An old CE article I wrote for my local agency, I also use in training.

1. Capnography : An Overview for EMS personnel
By Robert S. Cole, CCEMT-P
About the Author: Steve Cole has been involved in EMS and EMS education since 1990. He has worked
for a variety of EMS agencies including volunteer fire, military, hospital based, private, and third service.
He is currently employed by Ada County Paramedics (Boise, Idaho),a top-tier and often besieged Third
Service EMS.
He has no current conflicts of interest with any portion of this article, and has not received from any
commercial interest for its production. He would like to mention that he is open for some money, should a
sufficiently insane commercial enterprise wish to invest in his expansive, eclectic, and somewhat
haphazard quest for EMS excellence.
Comments are welcome. He can be reached by email at: colemedic@hotmail.com
“CO2 is the smoke from the flames of
metabolism.”
– Ray Fowler, M.D. Dallas, Street Doc’s Society
Introduction
Capnography, while seen as a relatively new tool for EMS, was first introduced in 1943.
Using infrared technology, EMS has seen the advancement of many tools and
technologies that have promised revolution in medicine. Ultrasound, cardiac markers,
CO-oximetry, and pulse oximetry to name a few. Some have panned out and some have
not. One of the more promising technologies to break into EMS practice has been End
Tidal Capnography (ETCO2). Originally conceived as a method of monitoring
ventilation, it has gained wide acceptance in EMS for airway maintenance and
monitoring in intubated patients. The availability of waveform capnography, as well as
recent breakthroughs in the understanding of capnography as a marker for overall
physiologic function, have broadened the role of capnography in EMS further. This
article will attempt to discuss both the standard and the new roles for ETCO2.
Types of Capnography
Most ETCO2 devices usually work on the principle that CO2 absorbs infra-red radiation.
A beam of infra-red light is passed across the gas sample to fall onto a sensor. The
presence of CO2 in the sample leads to a reduction in the amount of light falling on the
sensor. The analysis is typically rapid, accurate, and in EMS, fairly accurate. In HAZ-
MAT and anesthesia situations, the presence of high concentrations of other gasses (such
as nitrous oxide) will adversely effect the accuracy of the ETCO2.
There are three major types of ETCO2 devices, side stream and main stream sampling.

2. Colorimetric ETCO2
This is better described as an ETCO2 detector, rather than capnography. These devices
provide CO2 detection based on PH changes in airflow. Colorimetric ETCO2 is a
disposable, single use ETOC2 strictly for use in tube confirmation. In this role is cost
effective, easy, and quick. It does have some limitations. It does not provide any type of
waveform, quantitative analysis, or even changes specific to CO2. It loses its
effectiveness rapidly. It cannot be used to determine different types of respiratory
abnormalities. Presence of some fluids and moisture in the ETT will give false readings
as well.
“End Tidal CO2 reading without a waveform is like a heart rate without an
ECG recording.” – Bob Page “Riding the Waves”
Main-stream ETCO2
With mainstream capnography,
an infrared sensor is placed
directly over the passageway of
gas to reflect light across the gas
sample. The most common use
of this method is monitoring the
placement and function of an
endotracheal tube (ETT). While
uncommon, it is possible to hook
up mainstream capnography to a
breathing patient.
Side-stream ETCO2.
Side-stream capnography is more versatile than
mainstream, as it can be used in a variety of situations
besides placement and monitoring of an ETT. In side-
stream ETCO2, a sample of gas is extracted from the
pathway of gas, and sampled remotely from the
airway. It is also quicker to use, requires less
calibration by the user, and is more durable than
external sensors.
Side-stream technology can be used to monitor ETT,
as well as respiratory function in patients without an ETT. The most common example of
this is the nasal cannula ETCO2, which can be used to monitor patients with various
respiratory conditions, or those receiving treatments that might have respiratory effects.
The two main disadvantages of this method are (1) that it is considered slightly less
accurate, and (2) there is a slight delay in time for the sample to proceed up the tube to
the monitor. Neither of these concerns is considered clinically significant in EMS (yet).

