anaesthetic gas and blood measurements

Question 1

draw a capnography trace and explain the phases…

Answer 1

Capnography displays a trace of exhaled CO2.
phase 1 = initially deadspace is expired which contains no CO2
phase 2 = CO2 expired from alveolar gas and mixed with dead space gas, sharp rise as more alveolar gases empty and mix
phase 3 = pure alveolar gas - platau with slight increase. The peak gives a measurement of ETCO2
phase 0 or 4 = inspiration, no CO2 so sudden fall.

Question 2

what might cause and increase/ decrease in ETCO2

Answer 2

Question 3

what does this trace show..

Answer 3

Patient making own resp efforts over the top of the ventilator

either switch to spontaneous mode or give more muscle relaxant or may tolerate pressure support better.

Question 4

draw a capnography to show airway obstruction..

Answer 4

Question 5

what is the importance of CO2 monitoring?

Answer 5

Allows useful info about airway and breathing.

airway - placement of tube e.g. in intubation, throughout operation.

breathing - gives info on level of arterial CO2 e.g. if high or low can help diagnosis. Adequacy of ventilation. pattern may indicate bronchospasm

circulation - also info on circulation e.g. increase in CO2 is positive in cardiac arrest. sudden drop may be P.E

detection of rebreathing and needing to change soda lime/ increase flow.

Question 6

if you could have one piece of monitoring equiptment what would it be?

Answer 6

capnography - gives a lot of useful info and not invasive, simple to use and set up.

Question 7

what are the various methods for measuring CO2 in a gas mixture?

Answer 7

most common = infrared spectrometry
others..
* Raman spectrometry
* mass spectrometry
* colourimetry - use of litmus paper
* gas chromatography
* savinghaus electrode

Question 8

how does an infrared analyser work?

Answer 8

Based on fact that all molecules with 2 dissimilar atoms will absorb infrared light at a particular wavelength, for CO2 this is 4.3um - knowing this wavelength we can expose the molecules to this and measure how much light was absorbed using beers law.
Beers law = amount absorbed is proportional to concentration of gas molecules in the mixture

Uses a light source which emits infrared light from hot wire, filter to produce a specific wavelength of light, exposes this to a gas chamber of known constant size (lamberts is constant) and then has a photodetector to measure amount of light leaving the chamber. ( a lens focuses light onto this photodetector)

Reference chamber
during the process the light also passes through a reference chamber of known concentration. The absorptions are compared by the microprocessor this helps to remove any errors such as non-continous outputs by light source, ambient light influence, changes to functioning of the detector.

microprocessor displays info on a screen.

Question 9

what is beers and lamberts law? including equation

Answer 9

Beers law = amount absorbed is proportional to concentration of gas molecules in the mixture

Lambert law = amount absorbed is proportional to the length the light has to travel - this is kept constant

amount absorbed = ebc
c = conc, b = length passed,
e = molar absorbance - constant per substance per mole.

Question 10

what is the role of the interference filter..

Answer 10

A filter is placed between hot wire infrared source and reference chamber
this ensures only a specific wavelength of light is allowed to pass that is absorbed by the gas of interest.
therefore the amount absorbed can be said to be directly proportional to the conc of that gas.

otherwise, light of other wavelengths will be absorbed by other gases and this wont be an accurate measure

Question 11

what are the sample chamber walls made from?

Answer 11

crystals of silver bromide or salphire
doesnt absorb light of this wavelength e.g. glass would absorb infrared light.

Question 12

how does the photodetector work?

Answer 12

using a thermopile
as light shines on the thermopile, it is heated. The seabek effect is used where 2 dissimilar metals placed next to eachother which when exposed to a temperature will produce a voltage. this voltage is proportional to the temp change.
hence depending on the output voltage, the amount of light detected and hence absorbed can be deduced.

e.g. less CO2 in sample, the more light passing through, more temp at thermopile, higher voltage.

Question 13

what is the difference between capnography and capnometry?

