anaesthetic gas and blood measurements Flashcards
draw a capnography trace and explain the phases…
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.
what might cause and increase/ decrease in ETCO2
what does this trace show..
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.
draw a capnography to show airway obstruction..
what is the importance of CO2 monitoring?
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.
if you could have one piece of monitoring equiptment what would it be?
capnography - gives a lot of useful info and not invasive, simple to use and set up.
what are the various methods for measuring CO2 in a gas mixture?
most common = infrared spectrometry
others..
* Raman spectrometry
* mass spectrometry
* colourimetry - use of litmus paper
* gas chromatography
* savinghaus electrode
how does an infrared analyser work?
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.
what is beers and lamberts law? including equation
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.
what is the role of the interference filter..
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
what are the sample chamber walls made from?
crystals of silver bromide or salphire
doesnt absorb light of this wavelength e.g. glass would absorb infrared light.
how does the photodetector work?
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.
what is the difference between capnography and capnometry?
capnography = display of the ETCO2 waveform
capnometry is the measure of CO2 conc
what are the 2 methods gases can be sampled from the breathing system?
Side stream or Main stream analysers.
Tell me about side stream gas analysis…
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
tell me about the mainstream gas analyser for CO2
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.
what determines the delay in gas conc analysis
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
what are the problems with infrared spectroscopy?
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.
what are the advantages of IR spectrometry?
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
what is Raman Spectrometry?
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
how does Mas spectrometry work?
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
what are the pros and cons of mass spec?
very reliable, accurate and can measure many gases including O2
however bulky, expensive, time consuming, changes composition of gas
give an overview of how each anaesthetic gas is measured?
usually infrared absorption spectrometry including CO2, volatile, N20
however O2 by paramagnetic O2 analysis usually and sometimes by galavanic fuel cell
how does infrared spectrometry measure anaesthetic gases e.g. sevo
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
how is CO2 measured in anaesthetic practice?
Gas analysis - infrared spectrometry and capnography
blood gas analysis - servinghaus electrode
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
what does this represent…
cardiac oscilations - due to heart beat
how does the fuel cell work..
2 electrodes with shared solution completing a circuit. Ammeter to measure current.
(LAG = lead anode, gold cathode)
- anode - lead. reacts with OH in solution to produce lead oxide and electrons. e- flow around to cathode
- cathode - reduces oxygen to produce OH- ions.
- 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
how does a paramagnetic O2 analyser work?
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
what is a pulsed field paramagnetic analyser…
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
explain how a polographic clark cell works..
(polargraphic clark = PC = platinum cathode )
2 electrodes - anode and cathode, connected in a circuit with common ionic solution ( KCl ) and wire containing ammeter.
- anode = Ag/AgCl
Cl ions react with silver, to release e-. these flow around to cathode - 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
explain how a pH electrode works?
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.
tell me about the CO2 servinghaus electrode..
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
draw Capnograph for emphysema..
due to impaired alveolar ventilation and gas trapping/ closure of airways