3. Micro-stream ETCO2
Micro-stream technology is a new variation of side-
stream technology. The main difference of this
technology is that only very small amounts of the gas
sample are required to accurately detect and analyze
CO2 readings.
This technology was originally developed for use in
neonates with small tidal volumes, but has shown
promise in other patients as well, such as those with a
decreased tidal volume or in respiratory extremis.
Accuracy of types of ETCO2
Generally speaking, mainstream ETCO2 is considered slightly more accurate, but takes
longer to set up (“warm up”) and requires frequent calibration. The differences in
accuracy have only been demonstrated in spontaneously breathing patients, and are not
considered significant. In addition, both methods are considered accurate for monitoring
ETT placement. Micro-stream ETCO2 technology shows promise in overcoming these
differences in accuracy.
Many things can “spoof” ETCO2: The most common is excessive water vapor in the
ETT, which can cause sensors (especially main stream sensors) to provide “false highs”.
Leaky ETCO2 tubing can give false lows. Finally, repeated use of epinephrine (such as
seen in cardiac arrest) by ET or IV routes, can cause ETCO2 to drop. Typically a
decrease of about 5 torr has been seen. This is not constant through out the study group,
and varies by patient. The clinical significance of this beyond its effect on ETCO2
monitoring is the subject of several ongoing studies.
Basics of Capnography
Simply put it is a measurement of the byproduct of cellular metabolism in the human
body. CO2 is produced by cellular metabolization through the process known as the Kreb
Cycle. It is then exhaled through the respiratory process where we can measure it. The
point where it is measured is at the end of the airway, therefore it is called end tidal
measurement. ETCO2 can be measured in either torr, or mmHg.
Basic capnography is not only the measurement of ETCO2, but analysis of the phases
of exhalation (measured indirectly by monitoring ETCO2) in the respiration cycle.
It is also affected by a variety of respiratory states and perfusion conditions; which can be
indirectly assessed by detailed analysis of ETCO2.
Key Point
Basic capnography is not only the measurement of ETCO2, but analysis of the phases of
exhalation (measured indirectly by monitoring ETCO2) in the respiration cycle.
What is normal?
ETCO2 30-45 mm Hg is the normal value for capnography. Many iatrogenic factors can
alter this reading. For example, imperfect positioning of nasal cannula capnofilters may
cause distorted readings. Unique nasal anatomy, obstructed nares and mouth breathers
may skew results and/or require repositioning of cannula. In addition, oxygen by mask

4. may lower the reading by 10% or more. Finally, as discussed elsewhere, use of nitrous
oxide or excessive water vapor may cause false high readings.
What is the waveform?
Similar to the ECG, the waveform in ETOC2 describes function over time and volume.
The waveform can be divided into inspiratory (phase 0) and expiratory segments. The
expiratory segment, is divided into phases I, II and III, and occasionally, phase IV, which
represents the terminal rise in CO2 concentration. The angle between phase II and phase
III is the alpha angle. The nearly 90 degree angle between phase III and the descending
limb is the beta angle.
A waveform illustrating phase 4 of the capnogram:
Another view of the representative parts of the waveform (using different terminology):

5. When looking at ETCO2, 5 things are measured:
• Frequency: ETCO2 provides an objective analysis of
the rate of breathing.
• Rhythm: By assessing the rhythm in ETCO2
periodic apnea, cheyne stokes, boyts, and other
respiratory patters become easily identifiable.
• Baseline: Evaluation of the baseline can give clues
regarding inter-thoracic pressure and other resistance
to ventilation issues.
• Shape: The shape of the waveform gives clues
regarding the exhalation cycle itself. An asthmatic with an acute asthma attack,
for example, will present with a “shark fin” shape to their waveform.
• Height: The height of a waveform is representative of the level of CO@ being
measured. Various factors result in either increased or decreased/absent ETCO2.
ETCO2 increased :
Technical errors
CO2 output Pulmonary perfusion Alveolar Ventilation
Machine faults
Fever
Malignant Hypoventilation Inadequate O2 flows
hyperpyrexia Increased cardiac Bronchial intubation Leaks in breathing
Sodium bicarbonate output Partial airway system
use Increased blood obstruction Faulty ventilator
Tourniquet release in pressure Rebreathing of airway Faulty valves
compression injuries gas
ETCO2 decreased:
Technical errors
CO2 output Pulmonary perfusion Alveolar Ventilation
Machine faults
Hypothermia Reduced cardiac output Hyperventilation Circuit disconnection
Epinephrine Use Hypotension Apnea Sampling tube leak
Hypovolemia Total airway obstruction
Pulmonary embolism Partial airway obstruction