Answer 13

capnography = display of the ETCO2 waveform
capnometry is the measure of CO2 conc

Question 14

what are the 2 methods gases can be sampled from the breathing system?

Answer 14

Side stream or Main stream analysers.

Question 15

Tell me about side stream gas analysis…

Answer 15

Consists of a small tube of 1.2mm internal diameter, which removes a small samples from the expiratory limb of the breathing circuit.

This is taken at a flow of 150ml/min to be anlysed

it is made from teflon which is impermeable to CO2 and doesnt react with anaesthetic gases.

The closer this is placed to the trachea, the more accurate.

The gas is later added back to the circuit

PROS:
* less monitoring equiptment needed near patient end so more practical
* allows room for larger analyser that can measure other agents too.
* less bulky so can be used in prone.
* no deadspace

CONS:
* water vapour can interfer with CO2 and condense in the circuit. water trap required
* pressure drop may effect measurements
* delay in measurement readings - 2 to 3 secs

Question 16

tell me about the mainstream gas analyser for CO2

Answer 16

A gas analyser with sapphire windows is attached directly to breathing circuit
PROS:
therefore more bulky and limits positioning e.g. prone.
Also limited to size and so usually only for capnography.
CONS:
however there is less of a time delay and can be good for transport.
Adds deadspace
It is also affected by water vapour making it unreliable, by heating the element is can reduce this problem however this poses a risk to the patient.

Question 17

what determines the delay in gas conc analysis

Answer 17

side stream vs main - side stream has more delay
transit time - length of tube and flow rate
diameter of the tube - the smaller, the less delay

Question 18

what are the problems with infrared spectroscopy?

Answer 18

water vapour
* water has a wide infrared absorption so can absorb certain wavelengths and cause inaccuracies
* use of a water trap needed and hygroscopic tubing
* also can condense and physically block tube - position tube upwards to prevent droplets getting in

N20
* collision broadening
* CO2 absorbs infrared light, this molecule can collide with N20 and energy is transferred to N20 and CO2 can absorb more light - hence appears as if there is more CO2 than there is.
* modern machines compensate for this by knowing how much N20 is used.

O2
* cant be measured by infrared spec

atmospheric pressure
* changes to ambient pressure will change the partial pressure of CO2

lag time - to get to analyser and be analysed.

Question 19

what are the advantages of IR spectrometry?

Answer 19

Can simulataneously measure different gases at same time - use a filter that rotates

relatively inexpensive e.g. compared to mass spec

continuous measurements

gases can be added back into the circuit - i.e. doesnt change composion

Question 20

what is Raman Spectrometry?

Answer 20

Gas from breathing system is exposed to a monochromatic light from an argon laser

the energy absorbed from the intermolecular bonds is re-emitted at a new wavelength

the wavelength shift and scattering can be used to measure conc because the wavelength is specific to each gas.

can also be used to measure O2 as well as N20 and voltatiles

however expensive and complex

Question 21

how does Mas spectrometry work?

Answer 21

sample of gas taken and bombarded with electrons to cause it to fragment and ionise. (using anode and cathode)

These molecules are then accelarated by a magnetic field in a vacuum chamber

the distance travelled is proportional to their charge:mass ratio i.e. smaller charged ones will travel further.

By comparing amounts of different fragments, the amount of the parent compound can be determined.

Each gas usually has its own finger print

Question 22

what are the pros and cons of mass spec?

Answer 22

very reliable, accurate and can measure many gases including O2

however bulky, expensive, time consuming, changes composition of gas

Question 23

give an overview of how each anaesthetic gas is measured?

Answer 23

usually infrared absorption spectrometry including CO2, volatile, N20

however O2 by paramagnetic O2 analysis usually and sometimes by galavanic fuel cell

Question 24

how does infrared spectrometry measure anaesthetic gases e.g. sevo

Answer 24

same method as CO2 - i.e. infrared spectrometry however each gas has its own specific wavelength
CO2 - 4.3um
N20 - 4.6um
H20 - multiple wavelengths from 1.4 to 6.3
anaesthetic gases 8-9um

either measured via separate chambers and filters
or by the same chamber exposed to a rotating filter of different wavelengths

each anaesthetic gas has its own absorbance spectral shape e.g. absorbance of y and wavelength on x axis. this can be compared to a memory stored in the machine to detect the agent present and amount of absorbance is proportional to amount of agent

Question 25

how is CO2 measured in anaesthetic practice?

