CAPNOGRAPHY – END-TIDAL CO2 MONITORING
(References: “10 Things Every Paramedic Should Know About
Capnography” website, UpToDate, Brady & Caroline Paramedic
Textbooks, paramedicine.com, “Capnography for Paramedics”
website, emsstaff.buncombecounty.org, AHA 2010 Guidelines,
alibaba.com)
Capnography is the measurement of carbon dioxide
(CO2) in exhaled breath, often referred to as the
“ventilation vital sign”. Capnography is an objective
measurement of ventilation and hemodynamic status.
The term “End Tidal CO2 (ETCO2), refers to the level of
CO2 released at the end of exhalation, expressed as a
percentage or in millimeters of mercury (mmHg). Normal values are 5-6% CO2, equivalent to 3545mmHg. CO2 reflects 4 metabolic processes: alveolar metabolism, cellular metabolism, cardiac output
and pulmonary blood flow as gas is transported by the venous system to the right side of the heart then
pumped to the lungs by the right ventricle. When CO2 diffuses out of the lungs into exhaled air, a
“capnometer” measures the partial pressure or maximal CO2 concentration at the end of exhalation.
There are 4 main physiologic CO2 components:
1. Production: CO2 is a metabolic byproduct of aerobic cell metabolism. As intracellular CO2
increases, CO2 diffuses into tissue capillaries and carried by venous circulation to the lungs,
where it diffuses from pulmonary capillaries into the alveoli. Ventilation is one way the body rids
itself of this toxic byproduct as CO2 carried through the blood to be lungs to be exhaled.
2. Transport: maintaining CO2 tension of arterial blood at 35-45mmHg.
3. Buffering: buffering action of hemoglobin and pulmonary blood flow maintains normal CO2 levels
by eliminating excess CO2. CO2 either carried, dissolved or combined with water (H20) to form
carbonic acid (H2CO3), which dissipate into hydrogen ions (H+) and bicarbonate (HCO3-) ions per
the “Henderson Hesselbalch Equation”: (CO2 + H20 <=> H2CO3 <=> H+ + HCO3-). Hydrogen
ions and bicarbonate ions buffered by hemoglobin account for 90% of CO2 transport.
4. Elimination: CO2 elimination by alveolar ventilation under control of the respiratory center. This
process allows CO2 diffusion from blood to the alveoli where the partial alveolar pressure of CO 2
is lower than the tissue pressure.
Any increased metabolic state increases CO2 production. Decreased cardiac output lowers CO2 delivery to
the lungs decreasing ETCO2 levels. Any change in ETCO2 reflects changes in systemic and pulmonary
blood flow and therefore is an objective tool to evaluate adequacy of ventilation and pulmonary blood
flow during low cardiac output states, i.e. hemodynaminc instability, cardiac arrest, or to monitor
effectiveness of compressions during CPR.
MEASURING ETCO2
ETCO2 measured with a capnometer containing a source of infrared radiation, a
chamber with a gas sample, and a photodetector. When expired CO2 passes
between the beam of infrared light and photodetector, the absorbence
proportional to the concentration of CO2 in the gas sample. The gas samples
are analyzed by mainstream (in-line) or sidestream (diverting) techniques.
ETCO2 attachments available are nasal w/O2 tubing, nasal w/o O2 tubing for
use with a non-rebreather, and in-line for intubated patients.
CAPNOGRAPHY VALUES AND WAVEFORM INTERPRETATION
Normal
capnography
ranges
between
35
to
45mmHg.
ETCO2
<35mmHg
means
hyperventilation/hypocapnia; ETC02 >45mmHg means hypoventilation/hypercapnia. As the respiratory
rate increases, the CO2 level decreases (and conversely). The ETCO2 wave form begins just prior to
exhalation and ends with inspiration.






A-B:post-inspiration/dead space exhalation
B:start of alveolar exhalation
B-C: exhalation upstroke where dead space
gas mixes with lung gas
C-D: plateau of exhalation
D: end-tidal “peak” CO2 concentration
D-E: inspiration washout
Capnography provides a breath-to-breath trend of
respirations and an early warning of respiratory
distress. There are only a few abnormal waveform
patterns to recognize: hyperventilation, hypoventilation, esophageal intubation and obstructive. A benefit
of capnography over oximetry is that within a few breaths, capnography will change patterns, vs often
over a minute for the PaO2 to change. Pay more attention to the ETCO 2 waveform trend than the actual
number. A steadily rising ETCO2 (i.e. as patient becomes sedated and hypoventilates) can help a provider
anticipate when assisted ventilations or intubation may be necessary.
