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
Draw a clark electorde
Write a chemical equation desribing the clark electrode
Draw a pH measurement device
What is the underlying equation upon which the glass electrode is based?
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
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
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.
Draw an infrared gas analyser
Label the features of a normal capnograph
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
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
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-
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
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`