Answer 25

Gas analysis - infrared spectrometry and capnography blood gas analysis - servinghaus electrode

Question 26

Answer 26

A = normal B = sloped phase 3 , no plataeu -airway obstruction e.g. COPD, bronchospasm C = resp efforts over the top D = raised baseline, exhausted soda lime or rebreathing E = increasing ETCO2 - hypoventilation, MH, laparoscopic surgery

Question 27

what does this represent...

Answer 27

cardiac oscilations - due to heart beat

Question 28

what are the uses of capnography in cardiac arrest?

Answer 28

placement of ET tube monitoring ventilation rate monitoring quality of chest compression - should be more than 2.5kpa monitoring ROSC

Question 29

what are the uses of capnography in non-theatre areas...

Answer 29

ET in correct place vs oesophageal intubation in ITU adequacy of ventilation and cardiac output in ITU. also during percutaneous tracheostomy insertion can be used as a surrogate marker for arterial CO2 for those with head injuries monitoring for sedation either in ED or scan

Question 30

outline different methods of O2 gas conc measurement...

Answer 30

Can be classified into electrical * Fuel cell * clark electrode non electrical * paramagnetic O2 analyser - main one used on anaesthetic machine insp and exp limb * mas spec - used in research

Question 31

how does the fuel cell work..

Answer 31

2 electrodes with shared solution completing a circuit. Ammeter to measure current. (LAG = lead anode, gold cathode) 1. anode - lead. reacts with OH in solution to produce lead oxide and electrons. e- flow around to cathode 2. cathode - reduces oxygen to produce OH- ions. 3. OH- ions carry current to anode to complete circuit reactions... anode: Pb + 2OH- --> PbO + H20 + 2e- cathode: O2 + 2H20 + 4e- --> 4OH- the more O2 , the more reduced at the cathode, the quicker electrons can flow hence current is proportional to O2 conc

Question 32

how is the fuel cell calibrated?

Answer 32

against 2 known samples of 21% and 100% O2 daily this is because current output varies with life of cell.

Question 33

how quick is a fuel cell?

Answer 33

takes around 20-30seconds

Question 34

what limits the lifespan of a fuel?

Answer 34

the lead anode is oxidised with time/ exposure to O2 if left exposed to high O2 it will deplete in a few weeks

Question 35

pros and cons of the fuel cell..

Answer 35

pros: * cheap, compact, no power source needed cons: * gas mixtures containing N20 can damage it as it reacts with lead anode and produces N2 gas which exposes components to pressure * redox reaction at cathode is temp sensitive so need to keep temp constant * slow response time. * needs regular calibration, maintainance and replacement of lead anode.

Question 36

how does a paramagnetic O2 analyser work?

Answer 36

O2 is paramagnetic i.e. attracted to strong magnetic field. Due to its unpaired e- in outer shell. The O2 is exposed to a strong magnetic field and therefore is attracted to the other side and gas flows. this will cause 2 suspended dumbells, connected to a bar and filled of N2 gas to rotate. the degree of rotation is proportional to amount of O2 present. there are a number of ways to measure this rotation 1. mirror and light beam - the angle of reflection of the light beam will tell you the degree of rotation of dumb bells. 2. null deflection - a rotating motor preventing rotation, the current needed to resist rotation is proportional to amount of O2

Question 37

what is a pulsed field paramagnetic analyser...

Answer 37

newer version of paramagnetic - no longer use dumbells but a pressure transducer reference chamber and compare pressure changes... this works via having 2 chambers - one unknown and one reference of known O2. pulsed magnetic field is applied - when O2 molecules move, the pressure drops, displacing the diaphragm. A pressure transducer can compare the pressure drop in each chamber to determine O2 conc - the pressure drop is proportional to speed of O2 flow. accurate and continuous, less prone to drift and more robust than classic paramagentic analysers however can still be affected by water vapour

Question 38

pros and cons of paramagnetic O2 analyser..

