.5kpa)
Advantages and disadvantages of transcutaneous co2 monitoring
Continuous
Risk of burns, slow response time, not exact correlation with arterial
How can Optodes be used to measure in vivo co2
Similar to o2 but changes the intensity of fluorescence are caused by changes in pH thus is indirect for CO2 (probe tip covered in cO2 permiable membrane)
How does etco2 relate to PaO2
What alters this relationship
Usually 0.5-0.8kpa lower
Increased difference with respiratory disease where there is a larger VQ mismatch
How do most capnographs work
Ir spectrophotometers
What types of capnograph are there? How do they work
Side stream - gas draw from main flow into a side port to sample curvette post measurement gas either returned or scavenged.
Main stream - capnograph in main gas flow, transiluminated through side window.
Advantages of sidestream capnograph
Advantages of mainstream capnographs
Side stream - Convenient, less bulky, more robust
Main stream - less complex, no error due to sampling method, faster response time, no need for water trap
Causes of decreasing etco2
Increased alveolar ventilation
Decreased co2 production eg hypothermia
Increased dead space eg shock, pe,
Technical error
Causes of increasing etco2
Decreased alveolar ventilation
Increased co2 production eg hyperpyrexia, sepsis
Increased inspired co2 eg rebreathing
What IR wavelength does co2 absorb
4.3 micrometers
How does a ph electrode work
2 silver /silver chloride electrode
One in KCl as reference
One in glass permiable only to H+
Redox at measuring electrode proportional to h+ concentration
Small current flows which is measured with galvanometer
Types of oximetry
Transmission (absorbance)
Reflectance
How does transmission oximetry work?
Transilluminate tissue with ir light, monochromic at peak absorptions for oxy and deoxy Hb
Varied absorption based on concentrations of HbO2 and Hb present by beer lamberts law
What is the isobestic point in transmission oximetry
805nm, where both Hb and HbO2 absorb the same amount
What are the absorption peaks for Hb to ir light
HbO2 at 660nm
Hb at 940nm
Why is the result of transmission oximetry with 2 wavelengths termed functional saturation
Only accounts for HbA with oxygen
Ignors HbF, carboxyHb, methyamoglobin etc
How can fractional oxygen saturations be measured?
With a cooximeter
At least 4 different wavelengths measuring measurement of HbCO and HbMet etc.
When are cooximeters used
Why
In blood gas machines
Need haemolysis of the sample prior to measurement
How does pulse oximetry work
Peripheral probe with 2 leds transilluminating with red (660nm) and IR (940nm), cycles between them (alternating) so photodiode can tell which is which
Photodiode detects transmitted lite
Functional hb saturation then calculated in transmission unit
How does pulse ox imitate minimise error
Period where neither led on to correct for ambient light
Fires multiple pulses of light in each cycles to minimise artefact
What signals do pulse oximeters receive over time
Which is most important
The AC component - short (5% of total time) rapidly changing during pulsetyle portion of flow.
DC - 95%, constant absorption by resting state/volume of the tissues.
AC Is most important
What is the range of accuracy of pulse oximeters? Why?
Mainly accurate about 80%, calibrated on human studies and couldn’t ethically go lower, thus lower obtained by extrapolation
What can cause delay in response time in pulse oximetry?
Instrumental delay - the time needed to reduce artefact
Circulatory delay - time for changes in central saturations to reach the periphery
What are examplar circulatory delay times for varied pulse oximeters positions?
Ear probe - 10-15s
Finger - 60s
Both increased 2-3 fold if areas cold.
How does reflectance oximetry work?
Fibre optic cable passes light into tip of pulmonary artery catheter
Reflected back by the blood and analysed by photodiode.
Sources of error in oximetry
Variation in wavelength produced (eg 660nm led and 940nm led both are +/-15nm)
Ambient light - compensated with led cycling
Low perfusion states - oximeters tries to compensate by boosting signal but this decreases signal to noise ratio
Motion artefact
External dyes - eg methylene blue can drop sats read falsely to 45%!
Additional hb species
Anaemia
Bilirubinaemia
Diathermy
Probe performance
Effect of HbMet, CO, F and S on pulse oximeters
Hb met - greater absorption at 940 and less at 660, thus underreads at high values and overreads at low values
Co - similar absoption at 660 thus over reads
HbF - no effect
HbS - no reported effect though note odc right shifted, so for any PaO2 corresponding Spo2 would be less than expected
Effect of anaemia on pulse oximetry
Linear trend to underestimate
At hb 80 spo2 could be underreading by 10-15%
Effect of bilirubinaemia on spo2
No effect on pulse ox but may result in elevated hb met and hb co from art sample
What determins a muscle response to neuromuscular stimuli
Current not voltage
What are the characteristics of a stimulator pulse used in nm monitoring?
