Respiratory measurement Flashcards
Define pulse oximetry
Pulse oximetry utilises the Beer-Lambert Law to isolate the pulsatile arterial signals and isolates different haemoglobin species by their differential absorption of light wavelengths
Define oxygen saturation
The ratio of reduced haemoglobin to oxyhaemoglobin
What 2 wavelengths of light are used in pulse oximetry
660nm and 940nm
Deoxyhaemoglobin absorbs light at what wavelength
660nm - shorter wavelength, higher frequency i.e. more blue
Oxyhaemoglobin absorbs more light at what wavelength in pulse oximetry
940nm - longer wavelength, more red
What wavelengths are used in pulse oximetry and which wavelengths are predominantly absorbed by each haemoglobin species
◦ Two wavelengths (660 and 940 nm) are used in pulse oximettry
◦ Deoxyhaemoglobin absorbs more light at 660nm and oxyhaemoglobin absorbs more light at 940 nm.
Which haemoglobin absorbs more visible light and whcih more infrared
‣ Oxygenated haemoglobin absorbs less red light than deoxygenated haemoglobin, BUT more infrared
What is an isobestic point in the context of pulse oximetry
‣ Note that at two points on the diagram the extinction coefficients are equal (590, 805nm) called isosbestic points, this allows for the measurement of total haemoglobin absorbance proportional to total haemoglobin concentration allowing correction of absorbance data accounting for anaemia, different types of haemoglobin
What is Beer’s law
◦ Beer Law: the concentration of a given solute in a solvent is determined by the amount of light that is absorbed by the solute at a specific wavelength
How does the pulse oximeter derive arterial saturations given tissue and concurrent venous blood in the same area?
◦ Absorption-over-time signal from arterial blood is pulsatile, whereas signal from venous haemoglobin and tissue is not.
◦ When the arteries pulsate, the distance travelled by light though them changes
◦ One can therefore use Lambert’s Law (equal parts in the same absorbing medium absorb equal fractions of the light that enters them).
◦ Thus, one can compare the ratio of pulsatile and nonpulsatile absorbance to produce R, the ratio of absorbance at any given time
What is Lamberts law
equal parts in the same absorbing medium absorb equal fractions of the light that enters them
Draw the absorption graph for oxyhaemogl;obin, deoxyhaemoglobin, carboxyhaemoglobin and methaemoglobin
What is R
Give the equation for R
In the context of pulse oximetry
◦ Thus, one can compare the ratio of pulsatile and nonpulsatile absorbance to produce R, the ratio of absorbance at any given time
◦ R = (AC660 / DC660) / (AC940/DC940)
Explain the utilisation of Lambert’s law in pulse oximetry
◦ This pulsatile signal change is not due to some change in the arterial oxygenation which occurs with every heartbeat (arterial blood stays uniformly oxygenated between beats), or with the arrival of extra haemoglobin in front of the sensor (arterial haematocrit is also sable between beats). The main reason for the change in absorbance is the optical distance. As arteries expand with the arterial pulse, the distance between the probe and the sensor increases, and the absorbance increases proportionally (this is where Lambert’s law comes in).
Combine the Beer and Lambert law into one definition
◦ The measured absorbance for a single compound is directly proportional to the concentration fo the compound and the length of the light path through the sample
beer’s law
‣ Beers law deals the the concentration measurement - absorption or attenutation of light is proportional to concentration of the substance
Lamberts law
‣ Lamberts law deals with identification fo the pulsatile signal - ababsorption or attentuation is proportional to the distance the light has ti travel
How is the R value in pulse oximetry converted to saturations
- Calibration with empirically measured data
◦ R is meaningless unless it can be related to oxygen saturation;
◦ R is compared with a set of standardised values to deliver a calculated SpO2
◦ An R of 1 gives an SpO2 of 85%
◦ An R of 0.4 gives an SpO2 of 100%
◦ An R of 2 gives an SpO2 of 50%
◦ A series of saturation measurements and R values have been collected from healthy individuals in the 100-75% saturation range, and extrapolated to 0%
Over what range is pulse oximetry accurate
100-70%
Define isobestic point
◦ The isobestic point is the wavelength at which light is absorbed equally by both haemoglobin species
◦ Light absorption is therefore independent of saturation, and is instead a function of haemoglobin concentration
What are the values for the isobestic point
590nm
805nm
Plethysmograph means?
graph of the change in volume
How does the pulse oximeter correct for ambient light?
- Correction for ambient light
◦ The pulse oximeter LEDs strobe at a high frequency (400-900 Hz)
◦ When the LED is off, the photometer measures the absorption of ambient light, and subtracts this from the signal measured when the LEDs are on.
◦ This eliminates the contribution of (most) ambient light
What are the essential features of a pulse oximeter
◦ LED light sources x 2
◦ A photometer/photodiode as a light detector
◦ Opaque probe housing to minimise ambient light
◦ Signal amplifier and noise filter
◦ A control circuit
◦ Electronic storage which contains calibratino data, compliant connector to a user interface with display and alarm functions
What are the limitations of pulse oximetry
- Inevitable difference due to processing artefact - ABG machines lyse RBC shifting the saturation measurement due to pH change, temperature difference as ABG measures at 37 degreees
- Interference with absorbance - carboxyhaemoglobin, methaemoglobin or intravascular dyes
- Signal processing - outside of the pulse oximeter range, falsely lw reading in dark skin
- Pulse detection dependent - poor signal due to shock, tourniquet, ECMO, erratic movement, arrhythmia, venous pulsation in TR
- Signal measurement - ambient light, nail polish, oedema
What is a co-oximeter
- A co-oximeter is a device using lamberts law to identify arterial haemoglobin from venous
What is a co-oximeter different to a pulse oximeter
◦ Multiple wavelengths
◦ Does not require pulsatile flow
◦ Measures MetHb, COHb
◦ Heats to 37 degrees
What is the fractional concentration of total haemoglobin
◦ Fractional concentration is the fraction of total haemoglobin which happens to be oxygenated - calculated from values directly measured by absorption spectrophotometry expressed as FO2Hb
‣ Concentration of oxyhaemoglobin/concentration of total haemoglobin
Functional saturation is?
