Week 8 Capnography Flashcards
Describe infared absorption spectrophotometry as a measurement of CO2 in expired gases.
- most common method of measuring CO2 in expired gases
- gas mixture analyzed
- a determination of the proportion of its contents
- each gas in mixture absorbs infared 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
List the 5 characteristics of the capnogram for tracing interpretation.
- frequency
- rhythm
- height
- baseline
- shape
Causes of rising CO2 when ventilation unchanged.
- malignant hyperthermia
- release of tourniquet
- transient
- metabolic process still occuring distal to tourniquet, but no blood flow to exchange CO2
- release of aortic/major vessel clamp
- transient
- IV bicarb administration
- insufflation of CO2 in peritoneal cavity
- equipment defects
- expiratory valve stuck
- CO2 absorbent exhausted
Causes of decrease in EtCO2.
- hyperventilation
- gradual derease reflects increased minute ventilation
- rapid decrease
- PE (thrombus, fat, amniotic fluid, air)
- V/Q mismatch
- increase in PaCO2 - PEtCO2 gradient
- cardiac arrest
- sampling error
- disconnect (s)
- high sampling rate with elevated fresh gas flow
Describe phase IV of the capnogram.
- Beginning of Inspiration
- CO2 concentration - rapid decline to inspired values
List two clinical uses of capnography.
- may be used as estimate of PaCO2
- PaCO2 > PEtCO2
- average gradient = 2 - 5 mmHg under GA
- used as an evaluation of dead space
- (gas in alveoli, but no blood flowing past)
- exhibited by gradient > 5 mmHg difference between PaCO2 and PEtCO2
cardiogenic oscillations at the end of exhalation as flow decreases to zero and the beating heart causes emptying of different lung regions and back and forth motion between exhaled and fresh gas
- if value remains above baseline (zero) at end of phase IV → rebreathing
-
causes of rebreathing
- equipment dead space
- exhausted CO2 absorber
- inadequate fresh gas flows
- Obstructive lung disease pattern:
- COPD
- Asthma
- Bronchoconstriction
- acute obstruction
- slow rate of rise in phase II
- steep upslope of phase III
- (in extreme cases may not see phase III)
- increased alpha angle
rebreathing of CO2, as may occur with a faulty expiratory valve or exhausted absorber system. Inspiratory CO2 is consistently above 0.
What are the two methods of measuring CO2 in expired gases?
- colorimetric
- infared absorption spectrophotometry - most common
cardiac oscillations
Describe phase III of the capnogram.
- alveolar plateau
- constant or slight upstroke
- longest phase
- alveolar gas sampled
- **peak at end of plateau is where the reading is taken - End Tidal Partial Pressure of CO2 (PEtCO2)
- normal value = 30 - 40 mmHg
- reflection of PACO2 and PaCO2
spontaneous ventilation/recovery from neuromuscular blockade
**curare clefts**
Describe mainstream capnography. What is it? Advantages/Disadvantages?
- aka Flow through
- sample cell is place directly in patient’s breathing circuit
- ADVANTAGES
- less time delay
- sample cell is place directly in patient’s breathing circuit
- DISADVANTAGES
- heated infrared measure device placed in circuit
- heated 40 C
- potential burns
- from increased temp and proximity to face
- sensor window must be clear of mucous
- weight
- kinks ETT
- increased dead space
- less of an issue with newer technology
- heated infrared measure device placed in circuit
Describe phase I of the capnogram.
- an inspiratory baseline
- should have no CO2 reading
- should be zero
- inspiration and first part of expiration
- dead space gas exhaled
dual plateau in phase III, suggesting the presence of a leak in a sidestream sample line. Early portion of phase III abnormally low due to dilution of exhaled gas with ambient air. The sharp increase in CO2 at the end of phase III reflects a diminished leak resulting from the increased circuit pressure at the onset of inspiration
Describe phase II of the capnogram.
- an expiratory upstroke
- sharp upstroke represents rising CO2 level in sample
- slope determined by evenness of alveolar emptying
- mixture of dead space and alveolar gas
Normal capnogram during spontaneous breathing
What is a capnogram?
- graphic display that the capnograph generates
- level of CO2 over time
faulty inspiratory valve, resulting a slower downslope, which extends into the inhalation phase (phase 0) as CO2 in the inspiratory limb is rebreathed
Describe sidestream capnography. What is it? Advantages/Disadvantages?
- Aspirates fixed amount of gas/mixture (30 - 500 ml/min)
- pediatric sampling - lower Vt = dilution
- best location for sampling → near ETT
- tranport expired gas to sampling cell via tubing
- time delay
- about 30 seconds
- potential disconnect source
- water vapor
- condensation
- traps/filter used
clefts during phase III indicating spontaneous breathing efforts during controlled mechanical ventilation
faulty inspiratory valve
Normal capnogram during controlled mechanical ventilation
Describe the alpha angle of the capnogram.
- normal: 100 - 110 degrees
- if increased
- indicates expiratory airflow obstruction
- COPD
- asthmatics
- bronchospasm
- kinked ETT
- indicates expiratory airflow obstruction
Describe the beta angle of the capnogram.
- normal: 90 degrees
- fresh gas does not contain CO2 therefore capnogram should return to baseline (zero)
- If increased
- typically indicates rebreathing
- very specific to faulty inspiratory valve
- exhausted CO2 absorbent
- typically indicates rebreathing
esophageal intubation
two peaks in phase III suggestive of sequential emptying of two heterogenous compartments, as may be seen in a patient with a single lung transplant
Increased upslope of phase III, as may occur during bronchospasm (asthma, COPD), or partially obstructed endotracheal tube/breathing circuit
esophageal intubation
sudden shortening of the duration of phase III during controlled mechanical ventilation, suggesting the abrupt onset of a ruptured or leaking endotracheal tube cuff