Square waveform with uniform amplitude
Appropriate amplitude for application
Pulse length of 0.2ms
Varied pattern and frequency depending on application
What amplitudes of electric pulse are used in nm monitoring
Needle electrodes - 0.5 to 5mA
Skin electrodes 10-40mA
Feratures of nerve stimulator
Battery powered
Electrically isolated
How can the response to a neuromuscular stimulation be assessed
Vision or touch
Mechanomyography
Accelomyography
Electromyography
How does mchanomhography work for assessing response to neuromuscular stimulation?
Small preload applied to muscle to maintain isometric conditions
Electrical stimulation causes contraction against preload generating tension proportional to force - this is converted to electrical signal that can be measured
Disadvantages of mechanmyography and accelomyography for measuring neuromuscular stimulation
Needs correct positioning and immobilisation of hand
How does accelomyography work to measure nm stimulation
Position joint of interest so distal end free
Stimulate motor nerve causing movement of joint
Acceleration proportional to force of contraction, measured with piezoelectric wafer on distal part of digit
How does electromyography work for measuring nm stimulation
Measures magnitude of evoked compound APs from the electrode overlying a particular muscle
Active electrode placed over nerve
Indifferent electrode placed over tendon insertion
Advantages and disadvantages of electromyography for measuring nm stimulation
Avoids mechanical issues of joint positioning and calibration or transducers
Altering hand positio;can still alter response
What is post tetanic facilitation
Why does it occur
Increase in response to stimuli following tetanic stimulation due to mobilisation of ach
What patterns of neuromuscular monitoring can be used
Single twitch
Train of four
Tetanic stimulation
Post tetanic count
Double burst
Process and use of single twitch stimulation, range of receptor occupancy detected
Single supramaximal stimuli at 1Hz
Useful for assessment of sux
Fade begins to be noticable at around 75% occupancy, fading to absent at 100% occupancy thus can only differentiate a wide range (little accuracy)
How does train of four work in nm monitoring
What is the % blockade for number of detectable twitches
Delivers four identical stimuli at 4Hz
Non depolarising muscle relaxants cause successive reduction in height of twitches
0 - 100%
1 90
2 80
3 75
4 <75
Why can TOF be more specific than single twitch (even with pre and post blockade comparison)
Pre post single twitch comparison only picks up blockade >75% occupancy
Tof shows occupancy <75% with 4 twitches then ratio of T4:T1 can be calculated to further quantify response beneath 75% occupancy
What t4:t1 ratio on tof is adaquate for resp function
70%
What level of tetany produces a response similar to maximum voluntary muscular effort
50Hz for 5s
How does post tetanic count work
When no chain of 4 apply tetanic stimulation to increase ach mobilisation
Then twitch at 1s intervals, number of twitches cororlates with receptor occupnancy
<5 is profound blockade, >15 is equivalent to 2 twitches on normal tof.
How does dbs work
Two tetanic bursts of three twitches at 50Hz separated by 0.75s
The ratio of t2:1 is approximate to tof 4:1 but easier to assess visually
How is dbs fade best appreciated
By feel
Historical methods of assessing depth on anaesthesia
Integrated clinical scores (eg evans, gudel)
Tunstalls isolated arm
Lower oesophageal contractility
Frontalis electromyography
Components of Evans depth of anaesthesia score
P - systolic blood Pressure
R - heart Rate
S - Sweating
T - Tears
How does lower oesophageal contractility monitoring work for depth of anaesthesia
Two types of contractions
- provoked - from distension, involuntary
- spontaneous - arise due to emotion and stress
Probe monitors these and anaesthesia depth determined by a index of them
How did frontalis electromyogram work
Why
Monitoring frontal is with two surface electrodes
Frontalis recieves both viceral and somatic input from the facial nerve
Action potential monitored indicating anaesthetic depth
Current ways of measuring anaesthetic depth
EEG
Evoked Responses
BIS
What waves catagories are made on EEG
What associated frequencies
Delta <3
Theta 4-7
Alpha 8-12
Beta >13
Gamma 40
Which bands of eeg activity are associated with waking, how do they change as anaesthesia is applied
Beta and gamma (high frequency) associated with waking, but have low amplitude
Under anaesthesia progresses to delta theta with larger amplitude
What controls eeg activity
Subthalmic nucli
Amplitude range of eeg
1-500microvolts
How can assitance be given to interpretation of eeg
Cerebral function analysing monitor (cfam) produces display of eeg from two symetrical scalp electrodes showing mean power in each frequency band
Compressed spectral array plots sequential segments of eeg power spectrum creating a 3d (amplitude vertically, frequency horizontally, time on a z axis - shows how eeg changes over time
What can depress eeg output
Hypoxia
Hypotension
Cerebral oedema
Metabolic encephalopathy
How does evoked response potential work
Provide stimuli (eg brainstem auditory or visual)
Evoked signals are very low amplitude compared to eeg (1-2microvolts)