◦ Functional saturation is the fraction of the effective haemoglobin which is oxygenated
‣ Saturations (SO2) = concentration of oxyhaemoglobin/concentration of effective haemoglobin
What is the difference between fractional and functional saturations?
◦ Typically very little difference between these values - dyshaemoglobin accounts for the differenc ein functional Hb however the SO2 will consistently be a little higher because everyone has a tiny by 1% of methaemoglobin and carboxyhaemoglbin
◦ If one does not have the ability to measure abnormal haemoglobin species, one is served with the ctHb value. This value represents the total haemoglobin. The FO2Hb can be used together with the total haemoglobin to calculate the total oxygen content; and one would not need to know the concentration of dyshaemoglobin species.
◦ However, one would not be able to determine the total oxygen carrying capacity in this way.
◦ For that, the sO2 would be the more relevant measure. The sO2 value excludes haemoglobins which have lack the capacity for oxygen transport (it uses oxyhaemoglobin and deoxyhaemoglobin only).
What are the advantages of a co-oximeter?
Not affected by
- ambient light
- Pulsatility or lack thereof
- TR
- Carboxyhaemoglobin or dyes
What are the disadvantages of a co-oximeter
Handheld
Non disposable
Increased cost
Causes of high co-oximetry and low pulse oximeter
◦ Poor peripheral perfusion
◦ Ambient light
◦ Poor probe contact
◦ Dyes - methylene blue
◦ Tricuspid regurgitation
◦ Methaemoglobinaemia
Causes of low co-oximetry but high pulse oximetry
◦ Carboxyhaemoglbin
◦ Radiofrequency interference
◦ Leukocyte larceny - oxygen consumption in the collection tube
Define capnograph
capnograph measures how much carbon dioxide is present in the patients breath
Capnometry - measurement of the concentraiton of CO2
Capnography - graphic display of measurement over time
Define capnometry
capnograph measures how much carbon dioxide is present in the patients breath
Capnometry - measurement of the concentraiton of CO2
Capnography - graphic display of measurement over time
What are the principles of CO2 measurement in capnography
- Uses infrared waves (infrared spectroscopy) to measure CO2 by the Beer Lambert Law- as the amount of infrared absorbed passing through a substance is proportional to the concentration of the infrared absorbing substance. This requires
◦ An infrared source/light emitting diode - or a radiation source with a filter
◦ Sample chamber - fixed side and allows infrared light to pass thorugh
◦ Photosensitive IR Detector -an infrared absorption spectrophotometer
◦ Circuit allowing IR absorption signal to be related to CO2 concentration using calibration values
◦ Output device designed to represent absorption data as a graph over time on a monitor - CO2 absorbs infrared radiation as does any gas which two or ore different atoms
What components are required in a capnometer
- Uses infrared waves (infrared spectroscopy) to measure CO2 by the Beer Lambert Law- as the amount of infrared absorbed passing through a substance is proportional to the concentration of the infrared absorbing substance. This requires
◦ An infrared source/light emitting diode - or a radiation source with a filter
◦ Sample chamber - fixed side and allows infrared light to pass thorugh
◦ Photosensitive IR Detector -an infrared absorption spectrophotometer
◦ Circuit allowing IR absorption signal to be related to CO2 concentration using calibration values
◦ Output device designed to represent absorption data as a graph over time on a monitor - CO2 absorbs infrared radiation as does any gas which two or ore different atoms
What features of carbon dioxide underly the principles of capnometry
Calibration
* Using infrared light of wavelength 4.26 - in order to absorb IR radiation a molecule must be assymetrical, polyatomic because IR absorption occurs as a conseqeuence of atomic vibration in a molecule with a dipole moment
What wavelength is used in capnographs
Calibration
* Using infrared light of wavelength 4.26 - in order to absorb IR radiation a molecule must be assymetrical, polyatomic because IR absorption occurs as a conseqeuence of atomic vibration in a molecule with a dipole moment
What sources of error in capnography are important
What are capnography limitations
Sources of error and limitations
* Nitrous oxide (wavelength 4.5), isofluorane, helium absorb infrared and can result in false readings especailly if infrared source does not have a tight wavelength released
◦ Collision broadening is the effect whereby the absorption of wavelengths broadens when CO2 is in the presence of another gas e.g. oxygen or nitrous oxide and can be due to collision between molecules altering the absorption of light
* Response time - the delay between physical change in gas flow and measurement and representation of change and is seen on the CO2 analyser as both transit time and rise time
◦ Transit time - time taken for CO2 to travel from sampling to analyser. Can be shortened by main stream analyser, short narrow sampling tube and high suction (flow rate)
◦ Rise time - How quickly the analyser responds to CO2, measurement of minimum to maximum CO2 transition and is measured on the straight section of the line to make measurement easier. Rise time shorter by using a smaller measurement chamber
◦ Up to a 4 second delay
* Water vapour condensation - blockage by secretions or condensation
◦ In side stream analysers water vapour can collect and enter the sensor- to minimise this before the tube enters the sensor there is a water trap
* ETCO2 not correlating with PaCO2
* Mainstream devices increase dead space
General limitations