6. Accidental tracheal
Cardiac arrest extubation
Ventilation/Perfusion mismatch and ETCO2
If ventilation or perfusion are unstable, a Ventilation/Perfusion (V/Q) mismatch can
occur. This will alter the correlation between Arterial CO2 (PaC02) and ETCO2.
This V/Q mismatch can be caused by blood shunting such as occurs during atelectasis
(perfusing unventilated lung area) or by dead space in the lungs (Ventilating unperfused
lung area) such as occurs with a pulmonary embolism or hypovolemia.
Uses of capnography
Capnography and intubation
“When exhaled CO2 is detected (positive reading for CO2) in cardiac arrest,
it is usually a reliable indicator of tube position in the trachea.” - The
American Heart Association 2005 CPR and ECG Guidelines
A relatively new concept, ETCO2 monitoring to confirm and monitor ETT placement,
was first reported in 1991. One early use was to prevent inadvertent placement of a
gastric tube into the airway instead of the esophagus. Since then, it has evolved to
become the standard of care for the confirmation of tube placement.
While most research has focused on maintenance of ETT, the ETCO2 has been shown
useful in studies involving the LMA. In theory it should also be acceptable in double
lumen airway devices such as the PTLA and Combitube, as well as new airway devices
like the King LTD.
ETCO2 is considered mandatory in many services for ETT. Therefore, EMS providers
should document their use of continuous ETCO2 monitoring and when possible attach
wave form strips to their PCRs. This provides an objective and concrete documentation
of tube placement. This is especially important if tube placement is later called into
question by other agencies or the ER. This will timestamp and document your tube as
good.
A good tube will produce a clearly identifiable waveform, but the details of the waveform
will depend on the underlying medical conditions of the patient. Vukmir et al reported a
sensitivity and specificity of 100% for endotracheal tube localization by capnography.

7. A Properly Placed ETT
An esophageal placement (AKA “Gut tube”) should not produce any waveform. Example
(A) below shows sudden dislodgement. Example (b) shows a gut tube from the start.
A:
B:
A misplaced ETT
In low cardiac output states (like shock, cardiac arrest or inadequate chest compressions)
ETCO2 may not be detected. If a patient has had carbonated beverages, or if mouth to
mouth ventilation has been attempted, CO2 may be detected after esophageal intubation
(false positive). The EtCO2 should rapidly decrease to zero (within 3-6 breaths) in this
situation, and the wave form will not be ‘Normal’ looking.
Capnography as a diagnostic tool
Broncheospasm/Asthma: ETCO2 has proven diagnostic value in the determination of
broncheospasm in reactive airway states (as opposed to CHF, hyperventilation, and other
states). This has increasing importance given that recent research calls into doubt the
ability of providers (and even physicians) to adequately differentiate between pneumonia,
asthma/COPD, and CHF based on assessment alone.
ETCO2 values change with the severity of the broncheospasm. With a mild asthma, the
CO2 will drop (below 35) as the patient hyperventilates to compensate. As the asthma
worsens, the C02 levels will rise to normal. When the asthma becomes severe, and the
patient is tiring and has little air movement, the C02 numbers will rise to dangerous levels
(above 60).
The characteristic waveform of broncheospasm is the “Shark Fin”. The Shark Fin is term
for the classic shape of the waveform seen in acute broncheospasm. It is manifested by a
more acute upward of phase II and III than is normally seen, making the ALPHA angle
less acute and the BETA angle more acute.