Answer 38

Pros * quick * no need for frequent calibration/ replacement * continous cons: * water vapour can cause inaccuracies through stickign and altering ease of movement of the balls. * needs calibration * needs power supply * magnetic properties of other gases could cause interference however not significant as O2 is much more paramagnetic

Question 39

what is the main differences between clark and fuel cell electrode..

Answer 39

clark - needs voltage different solutions and voltage

Question 40

how does gas chromatography work?

Answer 40

separation of gases by passing through a column with a mobile and stationary phase different gases travel at different speeds depending on solubility in each phase hence separated and detected by using known reference

Question 41

why are inspiratory and expiratory volume measurements not the same?

Answer 41

presence of water vapour from lungs

Question 42

give an overview of what blood gas analysers can measure..

Answer 42

direct measurements * O2 - fuel / clark * pH -pH electrode * CO2 - serveringhaus * other electrolytes - Na, K, glucose derived = HCO3, BE, O2 sats

Question 43

what is the difference between an amperiometric and potentiometric technique

Answer 43

measures a current vs voltage e.g. O2 measures current pH and CO2 - measure voltage

Question 44

what are the indications of blood gas monitoring?

Answer 44

respiratory or metabolic acid/ base derrangements see response to treatment in respiratory disease e.g. before/after NIV quick way to check K+ and Hb before official lab results are back

Question 45

how is an ABG sample obtained and analysed?

Answer 45

usually from radial arterial, also femoral asceptic tecnique - wash hands, gloves, chlorprep 22 G needle or via arterial line heparin filled syringe - remove heparin first can use a capillary tube or syringe after sample taken * remove air bubbles * reduce time to analysis * deliver via inlet probe of gas analyser * aspirates sample and heats to 37 degrees * input patient details, FiO2 and temp

Question 46

what is a co-oximeter?

Answer 46

works in a similar way to pulse oximetry but on a blood sample allows analysis of different forms of Hb e.g. oxy, deoxy, carboxy, methamoglobin using a spectrometer of 4 different wave lengths

Question 47

how can the saturations of arterial blood be measured in sample of blood...

Answer 47

1. co-oximetery measures directly 2. using O2 sats dissociation curve - known PaO2 so can derive saturated Hb

Question 48

what is the difference between pulse oximeter and co-oximeter?

Answer 48

pulse oximeter - in vivo, non invasive, also measures pulse rate co-oximeter also uses spectrophotometry but in vitro on blood sample. usually more types of hb

Question 49

explain how a polographic clark cell works..

Answer 49

(polargraphic clark = PC = platinum cathode ) 2 electrodes - anode and cathode, connected in a circuit with common ionic solution ( KCl ) and wire containing ammeter. 1. anode = Ag/AgCl Cl ions react with silver, to release e-. these flow around to cathode 2. cathode = platinum e- reduce O2 to form OH- ions reactions Ag + Cl- --> AgCl + e- O2 + H20 + 4e- --> 4OH- overall 4Ag + 4KCl + O2 + 2H20 --> 4AgCl + 4KOH the more O2 available, the quicker the reduction, more current can flow. hence the current measured is proportional to amount of O2. calibrated scales of known values used to determine ppO2 uses voltage of 0.6V teflon membrane permeable to O2

Question 50

pros and cons of the clark electrode

Answer 50

pros * compact * accurate * acceptable response time - but not quick enough for continous ins/exp gases * slower depletion of electrodes than fuel cell - lasts longer 3yrs. cons - needs voltage - needs frequent calibration - membrane delicate, prone to damage and protein deposits which affects performance - halothane can give high O2 readings, now membrane used impermeable to halothane.

Question 52

explain how a pH electrode works?

