Capnometry/Capnography Flashcards
What is capnometry? / Capnography?
Capnometry - measures CO2 concentration / Capnography - creates/interpretation of CO2 waveform
Why Measure PEtCO2?
Equates PEtCO2 to PACO2, which is near identical to PaCO2. Provides assessment of adequacy of: ventilation status, circulation, metabolic activity, equipment function.
Rules and Regulation of PEtCO2 monitoring
Required for correct position of LMA or ETT. Mandated use for all cases of general anesthesia. Not used during MAC or regional cases.
Rules and Regulation for Anesthetic Gas monitoring
We measure concentration of gas: indicates correct gas being delivered, remaining amount in vaporizer, provides info on uptake and elimination and indirect measurement of anesthetic depth. We document exhaled gas % (this is what pt is getting)
What is diff between PACO2 and PaCO2? / PACO2 and PEtCO2?
PA and Pa CO2 nearly identical as gas is very soluble, A-a gradient is < PACO2 as other gases in circuit dilute CO2 (via dead space). Exhaled gas included alveolar (CO2) and dead space gas (others).
Define Dead Space
Dead space is portion of tidal volume gas that does not participate in gas exchange. Mechanical and Physiologic (anatomical and alveolar) dead space. Normal 1cc per Lb of body weight (anatomical).
Physiologic dead space
Anatomical - fixed amount, always present, air in upper AW down to terminal bronch, around 150cc. Alveolar - present in disease like states, alveoli that are ventilated but not perfused (opposite of shunting), limited by HPV (inh by GETA)
Methods of capnography, 2 methods
Mainstream
Mainstream - sensor at end of AW close to ETT yields faster response, heavy/bulky, can move ETT, poss burn patient, doesnt req large volume sxn for analysis (for low vol pts and low flow)
Methods of capnography, 2 methods
Sidestream
sidestream - continually aspirates gas from circuit, 50-100 cc/min, no weight, sensors in cabinet/monitor, set up creates delay by few breaths (method avoided in pediatric and low flow anesthesia)
Techniques for Gas Analyzing: Five methods
Mass spectometry / Rama spectometry / Infrared gas analysis / Galvanic cell / chemical detectors
Mass Spec
gases are ionized then separated according to mass/charge ratio. Ion deflection forms basis of analysis. Measures any kind of gas (reg gases and noble gases) but is expensive, prolonged sampling.
Raman Spec
Laser emits light that interacts with inter-atomic bonds of gas. Won’t work with mono-atomic gases, must have 2 molecules, O2, N2, CO2 and volatile agents.
Infrared Gas analyzer
Gases with 2 or more dissimilar atoms absorb IR light in specific manner. Polar gas + 2 element. Can measure CO2, N20 and volatiles, cant measure N2 or O2 (single element)
Galvanic Cell
like fuel cell, contains anode & cathode, oxygen presence and concentration drives chemical reaction, with flow proportional to oxygen concentration. O2 sensor present on insp limp of circuit.
Chemical detectors
pH sensitive material, exposed to CO2 becomes acidic and produces color change. Purple = no CO2, beige/yellow = CO2 present. will revert color with Fresh gas present. Does require adequate amounts of circulation/metab to produce change (CPR use*)
End Tidal CO2 units of measurement
expressed as mHg, kPa, or %. kPa x 7.5 = mmHg. or PeTCO2 1% = 7.6mmHg (round to 8). kPa and % are roughly equal. Multiply by 7.5 to get mmHg.
Indications derived from CO2 monitoring: adequacy of….confirms placement??
Ventilation / tissue perfusion /intact metabolic pathways. Does not reliably indicate endobronchial intubation, CO2 alone is not 100% confirmatory of ETT placement. CO2 detectable in stomach but would wash out after few breaths.
Capnography, waveform characteristics: phases
Phase 1-begins at expiration?, expiration from dead space. phase 2-exp upstroke, consists of dead space and alveolar (majority) gas. Phase 3-plateau as all gas is now alveolar gas, small slope with cont alveolar CO2 dumping. no plateau=no good, slope indicator of v/q mismatch. EtCO2 measured at end of phase 3. Phase 0 is inspiration, void of any CO2, should be zero CO2
Capnography, waveform characteristics: angles
Phase 2-3 is alpha angle. 100-110 degrees. Slope of phase 3 depends on v/q status of lung. Angle at end of phase 3 is beta angle.
PEtCO2 gradient to PaCO2.
EtCO2 is close to blood CO2, but dead space increase the gradient. PaCO2 is 2-5 higher. Gradient increased by increasing dead space: PE, anesthetics, vasodilators (decr HPV response). Basically more CO2 left in body with less measured in circuit d/t underperfused alveoli (which pick up CO2 and take to monitor). Good idea to get baseline gradient assessment via ABG.
Changes in EtCO2 waveform: Rapid/sudden decreases
Sudden decr to near zero: eso intubation, circuit disconnect/malfxn, AW obs, apnea, monitor/line blocked (seem mostly equipment related). Sudden decr to low value: probably a leak in system, losing gas to atmosphere.
Changes in EtCO2 waveform: Exponential decreases
Decr over short time (12 breaths) - d/t serious cardio/pulm event - hypovolemia, arrest, emobolic event - require aggressive hand ventilation // Gradual decr over many minutes: slow trend down - decr metab activity, mild hypervent (decr MVe), decr global perfusion.
Changes in EtCO2 waveform: Increases
Rapid increase: acute increase in CO2 production - bicarb admin, rls of tourniquet/Xclamp // Slow increase over time: decr MVe (incr MVe), increased metabolism, absorption of exogenous CO2 (lap cases), possibly exhausted absorbents (would not reach zero baseline, need to incr FGF), or malfxn exp valve.
Abnormal Waves: Sustained low EtCO2 w/o good plateaus
Low amplitude/numbers with no slope/angles - d/t excess dead space, low alveolar gas contribution. r/t (severe) bronchospasm (see this before hearing wheeze) or hypoventilation. Need bronchodilator and/or incr Vt. Possibly high sampling rates (sidestream monitor)
Abnormal Waves: Sloping Plateau
Seen in pts with Obs Lung disease, shows resistance to exhalation with exaggerated upward slope/prolong exp time, slow flow exhale. Uneven ventilation with normal perfusion, CO2 just takes longer to exhale d/t narrowed air spaces. tx “shark fins” with bronchodilator.
Abnormal Waves: Cardiac Oscillations & Curare Cleft
Cardiac Oscillatios - mech effects of cardiac contraction, normal, called “feathering” // Curare Cleft - neg deflection with pt spont breath, inspiratory deflection.
