Week 3 - Gas Monitoring Flashcards
What is the Diverting (Side-stream) gas sampling mode?
Gas is aspirated from sampling site and through a tube to sensor located inside or on top of the machine
- Shorter tubing will decrease delay time and provide better waveforms
- ALL gases can be monitored this way
Oxygen: paramagnetic technology
CO2/Volatiles: infrared technology
What is Non-Diverting (in-line) gas sampling mode?
Sensor is located directly in the gas stream
-Only CO2 and O2 are monitored with this mode
Oxygen: Fuel Cell (electrochemical)
CO2: infrared
*CO2 sensor must be positioned between the pt and circuit, ideally closest to the pt end as possible
What molecules does infrared spectroscopy measure?
Molecules containing dissimilar atoms (DOES NOT work for oxygen and nitrogen)
Most molecules will absorb infrared at specific wavelengths and hence the molecule can be identified and its concentration measured (Beer-Lambert Law)
How does infrared diverting (side-stream) sampling work?
Continuously aspirates a sample of the gas from patient circuit, usually near where breathing circuit is connected to the airway device — 50 to 250 mL/min aspirated
- Sampling directed to place between infrared emitter, optical filter, and infrared detector, which outputs a signal proportional to remaining infrared energy not absorbed by the gases
- Multiple optical filters are required to quantify and identify multiple gases simultaneously
- Detected signal then amplified and interpreted via microprocessors
What are the pros and cons of diverting (side-stream) gas sampling mode?
Pros: Automatic calibration and zeroing, Minimal added dead-space, Low potential for cross-contamination between patients
Cons: Multiple places that leaks may occur, More variability in CO2 readings than with in-line sampling (accurate with RR 20-40, decreases with increased rate), Slower response to changes than with in-line sampling, Water contamination (water traps)
What are common issues with diverting (side-stream) gas analyzers?
- Relatively high sample flow (~200 mL/min)
- High flow rate can impede use with infants whose inspiratory/expiratory flows similar to that of analyzer unit (most units now return that flow to the system)
- Essential that water vapor, liquid, patient secretions be kept isolated from the system (managed via selection of sampling line and water trap on unit) (albuterol and other inhalants interfere)
What are the phases of a normal CO2 waveform?
Phase I (Baseline): full inspiration
Phase II (Expiratory Upstroke): represents the transition from dead space gas that does not participate in gas exchange to alveolar gas that contains CO2 (alveolar gas mixes with dead space)
Phase III (Expiratory Plateau): alveolar gas is exhaled – point right before phase IV = end-tidal point (max CO2)
Phase IV (Downslope): start of inspiration - abrupt fall to zero
What is the first line of defense against detecting hypoxic mixtures?
Inspired Oxygen Analysis
- FiO2 monitor is extremely important in pt safety
- But.. ventilation and oxygenation must be considered as two separate entities (Just because you are delivering O2 doesn’t mean pt is oxygenating)
High vs Low V/Q ratio
Low V/Q = Shunt Perfusion (alveoli perfused but not ventilated) – ET tube in mainstem bronchus
Normal V/Q = ~0.8 (alveoli perfused and ventilated)
High V/Q = Dead space Ventilation (alveoli ventilated but not perfused) – cardiac arrest or PE
What are the two types of oxygen analyzers?
Paramagnetic: no need to calibrate, more expensive, fast enough to differentiate between inspired and expired O2 concentrations, uses diverting gas sampling mode
Electrochemical: Galvanic Fuel Cell (calibration needed), uses non-diverting
*Always check to see if the FiO2 reads 21% when exposed to RA and 100% on 100% O2
How does paramagnetic oxygen analysis work?
- The unpaired electron in the oxygen molecule is attracted to a magnetic field
- When oxygen passes through magnetic field, it goes to strongest portion of that field
- Expansion, contraction of the gas creates a pressure wave that is proportional to the oxygen’s partial pressure
- This wave sensed by the measurement unit and eventually converted from a mechanical signal into an electrical one that displays either oxygen partial pressure or Vol%
How does electrochemical oxygen analysis (Fuel Cell) work?
Oxygen diffuses through sensor membrane and electrolyte to cathode ray tube
- Reduced there, allowing a current to flow
- Rate at which oxygen enters cell and generates current is proportional to the partial pressure of the gas outside the membrane
What is the purpose of End-Tidal CO2 Monitoring in Anesthesia?
- Validation of proper ET tube placement
- Detection and monitoring of respiratory depression
- Hyper/Hypoventilation
- Cardiac Function (decrease CO = decrease CO2)
- Adjustment of parameter settings in mechanically ventilated pts
- Estimate PaCO2
- Detect circuit disconnection
How does infrared CO2 analysis work?
- A beam of infrared light energy is passed through a gas sample containing CO2
- CO2 molecules absorb specific wavelengths of infrared light energy
- Light emerging from sample is analyzed
- A ratio of the CO2 affected wavelengths to the non-affected wavelengths is reported as ETCO2 value
How does diverting (side stream) method analyze CO2 vs non-diverting (in-line)?
Diverting (side-stream) = most often Non Dispersive Infrared Spectorscopy
Non-Diverting = Infrared
Label the Waveform
Normal CO2 Waveform:
A-B = Baseline
B-C = Expiratory Upstroke
C-D = Expiratory Plateau
D = ETCO2 value
D-E = Inspiratin begins
Interpret the waveform
Esophageal Intubation
- Little to no CO2 is present
- a normal capnogram is the best evidence that the ETT is correctly positioned
Interpret the waveform
What are possible causes?
Inadequate seal around ETT
- Leaky or deflated endotracheal or tracheostomy cuff
- Artifical airway too small for the patient
Interpret the waveform
What are possible causes?
Hypoventilation (increase in ETCO2)
- Decrease in respiratory rate
- Decrease in tidal volume
- Increase in metabolic rate
- Rapid rise in body temperature (hyperthermia)
Interpret the waveform
What are possible causes?
Hyperventilation (decrease in ETCO2)
- Increase in RR
- Increase in tidal volume
- Decrease in metaolic rate
- Fall in body temperature (hypothermia)
Interpret the waveform
What are possible causes?
Rebreathing
- Faulty expiratory valve
- Inadequate inspiratory flow
- Insufficient expiratory flow
- Malfunction of CO2 absorber system (main reason)
Interpret the waveform
What are possible causes?
Obstruction
- Partially kinded or occluded artifical airway
- Presence of foreign body in airway
- Obstruction in expiratory limb of the breathing circuit
- Bronchospasm
Interpret the waveform
What are possible causes?
Spontaneous Breathing Effort with Controlled Ventilation
“Curare Cleft”
- Appears when muscle relaxants begin to subside
- Depth of cleft is inversely proportional to degree of drug activity
Interpret the waveform
What are possible causes?
Cardiac Oscillations
-CO is so good it changes the thoracic pressure with each contraction
Interpret the waveform
What are possible causes?
Faulty Ventilator One Way Circuit Valves
- Baseline elevated
- Abnormal descending limb of capnogram
- Allows pt to rebreath exhaled gas