Computer extracts them from background noise by averaging
Delay between signal and response indicates where it is coming from (0-10ms brainstem, 10-100 early cortical, 100-1000ms late cortical eg frontal cortex)
How does bis work
Internal calculation and derived index based on eeg (time, frequency and spectrum)
How to convert f to oC
oC = (F x 9/5 or 1.8) + 32
Set points of f, oC and k
F - ice melting 32, water boiling 212
C - ice melting 0 water boiling 100
K - absolute 0 0, triple point of water 273.16
Points to measure temp and why/why not
Lower 25% of Oesophagus - good estimate central temp
Nasopharynx - less accurate
Tympanic - good measure of hypothalamic temp, short response time
Blood - very accurate continuous measurement of core temp
Rectal - slow response time, increased by gut flora so about 1% higher than core
Bladder - can be done as part of catherisation, slow to respond esp. with high urine flow
Main classifications of thermometers
Direct reading - display and site of measurement in direct contact
Indirect reading - display distant from site of measurement
Examples of direct reading thermometers
Liquid expansion thermometers
Chemical thermometers
Dial thermometers - bimetallic or pressure gauge
How do liquid expansion thermometers work
Advantages and disadvantages
Glass bulb filled with alcohol or mercury connected to narrow capillary tube
When warmed liquid expands up tube giving reading on scale
Linear relationship if tube diameter constant
Simple, no power needed
Poor visual display, slow, limited range, easy to break, can’t put in cavities
How do chemical thermometers work
Advantages and disadvantages
Cells containing mix of chemicals, colour of which is temp dependant
Usually liquid crystal technology, solid crystals that melt, realign and change colour
Fast response, no breakage, disposable
Not accurate, best accuracy around 0.5oc
How do dial thermometers work
Bimetallic - flattened spring made of two dissimilar metals, one end fixed, other pointer, as temp increases spring uncoils and pointer moves
Pressure gauge - based on bourdon gauge, hollow spiral tube forming a spring, end in a temp sensing bulb containing volatilise liquid, expands increasing pressure causing unwinding and dial to move.
Examples of remote reading thermometers
Resistance thermometers
Thermistors
Thermocouple
How does a resistance thermometer work
Based on change in resistance of coil of wire with temperature
Detected using a Wheatstone bridge
What is the wire of a resistance thermometer usually made of
Accuracy
Relationship between temp and resistance
Platinum
Accurate to 0.0001oC
In 0-100oC largely linear relationship
Disadvantages of resistance thermometers
Hard to make small probes, fragile, slow response time
How does a thermistor work for measuring temperature
Made of fused oxides of heavy metals (manganese, nickel, zinc, iron)
Semiconductor that possesses a negative temp coefficient of resistance.
Can be made very small minute beads
Advantages of thermistors for temp sensing
Disadvantages
Rapid response
Great sensitivity
Small size
Non linear, wide variation across range thus needs conditioning and calibration, exhibit hysteresis,
How does a thermocouple work for temp sensing
Two different metals joined to form two junctions.
If junctions are at different temp generates an electromotive force proportional to the difference - the seebeck effect
What metals are used in a thermocouple
Commonly copper constantin (compare nickel alloy) and platinum-rhodium
Advantages and disadvantages of a thermocouple
Small and versatile, rapid response, acceptable accuracy (0.1oC), cheap
Requires amplification thus vulnerable to noise, non linear so needs processing
Side effects of not using a heat moisture exchanger
Hypothermia
Mucosal drying
Increased viscosity of secretions
How can humidity be defined
Absolute - mass of water vapour in given volume of gas at defined temp and pressure
Relative - mass of water vapour in given volume of gas expressed as a percentage of the mass of water vapour required to saturate the same volume of gas at the same temp and pressure
What is the term for a device that measures humidity?
Hygrometer
Examples of hygromometers with brief description
Regnaults - silver tube in ether, sample bubbled through, dew point temp proportional to humidity
Hair - hair protein in beta sheets, elongates in presence of moisture, elongation proportional to humidity
Wet/dry bulb - compare dry thermometer with one surrounded by a wet wick. Temp of wet thermometer will depend on evaporation which is indirectly proportional to humidity thus difference proportional to humidity
Humidity transducer - change in electrical current based on change in humidity - can be as a resistor or a capacitor
Mass spectrometers
Definition of pain
Unpleasant sensory and emotional experience associated with actual or potential tissue damage
Examples of single dimension pain measurements
Body charts for location
Visual analogue scales (continuous data)
Numerical or pictorial ranking (discrete catagories)
Word descriptors - can go on to score words to give a score or index
Multidimensional pain measures
McGill questionnaire
Brief pain inventory
Memorial pain assessment card
Pain observation chart
Beck depression inventory