Hyperventilation:
When a person hyperventilates, their CO2 level drops (hypocarbia). Hyperventilation is caused by many
factors including anxiety, bronchospasm, pulmonary embolus and diabetic ketoacidosis. Hypocarbia may
also be present in low metabolic / low cardiac output states: cardiac arrest, hypotension, hypothermia,
sepsis, pulmonary edema.
Hypoventilation:
When a person hypoventilates, their CO2 rises (hypercapnia). Hypoventilation can be caused by sedation
states, including overdose, intoxication, post-ictal, head trauma, stroke, sepsis, and respiratory fatigue /
failure. CO2 may be chronically high in smokers and with underlying lung disease (i.e. COPD).
Obstructive:
Bronchospasm produces a “shark fin” waveform created when the patient exhales against an
“obstruction” producing the characteristic upstroke pattern. The sloping shape correlates to the level of
bronchospasm / obstruction – patients with severe underlying COPD / asthma may have at baseline a
small upstroke in their waveform pattern. With mild asthma, CO2 drops as the patient hyperventilates to
compensate for their respiratory distress. As the asthma worsens, the C02 levels start elevating. When
the bronchospasm becomes severe and patient tires and hypoventilates, they become hypercapnic. This
is known as “CO2 Narcosis”, as high CO2 blood levels have a sedating effect. Successful bronchospasm
treatment decreases the shark-fin slope returning the ETCO2 to a normal range.
The waveform pattern in patients with CHF or pulmonary edema is more upright, and less “sloped” than
in patients with COPD / asthma. This may help along with SAMPLE history and exam when differentiating
between obstructive airway wheezing vs “cardiac wheezing” of CHF.
Confirming, Maintaining , Assisting Intubation: Continuous ETCO2 monitoring can confirm a tracheal
intubation or rapidly determine if an ETT becomes dislodged or is misplaced as a stable waveform
ensures the ETT is in the trachea. One study evaluated prehospital intubations utilizing continuous
waveform capnography to confirm ETI, demonstrating 0% unrecognized misplaced intubations in the
waveform group versus 23% misplaced ETI in the group without waveform capnography (Silversti,
2005). An important caveat is in patients with cardiopulmonary arrest, good compressions are necessary
to generate a waveform. In patients with a prolonged downtime, ETCO2 reading may be so low that the
waveform looks “flat”, or appears to be consistent with an esophageal intubation. If the provider can
ventilate without resistance with fog in the tube, bilateral lung sounds and no epigastric sounds, do not
automatically pull the ETT; the patient may be “too dead” to produce anything but a scant amount of
CO2, which looks flat or minimally elevated from baseline. Capnography cannot detect right main-stem
intubations. Capnography can also be used for supraglottics, combitubes and LMAs.
CARDIAC OUTPUT MEASUREMENT DURING CARDIOPULMONARY RESUSCITATION
The 2010 AHA Guidelines endorse waveform capnography as a Level I recommendation for ETT
verification, a Level IIa recommendation for detecting return of spontaneous circulation (ROSC) and a
Level IIb for monitoring CPR quality. Colormetric ETCO 2 devices should only be used when waveform
capnography not available (Class IIa), or for initial confirmation. Utilizing waveform ETCO2 during
cardiopulmonary arrest with adequate compressions may show waveform oscillations immediately upon
intubation, replaced by larger wave forms once ventilations initiated. The oscillations demonstrate that
compressions alone produce some ventilation (further evidence behin AHA’s 2010 recommendations to
not provide advanced airway management during the 1st 8 minutes of witnessed OOHCA).
