CAPNOGRAPHY

Horia Hapca

Educational objectives
What you need to know
 what is capnography
 what are the normal values of capnography
 how the capnograph works
 types of capnographs
 stages of capnography
 capnography indications in intubated patients
 capnography indications in spontaneously breathing patients

Definition
Capnography (end-Tidal CO2, PETCO2 and ETCO2) is a
non-invasive monitoring means of the partial pressure of carbon
dioxide in the exhaled air.
Values may be plotted against time (CO2 concentration in
time) and against volume (CO2 volume concentration), the latter
being difficult to carry out in non-intubated patients. The
maximum level of carbon dioxide at the end of each exhaled
breath is the end-Tidal CO2 pressure. The shape changes
illustrated in the diagram serve as diagnosis for certain conditions,
whereas end-Tidal CO2 changes are used for establishing
severeness and treatment response.
Normal PETCO2 values
fall within the range 24-45 mm
Hg.
The role of capnogra-
phy is to indicate data about
each respiratory cycle in terms
of:
 ventilation, i.e. the effi- Figure 1. Capnograph: monitoring
ciency of carbon dioxide ETCO2 (39 mmHg, graph with
elimination from the exhalation plateau; pulse oximetry
airways with RR 8, SpO2 100%)
 perfusion, i.e. the efficiency of carbon dioxide transport
through the respiratory system
 metabolism, i.e. the efficiency of carbon dioxide production
 by cell metabolism.

Technology
Capnographs use infra-red radiations. Carbon dioxide
molecules absorb infra-red radiations with wavelengths
proportional with carbon dioxide concentration levels. For
recording carbon dioxide levels, 2 configurations are used:
 mainstream – used only with intubated patients having a
sensor inserted directly within the airways, in the intubation
cannula region extending beyond the vocal chords
 sidestream, which determines carbon dioxide concentration in
the exhaled air through the tube and sensor incorporated in the
monitor. The latter is used both with intubated patients and
non-intubated patients (by means of nasal or oral cannulas).

Figure 2. Mainstream capnograph Figure 3. Sidestream capnograph

Physiology
Capnography is made up of 4 phases:
Phase 1: ventilation of
anatomical dead space –
coincides with the start of the
exhalation when the air from
the dead space is expelled to
the upper airways. Figure 4. Capnography: AB – phase
Phase 2: the ascending phase – 1, BC – phase 2, CD – phase 3, DE –
coincides with the rapid growth phase 4
of
carbon dioxide concentration during respiration, upon reaching
the upper airways.
Phase 3: the concentration of the carbon dioxide reaches a
uniform level throughout the entire respiratory flow (from the
alveoli to the upper airways) and coincides with a maximum
carbon dioxide pressure point (which is the value shown on the
monitor).
Phase 4: is the inhalation cycle during which the carbon dioxide
level is 0, when oxygenated air reaches the airway.

Capnography indications in intubated patients:

1. Checking the position of the endotracheal tube
2. Continuous monitoring of the tube position during patient
transport
3. Assessing the success of the resuscitation manoeuvre and the
prognosis during cardiac arrest
4. Titration of PETCO2 values in patients suspected of increased
intra-cranial pressure
5. Predicts the prognosis of trauma patients
6. Assessing the ventilation quality

