Gas Analyzers Flashcards
Ways to Measure Oxygen
Electrochemical (Galvanic Sensors/Polargraphic Electrode)
Paramagnetic Technology (ETO2)
Ways to Measure CO2
IR, chemical colorimetric analysis
(Raman, Mass Spec)
Ways to Measure Volatile Agents
IR, Refractometry, Piezoelectric Activity
Ways to measure N2O
IR
Infrared Gas Analyzers
gases with two or more dissimilar atoms in molecule have specific, unique IR light absorption spectra
Amount of IR light absorbed proportional to concentration of absorbing molecules –> concentration determined by comparing IR absorbance and sample with known standard
Application of Beer Law
Blackbody Radiation Technology
- Analyzer selects appropriate IR wavelength using individual filter or filter wheel to maximize absorption by selected gas at peak wavelength
o Minimize absorption by other gases/vapors that interfere with measurement - After sensor detects transmitted IR energy, electrical signal produced, amplified concentration displayed
Elements of Blackbody Radiation Technology
Heated element = blackbody emitter
Source of IR – broad IR spectrum
Halogenated Gases and IR
separate chamber to measure absorption at several wavelengths
Typically single channel, 4 wavelength IR filter photometers with filter for each anesthetic agent and one for baseline
Each filter transmits specific wavelength of IR light - each gas absorbs differently in selected wavelength bands
Sidestream IR
IR continuously focused on spinning wheel
o Wheel: holes with filters for gases to be measured
Gases pumped continuously through measuring chamber with filtered/pulse light passed through sample, reference chambers
Light focused on photosensor
o Amount of light absorbed by sample gas proportional to partial pressure of gases whose IR absorption pattern corresponds to wavelengths selected by filters on chopper wheel
Monochromatic Sidestream IR
one wavelength to measure inhalation agent, unable to distinguish btw agents or detect mixture of agents
Must select agent
Polychromatic Sidestream IR
multiple wavelengths to identify and quantify various agents
Mainstream IR
Sensor houses both light source and detector, fits over cuvette
o IR light shines through window on one side of adapter
o Sensor receives light on opposite side
o After passing through sample chamber, light goes through three ports in rotating wheel - known high CO2, vented to sensor’s internal atmosphere, sealed cell with N2
o Radiation then passes through filter that screens light to correct wavelength to isolate CO2, directs onto photo detector - signal amplified and sent to display
Microstream IR
- Laser based technology to generate IR emission that precisely matches absorption spectrum of CO2
- Smaller sample cell, low flow rate (50mL/min)
- Emission source: glass discharge lamp without an electrode coupled with IR transmitting window
MOA Microstream IR
o Electrons generated by radio frequency voltage excite nitrogen molecules
o CO2 molecules excited by collision with nitrogen molecules, emit signature wavelength as drop back down to ground state
o Emission split: one directed to main optical detector via gas sample cell, other passes through reference detector (continuous reference detector)
o Amplitude of signals received by detector depend on amount of radiation absorbed from gas sample
Absorbed radiation proportional to CO2 concentrations
Advantages of IR Microstream
- Smaller sample cell, low flow rate (50mL/min)
o Advantageous in very small patients, high respiratory rates, low flow applications, on intubated patients
o Readings not affected by high concentrations of oxygen or anesthetic gases
o Measurements Q25 msec – rapid response time - Adapter able to be used in any orientation, prevents sampling line from occlusion by water/secretions, line contains hydrophobic filter
o No water trap necessary
Advantages - IR
- Multigas analysis: CO2, all volatile agents, N2O
- No need to scavenge gases, portable
- Quick response time
- Short warm up time
- Convenient, require only periodic calibration
- Lack of interference from other gases (argon, O2 concentrators)
- Warn of breakdown products/CO - display of wrong or mixed agents
Disadvantages IR
- Cannot measure nonpolar molecules: O2, argon, He, N2, Xe
- overlap and absorption Spectra between CO2, N2O - false elevations of CO2 with N2O (0.1-1.4Torr increase/10% N2O)
- Mixture of agents can cause erroneous readings
- Helium: IR analysis underestimates CO2
- Interference from other substances, esp methane (also Ethanol, diethyl ether) spuriously high volatile agent readings
- Interference from water vapor: CO2, VA readings
- Slow response time
- Radio frequency interference
- Difficult to add new agents
Piezoelectric
o Vibrating crystal coated with layer of lipid to measure VAs
When exposed to VA, vapor adsorbed into lipid –> resulting change in mass alters vibration frequency
Electronic system, two oscillating circuits: uncoated (reference) crystal, coated (detector) crystal
Electrical signal proportional to vapor concentration
Piezoelectric - Advantages
- High accuracy
- Fast response times
- No need for scavenging
- Short warm up time
- Compact
Piezoelectric - Disadvantages
- Only one gas measured
- No agent discrimination
- Inaccuracy with water vapor
Refractometry
Gold standard for inhalants
Primarily used for vaporizer calibration
Cannot be used to measure vapor levels of halogenated agents, typical anesthetic gas mixture - sensitivity to N2O
MOA Refractometry
light passes through gas, refracts it by certain degree - calibrated for specific gas
Portion split light beam passes through chamber into which sample gas aspirated
Other portion passes through identical chamber containing air
Vapor slows velocity of light so portion passing through vapor chamber delayed
Beams recombined to form interference pattern, consists of dark/light bands
* Position of bands viewed through eye piece against superimposed scale= vapor concentration
Requirement of Refractometry
Requires refractivity number of gas being analyzed
What is refractivity?