* Ent tidal CO2 is not pathology specific of diagnostic - dependent on perfusion, normal trace does not indicate normal patient, and if alveolar gas not involved in tiny breaths you get an aberant waveform
* Bias flow can dilute the sample
* False positvie measurements can occur
Sources of error in capnography
Sources of error and limitations
* Nitrous oxide (wavelength 4.5), isofluorane, helium absorb infrared and can result in false readings especailly if infrared source does not have a tight wavelength released
◦ Collision broadening is the effect whereby the absorption of wavelengths broadens when CO2 is in the presence of another gas e.g. oxygen or nitrous oxide and can be due to collision between molecules altering the absorption of light
* Response time - the delay between physical change in gas flow and measurement and representation of change and is seen on the CO2 analyser as both transit time and rise time
◦ Transit time - time taken for CO2 to travel from sampling to analyser. Can be shortened by main stream analyser, short narrow sampling tube and high suction (flow rate)
◦ Rise time - How quickly the analyser responds to CO2, measurement of minimum to maximum CO2 transition and is measured on the straight section of the line to make measurement easier. Rise time shorter by using a smaller measurement chamber
◦ Up to a 4 second delay
* Water vapour condensation - blockage by secretions or condensation
◦ In side stream analysers water vapour can collect and enter the sensor- to minimise this before the tube enters the sensor there is a water trap
* ETCO2 not correlating with PaCO2
* Mainstream devices increase dead space
General limitations in capnography
General limitations
* Ent tidal CO2 is not pathology specific of diagnostic - dependent on perfusion, normal trace does not indicate normal patient, and if alveolar gas not involved in tiny breaths you get an aberant waveform
* Bias flow can dilute the sample
* False positvie measurements can occur
Advatnages of capnography
Advantages of capnography:
* Helps assess a variety of problems , from the cell all the way to the breathing equipment
* Non invasive
* Rapid - feedback on cardiac output and ETT position
* Provide continuous measurement
* Physically small
* Cheap
* Waveform analysis
Disadvantages of capnography
- False alarms
- ETCO2 not correlating with PaCO2
- Dead Space - in line connector
- Adaptor fitted tot he end of the ETT may be heavy
- Gas sampling diminish delivered minute volume - as they access the circuit gas at a rate of 200ml/min
- Nitrous oxide can confuse some capnometers
- Helium can cause ETCO2 to be incorrectly elevated in some capnometers
What explanations are there for a capnography trace going flat?
- Disconnection
- Airway obstruction - secretion, bit down on tube, ETT perforation
- Capnometry disconnection or obstruction
- Cardiac or respiratory arrest
Why would PaCO2 and EtCO2 be different?
- Pulmonary perfusion
◦ Regional
‣ PE
‣ Fat embolism
‣ Air embolism
◦ Global
‣ Cardiac failure - RHF, pulmonary hypertension
‣ Cardiac arrest
‣ Extreme hypovolaemia
‣ Very high PEEP - Ventilation
◦ Increased V/Q mismatch use to high PEEP/positive airway pressure
◦ Increased alveolar dead space
◦ Shunt
‣ due to high FIO2
‣ Veyr large shunt fractiono >30%
◦ Oesophageal intubation - Artifact
◦ Helium - diluting expired gas
◦ Nitrous oxide
◦ HME
What types of capnography devices are there
- Mainstream - analyser near the CO2 expired and attached to the aptient, with the analyser connected to the monitor by long electrical wire
- Side stream - long narrow tube connected to patient end and a small pump suctions 150mL per minute of the patients respiratory gasses
What are the differences between mainstream and sidestream CO2 analysers
- Comparing
◦ Response time
‣ Side stream has a longer transit time
◦ Weight of device at patient end increased in mainstream samplers
◦ Removal of gas - side stream continuously suction 50-150mL/min
◦ Obstruction of tubing to side stream analysers can become blocked
◦ Dead space increased to greater degree with mainstream
◦ Accuracy - side stream less accurate especially at low expiratory flow rates or low volumes as entrained gas from circuit diluted by frash gas from bias flow from ventilator
Uses of CO2 analysers
- COnfirmation of ETT placement
- Disconnection alarm
- Monitoring during transport
- During CPR to assess adequacy of cardiac compressions
- Recognition of spontaneous breath during apnoea test
- Neurosurgical patient to provide protection against unexpected hypercapnoea
- Quick bedside assessment o bronchospasm
- Changes to pulmonary perfusion
- Accurate respiratory rate
Draw a normal capnography trace and describe each part of the trace
What methods other than IR absorption are availabel for CO2 measurement
Colour change colourimetry
Mass spectroscopy
Raman spectroscopy
Acoustic capnometry
What is the mechanism by which colour change colourimetry works
◦ CO2 changes the pH of the a solution it passes through it can be sued to detect CO2 in expired gas
What limitations are there to colour change colourimetry
‣ Not quantitative - also measures pH not CO2
‣ Only useful over a short lifespan of detector
‣ Ambient light conditions can make it difficult to see the change
‣ Do increase resistance to airflow and dead space - increasing work of breathing
‣ Require increased disconnections
‣ Highly sensitive to CO2 potentially leading to changes with oesophageal intubation; but equally small tidal volumes or low cardiac output states can lead ot false negtives and belief your ETT is in the wrong spot
‣ Requires humidified air to work
Mass spectroscopy for CO2 - explain its key features
◦ Charged particles move in different ways depending on mass and charge when subjected to electromagnetic field.
