Capnography
Tom Archer, MD, MBA
Clinical Professor
Director, Obstetric Anesthesia
UCSD
July 3, 2012
Capnography
• Detection, display and measurement of
CO2 in respiratory gases.
• CO2 vs. time (commonest, what we have).
• CO2 vs. expired volume (useful, but rarely
used).
Gas is sampled from the airway (ETT, nose, mask, etc.) and
CO2 concentration is displayed over time.
Normal capnogram,
intubated patient,
controlled ventilation
Capnography is mostly
about expiration
• This is logical since inspired gas should
not contain CO2.
• Inspired gas does contain CO2 with
exhausted CO2 absorbent or faulty valves
in the circle system.
• So both phases matter.
Capnography can detect disorders of CO2 production or elimination due to abnormal
metabolism (fever, MH, thyrotoxicosis), cardiac arrest, hypotension, pulmonary
embolism of clot or air, V/Q mismatch (bronchospasm, COPD), CO2 rebreathing
(bad valves in circle system), exhausted CO2 absorbent, ventilator disconnect, etc.
Picture from Capnography.com
Sidestream capnography
Advantages—
Sensor does not get damaged
or discarded.
Heavy sensor does not pull on
the ETT.
Easier to clean and discard
tubing
Mainstream capnography
Advantages—
Does not take gas from the
airway (pediatrics).
Rapidity of response.
Pictures from Capnography.com
Normal capnogram. Note that expiration begins before CO2
appears, because of anatomical dead space gas, which
contains no CO2 and comes out before alveolar gas.
ETCO2
Normal capnogram, controlled
ventilation
Hyperventilation
Normal capnogram, spontaneous
ventilation
Hypoventilation
V/Q mismatch (bronchospasm, COPD):
Alveoli emptying early have less CO2.
Alveoli emptying late have more CO2.
“Curare cleft”: muscle relaxant is wearing off and patient takes
a weak breath during exhalation, temporarily decreasing
sampled CO2 concentration.
“Cardiogenic oscillations”: Cardiac activity moves tracheal gas
column enough to vary the sampled CO2 concentration.
Absent capnogram
• No CO2 production (death).
• Cardiac arrest (no CO2 transport to lungs
despite ongoing metabolism).
• No ventilation (circuit disconnect, esophageal
intubation, upper airway obstruction).
• Capnograph dysfunction: sampling tube
disconnect, loose or blocked filter, etc.
Capnography
• Detects gross changes in ventilation such as esophageal
intubation, disconnection or airway obstruction.
• Morphology picks up bronchospasm, patient breathing against
ventilator, cardiogenic oscillations, etc.
• End-tidal CO2 (ETCO2) has special importance because
abrupt decreases in this value can alert us to dangerous
events.
ETCO2 is the highest CO2 value, which occurs at endexpiration.
ETCO2
ETCO2
• This measurement is important because in
healthy, well-perfused people, ETCO2
approximates arterial PaCO2.
• ETCO2 is also important because when it
decreases rapidly it suggests an abrupt
change in pulmonary perfusion, due to
increased alveolar dead space.
Significance of ETCO2.
• PaCO2 is always higher than ETCO2– sometimes much higher
(see below).
• Commonest cause of abrupt fall in ETCO2 is hypotension
causing decreased pulmonary artery (PA) pressure.
• Low PA pressure cannot perfuse upper parts of lung, creating
increased alveolar dead space (DS).
• Much less common cause of increased alveolar DS is
pulmonary embolism.
What is dead space (DS)?
• Volume of expired gas which has not
participated in gas exchange (i.e. carries
no CO2).
• Physiological DS = Anatomical DS +
Alveolar DS.
• Unfortunately, you need to know this.
Anatomical dead space consists of the volume of the conducting airways-which never participate in gas exchange and which always empty
before the alveoli. Gas from anatomical DS contains no CO2.
faculty.alverno.edu
Normal capnogram. Note that expiration begins before CO2
appears, because of anatomical dead space gas (AnatDSG),
which contains no CO2.
ETCO2
AnatDSG
Alveolar dead space consists of the volume of the alveoli which are not
currently participating in gas exchange. These unperfused alveoli empty at
the same time as the alveoli which are participating in gas exchange and
the gas inside them dilutes the CO2 in expired gas and decreases ETCO2.
www.lib.mcg.edu/.../section4/4ch3/s4ch3_15.htm
Normal capnogram. Note that alveolar dead space gas (AlvDSG)
comes out at same time as other alveolar gas containing CO2
and serves to dilute expired CO2 and to lower ETCO2.
AlvDSG
Anatomic and alveolar dead space
• Anatomic dead space gas comes out before
alveolar CO2.
• Alveolar dead space gas comes out at the
same time as CO2 from perfused alveoli.
• Alveolar dead space gas dilutes CO2 from
perfused alveoli. This is why PaCO2 > ETCO2.
Rapid decrease in ETCO2 implies rapid decrease in
pulmonary perfusion and rapid increase in alveolar
dead space, possibly due to:
– Hypotension (hemorrhage, pump failure,
anaphylaxis, etc.)
– Clot or gas embolus
– Cardiac arrest
40
ETCO2 =
40 mm Hg
ETCO2 = 20
mm Hg
With no
alveolar
dead space
With 50%
alveolar
dead space
(e.g.severe
hypotension)
20
40
20
Alveolar dead
space gas
(with no CO2)
dilutes other
alveolar gas.
40
46
40
46
0
0
40
46
Hemorrhage abruptly reduces pulmonary perfusion and
increases alveolar dead space, abruptly reducing ETCO2.
Hemorrhage is a common cause of rapid decrease in ETCO2.
Thromboembolus abruptly reduces pulmonary perfusion and
increases alveolar dead space, abruptly reducing ETCO2.
Air embolus abruptly reduces pulmonary perfusion and
increases alveolar dead space, abruptly reducing ETCO2.
Why does increased alveolar
dead space reduce the ETCO2?
• The gas in alveolar dead space does not
have CO2 in it.
• Therefore, alveolar dead space gas dilutes
the CO2 coming from the perfused alveoli.
• This dilution of expired CO2 during the
“alveolar plateau” is the key idea.
So, capnography is really cool
and useful, but….
Limitations of capnography
• “Mesmerism”– don’t forget to watch and examine
the patient. Good advice for the rest of your
career.
• Watch the chest rise and fall (Symmetrical?).
• Listen to the chest (bilateral BS? Wheezing?
Rales?).
• Feel the bag as you hand ventilate (Easy air flow
in and out?)
Limitations of capnography
• Presence of expired CO2 does not protect the
airway from aspiration of stomach contents!
• During MACs and blocks (including neuraxial),
watch the patient! “Never turn your back on a
spinal.”
• Talk with the patient if she is awake. How does
she feel she is breathing?
Summary
• Capnography is very useful, but hard to
understand thoroughly.
• Capnography deserves extended and
careful study.
• Close and informed examination of the
capnogram can detect breathing problems
early and help prevent patient harm.
Capnography.com
• An amazingly cool and useful website by
Bhavani Shankar Kodali, MD. This website has
won many awards and is worth hours of study.
• Please use this website if you want to
understand capnography in greater depth!
The End