8. Successful treatment will lessen or eliminate the shark fin shape and return the ETCO2 to
normal range. Therefore, documenting ETCO2 before and after a breathing treatment is
useful in documenting treatment.
It is important to note that a recent study published in the Annals of Emergency Medicine
failed to show the efficacy of ETCO2 in detecting asthma in pediatrics. Future studies are
pending.

9. KEY POINT:
ETCO2 is not an effective diagnostic tool for asthma in pediatrics.
Hypoventilation: This may be due to over sedation, head injury, or other issues.
A real example of hypoventilation. Note the normal shape of the waveform, but the
elevated levels of CO2.
Hyperventilation: This may be due to anxiety, a panic attack, or well compensated
respiratory distress.
Return of spontaneous ventilation during paralysis: ETCO2 is useful for monitoring a
patients level of paralysis, and may detect the need for further paralysis or sedation
before other methods commonly used in EMS. This is usually evident by “notches” in
the waveform.

10. In this capnogram, the notches are notable between the normal waveforms.
Air trapping/overventilation: Mechanical over ventilation in some patients causes a
number of issues effecting coronary perfusion, ROSC, and barotraumas. This will
manifest itself by an elevation of the baseline over time. When this is seen, then
prolonged exhalation times are needed.
Equipment Malfunction: The ETCO2 can detect a failure in the ventilation circuit,
particularly a leaky cuff, detached BVM, or failing one-way valve.

11. Detection of sudden cardiac arrest: While uncommon, the sudden loss of pulses, while
an electrical rhythm remains (such as seen in a tension pneumo-thorax) does happen. This
is even harder to detect in an intubated and paralyzed patient.
Return of Spontaneous Circulation: A sudden increase in the height in an ETCO2
waveform may indicate a sudden increase in blood flow in cardiac arrest, usually from
ROSC.
PEA vs. Severe Shock: White R.D. et al used EtCO2 measurement in out-of hospital
cardiac arrest. They concluded that capnography can detect the presence of pulmonary
blood flow even in the absence of major pulses (pseudo-pulseless electrical activity-
PEA) and also can rapidly indicate changes in pulmonary blood flow (cardiac output)
caused by alterations in cardiac rhythm. This can be helpful in determining the difference
between a true low-flow state and actual PEA.
Capnography as a monitoring tool
ETCO2, like any “tool” is not, and should not, be considered the ultimate monitoring
device. It is however very useful in select situations. Its effectiveness is increased when
used with other monitoring devices, such as SPO2. One anesthesia study showed that use
of both ETCO2 and SPO2 together would have prevented 93% of anesthesia “mishaps”.
This is not to imply that SPO2 is accurate or reliable by itself either, and as a general rule
ETCO2 is more “timely” in its detection of problems than SPO2.
Another study used ETCO2 as a detector for compressor fatigue in cardiac arrest. This is
based on the thought that quality compressions produce better ETCO2 readings. When
the quality of compressions fall, then ETCO2 will fall as well. Therefore as a matter of
troubleshooting, when ETCO2 decreases, switching compressors should be considered.
What does the literature say?
In a study of 291 intubated head injured patients, 144 had ETCO2 monitoring. Patients
with ETCO2 monitoring had lower incidence of inadvertant severe hyperventilation
(5.6%) than those without ETCO2 monitoring (13.4%). Patients in both groups with

