Capnography Flashcards
What is the difference in PCO2 between alveolar/arterial and end-tidal?
2-5 mm Hg
Explain how a mainstream capnograph works
inserted between ET tube and patient circuit –> infrared light being emitted in the window of the sample cells - light reaches the photodetector on the opposing site - measures PCO2
What are the advantages and disadvantages of a mainstream capnograph?
advantages:
* CO2 in real time
* no deformities due to dispersion of gas in sample line - crisper wave form
* do not need large monitor, usually small hand-held devices
disadvantages
* condensation can cause falsely high CO2 reasings
* bulky - can cause traction on ET tube - risk of extubation
* increases apparatus dead space
Explain how side stream capnographs work
measuring unit is remote from the patient - connected via sampling tube to small port added usually at the end of ET tube - actively sucks air out of circuit
What are the advantages and disadvantages of side stream capnographs?
Advantages:
* monitor can often measure other gases as well, e.g., inhalant cc
* can use it in nonintubated patients - e.g., nasal cannula
Disadvantages:
* delayed reading
* slurred/rounded capnograph waveforms if long line, slow rate of aspiration, or large bore sample line - from CO2 mixing with non-CO2 gas
* may pose infectious disease risk from contaminated lines
* lines can become obstucted with moisture/secretions - need water/moisture trap
If you don’t use water traps how would that affect the CO2 measurement?
- water vapor can absorb infrared light - falsely increases CO2 reading
List 3 methods of measuring CO2
- Infrared light absorption
- Raman scatter
- Mass spectrometry
List 6 indications/uses for capnography
- ensure ET tube placement
- detect apnea
- monitor adequate ventilation
- monitor pulmonary perfusion and CO
- assess for correct nasogastric tube placement
- detect equipment problems (e.g., incorrect breathing circuit assembly etc., absorbent saturation etc.)
Describe the 4 phases of the capnograph
Phase I: flat baseline, should be zero, inspiring gas with virtually no CO2, end of this already includes beginning of expiration –> anatomic dead space exhaled doens’t have CO2
Phase II: upstroke, shows mixture of alveolar and anatomic dead space CO2
Phase III: plateau, expiration and pause after expiration, should be flat, represents alveolar CO2, highest part: ETCO2 (end of this phase)
Phase IV: rapid downstroke, shows start of inspiration
What are the two angles measured on a capnograph and what are their normal values?
alpha angle - between Phase II and III - normal 100-110 degrees
beta angle - between Phase III and IV - normally close to 90 degrees
What does this waveform indicate?
sine waveform
with high RR but slow analyzer response time
shows falsely elevated baseline and underestimated peak expired CO2
What are your differentials for elevated phase I (i.e., baseline CO2)?
- sine wave form
- exhausted CO2 absorbant
- faulty inspiratory valve
- inadequate gas flow rate
(last 3 all lead to high inhaled CO2)
What does this capnograph indicate?
wider alpha angle and sloped plateau with normal beta angle (note if both angles wider usually from slow sampling rates of a side stream capnograph)
indicates slow expiration, i.e., obstruction
* bronchoconstriction
* ET tube kinking or obstruction
What does this capnograph indicate?
decreasing pulmonary blood flow (note gradual decrease as opposed to abrupt, i.e., extubation etc.)
* PTE
* drop in CO
List differentials for decreased ETCO2
- PTE - increased dead space, decreased pulmonary perfusion
- CO decreased –> decreased pulmonary perfusion
- leak in side stream capnograph - will dilute expired air with room air from aspiring room air into the sensor
*
What could this capnograph indicate?
uneven alveolar emptying (V/Q mismatch) can cause wider alpha angle and slanted plateau
Note: may be difficult to differentiate from airway obstruction causing slow exhalation
What does this capnograph indicate if this patient is on a mechanical ventilator?
patient is trying to breathe in during exhalation
What does this capnograph indicate?
wide beta angle and prolonged phase IV
- usually from rebreathing CO2
- uncommon: from abnormally slow inspiraiton
