Capnometry Flashcards
Capnometry
measurement of CO2 in gas mixture
Capnometer
device that performs measurement
Capnography
recording of [CO2] vs time
Capnograph
Machine that Generates Waveform
Capnogram
Actual Waveform
ETCO2
- Normal PaCO2 35-45mm Hg, ETCO2 2-5mm Hg (LJ 3-6 in dogs) less than PaCO2
o NRB: 3-6mm Hg
o Horses: PaCO2 10-15mm Hg less than PETCO2
o Gradient may be less or even negative if reduced FRC (obese, pregnant)
Consequences of Hypoventilation
o Increased mean PAP
o Increased HR, SV, Q, MAP
o Displacement of alveolar gas – hypoxia
o Right shift of oxygen dissociation curve
o Stimulation of catecholamine release
o Depressed mentation (awake), reduced MAC (under GA)
ETCO2 as Indicator of CO
o Stable conditions of patient minute ventilation, body temp; in absence of airway obstruction or extra-metabolic source of CO2 – lap sx, NaHCO3 admin
o Sudden change in ETCO2: reflect linearly proportional alteration in CO
Decrease ABP + ETCO2: primary reduction in CO
Decrease ABP, no change ETCO2: decrease in SVR
MOA Capnography
o Infrared absorption proportional to PCO2
o Non-rebreathing circuits dilute sample, reduced accuracy
Mainstream Capnometer
Sensor located directly in gas stream, CO2 via IR technology, O2 via electrochemical energy
Advantages - Mainstream Capnometer
- Fast response time, no delay time
- More accurate waveform
- No gas removed from BS: not necessary to scavenge or increase FGF to compensate for sampling
- Water, secretions seldom problematic with analyzer
- less likely to have sample contamination
- CO2: standard gas not required for calibration
- O2: calibrated with room air
Disadvantages - Mainstream
- Secretions on windows of cuvette can cause erroneous readings (interference with light transmission
- Adds weight to BS, traction on airway device or breathing tubes
- Increases VD
- Become dislodged, leaks disconnections, circuit obstructions
- Only measure oxygen, CO2
- Expensive
- Thermal burns
Sidestream Capnometers
o Pump aspirates gas from sampling site through tubing to sensor located in main unit
Shorter sampling tube: decrease delay time, more satisfactory wave forms
Pump rates 50-200mL/min – sample rate should be matched to patient size/VT to maintain accuracy
Zeroed using room air, calibrated using gas of known composition
Sample evaluated in capnography unit
Delay with Sidestream Analyzers
Approx 3s delay btw aspiration, measured/displayed
* Affected by length/diameter of aspiration tube, flow rate
How prevent secretion accumulation with side stream
o Traps, filters, hydrophobic membranes, special tubing to avoid water or particulate contamination in the monitor
o Dorsal position: minimize contamination with secretions
Other Considerations with Sidestream Analyzers
o Adaptor in circuit adult vs pediatric – matched to ETT internal diameter
o Aspirated sample must be returned to circuit to minimize depletion of circuit vol with low flow techniques or scavenged to minimize environmental pollution
Advantages - Side Stream
- Less interference with ETT
- Ability to monitor multiple gases
- Automatic calibration in zeroing
- Minimal added dead space
- Low risk of cross contamination btw patients
- Administration of bronchodilators via sampling port
- Remote monitoring
Disadvantages of Sidestream Analyzer
- Accuracy effected by contamination
- Active gas sampling
- May need to increase fresh gas flow
- Increased risk of environmental contamination of sample (leaks)
- Less accuracy with higher respiratory rates, long sampling lines
- lag time in readings
- May have deformation of waveform, erroneously low readings from fresh gas dilution
- More variable difference between arterial, ETCO2 levels
Time Capnography
CO2 expressed as function of time
Volumetric Capnography
expired CO2 vs volume
o Allows determination of: VT, anatomic/alveolar dead space, physiologic dead space, effective alveolar tidal volume, end tidal PCO2, alveolar PCO2, eliminated CO2 vol, mixed-expired CO2
Volumetric Capnography: flattened curve
less steep phase II + less plateaued phase III = increased physiologic dead space
* Lower airway obstructive dz (narrowing airway diameter)
* Increased alveolar dead space ventilation DT hypovolemia, PTE
Volumetric Capnography: PPV
flatten slope (increased alveolar dead space ventilation) or normalize slope (recruitment of collapsed lung units)
Volumetric Capnography: Phase I of exhalation
inspired gas of that breath, above 0 PCO2 = increased inspired PCO2(excessive apparatus dead space or exhausted soda lime)
Phase I - baseline
–Start of expiration
–Normally zero, reflects gas that normally devoid of CO2 from anatomic/mechanical dead space
Phase II - expiratory upstroke
Emptying of connecting airways, beginning of emptying of alveoli
Exhalation continues: gas from alveoli in regions with relatively short conducting airways appears, mixes with dead space gas from regions with relatively long conducting airways –> increased CO2
Phase III - Alveolar Plateau
Gentle slope due to uneven emptying of alveoli (different time constants)
* Alveoli with longer time constants = more CO2, hence positive slope
Slope depends on lung’s VQ status – airway obstruction, PEEP increases slope
Prolonged expiratory upstroke: “shark fin” of ETCO2
Last portion = end-tidal point, maximum CO2 level
Phase 0/IV Inhalation
Patient inhales, CO2 abruptly falls to zero and remains there until initiation of subsequent exhalation
alpha angle
Take off or elevation angle; btw phase II, III – normally ~100-110*
Increased: airway obstruction, PEEP
Decreased: obstructive lung dz (VD longer to be exhaled)
Other factors: capnometer’s response time, sweep speed, resp cycle time
beta angle
Btw phase 3, descending limb – normally approx 90*
Angle increased with rebreathing, prolonged response time
Decreased with decreased slope of phase III (inspiration)
Advantages of Capnometry
- Continuous
- Assessment of metabolism, circulation, ventilation, proof of life
- Detection of equipment, patient problems ie leaks, inadvertent extubation/missed intubation, obstructed airway, CPA, MH
- Portable, battery operated models
Disadvantages of Capnometry
- Sample rate may be higher than VT of patient – underestimates
- Adds deadspace to circuit
- Mainstream: addition of weight to circuit –> traction on ETT –> damage to respiratory epithelium
- Expensive
Physiologic Factors that Can Affect Cap Waveform
Production: metabolism, drugs, temp
Transport: CO, pulmonary perfusion
Ventilation: obstructive/restrictive dz, RR
Mechanical Factors that Can Affect Cap Waveform
Breathing equipment: ventilator settings, malfunctions, tubing obstructions, disconnections
Measuring equipment: sampling method, rate
DDX - Rebreathing
- Faulty expiratory valve
- Exhausted absorbent
- FGF too low
- Insufficient expiratory time
- Increased dead space
How Fix Rebreathing
- Dry, replace valves
- Calculate appropriate oxygen flow rate
- Change absorbent
- Change IE ratio
Ddx: sharking - increased alpha angle, decreased beta angle
- Mucous plug
- Blood clot, trauma
- Kinked, bent tube
- Physiological causes: asthma, COPD/BC
Fixes for Sharkfin
- Armored Tube
- Scope/suction
- Reintubate