◦ Gas cna be aspirates into a vaccuum chamber, ionised by an electric beam and allows to separare where a detector measures them
◦ Precise
Raman spectroscopy in CO2 measurement - explain its key features
◦ Relies on Raman scattering to detect specific gasses
◦ Gas sample exposed to hihg intensity light source and molecules are excited into unstable energy states and collapse back by releasing raman scatter radiation which is characterstic for each molecule
Acoustic capnometry - describe its keyf eatures in CO2 measurement
◦ Photoacoustic spectroscopy uses combination of infrared light shining through gas mixture exciting absorbing molecules which vibrate, collide more and increase the pressure. The light is pulses so during dark period molecules relax
◦ A sound detector detects the changes in pressure specific to eery gas
◦ No commercial application
How is Hb measured in a blood gas machine?
- Absorption spectroscopy is based on Beer-Lamberts Law relating the properties of transmitted light to the properties of the substance it passes through
◦ The measured absorbance for a single compound is directly proportional to the concentration of the compound and the length of the light path through the sample - Method
◦ Haemoglobin liberated from RBC by 30Hz ultrasound beam shredding the RBC
◦ 128 beams of light created using a concave mirror and 128 individual light sensitve diodes measure the light intensity which will inversely reflect the absorbance (wavelength range 478 - 672nm
◦ Total absorbance will be a sum of all the absorbance form all the Hb species (ctHb)
◦ The relative contributions of different varieties of haemoglobin are then extrapolated using multivariate data analysis
How are oxygen saturations measured in a blood gas machine?
Spectrophotometrically - * The oxygen saturation does not take into account methaemoglobin or carboxyhaemoglobin - instead reflects the total oxygen carrying capacity
* This value can however misrepresent effectiveness of oxygen transport if there is minimal haemoglobin available
* Some gas machines do not have the ability to measure the ceHb so instead calculate it using pH and pO2 a sO2 is calculated from a loook up table
What flaws are there in O2 sats as measured by a ABG
- The oxygen saturation does not take into account methaemoglobin or carboxyhaemoglobin - instead reflects the total oxygen carrying capacity
- This value can however misrepresent effectiveness of oxygen transport if there is minimal haemoglobin available
- Some gas machines do not have the ability to measure the ceHb so instead calculate it using pH and pO2 a sO2 is calculated from a loook up table
Oxygen saturation equations (not fractional saturations)
What methods are there for measuring PaO2
- Polarographic analysis - the clark electrode - an amperometric electrode
- Paramagnetic analysis
- Fuel cell
- mass spectrometry - expensive therefore only one exists per theatre complex, delay in results due to long sampling lines, lack of flexibility if the machine breakds down all theatres lose their gas analysis. it does however mean multiple gasses including anaesthetic gasses cna be monitored
- Raman scattering analysis q
What is used generally to measure PaO2
Clark electrode
What does the clark electrode do in one sentance
- The Clark electrode measures the change in current flowing through a reaction chamber where O2 is reduced to OH- ions by a change in voltage.
Describe what happens to oxygen in the clark electrode
- How does it get into the electrode
- What does solution is around the electrode
- What i the potential difference used
- What happens to oxygen
- What node does oxygen interact with? Is it reduced or oxidised?
- Why is this voltage used and not another?
- O2 from the blood sample diffuses through a semipermeable membrane into an aqueous buffer.
- In the aqueous buffer it is reduced to OH- ions with the application of a potential difference (600-800mV/0.6V); this causes a current to flow between two submerged electrodes - a silver/silver chloride anode and platinum wire cathode sitting in solution of sodium chloride. Increasing the voltage across this system also increases the current - up to a plateau. The plateau level depends upon, and is proportional to, the concentration of oxygen.
◦ Oxygen si reduced at the cathode
◦ Silver is oxidised at the anode
◦ The current reaches a plateau when the rate of reaction is determined by diffusion of oxygen reather than the voltage and at this stage correlates with oxygen tension - The rate of increase of current in proportion to increase in voltage becomes non-linear at a PaO2 above 150mmHg, and the ABG machine is usually clever enough to compensate for this known fact; additionally as the potential difference or voltage is increased further above the plateau a new rise occurs with reduction of H2O
Draw a curve relating current to voltage in the context of oxygen measurement at the clark electrode
Draw a curve relating PaO2 to current as per the clark electrode
Is the curve relating current flow to PaO2 linear?
Yes until the PaO2 is >150
- The rate of increase of current in proportion to increase in voltage becomes non-linear at a PaO2 above 150mmHg, and the ABG machine is usually clever enough to compensate for this known fact; additionally as the potential difference or voltage is increased further above the plateau a new rise occurs with reduction of H2O
Why is there a membrane for oxygen to diffuse past in the clark electrode?
- The membrane both protects the platinum from becoming encrusted and offers predictable diffusion distance for oxygen without chances of convection protecting it from some sources of error
What is the problem with using higher and lower voltages in the clark electrode
Firstly - the change in current varies greatly with small changes in potential difference at lower and higher levels which means that unless the deviece creating potential difference is highly specific alterations in current may be due to potential difference alterations rather than PaO2
At higher voltages water becomes to be reduced thus the plateau rises again
- At lower voltages the reaction rate is limited by the avilability of energy - few oxygen molecules are reduced at such a low voltage so little current is seen; as the current is increased the plateau develops where the failure of increasing potential difference to drive oxygen reduction occurs not because of available energy but now because of available oxygen
What does the clark electrode actually measure?
The electric activity equivilant of oxygen tension/equivalent partial pressure - it does not measure PaO2
Inaccuracies of the clark electrode may be encountered due to
◦ The other elements equally electroreducible are copper, silver and lead which should not be in blood samples
◦ Electrode barnacles - hygeine issues of exposing biological systems to current as the silver anode will become encrusted with AgCl requiring scraping and chloride depletion occurs
◦ The electrode depends on steady flow therefore with a stagnant sample you have about 25 milliseconds to get the measurement before it is inaccurate as diffusion from the sample is exhausted
Draw a clark electorde
Write a chemical equation desribing the clark electrode
Define pH
a number expressing the acidity or alkalinity of a solution as the logarithm of the reciprocal of hydrogen ion concentration
◦ pH is the negative logarithm (base 10) of the hydrogen ion acitvity in a solution
How is pH measured
- pH is measured with a glass electrode suspended in the blood sample.
