Clinical Tips and Tricks: Capnography in the
Emergency Department: A primer
Hugh Greenbaum a
a Forest View Volunteer Rescue Squad, Richmond, Virginia, USA
Corresponding author: Hugh Greenbaum; Email: hugh_greenbaum@fvrs.org
Abstract
Capnography is an underappreciated tool in the emer-
gency department. Continuous monitoring of a patient’s
exhaled carbon dioxide (CO2
) provides insights into
their ventilatory, perfusion, and metabolic status at the
bedside. Since it is a continuous process, capnography
can often warn of patient deterioration seconds to
minutes before other modalities (e.g., pulse oximetry
and blood pressure). This article briefly reviews the
physiology behind CO2 production, how capnography
monitors work, and what they tell us. Common and
important waveforms are reviewed, with emphasis on
those that may impact immediate patient care decisions.
The remainder of the article briefly discusses various
clinical presentations and how the associated capnogra-
phy measurements, along with patient history and other
diagnostic tools, can provide early notice of life-threaten-
ing conditions or deterioration.
Keywords: capnography, emergency capnography, EMS,
emergency department, ETCO2
Background
C apnography is a system that continuously monitors exhaled
carbon dioxide (CO2
). Typical capnography monitors
used in emergency medical services (EMS) and emer-
gency departments have three displays (Ward & Yealy, 1998):
a) A waveform that displays the patient’s ventilatory pattern over
time. his is referred to as a capnogram.
b) A numeric indication of the amount of exhaled CO2 at the
end of each breath. This is referred to as the End-Tidal CO 2
(ETCO2
) measurement.
c) The number of ventilations per minute. These two numeric
displays are collectively referred to as capnometry.
CO2 is an acidic by-product of aerobic metabolism. Aerobic
metabolism depends on cells being well perfused with oxygen and
nutrients, with sufficient blood flow to remove the CO2 (Kamel &
Halperin, 2017). Since CO2 is a gas, its excretion depends on the
patient’s ability to ventilate adequately. Capnography, therefore, is
a tool that allows providers to non-invasively monitor the patient’s
metabolism, perfusion, and ventilations (Ward & Yealy, 1998).
Capnography measurements must be interpreted in the context
of the patient’s history and other diagnostic signs. For example,
ETCO2 measurements above 45 mmHg (6 kPa) could indicate
CO2 trapping (e.g., bronchospasm) or a hypermetabolic state (e.g.,
fever, stimulant overdose). Similarly, ETCO2 measurements below
35 mmHg (4.7 kPa) could indicate a hypometabolic state or an
acidotic state (e.g., shock).
As acidosis progresses, the body’s stores of bicarbonate (HCO3
)
are consumed, buffering the abnormal acids. Unfortunately,
HCO3 is also necessary for removing CO 2 from the blood stream,
so it may be exhaled via the lungs. This results in seemingly par-
adoxical low ETCO2 measurements as acidosis increases. These
low measurements, along with the patient’s history and other
diagnostic indicators, provide clues as to the depth and nature of
the patient’s illness (Ward, 2011).
The following sections illustrate how capnography can benefit
emergency department patients.
Airway Management
The patency of an advanced airway (i.e., endotracheal tubes or
supraglottic airways) can be quickly assessed by observing the
patient’s capnogram. A normal wave form is shown in Figure 1
(Ward & Yealy, 1998). Figure 2 shows the waveform of a patient
with a kinked endotracheal tube or partially clogged advanced
airway (Ward & Yealy, 1998).
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42 Vol. 48, No. 2, Summer 2025 · Canadian Journal of Emergency Nursing
A leak in an endotracheal tube’s distal cuff or a supraglottic air-
way’s seal will result in a waveform like that shown in Figure 3
(Ward & Yealy, 1998).
Respiratory Management
Apnea detection is an application of capnography that is
potentially, and immediately, lifesaving. It can take minutes
for a patient’s oxygen (O 2
) levels to desaturate below the usual
pulse oximeter oxygen saturation (SpO2 ) alarm settings. Since
capnography monitors patient ventilation in real-time, it can
detect bradypnea and apnea in seconds, allowing the clinician
to quickly restore ventilation and perfusion to prevent patient
deterioration (Cacho et al., 2010).
We need to ensure that our intubated patients are properly oxy-
genated and sedated. A capnogram like that shown in Figure 4,
sometimes referred to as a “curare cleft”, indicates that the patient
is inhaling prematurely during exhalation. This may indicate that
the patient is hypoxic or that the patient’s sedation is wearing off
(Ward & Yealy, 1998).
Interestingly, non-intubated patients may also present with this
waveform, which may indicate hypoxia or unmanaged pain
(Greenbaum, 2024).
Cardiac Emergencies
American Heart Association (AHA) guidelines suggest that
ETCO 2 measurements of at least 10 mmHg (1.33 kPa) during
cardiopulmonary resuscitation (CPR) indicate adequate com-
pressions and ventilation (American Heart Association, 2020).
A sudden rise of ETCO2 of 10 mmHg (1.33 kPa) or more indi-
cates that return of spontaneous circulation (ROSC) has been
achieved (Kodali & Urman, 2014).
Metabolic Emergencies
Low ETCO 2 measurements, lacking a ventilatory disturbance
(e.g., hyperventilation), may indicate either an abnormally low
metabolic level or that the patient is suffering from a metabolic
acidosis. For example, a patient presenting with an ETCO 2 mea-
surement of 25 mmHg (3.3 kPa) with blood glucose measure-
ments greater than 16.7 mmol (300 mg/dl) and a history of
