Anesthesia Gas Monitoring Flashcards
Why do we care?
- inadvertent agent OD
- timing to reach MAC/MAC awake
- error detection in vaporizer filling
- monitor uptake/distribution
- monitor low-flow anesthesia
How is gas monitoring measured?
Infrared analysis- gasses with 2+ dissimilar molecular atoms in molecule have specific/unique infrared absorption spectra
Can O2 be measured with infrared analysis?
NO! 2 similar atoms in molecule.
How is O2 monitoring measured?
-requires paramagnetic/fuel-cell analysis
Infrared photospectrometry
- amt of IR rays absorbed is proportional to the concentration of the absorbing molecules
- compared to known standard
- calculated
Diverting =
Sidestream sampling
How does diverting: sidestream sampling function?
- continuously aspirates a sample of gas from patient circuit near the breathing circuit and airway device connection (as close to the patient as possible)
- detects inspired/expired gases simultaneously
How many mls aspirated from diverting: sidestream sampling?
50-250 mls
Diverting: Sidestream sampling
- sample directed between infrared emitter, optical filter, & infrared detector -> outputs a signal proportional to remaining infrared E not absorbed by gases -> detected signal amplified/interpreted via microprocessors
- multiple optical filters required if identifying multiple gases simultaneously
Advantages of diverting: side stream sampling
- automatic calibration/zeroing
- minimal dead-space
- minimal cross contamination between patients
Disadvantages of diverting: side stream sampling
- multiple places leaks could occur
- more variability in CO2 reading
- slower response to changes
Non-diverting =
in-line gas sampling
How does non-diverting: in-line sampling function?
-measures gas concentration by using sensor that is located DIRECTLY in the gas stream
What gases are measured with non-diverting: in-line sampling?
O2 & CO2
How does non-diverting: in-line sampling measure CO2?
infrared analysis with sensor between breathing system and patient
How dose non-diverting: in-line sampling measure O2?
electrochemical analysis with sensor in the inspiratory limb of breathing circuit
Issues with diverting: side stream sampling monitoring
- high sample flow (200 ml/min)
- high flow rate impedes use with infants d/t inspiratory/expiratory flows similar to analyzer
- water vapor, liquid, secretions remain isolated from system (Dfend water trap)
Increase EtCO2:
- increased muscular activity (shivering)
- MH
- increased CO (resuscitation)
- Bicarb infusion
- Tourniquet release
- effective bronchospasm therapy (bronchodilation)
- decreased minute ventilation (hypoventilation)
Decrease EtCO2:
- decreased muscular activity (NMBA)
- hypothermia
- decreased CO (cardiac arrest)
- pulmonary embolism
- bronchospasm
- increased minute ventilation (hyperventilation)
Esophageal intubation- little or no CO2 present
Inadequate seal around ETT
possible causes:
- leaky/deflated cuff
- airway too small for patient
Hypoventilation
possible causes:
- decreased RR
- decreased TV
- increase metabolic rate
- hyperthermia
Hyperventilation
possible causes:
- increased RR
- increased TV
- decreased metabolic rate
- hypothermia
Rebreathing
possible causes:
- faulty expiratory valve
- inadequate inspiratory flow
- insufficient expiratory flow
- malfunction of CO2 absorbant
Obstruction
possible causes:
- kinked/occluded airway
- foreign body in airway
- obstruction in expiratory limb of circuit
- bronchospasm
Muscle relaxants
“curare cleft” appears when NMBA begin to subside
depth of cleft is inversly proportional to degree of blockade
faulty ventilator/one way circuit valves
- baseline elevated
- abnormal descending limb of capnogram
- patient rebreathing exhaled gas
post-bronchospasm
“shark fin” waveform
What are the normal EtCO2 values?
30-43 mmHg
What is capnography?
- measurement and display of EtCO2 value and capnogram (waveform)
- measured via capnograph
What is capnometry?
- measurement and display of EtCO2 value
- measured via capnometer
What is deadspace?
ventilated areas that do not participate in gas exchange
total deadspace = anatomic + alveolar + mechanical
Normal EtCO2 waveform (A-E)
A-B- baseline
B-C- expiratory upstroke
C-D- expiratory plateau
D- EtCO2 value
D-E- inspiration begins
Sudden loss of EtCO2 waveform
Airway disconnection
apnea
airway obstruction
disloged airway (esophageal)
ventilator malfunction
cardiac arrest
Qualitative measure of EtCO2
Colorimetric “detector”
chemically treated paper changes color when exposed to CO2
requires 6 breaths before determination made
gold is “golden”
