Dr. Jeffrey Elliot Field
HBSc, D.D.S. , Diplomat of the National
dental Board of Anesthesia,
Fellow of The American Dental Society of
Anesthesia
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Carbon dioxide absorbs infrared light with a
wavelength of 4.3m m.
Light at this wavelength is shone through a gas
sample and the absorption is proportional to
the carbon dioxide concentration.
A sample of expired gas is withdrawn from the
anaesthetic circuit by a pump and analyzed
inside the machine.
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Valuable information can be obtained from
the continuous measurement of carbon
dioxide.
At the most basic level the regular rise in
carbon dioxide at the end of respiration can
be used to determine respiratory rate, and
regularity of respiration.
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2)The shape of the capnograph (the plot
of carbon dioxide against time) is used to
assess pulmonary function.
The carbon dioxide level should rise rapidly during the first
part of exhalation and then flatten off - the "alveolar
plateau". If there is pulmonary disease or poor lung
perfusion (secondary to poor cardiac output) the alveolar
plateau disappears.
The level of carbon dioxide at the end of expiration (end
tidal carbon dioxide, etCO2) is normally within a few mmHg
of the arterial carbon dioxide level and therefore a
predictable measure of arterial CO2.
EtCO2 is very useful for assessing adequacy of ventilation
both during spontaneous respiration and when using a
ventilator.
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EtCO2 is very useful for assessing adequacy of
ventilation both during spontaneous respiration and
when using a ventilator.
But the most important thing ETCO2
tells us in a sedated patient is :
 ARE THEY BREATHING
In an intubated patient ETCO2 will initially confirm
proper placement of the endotracheal tube.
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As the case progresses ETCO2 will continue to confirm that the
tube is placed properly and has not moved.
Finally the adequacy of ventilation can be continuously
assessed.
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In a sedated patient it is not so much a normal
value but a baseline value for that particular
patient that we look at. Changes in this
baseline value will let us know whether our
patient is breathing normally, hyperventilating
or hypoventilation.
In an intubated patient a normal end tidalCO2
value is 40 mm of mercury.
ETCO2 Less Than 35 mmHg =
"Hyperventilation" ETC02 Greater Than 45
mmHg = "Hypoventilation"
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There are two main graphs that we look at which are a
function of the sweep speed.
At high sweep speed we get a wave form of the CO2
from each breath which is known as the capnogram.
There is only one normal shape for a
capnogram:
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At first there is a rapid rise as the dead space gas comes
out of the major airways.
Then there is a plateau which has a slow rise.
Finally there is a rapid decline as the next breath enters
the patient.
A to B is post inspiration/dead space exhalation, B is the start
of alveolar exhalation, B-C is the exhalation upstroke where
dead space gas mixes with lung gas, C-D is the continuation of
exhalation, or the plateau(all the gas is alveolar now, rich in
C02). D is the end-tidal value – the peak concentration, D-E is
the inspiration washout.
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Kinked tube (intubated patients only)
Herniated cuff (intubated patients only)
Bronchospasm (intubated and non-intubated
patients)
Any obstruction that limits expiration( eg
mucous on the tube, COPD, asthma, foreign
body obstruction.
Causes1)Unequal emptying of lungs( pnemothorax). Seen in
intubated and non-intubated patients
2)Lateral position
3)Tube touching carina
Note what happens when the patient
stops breathing.
The pulse oximeter lags way behind the
capnograh in picking up a cessation in breathing.