Capnography Flashcards
What is end tidal CO2?
ETCO2 is the partial pressure or maximal concentration of CO2 at the end of an exhaled breath expressed in mmHg.
The partial pressure of CO2 in the capillary blood (PaCO2) is normally?
35-45 mm Hg
The partial pressure of CO2 in the alveoli (PACO2) is usually?
30-40 mmHg
The 5mmHg difference between PACO2 and PaCO2 is useful because?
It creates a gradient encouraging gas exchange out of the capillary and into the alveoli to be exhaled
In a normal V/Q relationship PaCO2 and PACO2 are close to each other, therefore ETCO2 is very close to and reflective of _____________
PaCO2 values
What is a normal ETCO2?
30-40mmHg
3 principle physiological determinants of ETCO2 are:
- alveolar ventilation 2. pulmonary perfusion (cardiac output) 3. CO2 production (cellular metabolism)
It is important to remember that you must have good ___________ in your patient to get a good CO2 read.
perfusion
For which cases is ETCO2 contraindicated?
None, it can be used in any and every case.
Can capnography be used to determine ETT placement?
Yes. It is considered the gold standard for ETT placement
The purpose of using capnography in the anesthesia setting is to:
determine if the patient is being ventilated, guide ventilator settings, and detect abnormalities (PE, MH, disconnect, obstructed airway)
What is happening with your patient if the A-a CO2 gradient is widening?
There is an increase in dead space somewhere
What 2 methods are used to measure CO2 in expired gases?
- Colimetric method and more commonly: 2. infrared absorption spectrophotometry
Describe the colorimetric method of CO2 detection.
The colimetric method of CO2 detection is for rapid assessment of CO2 presence. It uses metacresol purple impregnated paper that changes color in the presence of acid. CO2 combines with H2O and makes H2CO3 (carbonic acid) which makes the paper change color.
Describe infrared absorption spectrophotometry
This is the method used in the OR. It can measure other gases, not just CO2. The entire gas mixture is analyzed. Each gas absorbs infrared at different wavelengths. Gas proportions can then be determined. CO2 specifically is measured by detecting its absorbance at specific wavelengths and filtering the asorbance related to other gases.
Name the 2 measurement techniques for capnography.
Mainstream Capnography and Sidestream Capnography
Which capnography measurement technique is depicted in this picture?
Mainstream capnography. It measures in line, the amount of CO2 you are exhaling right there.
What are some the advantages of mainstream capnography?
there is no time delay, it reads an immediate sample. It does not divert gases away from the circuit.
What are some of the disadvantages of mainstream capnography?
It is a heated infrared measuring device that is placed on the circuit (be careful not to let pt. get burned), The sensor window must be clear of mucus, it adds weight to the circuit, could pull on ETT.
This type of set up would be used to perform capnography using which technique?
Sidestream. The tube is a sample port that pulls gases into the machine for analysis.
When using sidestream capnography, where should the sampling line be?
As close to the patient as possible to get the most accurate measurement of expired gases.
What is a benefit to using sidestream capnography?
You have the ability to measure multiple gases including your anesthetic gas.
What are some disadvantages to using sidestream capnography?
It diverts about 50mL a minute that then needs to be scavenged. In a pediatric patient this is more serious because it depletes tidal volume and alters the concentration of your anesthetic, there is a time delay for sample results, it is a potential disconnect source, water vapor and condensation will get trapped and clog the filter altering your measurements.
Label the 4 phases of this normal capnography waveform.
What happens during phase 1 of the capnography wave?
Phase 1 is an inspiratory baseline. There is no CO2. It runs from inspiration to the first part of expiration. This is where dead space gas begins to be exhaled.
What happens during Phase II of the capnography waveform?
Phase II is a sharp expiratory upstroke. The sharp upstroke represents CO2 rising in the sample. The slope of the upstroke wave is determined by the evenness of alveolar emptying. It is mixture of dead space and alveolar gas.
What happens in Phase III of the capnography waveform?
Phase 3 is the alveolar plateau, it has a constant or a slight upstroke. It is the longest phase of the capnography waveform. At the very end of the waveform is the peak expiration. This is where end tidal CO2 is sampled and read from.
What happens in Phase IV of the capnography waveform?
Phase IV is the beginning of inspiration. CO2 rapidly declines to the inspired value.
To accurately interpret a capnography waveform you must assess what 5 characteristics?
frequency rhythm height baseline shape
What is the primary use of capnography waveform?