It has well documented that quality compressions are associated with elevated ETCO 2 levels
(Gravenstein, Cambridge Press 2004). With the new AHA Guidelines calling for “hard, fast and deep”
compressions, rescuers should switch places every two minutes. Set the monitor so providers can see the
ETCO2 and cardiac monitors, with the goal of providing 100 compressions / minute and maintaining a
stable and high ETCO2 waveform. A “spike” in ETCO2 during resuscitation is often the 1st sign of return of
spontaneous circulation (ROSC). ETCO2 will elevate >35mmHg post ROSC due to CO2 tissue washout. Per
AHA Guidelines, “ETCO2 monitoring during cardiac arrest is a safe and effective noninvasive indicator of
cardiac output during CPR and may be an early indicator of ROSC in intubated patients.” Conversely, in a
recently resuscitated or hemodynamically unstable patient, if the ETCO2 number dropping, immediately
check pulses as this drop signals an acute loss of cardiac output, and compressions may be required.
ETCO2 monitoring can confirm resuscitation futility in addition to predicting likelihood of resuscitation
outcomes. Per Levine an ETCO2 <10 mmHg 20 minutes post initiation of resuscitation accurately predicts
death in patients with PEA or VF arrest leading to the recommendation to terminate resuscitation ( NEJM,
1997). Multiple studies have noted that patients with a high initial ETCO2 level are more likely to be
successfully resuscitated than those with lower initial ETCO2 levels. Levine’s study demonstrated that no
patient with an initial ETCO2 <10mmHg survived while in patients with ROSC, ETCO 2 rose to at least 18
mmHg before any clinically detectable return of vital signs. Other studies have shown rare patients
surviving resuscitation with an ETCO2 initially <10mmHg, especially in cold-water drowning. Like any vital
sign or lab finding, the ETCO2 is one of many factors taken into consideration when terminating or
continuing resuscitation efforts.
MONITORING SEDATED PATIENTS
Capnography should be used to monitor sedated patients for evidence of hypoventilation or apnea.
ETCO2 monitoring flagged a problem before changes in oximetry or observed changes in respiratory rate.
Capnography essential in intubated patients as small changes wave form indicates the patient is
beginning to arouse from sedation or starting to breathe on their own.
VENTILATING HEAD TRAUMA PATIENTS
Capnography helps avoid hyperventilation in intubated head injured patients, avoiding secondary “insult”
of excessive free radical formation cause by hyperoxygenation on the vulnerable ischemic brain
“penumbra”. Hyperventilation decreases intracranial pressure by decreasing intracranial blood flow,
resulting in cerebral ischemia emphasizing the importance of prehospital “appropriate” ventilation. One
study showed patients with ETCO2 monitoring had a lower incidence of inadvertent hyperventilation (6%)
than those without ETCO2 monitoring (13%). Patients in the hyperventilation group had a significantly
higher mortality rate (56%) than those with “normal” ventilation (30%) (Davis, J Trauma 2004)
OTHER CLINICAL SITUATIONS WHERE ETCO2 HELPFUL
 Patients with acidosis (i.e. diabetic ketoacidosis) hyperventilate to “blow off’ CO2.
 A pulmonary embolus increases pulmonary dead space decreasing alveoli available to offload CO2,
causing a drop in ETCO2.
 Metabolism rises in fever & neuroleptic malignant syndrome, causing an elevation in ETCO2.
 A study of blunt trauma patients showed 5% with ETCO2 <26mmHg after 20 mins survived to
discharge vs a median ETCO2 for survivors was 30mmHg (Deakin, J Trauma 2004).
 Capnography used as a triage tool to assess respiratory status in MCIs involving chemical attacks.
 ETCO2 provides warning to anaphylaxis reoccurrence after initial treatment, as the waveform starts
sloping upwards before wheezing clinically noted or pulse oximetry drops.
 Capnography provides an accurate respiratory rate, as well as documents respiration trends.
 ETCO2 monitoring can provide an early warning sign of shock. A patient with a sudden drop in
cardiac output will show a drop in ETCO2 numbers that may be regardless of any change in
breathing. This has implications for trauma patients, cardiac patients – any patient at risk for shock.

DOCUMENTATION
Paramedics should document their use of continuous ETCO 2 monitoring and attach waveform strips to
their PCRs for the following points of care: prior to intubation, immediately post-intubation, periodically
during care and transport, and upon and after any patient transfer. This serves as both a timestamp and
proof that your intubation was correctly placed.