1. Checking the correct position of the endotracheal tube

 A waveform with all the 4 capnography phases appears
following a correct intubation.
 A linear waveform following intubation indicates the
protrusion of the tube in the oesophagus. There are certain
situations which trigger a linear waveform even when the tube
is inserted in the trachea:
o the obstruction of the tube in the endotracheal section
o the complete obstruction of the airway distal to the tube
o improper pulmonary blood flow (due to improper
compression during resuscitation)
o prolonged cardiac arrest without carbon dioxide flow due
to the interruption of cell metabolism.
2. Continuous monitoring of the tube position during patient
transport
 Failing to acknowledge the correct position of the tube either
due to its incorrect introduction or to its dislocation during
transport may have catastrophic consequences.
 Continuous monitoring of the tube position during transport is
essential for the patient's safety.
 PETCO2 confirmation upon initial positioning of the tube and
during transport is a widely accepted norm by the American
Society of Anesthesiologists.
3. Assessing the effectiveness of cardiopulmonary resuscitation
 During cardiac arrest, PETCO2 reflects the degree of
pulmonary vascularization.
 In the mean time, it may be used as a quality standard for
thoracic compressions. Effective cardiac compressions lead to
an increase of cardiac flow and subsequently to the increase of
perfusion, which contributes to the increase of PETCO2 from
the baseline.
4. Cardiac arrest recovery indicator
 The level of carbon dioxide reflects the recovery of the heart
rate and of the pulmonary blood flow. Carbon dioxide is
carried through the venous system towards the right half of
the heart and then is pumped towards the lungs by the right
ventricle.
 A PETCO2 peak is an early sign of resuscitation success and
may occur before palpation or assessment of hemodynamical
signs (pulse, blood pressure). AHA guidelines recommend
continuing chest compressions until the rhythm is restored on
the monitor. Capnographic monitoring is always useful
because it prevents the stopping of compressions for pulse
checking. The restoration of rhythm is associated with a
dramatic increase of PETCO2, at which point chest
compressions should stop and the ECT rhythm and blood
pressure are assessed. Despite these considerations, the 2015
resuscitation protocol maintains that there is no clear
indication that the use of capnography during CPR contributes
to the improvement of patient results. Nevertheless, it has
been very clearly established the relevance of capnography in
detecting the oesophageal position of the endotracheal tube.
 PETCO2 is most feasible in intubated patients but may also be
used in patients with supraglottic airway devices.
 PETCO2 may indicate the recovery of spontaneous circulation
and may prevent useless dosing of additional adrenaline.
 The precise determination of en-Tidal CO2 depends on a
series of factors such as: cardiac arrest, the quality of
resuscitation carried out by the assistant, the ventilation rate
and volume.
5. Establishing prognosis after initiation of cardiopulmonary
resuscitation
 It may be used as a prognosis factor during resuscitation: a
low level of end-Tidal CO2 (i.e. < 10 mm Hg 20 minutes into
the resuscitation procedure) may indicate a bad prognosis with
low chances of spontaneous circuation recovery.

6. Identifying the cause of cardiac arrest

 Studies conducted in animals have revealed low end-Tidal
CO2 during cardiac arrest secondary to respiratory arrest as
compared to values occurred during primary cardiac arrest
caused by ventricular fibrillation.
 Similar results were obtained in patients with respiratory
arrest (caused by a foreign object in the airways, aspiration,
asthma or drowning), followed by cardiac arrest through
asystole or pulseless electrical activity.

7. Serial determinations of PETCO2 levels in patients suspected

of increased intra-cranial pressure
 PETCO2 is important for the control of ventilation in patients
with brain injuries and suspected increased intra-cranial
pressure.
 The level of carbon dioxide affects the cerebral blood flow:
o high levels of carbon dioxide lead to cerebral
vasodilation
o low levels of carbon dioxide lead to cerebral
vasoconstriction
 Sustained hypoventilation is harmful in patients with high
intra-cranial pressure since it causes the increase of cerebral
blood flow and – probably – the deterioration of intra-cranial
pressure.
 Sustained hyperventilation is also harmful and is associated
with the deterioration of the neurological status.
 Therefore, monitoring of the carbon dioxide level may prevent
hyper- or hypoventilation, which may be harmful for the
underlying cerebral injury.
 Capnography indications in spontaneously breathing
patients
1. The quick assessment of the critical patient with injuries or
convulsions with the aid of airways, respiration and
circulation
2. Assessment and sorting of terrorist attack or major accident
victims
3. Appraisal of severity and response to treatment in patients
with acute respiratory distress
4. Assessing appropriate ventilation in altered mental status
patients
5. Assessing metabolic acidosis in diabetes patients and children
with gastroenteritis.

1. The quick assessment of the critical patient with injuries or

convulsions by means of ABC:
 Primary ABC assessment may be carried out with the help of
capnography.
 A normal capnographic value indicates spontaneous
respiration and a permeable airway.
 Capnography is not affected by motion artifacts and generates
trustworthy values in patients with abnormal perfusion status.

2. Assessment and sorting of terrorist attack or major accident

victims
 Capnography may serve as a non-invasive method to quickly
assess life-threatening complications caused by chemical
terrorist acts.
 In the mean time, it may quickly detect respiratory, airway
and central nervous system related adverse reactions
assoicated with nervous agents, including apnoea, upper
airway obstruction, laryngospasm, bronchospasm, respiratory
depression, convulsions and coma.