- The blood sample acts a a conducting electrolyte.
- The potential difference across the electrode is proportional to the pH difference, and this can be measured.
Draw a pH measurement device
Describe the base prinicples to the glass electrode for measuring pH
- Differences in hydrogen ion activity between two solutions can generate a potential difference between these solutions and this potential difference can be compared to the potential difference of a reference samplew ith a known pH allowing pH of an unknown sample to be determined - note this measures acitvity and not concentration
What actually is measured by the glass electrode in pH testing
measures acitvity and not concentration
* An electrical potential develops across a glass membrane proportional to the pH difference across it
How does glass conduct in pH testing equipment
Ionic impurities
* Hydrated gel interface is water saturated glass and is incredibly thin - and the only reason it functions as a pH measurement device is this capacity - as the solution of increased H+ activity encounters the hydrated glass a cation exchange occurs
What is the underlying equation upon which the glass electrode is based?
What is the gold standard of pH measurement
Harned cell
What is a Harned cell?
- Cell composed of two half cells - one with standard platinum/hydrogen electrode and the other with a siler/silver chloride electrode
- Electrodes are immersed in the same reference buffer and a spontaneous reaction takes place generating a current E whcih together with a known concentration of variables is plugged into the Nernst equation to arrive at pH
What is the problem with the Harned cell and why it is not used commercially
◦ Experimental uncertainty of 0.004
◦ Comparison of pH measurements between laboratories do not perfectly agree
◦ Takes hours to rpepare and a considerable level of skill and methodological precision to replicate - i.e. potentiometric method too cumbersome
What is PaCO2 measured by
Severinghause electrode
What is the severinghause electrode measuring? What is it composed of?
CO2
Glass
Describe the workings of the severinghaus electrode
- The electrode is bathed in a solution which contains some sodium bicarbonate reacting with CO2, and generates a change in pH in the electrode corresponding to a change in known potential difference.
- The CO2 from the blood sample diffuses across a semipermeable membrane into the bicarbonate solution –> across the silicone membrane into an acqueous solution or a bicaronate solution –> carbonic acid dissociation –> pH change which is completely PCO2 dependent provided temperature and pressure are constant
- The reaction changes the pH in the electrode, which corresponds to a change in potential difference, and this is measured.
- The CO2 is then inferred from the change in pH.
Draw a Severinghaus electode
What is the equation to solve pH when CO2 is dissovled
How is bicarbonate calculated on the ABG machine?
alculated using the henderson Hasselbach equation
* The ABG machine derives its value from pH and pCO2
Draw the equation for calculating HCO3 concentration
Which is more accurate
- Biochemical lab measured HCO3
- ABG machine
- The answer to which is more accurate is which got the analyser faster - as the measurements are within 3mmol 96% of the time and discrepancies are more likely to be due to pre-analytical error and sample handling
What is the normal PaCO2 difference
ETCO2 difference is 2-5mmHg
* This is due to alveolar dead space which is usually small in healthy adults
◦ If there was no alveolar dead space these values would be identical
◦ Alveoli which are ventilated and not perfused contain a gas mixture identical to inspired gas and this dilutes end expiratory CO2 concentration and decreases the ETCO2 value
How are PaCO2 and EtCO2 measured
Measurement of end tidal and arterial PaCO2 is performed by different mechanisms
* PaCO2 is measured using the partial pressure of CO2 and its effect on carbonic acid. The change in hydrogen ion concentration in response to CO2 partial pressure is measured in a carbonic solution. The apparatus is called a Severinghaus CO2 electrode
* End tidal CO2 measurements are performed using an infrared analyser where specific 4.28 micrometer wavelength infrared light is passed through the gas supply continuously and the absorption fraction at end expiration reflects sample CO2 concentration. As end expiration is used it closely approximates alveolar gas as the earlier phases of expiration reflect some combination with anatomical dead space.
What factors can accoutn for a difference in PaCO2 and EtCO2
- Patient factors
◦ Changes in pulmonary perfusion
‣ Regional - emboli (fat/air/pulmonary)
‣ Global
* Pulmonary hypertension
* Cardiac failure
* Cardiac arrest
* Severe extreme hypovolaemia
* Very high PEEP or positive inspiratory pressure
◦ Changes in ventilation
‣ Increased V/Q mismatch or increased alveolar dead space - high PEEP or positive airway pressure
‣ High FiO2 - causing shunt to poorly ventilated alveoli
‣ Oesophageal intubation
‣ Very large shunt fraction >30% - Equipment factors and measurement error
◦ DIlution of expired gas e.g. presence of helium
◦ Presence of nitrous oxide is absorbed at a similar wavelength to CO2 and can cause errors in ETCO2 measurement
◦ In-line HME filter can reduce ETCO2 concentration
◦ Timing of measurement may be wrong - measurement is only valid if truly end tidal
◦ Leaks or occlusion of the capnometry
Is it possible to invert the relatinoship between PaCO2 and EtCO2
Reverse relationship where PaCO2 lower than end tidal
* Very small FRC —> means at end expiration CO2 from circulation added to reduced lung volume and becomes concentrated in those alveoli still open at end expiration
* High VT and low respiratory rates - at slow expiration deeper slower alveoli are able to share gas with capnometer whereas previously it would remain in the airway
* Strenuous exercise - large CO2 variation seen over the course of a breath
* Rebreathing of expired CO2 may initially increase end tidal CO2 above arterial
* Addition of CO2 to expired gas e.g. intestinal CO2 used for insufflation, escaping following gastroscopy
Can oxygen be measured using infrared techniques?
