What Does This Channel
Measure?
This channel measures the partial pressure of carbon dioxide (CO2) in
the subject’s
expired air (pCO2). Especially relevant is the end-tidal value at the
end of a complete expiration, which corresponds closely to arterial
values at the arterioles deep inside the lung where gas exchange
between outside air and the blood takes place. End-tidal pCO2 (ETPCO2)
is the gold-standard non-invasive measure of hypocapnia (low pCO2) and
hypercapnia (high pCO2), corresponding to hyper- and hypoventilation.
As the first step in the ANSLAB analysis, the end-tidal pCO2 values
over
the duration of the file are displayed.
Normal healthy baseline values for
ETPCO2 are approximately 38 +/- 5 mmHg. Consistently lower values often
indicate hyperventilation due to emotional activation. For editing, you
should be mainly concerned with sudden spikes in the data. Slower,
relatively consistent trends in the data are typically due to changes
in the breathing pattern producing blood gas changes.
What Kind of Artifacts are
Common in this Channel?
The most common problem with ETPCO2 measurement is an incomplete
exhalation through the nose (where the sampling line is attached),
e.g., due to partial breathing through the mouth, speaking, shallow
breathing, or early termination of exhalation. Many values from
instances of these kinds of breaths are automatically excluded by the
program using a strict criterion for a stable end-tidal plateau of
expired pCO2. Only values from such plateaus can be assumed to
approximate gas exchange levels at the arterioles. The end-tidal level
of a shallow or interrupted expiration will mostly reflect levels of
pCO2 in the dead space or upper parts of the lung. Plateaus from such
breaths are typically not flat enough to reach criterion. The same
applies to many end-tidal levels of speech breaths, which are also
typically excluded automatically. However, some of such invalid breaths
might not be screened out automatically and will then occur
intermittently in the end-tidal pCO2 display as easily discernible
spikes (e.g., in the figure above at about 1.5 min).
However, a prolonged series of very shallow breaths might appear as a
rather
stable period of low ETPCO2, and needs to be judged in the context of
the results of the respiratory pattern analysis.
How Are Artifacts Removed?
The suspicious intervals should be marked using option 2 (Editing of
pCO2 plateaus) and clicking twice to mark intervals. By pressing ‘0’
the raw expired pCO2 data will come up.
By pressing ‘0’ again, the first marked interval appears, indicating
with a vertical blue line where the end-tidal plateaus met the
stability criterion of the detection algorithm. The sudden drop of the
pCO2 value in the example below at around 45 seconds is due to an
incompleted expiration and
was automatically excluded. If the automatic exclusion did not work,
however, you would need to exclude it manually using '#'. Most sudden
drops in pCO2 of more than 4 mmHg in one isolated breath without an
obvious change in respiratory rate or pattern preceding or following
that breath (e.g., sigh, yawn) are invalid and should be excluded.
Note the breath starting at second 36. It shows how a long release of
air coupled with pulsations of the blood
vessels in the nostrils result in a rhythmic fluctuation of the expired
pCO2 curve. Only the stable plateau at the beginning of these
fluctuations should be included as valid ETPCO2 measurement for this
breath.