12. severe hyperventilation had significantly higher mortality (56%) than those without
(30%). –Davis, The Use of Quantitative End-Tidal Capnometry to Avoid Inadvertant
Severe Hyperventilation in Patients with Head Injury After Paramedic Rapid Sequence
Intubation,
-Journal of Trauma, April 2004
Trauma - A 2004 study of blunt trauma patients requiring RSI showed that only 5 percent
of patients with ETCO2 below 26.25 mm Hg after 20 minutes survived to discharge. The
median ETCO2 for survivors was 30.75.
- Deakin CD, Sado DM, Coats TJ, Davies G.
“Prehospital end-tidal carbon dioxide concentration and outcome in major trauma.”
Journal of Trauma. 2004;57:65-68.
Use of ETCO2 in intubated patients is not just useful in tube confirmation. It can be used
as a guide to ventilation rates. A target ETCO2 of 30-35 is acceptable and desirable.
Decreasing the ventilatory rate in an intubated patient is acceptable to achieve this goal.
Key Point
An ETCO2 of 30-35 is a desirable goal of ventilation
Capnography as a predictor of survival in SCA
Several studies have looked at ETOC2 as a predictor of survival of cardiac arrest. One of
larger studies was conducted overseas with a study population of nearly 400 patients. In
this study, the researchers looked at both initial and final ETOC2. They found that the
average initial ETCO2 in the ROSC was 18 torr vs 6.75 torr in the non-ROSC group. In
addition, the final ETCO2 for ROSC group was on average 26 torr vs 7.5 for the non-
ROSC group. The most important finding was that no patient with an initial ETCO2 of 9
or less survived to admission.
Another study noted that "An end-tidal carbon dioxide level of 10 mmHg or less
measured 20 minutes after the initiation of advanced cardiac life support accurately
predicts death in patients with cardiac arrest associated with electrical activity but no
pulse. Cardiopulmonary resuscitation may reasonably be terminated in such patients.”
(Levine, 1997) It should be noted than most normal arrests that fail to be resuscitated
after 20 minutes survive, regardless of ETCO2.
Several other studies have found dramatic differences in ETCO2 in survivors and non
survivors of SCA, but no consensus has yet been reached on what constitutes a survivable
ETCO2. In addition, it remains unclear if ETCO2 is truly an independent predictor, or
simply a result of other factors (such a quality of CPR) that affect ROSC.
Therefore, while future studies will be needed before ETCO2 alone can be used as
justification to terminate resuscitation (especially in special resuscitation groups like
hypothermia), use of this technology does hold promise, and should be considered
important in the termination thought process and included in discussion with medical
control.
KEY POINT:

13. LOW ETCO2 alone is not indication to terminate resuscitation, but it is an
important consideration. High ETCO2 may be an indicator to continue
prolonged resuscitation.
Pitfalls of capnography
The greatest potential for failure is focusing on the ETCo2 and not the patients overall
clinical picture. The ages old mantra “Treat the patient, not the monitor” is more true than
ever.
Other pitfalls include not using ETCO2 on ETT patients. This is now considered by many
experts as the “gold standard” and failing to do so opens up the medic to accusations of
tube displacement.
Summary
Basic capnography is not only the measurement of ETCO2, but analysis of the phases of
exhalation (measured indirectly by monitoring ETCO2) in the respiration cycle. In this, it
is as useful, and as complex, as electrocardiography. It has been proven accurate and in
some cases definitive in many clinical concerns facing the EMS community today. While
still an emerging technique, it is likely that future paramedics will be stressing over
capnography classes as much as they do over cardiology or pharmacology today.

14. Glossary
Capnography – A graphic representation of the measurement of carbon dioxide (CO2) in
exhaled breath.
Capnometer – The numeric measurement of CO2.
Capnogram – CO2 waveforms which can be of two types: FCO2 can be plotted against
expired volume (SBTCO2 curve /volume capnogram/ CO2 expirogram) or against time
(time capnogram) during a respiratory cycle. Most EMS monitors are TIME capnograms.
End Tidal CO2 (ETCO2 or PetCO2) - the level of (partial pressure of) carbon dioxide
released at end of expiration.

15. References
Lisa Evered, MD;Francine Ducharme, MD;G. Michael Davis, MD; Martin Pusic, MD,
Can we assess asthma severity using expiratory capnography in a pediatric emergency
department? Can J Emerg Med 2003;5(3):169-170
Levine R, End-tidal Carbon Dioxide and Outcome of Out-of-Hospital Cardiac Arrest,
New England Journal of Medicine, July 1997
Bhavani Shankar K, Moseley H, Kumar AY, Delph Y. Capnometry and
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Kalenda Z: Capnogram as a guide to the efficacy of cardiac massage.
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