No
It is a molecule with 2 similar atoms
How is oxygen concentration measured
- Paramgnetic analysis
- Fuel cells
How does paramagnetic measurement of oxygen work?
- Oxygen is paramagnetic - meaning it is attracted by magnetic fields but does not propagate the field (diamagnetic = repels)
◦ This is because its two unpaired valent electrons have the same spin - Many other gases weakly repelled by magnetic fields (diamagnetic)
- The attraction of a gas mixture to a magnetic field is therefore proportional to its oxygen content
- Many different methods exist which use this property to determine oxygen content
Draw a paramagnetic oxygen analyser circuit
- Gas tested flows into a tube
- A reference gas flows into a parallel tube
- Both gases then pass through:
◦ Flow restrictors
◦ Magnetic field
‣ This is being turned on and off at ~100Hz. - The gases combine in the magnetic field
- The greater the oxygen content of the gas, the more it will move into the magnetic field
- This movement creates a negative pressure behind the gas.
- The pressure difference between the tested gas and the reference gas is proportional to the oxygen content of the test gas.
What are the pros and cons of paramagnetic oxygen analysis
- Advantages
◦ Accurate
◦ Rapid response time - breath to breath variation
◦ Don’t require regular calibration - Cons
◦ Water vapour reduces accuracy
◦ Other paramagnetic gasses interfere e.g. nitric oxide - weakly interferes
◦ Needs a side stream line if used on a ventilator
Describe a fuel cell measurement of oxygen concentration
- Fuel cells rely on reduction of oxygen to measure oxygen partial pressure. They consist of:
◦ Oxygen permeable membrane
◦ KOH solution - This contains:
◦ Lead anode
‣ Lead is consumed as the fuel cell operates.
◦ Gold cathode - Method
◦ Oxygen diffuses across the membrane into the potassium hydroxide solution
◦ At the cathode:
‣ O2+4e−+2H2O→4OH−
◦ At the anode:
‣ Pb+2OH−→PbO+H2O+2e−
◦ The oxygen consumption is proportional to the current generated, which is measured with an ammeter
Advantages and disadvantages of fuel cell oxygen measurement
- Advantages
◦ No power needed
◦ Small
◦ Accurate - Disadvantages
◦ Accumultates nitrogen ni the presence of N2O and underreads PO2 in this instance
◦ 6-12 month replacement
◦ Requires regular 2 point calibration at 21% and 100%
◦ Slow response time 20seconds
Psychrometry is?
SCience of humidity measurement
Hygrometer
Device for measuring humidity
Methods of measuring humidity
- Wet and dry bulb hygrometer
- Hair tension hygrometers
- Dew tension hygrometers
Explain how a wet and dry bulb hygrometer works
2 glass thermometers
- 1 measures ambient temperature
- 2nd one has its bulb surrounded by a water saturated wick which will be subject to evaporation determined by surrounding air temperature AND relative humidity. Evaporation will cause a temperature drop in the wet thermometer and the degree of drop can be compared to reference tables
Explain how a hair tension hygrometer works
The cytokeratin molecules in human hair are held together by disulphide bridges and hydrogen bonds. Humidity does not affect sulphur - sulphur bonds; it does effect the hydrogen bonds which readily absorb water and lengthen/relax as the hydrogen bonds weaken. Hair lengthens in a consistent manner to water and therefore is relative tot he surrounding humidity. Therefore if hair is held at a constant tension, the length will varying with humdiity and is coupled ot a needle gauge.
Issues - slow response difficulty coupling to an electrical circuit.
Accuracy highest 20-90% humidity
Explain how a dew point hygrometer works
Dew point - temperature at which water in a sample of moist air is at its saturated vapour pressure and below which condensation will occur
If ambient temperature and saturated vapour pressure are known for a temperature then relative humidity can be calculated from the dew point. Regnaults hygrometer consisting of a silver tube (good thermal conductivity so the temperature adjcaent to the outside of the tube is the same inside) contains diethyl ether - air is bubbled through the ether causing ti to vapourise (latent heat of vapourisation) and therefore cool the tube. The temperature at which condensation begins to form on the outside surface of the tube is the dew point. Chilled mirror is used instead of a glass tube, and electornic sensors detect condensation
Equation for relative humidity
= absolute humidity x 100/ absolute humidity at saturated vapour pressure for that temperature
Is absolute humidity temperature dependent
no
It is simply the amount of water vapour present per unit volume and does not change unless water vapour is added or removed
The only caveat being is if you are at saturated vapour pressure and drop the temperature the volume will drop because condensation occurs
Is relative humdiity temperature dependent
Yes
SVP of water and thus absolute humidity when saturated is temperature dependent. The demoninator for the calculation changes with temperature
What is a regnaults hygroemter
Dewpoint hygrometer measuring humidity
How much flow is lost in a side stream end tidal CO2 analyser?
150-200ml/min
Carbon dioxide peak wavelngth absorption
4.28 micrometers
Collision broadening effect is?
Mixtures of gasses alter the absorption wavelngths boradening the spectrum of absroption but also potentially shifting the peak absorption wavelength due to itneractive forces between moelcules
Draw an infrared gas analyser
Label the features of a normal capnograph
Is oxygen attracted or repelled by a magnetic field?
Attracted weakly - paramagnetic property because electrons in the outer shell are unpaired
Explain a null deflection paramagnetic oxygen analyser
Sample chamber, containing a beam connecting two nitrogen containing spheres. This dumb bell is suspended by a torisional balance, when no oxygen is present the psheres rest over the poles of a strong magnet, but oxygen when present dispoalceds or pushes the weakly diamagnetic nitrogen filling the suspended spheres away from the mangetic causing rotation o the beam whcih is measured by a mirror attached to the beam reflecting light onto a scale.