3. Assessing severity and response to treatment in acute

respiratory distress:
 Capnography allows the dynamic monitoring of the
ventilation status in acute respiration distress patients
regardless of the etiology.
 By determining the PETCO2 and the breathing rate of each
respiratory cycle, capnography can acquire instant data
regarding the chemical status of the patient.
 The assessment of the breathing rate through the oronasal
mask is more relevant than the assessment through the
transthoracic electrode by means of the respiratory impedance.
o E.g. In the case of upper airway obstruction, detecting the
breathing rate through transthoracic impedance may
generate normal values, as opposed to capnography, which
will indicate a linear waveform.
 PETCO2 may be useful in diagnosing the effectiveness of the
treatment in tachypneic patients.

4. Procedural sedation and analgesia

 Although pulse oximetry is the standard ER monitoring
technique of procedural sedation, capnography has a crucial
role since it can detect the common airway and breathing
related adverse events associated with procedural sedation and
analgesia and is considered an early indicator of compromised
airway and breathing.
 Additionally, it assesses the breathing rate without using
impedance, directly through the airways, especially in patients
with obstructive pulmonary disorders or laryngospasm.
 Central apnoea equates with the loss of the capnogram, of the
thoracic motion amplitude and as the lack of breathing sounds
during auscultation.
 Obstructive apnoea equates with the loss of the capnogram,
but with the persistence of thoracic motions and the lack of
breathing sounds.
 There are 2 types of anaesthetic-induced hypoventilation
during procedural sedation and analgesia:
o Bradypneic hypoventilation – induced by opioids:
 PETCO2  wide and high amplitude capnography
waveforms
 PaCO2 increases
 The breathing rate (BR) is depressed to a higher extent
than the Tidal volume and is ensued by bradypnoea,
the increase of the exhaling duration and an increase of
 PETCO2, graphically represented by a high waveform
amplitude and a wide capnogram.
 A predictable course ensues with the progressive
increase of PETCO2 before the onset of apnoea.
 Although there is no threshold for apnoea onset,
patients with high PETCO2 volumes, of over 80 mm
Hg are at a high risk of apnoea.
o Hypopnoeic hypoventilation – induced by hypnotic
sedatives
 PETCO2 decreases
 PaCO2 increases the air in the dead space is constant
 Tidal CO2 volume decreases more than the breathing
rate, which contributes to the increase of the dead
space, and to gradient increase between alveolar
pressure of CO2 and the end-Tidal CO2.
 3 possibilities ensue:
 Ventilation remains stable with a low Tidal level
during breathing which resolves once the sedative
concentration decreases in the CNS.
 Hypoventilation progresses towards periodic
breathing with apnoea intermittent pauses (which
may resolve spontaneously or may advance
towards central apnoea).
 Hypoventilation may regress directly towards
apnoea.

5. Assessing appropriate ventilation in altered mental status

patients
 Patients with altered mental status due to alcohol abuse,
intentional or accidental overdose and post-critical patients
(especially those treated with benzodiazepines) may develop
an abnormal ventilation function.
 Capnography may differentiate efficiently ventilated patients
from non-efficiently ventilated patients.
6. Determining metabolic acidosis
 Capnography is performed in order to assess metabolic status
with the aim of obtaining data regarding carbon dioxide
production by cell metabolism.
 Recent studies have shown that the level of PETCO2 and
bicarbonate correlates very well in diabetes and gastroenteritis
patients.
 When the patient develops acidosis (with bicarbonate
decrease), compensatory respiratory alkalosis ensues with the
increase of minute ventilation and subsequent decrease of
PETCO2 followed by the decrease of carbon dioxide
concentration in arterial blood, in order to aid acidemia
correction.
 There is a similar association between PETCO2 and the level
of bicarbonate in children with gaostroenteritis.

(Self-)Assessment form

Stage/Criterion Correct Incorrect

Ahat is capnography?
Ahat are the normal values of capnography?
What are the capnography indications in intubated
patients?
What are the capnography indications in
spontaneously breathing patients?
Which are the 4 stages of capnography?