diabetes strongly suggests that our patient has progressed from
simple hyperglycemia into diabetic ketoacidosis. A rapid bed-
side diagnosis allows treatment to begin sooner, relative to wait-
ing for blood tests to be completed (Kamel & Halperin, 2017).
High ETCO 2 measurements, lacking a ventilatory disturbance
(e.g., bronchospasm, respiratory failure), may indicate an abnor-
mally high metabolic level. For example, a patient presenting
with recent sedation by EMS personnel for delirium and violent
behaviour secondary to a suspected phencyclidine (PCP) over-
dose presents with increasing ETCO2 measurements, the last of
55 mmHg (7.3 kPa). This suggests that the patient is developing
malignant hyperthermia. Capnography allows providers to see
the increasing metabolic activity before it results in potentially
lethal temperature levels (Gupta & Hopkins, 2017).
Shock
Shock is (by definition) a systemic disruption in perfusion
(Ward, 2011). Cells that are inadequately perfused switch to
anerobic metabolism, which produces lactic acid and little to
no CO 2
. The lactic acid is buffered by HCO 3 , which reduces the
amount available for removing CO 2 from the blood stream. The
reduced production and removal of CO 2 , and thus shock, can be
detected by monitoring ETCO 2 measurements. Since capnogra-
phy is a continuous measurement process, shock can be detected
sooner than using other vital signs.
Compensated shock can be recognized with ETCO2 measure-
ments of 28–35 mmHg (3.7–4.7 kPa) with an appropriate his-
tory. Decompensated shock can be recognized with ETCO 2
measurements 13–27 mmHg (1.7–3.6 kPa). ETCO2 measure-
ments below 12 mmHg (1.6 kPa) imply irreversible shock has
set in (Kheng & Rahman, 2012). At this stage, treatment must
be rapid and aggressive if the patient’s life is to be saved.
Trauma
Capnography can assist in managing patients with traumatic
brain injuries (TBIs). Hyperventilation of TBI patients improves
oxygenation but reduces cerebral blood flow. Hypoventilation
improves blood flow but reduces oxygenation. Maintaining
Figure 1
Normal Capnogram
Figure 2
Kinked Endotracheal Tube Capnogram
Figure 3
Leaky Endotracheal Tube Cuff Capnogram
Figure 4
Gasping Capnogram
Canadian Journal of Emergency Nursing · Vol. 48, No. 2, Summer 2025 43
ETCO2 between 30 and 35 mmHg (4.0 and 4.7 kPa) provides
a healthy balance of these two competing priorities (Frakes,
2011).
Conclusion
Capnography provides continuous, non-invasive monitoring
that provides insights into a patient’s perfusion, metabolic, and
ventilatory status. In conjunction with other diagnostic tools,
capnography can help emergency department providers recog-
nize potentially life-threatening problems earlier, and monitor
the effectiveness of related patient treatment.
Implications for Emergency Nursing Practice
• Capnography allows for rapid assessment of the patient’s ven-
tilatory, perfusion, and metabolic status at the bedside.
• Capnography changes with potentially life-threatening condi-
tions much more quickly than other diagnostic tools.
• Capnography supplements, but does not replace, other diag-
nostic modalities.
About the Author
Hugh Greenbaum BS, NRP, is an 9-1-1 response paramedic in the
Greater Richmond, Virginia, United States of America area, and
is the author of Emergency Capnography, published by CRC press.
He has more than 39 years of volunteer EMS experience. Hugh is
an EMS educator and preceptor for his agency. He holds a Bachelor
of Science degree in Information and Computer Science from the
University of California, Irvine.
Acknowledgments
I thank Nancy Davis, MSPT, for her editorial assistance.
Conflicts of Interest
None reported.
Funding
None reported.
REFERENCES
American Heart Association. (2020). Advanced cardiovascular life
support. American Heart Association.
Cacho, G., Pérez-Calle, J. L., Barbado, A., Lledó, J. L., Ojea, R., &
Fernández-Rodríguez, C. M. (2010). Capnography is superior to
pulse oximetry for the detection of respiratory depression during
colonoscopy. Revista Espanola de Enfermedades Digestivas, 102(2),
86–89. https://doi.org/10.4321/S1130-01082010000200003
Frakes, M. A. (2011). Capnography during transport of patients (inter/
intra-hospital). In J. S. Gravenstein, M. B. Jaffe, N. Gravenstein,
& D. A. Paulus (Eds.), Capnography (2nd ed.). Cambridge
University Press.
Greenbaum, H. (2024). Emergency capnography (1st ed.). CRC Press.
https://doi.org/10.1201/9781003491576
Gupta, P. K., & Hopkins, P. M. (2017). Diagnosis and management of
malignant hyperthermia. BJA Education, 17(7), 249–254. https://
doi.org/10.1093/bjaed/mkw079
Kamel, K. S., & Halperin, M. L. (2017). Fluid, electrolyte, and acid-base
physiology (5th ed.). Elsevier USA.
Kheng, C. P., & Rahman, N. H. (2012). The use of end-tidal carbon
dioxide monitoring in patients with hypotension in the emergency
department. International Journal of Emergency Medicine, 5(1).
https://doi.org/10.1186/1865-1380-5-31
Kodali, B. S., & Urman, R. D. (2014). Capnography during
cardiopulmonary resuscitation: Current evidence and future
directions. Journal of Emergencies, Trauma, and Shock, 7(4), 332–
332. https://doi.org/10.4103/0974-2700.142778
Ward, K. R. (2011). The physiological basis for capnometric monitoring
in shock. In J. S. Gravenstein, M. B. Jaffe, N. Gravenstein, & D. A.
Paulus (Eds.), Capnography (2nd ed., pp. 231–238). Cambridge
University Press.
Ward, K. R., & Yealy, D. M. (1998). End-tidal carbon dioxide
monitoring in emergency medicine, Part 1: Basic principles.
Academic Emergency Medicine, 5(6), 628–636. https://doi.
org/10.1111/j.1553-2712.1998.tb02473.x