nonlinear function of partial pressure of gas
Chemical CO2 Detectors
o pH sensitive indicator enclosed in housing: indicator exposed to carbonic acid formed as product btw CO2 + water becomes acidic, changes color
o Returns to resting color unless used with rebreathing system
o Btw patient/BS or patient/RBB
What are the two types of chemical CO2 detectors?
- Hygroscopic
- Hydrophobic
Hygroscopic Chemical CO2 Detectors
- Hygroscopic filter paper impregnated with pH indicator
- Mean minimum CO2 for color change = 0.54%
- Usefulness: few minutes to several hours, prolonged with HME
- Very affected by humidity
Hydrophobic Chemical CO2 Detectors
- Faster response time, less affected by humidity vs hydroscopic
- Colorimetric device, color change
When might use a chemical CO2 detector?
confirm correct placement when no capnometer, unreliable in arrest
Advantages of Chemical CO2
- Easy to use, disposable, portable
- Small size, no power source needed
- Performance not affected by vapors, N2O, CO
- Low cost
- Accurate in identifying esophageal intubation
- Minimal resistance to airflow
- No cleaning
- Always ready for use
Disadvantages of Chemical CO2 Detector
- Several breaths before conclusions drawn (6 breaths recommended)
- False neg with low VT, low ETCO2
- Not helpful during CPR: no CO2 if no circulation to lung
- IT drugs or gastric contents: irreparable damage
- No alarm, waveform
- Difficult to interpret color changes
- Not cost effective for routine use
- Semiquantitative info only
Paramagnetic O2 Analyzer
O2 = paramagnetic gas, locates self in strongest portion of magnetic field
When gas that contains oxygen passes through switched magnetic field, gas expands/contracts –> pressure wave proportional to oxygen partial pressure
MOA of Paramagnetic O2 Analyzer
Sample gas compared to reference gas, passed by rapidly switching magnets
* Difference in magnetic potential generates force
* Difference detected by transducer, converted into electrical signal displayed as oxygen partial pressure or volumes percent
Advantages of Paramagnetic O2 Analyzers
rapid breath to breath analysis
can be combined with IR analysis of CO2/VA/N2O in same monitor using same diverted gas – single monitor for everything
Disadvantages of Paramagnetic O2 Analyzer
desflurane causes paramagnetic sensor to read higher than expected; nitrogen accumulation esp during closed circuit ax if air reference gas
What are the two types of electrochemical O2 Analyzers?
- Galvanic
- Polarographic
MOA Galvanic O2 Sensors
Oxygen diffuses through sensor membrane, electrolyte to cathode – reduced, causes current to flow
o Rate at which oxygen enter cell, generates current proportional to pp of oxygen in gas outside membrane
o Display usually in percent oxygen
o Calibrated with gas with known pp of O2 (room air)
Sensor of Galvanic O2 Sensor
anode, two cathodes surrounded by electrolyte
* Cathode: sensing electrode, not consumed
* Hydroxyl ions formed at cathode react with lead anode = lead oxide - anode gradually consumed
Useful Life Span of Galvanic Sensors
- Useful lifespan when package opened, cited in percent hours - product of hours of exposure, oxygen %
o Always measuring even when no flow passing through machine
o If exposed to high concentration life expectancy decreased
o Prolong life by removing from BS when not in use TIROS
Power Source of Galvanic Sensor
Power source not required to operate analyzer, required to power alarms
Reaction within Galvanic Analyzer
Reaction equals temperature dependent, usually paired with thermistor
Polarographic Sensor Components
anode, cathode, electrolyte gas permeable membrane with power source for inducing potential btw anode, cathode
Polarographic Sensors MOA
O2 molecules diffuse through membrane, electrolyte
o Polarizing voltage applied to cathode, electrons combined with oxygen molecules, reduce them to hydroxide ions
o Current proportional to oxygen partial pressure and sample flows btw anode and cathode
Polarographic Sensor - Location
inspiratory side of breathing system
* Service life of some galvanic analyzers reduced by exposure to CO2
Advantages - Electrochemical O2 Analyzers
- Dependable
- Accurate (galvanic > polarographic)
- User Friendly
- Low cost
- Compact
- No effect from argon (O2 concentrators)
Disadvantages - Electrochemical O2 Analyzers
- Maintenance (polarographic > galvanic) – frequent membrane, EL changes
- Calibration required Q8hr
- Not an integral part of AM, turned on by user
- Slow response time/cannot measure ETO2
Rama Spectroscopy
o Measurement of CO2
o Monochromatic argon light beam passed through gas sample
o Light absorbed, re-emitted at different wavelength specific to gas molecule that absorbed it