Answer 52

2 half cells reference electrode = silver/silver chloride + KCl / KOH solution. AgCl + e- --> Cl- + Ag (sometimes uses a calomel/ mercury electrode) measuring electrode = silver / silver chloride electrode + 0.1M HCl buffer solution. pH sensitive glass in contact with blood. pH sensitive glass is negativley charged on both sides(silica) , H+ ions bind and need to be equal on both sides, causes HCL to dissolciate and Cl free to react with Ag. produces AgCl + e- . kept at a constant temp potential difference betwen 2 half cells is measured.

Question 53

what are the pros and cons of the pH electrode..

Answer 53

small, compact, accurate, resonable response time cons * affected by temp changes * needs to be calibrated against 2 known solutions * regular maintainance as membrane and electrode can deteriorate * non continous

Question 54

tell me about the CO2 servinghaus electrode..

Answer 54

closely related to pH electrode uses fact than CO2 and H+ have a tightly linked relationship one added step - CO2 crosses CO2 permeable membrane (which is impermeable to H+) into a NaHCO3 solution CO2 + H20 --> H+ + HCO3 this solution is in contact with pH sensitive glass and reference electrode. H+ can be measured via pH electrodes. voltage measured is proportional to H+ and hence CO2

Question 55

pros and cons of CO2 electrode..

Answer 55

small, compact, accurate cons slower response time - 2 to 3 mins - extra step in reacting CO2 and H20 and diffusion across membrane membrane can become damaged/ covered in proteins regular calibration

Question 56

which method of O2 measurement is used in modern gas analysers?

Answer 56

clark (fuel cell used to be used on anaesthetic machine for gas measurements, now replaced by paramagnetic)

Question 57

tell me about transcutaneous O2 measurements..

Answer 57

continous non invasive measurement of O2 tension through skin. not used due to convience of pulse oximetry. clark electrode applied to skin, heating element causes vasodilation and O2 can diffuse from blood out of skin to clark electrode for measurement less accurate and can cause burns. but is a method of continuous non invasive measurement

Question 58

do you know any methods for continous blood gas measurements?

Answer 58

transcutaneous O2 measurements - clark on skin surface intra-artial placement of a clark electrode - risks of thrombosis, infection and reduces accuracy of invasive BP monitoring optical sensors coated in flurescent dyes - the intensity of the dye will veary with O2 and CO2 PP jugular venous oximetry - pulse ox in IJV to measure brain sats.

Question 59

how are gas machines calibrated and maintained?

Answer 59

calibrations.. 1 and 2 point calibrations at regular intervals automatically and whenever operator wants to perform. maintainance - electrodes, membranes, solutions. quality control - use known solutions to test accuracy

Question 60

what are teh sources of error for blood gas machines?

Answer 60

**temperature** important to tell machine, the temp of patient as temp influences how well gases dissolve in solution and dissociation of acids. e.g. in hypothermic patient, more CO2 will be dissolved in solution, if heated up this will come out of solution and give higher pp so will appear hypercarbic when not. similarly acidotic **air bubbles** contain 21% O2 so will give falsely high readings **heparin ** causes acidity of blood **delays in sample** respiration - production of CO2, use of O2 so changes **machine problems ** * drift and inaccuracies - need recalibrating * membrane damage - protein deposits

Question 61

how are errors in blood gas measurements reduced?

Answer 61

corrected for patients body temp minimise delays remove air bubbles regular calibration and maintainance

Question 62

AABGI standards for gas monitoring...

Answer 62

during general anaesthesia 02 monitoring + alarms Capnography - sedation and general vapour analysis when used.

Question 63

what is a UV gas analyser..

Answer 63

used to measure halothane as this absorbs light in UV region same principle as infrared spectrometry but UV light.

Question 64

what is meant by a double beam capnometer

Answer 64

the process used in infrared spectroscopy of capnography whereby a reference chamber is used. allowing the device to compensate for changes in the light source or detector sensitivity, changes in light source output, improving accuracy.

Question 65

draw Capnograph for emphysema..

Answer 65

due to impaired alveolar ventilation and gas trapping/ closure of airways