Capnography Flashcards
-What is the purpose of capnography
- confirms ETT and LMA placement
- determines adequate ventilation
- guides ventilator setting changes
- detects circuit disconnections
- detects circulatory abnormalities
- detects excessive aerobic metabolism
- *there are no contraindications
What are clinical use of capnography
-may be used to estimate the PaCO2-pETCO2 gradient
(Usually 2-5 mmHg under general anesthesia)
-used as an evaluation of dead space
What increases the PaCO2 > pETCO2 gradient?
- anything that inhibits the removal of CO2, or leak in the system
- V/Q mismatch
- increased dead space
- R to L shunt
- upper airway obstruction
- leak in the sample line, ETT, or LMA
- poor seal from ETT or LMA
What are two methods of measuring CO2 in expired gases
- colorimetric
- infrared absorption spectrophotometry (most common)
How does colorimetric assessment work?
-purple impregnated paper changes color to gold in the presence of acid (CO2 +H2O = H2CO3)
What is infrared absorption spectrophotometry work?
- analyzes mixtures of expired gas (CO2, volatile anesthetic etc)
- each gas absorbs infrared radiation at different wavelengths
- the amount of CO2 is measured by detecting its absorbance at specific wavelengths and filtering the absorbance related to other gases
What is Mainstream Capnography
- heated infrared measuring device placed in circuit
What are some issues with mainstream capnography?
- can cause potential burns/fire to the patient because it is heated
- adds weight and dead space to the breathing circuit
- sensor must be clear of mucous in order to work properly
What are some advantages to mainstream capnography?
-creates faster results than side stream
What is sidestream capnography?
-port that comes off the elbow of the ETT, that aspirates (30-550mL) of exhaled gas and transports to sampling cell for analysis (CO2, anesthetic gas)
What are some disadvantages of sidestream capnography?
- pulling off 30-500mL of gas is a lot of gas
- this large sample size may result in dilution of the ETCO2 sample
- there is a time delay (~30 sec) between exhalation and result
- *(if you are using sidestream in OR for confirmation of placement—utilize this time delay to listen for BBL, chest rise ETT fogging)
- potential for disconnection from source
- water vapor can get in the tube creating condensation —> traps or filters are sometimes required to prevent this
Capnography waveform
- has 4 phases
- displays PCO2 vs time
Phase I
- an inspiratory baseline
- should have zero mmHg
- reflects inspiration and first part of exhalation
- the exhalation is zero due to dead space
Phase II
- exploratory upstroke
- beginning portion of slope is dead space ventilation
- latter portion of the slope is alveolar ventilation
- slope determined by evenness of alveolar emptying
- prolonged in obstruction of exhalation: COPD, ETT kink, asthma, bronchospasm, acute obstruction
Phase III
- alveolar plateau
- constant or slight upstroke
- longest phase
- alveolar gas sampled
- peak portion at the end of plateau is where the reading is actually taken from
- normal value = 30-40 mmHg
- reflection of the PACO2 and PaCO2
Phase IV
- beginning of inspiration
- CO2 concentration experiences a rapid decline
Alpha Angle of CO2 waveform
- normally 100-110 degree
- if increased indicates airflow obstruction COPD, bronchospasm, kinked ETT
Beta angle CO2 waveform
- normal 90 degrees
- if increased, would signify rebreathing (faulty inspiratory valve, exhausted CO2 absorber)
What five characteristics do you interpret in an ETCO2 waveform?
- frequency (RR)
- height (how much CO2)
- shape (obstruction of exhalation or rebreathing)
- rhythm (abnormal - phase II of emergence)
- baseline (rebreathing)
What is the primary use of the ETCO2 waveform?
-confirm ETT placement
What indicates tracheal intubation?
3 waveforms > 30mmHg
Does it indicate proper placement in the trachea?
-NO! Listen for BBS
COPD, asthma, kinked ETT, bronchospasm waveform
- gradual rise in Phase II
- steep upslope of Phase III
- increased alpha angle
Esophageal intubation ETCO2 morphology
- little blips which indicate presence of acid
Rebreathing waveform
- baseline is above zero
- increased beta angle
- (think exhausted CO2 absorber or faulty inspiratory valve)
Spontaneous ventilation/recovery from NMBA on ETCO2 waveform
- create a QRA “cleft” in phase 3
- this might indicate they can be pressure supported
- this might indicate the patient needs more NMBA or sedative
Cardiac oscillations
- transient waves from heart beats
- as the heart is beating it motions the esophagus lying behind the trachea; cardiac oscillations are translated to the ETCO2 waveform
What factors affect the proper ETCO2
- CO2 production
- transport of CO2 to lungs via pulmonary (Q)
- ventilation to transport CO2 to the breathing circuit
- proper functioning equipment
What are some causes of elevated ETCO2?
Malignant hyperthermia (increased production)
Release of tourniquet (sudden increased in perfusion)
Release of aortic or major vessel clamp (sudden increase in perfusion)
IV Bicarb administration (increase production)
Insufflation of CO2 in abdominal cavity (increased production)
CO2 absorber exhausted (rebreathing)
Valve malfunction (rebreathing)
Increased dead space (decreases ventilation)
Hypoventilation (decreases ventilation)
How does IV sodium Bicarb increase ETCO2
- acidotic state = increased circulating H+ ions
- add base (NaHCO3-) to bind with H+
- creates H2CO3 which is a weak acid
- H2CO3 disassociates into H2O and CO2
What are some causes of depressed ETCO2
- hypothermia (decrease production)
- cardiac arrest (decrease production)
- V/Q mismatch (decrease perfusion or ventilation) (rapid)
- P.E (decrease perfusion) (rapid)
- hyperventilation (transient decrease)
- vent disconnect (equipment issue)
- sample line leak or high sample rate (equipment issue)
- esophageal intubation (equipment issue)
- poor ETT/LMA seal (equipment issue)