Pros - highly accurate
Cons - slow response time, interference from water vapour, vibrations, variations in entilation pressure and high flow rates
Thus not used in ventilators
Draw a pulse field paramagnetic oxygen analyser
Air is drawn from the breathing circuit, mixed with a reference gas within the analyser and a magnetic field is switched on which attracts oxygen molecules, causing oxygen to accumulate in the region of the field increasing prtessure on one side of a double sided pressure transducer. The pressure difference reflects difference in partial pressures between sample and reference gas. Cycling rapidly allows fast response time
Cation
positively charged
anion
negatively charged molecule
Which direction does current flow in a galvanic fuel cell
cathode to anode
Metal atoms within the anode are oxidised losing electrons forming cations (positively charged) that migrate towards the cathode
What is the name of the electrode measuring pH
Sanz
What are the terms used to delineate measurement of ABG variables at actual temperature vs 37 degrees
alpha STAT - measuring at 37 degrees irrespective of patients actual temperature
pH STAT measuring at the patients actual temperature
Draw a simplified clark electrode and explain
- What type of electrode
- Anode is
- Cathode is
- Solution
- What voltage is applied?
- Current depends on
Polarographic electrode
ANode is silver wire coated in silver chloride
Cathode is platinum wire
Solution potassiu, chloride
Small 0.6V fixed voltage
Cathode oxygen, electrons and water react to deliver hydroxide ions
O2 + 4e- + 2 H20 –> 4 OH-
Describe how mass spectrometry works
Used for measuring low concentrations of gasses
Creates positive ions from gas molecules by bombarding them with high speed electrons, knocking out electrons from outer shells of atoms, and the ions are then accelarated by passing through an electric field and deflected with a strong magnetic field with heavier ions being deflected less than lighter ones. Each ion is separated according to its charge to mass ratio allowing an array of detectors to collect these. Gas cannot be returned and they are large cumbersome machines, hgihly accurate however
Explain the priniciples of raman speectroscopy
Photons of electromagnetic energy interact with atoms by either elastically scattering where they retain their original frequency or wavelength; or a small minority are absorbed giving energy to the atom and raising an electrno to a higher energy level before being released with a different energy to the intiial photon. This is called inelastic scattering and each atom has a unique photon release, whcih can be analysed. An argon lazer of fixed monochromatic light is passed through a sampling chamber and raman scattered light measured at right angles - most suitable for carbon dioxide and nitrous oxide. Small portable, easy to use, low mantenance. Replaced by cheaper infrared
Define vapour
a subsatne in its gas phase at a temperature blow its critical temperature - the critical temperature being the temperature at which a substance can only exist as a gas no matter how much pressure is applied e.g. 100 degrees celcius for water
What is the saturated vapour pressure
a fraction fo the total atmospheric pressure at which an equilibrium is reached between liquid and gas phase
How to directly measure the oxygen content in a blood supply
release oxygen from the blood sample then use the Lloyd Haldane technique to measure the amount of oxygen in the evolved gas - add saponin and potassium, ferricynaide to blood causing haemolysis of blood cells and converts all haemoglobin to methaemoglobin causing oxygen release. Oxygen is then released and collected - measured using van slyke apparatus which is amnometric using pressure change in a constant gas volume. Oxygen partial pressure in the evolved gas can e measured using polarography, fuel cell for faster although less accurate results. Oxygen dissolved is not released but can be estimated from PO2
How to indirectly measure oxygne content in a blod sample
Oxygen contet = 0.003 x pO2 (mls O2/dL) + 1.34 x sats x Hb
What problems are there with indirect measures of oxygen content in blood samples
3 separate measurements required each with their own set of errors - pO2, Hb, Saturations - noting that saturations is often indirect based on pO2 ansd therefore subject to errors uncorrected e.g. oG and pCO2 effects on dissocitation curve accounted for but no others are
What methods of flow monitoring are used in a ventilator
Hot wire anemometry
Variable orifice flow meters
Screen pneumatacography
Ultrasonic flowmeters in modern ventilators
What methods of pressure monitoring are used in ventilators
- Aneroid manometers
- integrated silicon wafer transducers - wheatstone bridge
How is volume measured in a ventilator?
Extrapolated from flow over time
What system in modern ventilators is related to an aneroid manometer?
Bourdon gauge
Explain how a aneroid manometer works
Evacuated chamber with a flexible lid on top of it - deformation of the lid gives rise to some measureable change in the manometer whether electrical conductivtiy/resistance or mechnaical displacement of a lever on an analogue scale (Bourdon gauge)
How does a Bourdon gauge work
Coiled elastic tube to which a needle is coupled - as the pressur einside the tube increases it tries to straighten changing the possition of the needle (usually via a system of gears)
Problems with an anaeroid manometer
Time delay due to displacement and gear mechanisms - low frequency response - failing to detect brief changes in ressure but also reacting slowly
Fragility
Most ventilators now use what type of system to measure pressure
Electrical strain gauge - with vairbale inductance or strain gauge transducers. Measure the deformation of a diaphragm placed between two induction coils. The diaphragms inductance changes in proportion to the extent of deformation and therefore pressure
Strain gauges measure the resistance change in some component of the diaphragm which changes the current through a circuit - e.g. wheatstone bridge
How do modern ventilators measure flow?
Ultrasonic sensor array
Two transducers are used to analyse changesin ultrasound wave transit time caused by the velocity of the intervenign medium.
