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CO2 is the major by product of aerobic metabolism. Tissues consume Oxygen and Glucose
which produces CO2 and water as waist. There are 3 major mechanisms that eliminate
excess CO2 from the body:
1. Buffer – which mainly controls pH levels by shifting the H+ HCO3 <-> H2CO3 <-> H2O +
CO2 equation to the left or to the right...usually takes fragments of seconds to correct.
2. Respiratory – Controls the level of pH by eliminating CO2 from the lungs by increasing
or decreasing the rate and volume of respirations (usually the main method of the body
to control the pH). Usually effective for up to one our.
3. Renal – a much slower mechanism which utilizes the kidney to excrete or conserve H+
ions and or other elements in the blood. Can last for days.
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What is End Tidal CO2?
• Breaking the term apart, “carbon dioxide at the end of the tide”:
• Breathing is done in waves (in and out)
• EtCO2 is the amount of CO2 measured at the peak of the wave (the end of
exhalation)
• EtCO2 is measured at the nose, mouth, or hub of the ET tube
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Another, more easy to digest view of the major changes on gas concentration.
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2 totally different bio-tracks.
There is no correlation and or linear connection between the two procedures.
Patients can present high PaO2 and low ETCO2 and vice versa.
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Different than SPO2, capnography measures the current amount of exhaled CO2 from the
body in every breath. It’s not affected by ambient lighting, patient movement or other
disrupting factors.
With capnography, we can extrapolate and understand changes in our patients respiratory
and hemodynamic status at the exact time those changes occur.
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Briefly describe the history of ETCO2 measuring methods from the colourmetric single use
device to current day capnography.
In the beginning there was the “confirmation balloon” which was squeezed and connected
to the ETT. It the balloon was re-inflated by air from the lungs, it was a confirmation for
correct placement of the ETT… if not, usually the ETT was placed in the esophagus.
Then we were introduced to the colourmetric detectors. These devices acted like litmus
paper and one could tell that the ETT was properly placed if the device changed color from
blue to pink. Yet, the device could provide only gross information about the amounts of the
exhaled CO2.
The next generation was capmoneters (Nonin ©) which could provide only the ETCO2
numerical value of the exhaled CO2, yet could not provide any information about trends in
respiration.
Current capnography, provide us with comprehensive data that includes a waveform,
numeric value and the ability to monitor the patient and understand his respiratory and
hemodynamic state in intubated and non intubated patients.
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It is important to note that there may be a difference in the duration of the
waveform you see on the screen and the one on the printout. Capnography
waveforms on the monitor screen are condensed to provide adequate information
in the 4-second view.
Teaching Note: A patient with a RR of 8 would have a respiratory cycle of one
breath about every 7.5 seconds. If the waveform was not condensed, you would see
a full screen “flat line”, then one end of the waveform, etc. Press the PRINT button
anytime you need an accurate duration.
Although other gases are present in the airway, capnography detects only CO2 from
ventilation.
There is usually no CO2 present during inspiration so the baseline is normally on zero.
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There is no carbon dioxide at the beginning of exhalation because this air is from the
trachea, mouth and nose. Upper airway area is often called “dead space” because there is
no gas exchanged in the upper airway.
Teaching Note: An extension of the airway such as an ET tube / King LT expands the “dead
space” and can be critical when dealing with pediatrics.
Phase one on the capnograph is the ending of inhalation and the beginning of
exhalation. This baseline is normally at zero and shows the amount of carbon dioxide in
the dead space.
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In phase II, or the ascending phase, CO2 from the alveoli begins to reach the upper airway
and mix with the dead space air. This causes a rapid rise in the amount of CO2 that is now
detected in exhaled air.
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Phase three, the alveolar plateau is flat with a slight upward tilt toward the end. This
plateau illustrates the uniform concentration of carbon dioxide in the pulmonary system.
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The end of phase three is also the end of exhalation. This termination of
the breath cycle contains the highest concentration of CO2 and is labeled
the “end-tidal CO2” . This is the number seen on your monitor.
Normal EtCO2 is 35-45 mmHg
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Phase four shows the beginning of the next inhalation. Oxygen fills the
airway and the carbon dioxide level quickly drops to the baseline.
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A quick review of the four phases of the normal capnography waveform:
Phase I
• Inspiration ends and exhalation begins, dead space air is eliminated
first, no CO2 is present.
Phase II
• Alveolar air begins to mix with dead space air, a sharp upstroke is
produced
Phase III
• Alveolar air predominates and the CO2 level plateaus as exhalation
continues
• The ETCO2 is noted at the end of exhalation
• Normal range is 35-45 mm Hg (5% vol)
Phase IV
• Inspiration occurs, CO2 level quickly returns to baseline
• Normal baseline is at zero
• The pattern repeats with each breath.
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As mentioned, any increase in rate and volume of ventilations or patients respirations will
always decrease the numbers and the height of the ETCO2 waveform.
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* Basic life support for health care providers, Dallas, 1997, The Association.
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One of the most difficult decisions is determining when to stop resuscitation
efforts. Here again, capnography provides objectivity.
Capnography has been shown to predict probability of outcome following
resuscitation and may be used in the decision to cease resuscitation efforts.
Source: Levine RL, Wayne MA, Miller CC. End-tidal carbon dioxide and outcome of
out-of-hospital cardiac arrest. New England Journal of Medicine. 1997;337(5):301306.
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