To verify ETT placement is in the trachea. You must have 3 stable CO2 waveforms (3 breaths) but you also must listen to bilateral breath sounds. You cannot go on capnography alone.
Interpret this capnography waveform
Interpret this capnography waveform.
Interpret this capnography waveform
Interpret the capnography waveform.
Interpret the capnography waveform
Interpret the capnography waveform.
Interpret the capnography waveform.
Interpret the capnography waveform.
What is the difference between capnography and capnometry?
Capnography is the measurement and display of the ETCO2 waveform. Capnometry is just the digital number displaying ETCO2.
What is the difference between a shunt and deadspace?
What is the difference between anatomical, alveolar, and mechanical deadspace?
Name some factors that will either increase or decrease ETCO2?
Capnography tracing interpretation will help you determine:
the adequacy of minute ventilation, if a disconnect occurs, how well our CO2 absorber is working, and if there are changes in perfusion or dead space.
If the patient has severe hypotension, what will your capnography waveform show?
Low expired CO2 because of poor bloodflow.
If your CO2 absorber is not working correctly, what will your capnography waveform show?
an elevated baseline, this indicates that the patient is rebreathing CO2
If the patient has a massive PE, what will your capnography show?
A decrease in CO2, there is an entire region of lung not participating in gas exchange, so there will not be any CO2 released there.
If a patient has obstructive lung disease, what does this do to their I:E ratio?
Patients with obstructive lung disease need a longer time to exhale, so increasing their I:E ratio to allow for more expiratory time is appropriate.
Interpret this capnography waveform.
This is an osbtructive lung disease pattern. It would be seen in COPD, asthma, bronchoconstriction, and acute obstruction. It shows a slow rise in Phase II and little or no Phase III
Interpret this capnography waveform.
Esophageal intubation. Remember, you need 3 consecutive solid waveforms for ETT placement confirmation. A sample from the stomach would show a diminishing waveform with each breath given.
Interpret this capnography waveform.
Rebreathing. If you have a CO2 level that is above baseline at the end of Phase IV, then you have rebreathing happening.
What are 3 potential causes of rebreathing?
equipment dead space, exhausted CO2 absorber, Inadequate fresh gas flows.
Interpret this capnography waveform
Curare cleft. A patient who had been given neuromuscular blockade and it is now wearing off. At this point you would either redose, or prepare the ventilator to allow for more of the patient’s own spontaneous breathing. The blips of the curare cleft are the pt.’s attempts at respirations.
Interpret this capnography waveform.
Cardiac oscillations. There is nothing you can do about them, they are not harmful, just be aware if you see this, of what they are.
One of the very first signs of malignant hyperthermia is?
A rise in ETCO2
What are some potential causes of increased ETCO2?
We can cause this by hypoventilating the patient, if the pt. is wheezing or trapping CO2, if our CO2 absorber is exhausted. In laparoscopic surgeries, the abdomen is insufflated with CO2, patients do absorb some of it. malignant hyperthermia, the release of a surgical tourniquet, the release of the aorta or a major vessel clamp, IV bicarb adminstration, equipment defects like the expiratory valve getting stuck.
What are some potential causes of a decrease in ETCO2?
hyperventilation (gradual decrease indicates increased minute ventilation), rapid decrease could be PE, (thrombus, fat, amniotic, fluid, air), V/Q mismatch, increased PaCO2/ PEtCO2 gradient, cardiac arrest, sampling error, disconnect, sampling contaminated with fresh gas flow.
Capnography is also a good physiology tool because it shows us?
how well our patient is perfused. You need perfusion to get expired CO2
If you have 3 good capnography waveforms, why is this not enough to confirm ETT placement?
A capnography waveform can tell you if you are in the airway but not where in the airway. You could be in the right mainstem.
What is another name for mainstream capnography?
Flow Through
How much gas per minute is sampled in sidestream capnography?
A fixed amount, anywhere from 50-500 mL)
How does the IR sidestream capnography analysis work?
It compares its sample to a known quantity but it has to be calibrated to a known quantity of CO2, usualy 35 mmHg or 5%.
What is the ultimate fate of the gas that was diverted into the analyzer in sidestream capnography?
There is some debate about what happens to this gas. Nagelhout and Cathy say it is scavenged, however the TAs say they have seen older models where it is returned to the patient circuit.