The velocity of the gas throught he tube influences the propagation of sound waves in the gas in the same way movement of a sound emitter might do to a stationary gas e.g. doppler effect
They rely on the constant physical properoties of the gas flowing through them
Benefits - do not impede flow or add resistance; low volume
Problems - lower accuracy as compared ot other designs
What axes are used when describing pulse oximeter physiology for infrared and red light
Absorbance in lumens/ mmol per cm
Wavelength in nm
What is an extinction coeffcieint
The molar extinction coefficient is specific to every chemical and an important variable in the Beer-Lambert law. The molar extinction coefficient measures how much light a substance absorbs and is wavelength specific. It is also sometimes referred to as the molar absorption coefficient or molar absorptivity.
What are the 4 components to a pulse oximeter>?
(1) Diodes
within the probes produce light of the required wavelengths, usually in the red or infra-red range as absorbance by body tissue is small compared to blood
emitted light may alternate between wavelengths at several hundred Hz
(2) Photodetector
on opposite side of probe
detects transmitted light
(3) Signal converter
converted to a dc component = tissue background, venous blood & the constant part of arterial blood flow
converted to a ac component = pulsatile arterial blood flow
the dc component is disregarded/ subtracted
the ac component is amplified and averaged over a few seconds
(4) Display unit
signal is displayed ideally as a continuous trace
shows quality of signal and numbered value of SpO2
What do the AC and DC signals represent in a pulse oximeter
ac = pulsatile arterial blood
dc = tissue + capillary blood + venous blood + non-pulsatile arterial blood
What does the R ratio in pulse oximetry refer to?
R = (ac absorbance/dc absorbance) red / (ac absorbance/dc absorbance )IR
R corresponds to SaO2
— SaO2 100% = R 0.4
— SaO2 85% = R 1
— SaO2 0% = R 3.4
What does an R value of 0.4 correlate with
R = (ac absorbance/dc absorbance) red / (ac absorbance/dc absorbance )IR
R corresponds to SaO2
— SaO2 100% = R 0.4
— SaO2 85% = R 1
— SaO2 0% = R 3.4
What does an R value of 1 correlate with
R = (ac absorbance/dc absorbance) red / (ac absorbance/dc absorbance )IR
R corresponds to SaO2
— SaO2 100% = R 0.4
— SaO2 85% = R 1
— SaO2 0% = R 3.4
What does an R value >1 correlate with?
R = (ac absorbance/dc absorbance) red / (ac absorbance/dc absorbance )IR
R corresponds to SaO2
— SaO2 100% = R 0.4
— SaO2 85% = R 1
— SaO2 0% = R 3.4
What limitations are there to pulse oximetry? 5
- SpO2 and SaO2 are not measures of blood or tissue oxygenation (if [Hb] and cardiac output known, then oxygen dleivery (DO2) can be calculated from SaO2)
- insensitive to directional changes in PaO2 above 80mmHg
- relatively insensitive to perfusion due to gain
- reading failure
- lag time
Describe sources of error in saturation probes
Patient non pathology
- Skin
- Nail polish
- Movement
Patient pathology
- signal to noise ratio (shocked, hypothermia, vasoconstrictors) - as the pulsatile component is only 1-5% of total so vasoconstriction and a reduced volume of this makes it less accurate
- Dyshaemoglobinaemias
- IV dyes
- Arrhythmias and abnormal pulse - venous waves, ventilation, non pulsatile flow
Sensor
- Ambient light and light artefact
- Probe position
Processor
- Extrapolation of data below 80% with progressive inacccuracy
Why does methaemoglobinaemia give saturation readings of 85%
It has an R value of 1
Which means it absorbed 940nm light ot a greater degree than 660nm
Causes of high cooximetry and low pulse oximetry readings
Causes of HIGH Co-oximetery and LOW Pulse Oximetry Readings
Poor peripheral perfusion
Ambient light
Poor probe contact
Dyes – methylene blue
TR
Causes of low cooximetry readings and high pulse oximetry readings
Causes of LOW Co-oximetry and HIGH Pulse Oximetry Readings
COHb
MetHb
Radiofrequency interference
What is a plethysmograph
volume against time graph
Seen when using pulse oximeters to represent the pulsatile part of the trace to provide information as to the volume and therefore adequacy of the perfusion signal
What internal features does a pulse oximeter have to improve reliability of its readings
- Plastic jacket blocking ambient light
- 300-900Hz frequency to measure the effect of ambient light and cancel out
- Amplification of the pulsatile signal
- Averages measurements over a few seconds to allow for movement artefact
Does anaemia affect pulse oximetry
Anaemia does not affect measurements
Isobestic points allow sats probe to correct for Hb concentration
As it represents a percentage of xoy/deoxy t does not affect measurements
Does hyperbilirubinaemia affect pulse oximetry?
No
Does foetal haemoglobin impact oximetry
No
Does polycythaemia affect pulse oximetry?
No
Draw a clark electrode
Platinum cathode - oxygen combines with electrons and water to produce hydroxyl ions.
Silver/AgCl anode - silver reacts with KCl to produce silver chloride and liberate electrons
A potential differenc eof 0.6V is applied between electrodes
Surrounded by KCl
Battery
Ammeter - measuring current flow
Oxygen permeable memebrane and a blood chamber
Temeprature sensitive and therefore kept at 37 degrees
Two point calibration at 21% and 100% O2`
Why does a clark electrode use 0.6V
Cruve relating current flow to voltage is not linear for a particular oxygentension
0.6V is the middle of the plateau region for oxygen concentration meaning amount of current flow wont be altered for a small change in applied voltage making the reading more stable
The calibration curve of curretn vs PO2 is linear
Colorimetric CO2 devices have what level of sensitivitY/
Yellow when acidified with ETCO2 >2%
Purple when less than 0.5%