## Références

American Heart Association. (2020). Advanced Cardiovascular Life Support. American Heart Association.

G., Pérez-Calle, J. L., Barbado, A., Lledó, J. L., Ojea, R., & Fernández-Rodríguez, C. M. (2010). Capnography is superior to pulse oximetry for the detection of respiratory depression during colonoscopy. Revista Espanola de Enfermedades Digestivas, 102(2), 86-89. https://doi.org/10.4321/S1130-01082010000200003

Frakes, M. A. (2011). Capnography During Transport of Patients (Inter/Intra-Hospital). In J. S. Gravenstein, M. B. Jaffe, N. Gravenstein, & D. A. Paulus (Eds.), Capnography (Second ed.). Cambridge University Press.

H. (2024). Emergency Capnography (First ed.). CRC Press. https://doi.org/10.1201/9781003491576

P. K., & Hopkins, P. M. (2017). Diagnosis and management of malignant hyperthermia. BJA Education, 17(7), 249-254. https://doi.org/10.1093/bjaed/mkw079

Kamel, K. S., & Halperin, M. L. (2017). Fluid, Electrolyte, and Acid-Base Physiology (5th ed.). Elsevier USA.

C. P., & Rahman, N. H. (2012). The use of end-tidal carbon dioxide monitoring in patients with hypotension in the emergency department. International Journal of Emergency Medicine, 5(1). https://doi.org/10.1186/1865-1380-5-31

B. S., & Urman, R. D. (2014). Capnography during cardiopulmonary resuscitation: Current evidence and future directions. Journal of Emergencies, Trauma, and Shock, 7(4), 332-332. https://doi.org/10.4103/0974-2700.142778

Ward, K. R. (2011). The Physiological Basis for Capnometric Monitoring in Shock. In J. S. Gravenstein, M. B. Jaffe, N. Gravenstein, & D. A. Paulus (Eds.), Capnography (Second ed., pp. 231-238). Cambridge University Press.

K. R., & Yealy, D. M. (1998). End-tidal carbon dioxide monitoring in emergency medicine, Part 1: Basic principles. Academic Emergency Medicine, 5(6), 628-636. https://doi.org/10.1111/j.1553-2712.1998.tb02473.x