I&M I: Lecture 5 - Respiratory Gas Monitoring Flashcards
(68 cards)
What is the main difference between diverting and non-diverting gas monitors?
Diverting monitors use a pump to aspirate samples through a sampling line, while non-diverting monitors have the sensor directly within the gas stream
Diverting
Pump aspirated sample through sampling line to the sensor
Nondiverting
Sensor directly within the gas stream
Only measures CO2 and O2
What gases can non-diverting gas monitors measure?
CO2 and O2
Gas Monitors, Nondiverting (Mainstream) Advantages and Disadvantages
Diverting Gas Monitor
Delayed readings- keep gas line short
Zero using room air, calibrate with known gas composition
Water and secretions affect accuracy
Water traps, filters, hydrophobic membranes mitigate this
High RR or long sample lines decrease accuracy
What is a common application of nasal cannula in gas sampling?
It can be used to sample gas from the patient’s nostrils
Airway Devices and Gas Sampling
Nasal Cannula with sample line
Facemask
Circuit
SFM
Invasive Airways
LMA
Elbow connector sample port
ETT
Some have an incorporated sampling lumen
Most sample from elbow connector
Other
Place sample line near pt nostrils, edge of mouth, NPA, OPA, over trach stomas
Can place over NPA/OPA under SFM/nonrebreather
List some advantages of diverting gas monitors
- Automatic calibration and zeroing
- Minimal dead space
- Low patient cross-contamination
- Multiple gases can be analyzed
- OOD anesthesia (MRI, CT, etc.)… Monitoring from a distance
- Administration of bronchodilators
What are some disadvantages of diverting (sidestream) gas monitors?
- Tubing obstruction
- Sample line connected to wrong port
- Samples must be directed to scavenging
- Delay time
- Multiple disposable parts
- Fresh gas dilution (affects waveform and reading accuracy-false low)
- > difference between arterial and end-tidal CO2 readings
What technology is most commonly used in infrared analysis for gas monitoring?
Blackbody Radiation Technology
Produces broad infrared spectrum
Halogenated agents
Pulled through single-channel, 4 wavelength infrared filter photometers
1 filter per agent, 1 filter for baseline comparison
Underestimates inspired and overestimates end-tidal values at high RR
Infrared Analysis
Unique infrared absorption rates for each chemical
Compares known standard to the sample
Absorption proportional to [absorbing molecules]
Molecules that cannot be measured w/ infrared
O2
Xenon
Helium
Nitrogen
Argon
What is the purpose of the rotating wheel in a diverting infrared analyzer?
To continuously aim infrared light at a chopper and take hundreds of readings per respiratory cycle
Diverting Infrared Analyzer
Infrared light continuously aimed at chopper (spinning) wheel
Hundreds of readings per respiratory cycle (1000s of spins/minute)
Gas is continuously pumped through measuring chamber
Can range from 50 to >400ml/min flows
Avg 50-200ml/min
Filtered (from chopper wheel) light passes through reference and sample chambers
After transferring through chambers, infrared light hits photosensor
Electrical current running through photosensor changes with ∆ in light levels (pp of agents/gases)
What is a non-diverting CO2 analyzer?
A chamber placed between the patient and breathing system that measures CO2 levels
Sensor covers chamber (cuvette)
Light source and detector within sensor
Chamber has 2 windows
Infrared transmits through one window and hits sensor on opposite side
Chamber is heated > body temp
Prevents condensation
Rotating wheel
3 ports: High CO2, internal atmosphere of sensor, 100% nitrogen
Erroneous readings
Dislodged sensor (partial= good waveform, inaccurate reading), condensation, secretions
What is Microstream Technology used for in gas monitoring?
It is a laser-based technology that operates with a small sample cell and low flow rate
Small sample cell and low flow rate
Glass discharge lamp & infrared transmitting window
N molecules excite, collide with CO2, CO2 excites & falls back to ground state
Emits CO2 wavelength: gas sample cell (optical detector) and reference detector pick up wavelength
Absorbed radiation from gas sample is proportional to the [CO2]
No need for water trap
Ideal for tiniest patients, high RR, low-flow cases, and patent airways
Readings not affected by anesthetic gases or high [O2]
True or False: Infrared analysis can measure O2 and nitrogen.
False
Infrared Analysis Advantages
Measure CO2, nitrous, and all common volatile agents
Post-measurement, gases can return to breathing system
No need for scavenging
Part of main physiologic monitors or separate, compact solo unit
Quick measurements
Allows measurements for inspired and exhaled [ ]
CO2 = short response time
Volatiles and Nitrous = longer response times than CO2
Infrared Disadvantages
Cannot measure O2 or Nitrogen
Gas Interference
O2 broadens CO2 absorption spectra (⇩ readings)
N2O overlaps CO2 absorption peaks (⇧ readings)
Other substance interference
Ethanol, methanol, isopropanol, acetaldehyde, acetone
Albuterol
H2O interference
⇧ CO2 and agent readings
Slow response time
Rapid RR not enough time to analyze Insp and Exp agents accurately
Radio frequency interference
2-way radios may ⇧ readings
New volatile agent updates
Can be expensive
What are some applications of O2 analysis?
- Hypoxic/hyperoxic mixture detection
- Disconnect/leak detection
- Hypoventilation detection
- Preoxygenation/denitrogenation progress
- Malignant hyperthermia detection
- Air embolus detection
Paramagnetic O2 Analysis
Paramagnetic substances align near the strongest part of a magnetic field
Oxygen
O2 within a gas sample will expand and contract as it passes through a switched magnetic field
Rapidly switches on and off
Differential transducers detect the pressure difference between sample and reference gases
Electrical signal from transducer displays on monitor as O2PP or volumes %
Closed circuit anesthesia with air as reference gas may build up Nitrogen
O2 as reference reduces risk of Nitrogen accumulation
Des may affect sensor and give a falsely high O2 reading
Galvanic Cell Electrochemical Oxygen Analysis
Located within inspiratory limb of breathing system
Sensor: cathode/anode surrounded by gel electrolyte
O2-permeable membrane holds gel in place, but ions, proteins, etc. cannot permeate
Slow to respond to ∆ in O2 pressure
Anode, cathode (acts as sensing electrode), electrolyte, membrane
Generates enough voltage between 2 cathodes to operate the meter
Only requires separate power source for alarms
O2 diffuses through sensor membrane
O2 reduces at the cathode
Current then begins to flow
Rate O2 generates current is ∝ PP of O2 outside the membrane
Monitor displays the PP as a percent O2
Polarographic Electrode Sensor
Anode, cathode, electrolyte, and gas-permeable membrane
Power source creates potential between anode and cathode
O2 diffuses through membrane then electrolyte and is reduced at cathode
Current ∝ the PP of O2 flows between anode and cathode
Calibrate daily by exposing to room air and verifying 21% reading
Electrochemical O2 Analysis, Disadvantages
Electrochemical O2 Analysis, Advantages
No effect from Argon
Compact
Less expensive than other forms of O2 analyzers
User-friendly
Galvanic cell analyzers more reliable than polarographic analyzers
Piezoelectric Analysis
Lipid-coated crystals vibrate
Volatile agents adsorb into lipid, ∆ lipid mass, ∆ vibration frequency
Piezoelectric system: 2 oscillating circuits
1 reference crystal (uncoated)
1 lipid coated crystal
Difference between the 2 crystal readings gives level of anesthetic agent in sample being analyzed
