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CO₂ Monitoring and Capnometry - Clinical Examples

An easy way to follow ventilation, circulation and metabolism

CO2 monitoring and Capnometry banner


Easy way to follow ventilation, circulation and metabolism

Capnometry graphic

Some specific benefits of CO2 monitoring:

  • Continuous
  • Non-invasive
  • Easy
  • May help prevent ventilator related mishaps
  • Follow-up of optimized ventilation
  • Rapid verification of endotracheal intubation
  • Quick detection of problems in the airway or malfunction of the ventilator or gas supply
  • Objective, continuous and non-invasive indicator of adequacy of ventilation
  • Disturbances in gas exchange, circulation and metabolism may be easily recognized
  • May help reduce the need for frequent blood gas analyses, since EtCO2 trends approximate the changes in PaCO2

Response time of various parameters to respiration-related mishaps

That table* shows how fast detector both EtCO2 and Fi-EtO2 are in several critical situations. Even SpO2 is widely used, its response time is much slower than airway gases. SpO2 is measured on the peripheral site and EtCO2 represents the alveolar values.

Table showing the detection speed of CO2 and FiO2 in different critical situations

*Stenqvist O, Paloheimo M, Meriläinen P, Paloviita P. Patient Oxygen Appliguide - your margin of safety. Datex Division Instrumentarium Corp, 1992.

CO2 production, transport and elimination

Oxidation of Glucose is an essential part of the energy metabolism and CO2 is a waste product which must be continually removed. From the cells carbon dioxide diffuses into the capillary blood and subsequently to the venous circulation returning blood from the periphery to the right side of the heart. When the heart contracts, the mixed venous blood is pumped through the pulmonary circulation to the lungs for gas exchange. The exhalation phase of breathing eliminates CO2 from the body. Although the lungs are by no means emptied, the final part of the exhaled gas is almost pure alveolar gas.

CO2 production process

Using EtCO2 to Control Ventilation

During spontaneous breathing, our respiratory center located in the brain, automatically regulates PaCO2 being close to 40 mmHg by increasing or decreasing the minute ventilation according to the metabolism.

During mechanical ventilation, the use of sedatives can negatively affect respiratory drive. In that case, normoventilation cannot be adequately maintained without monitoring. In the presence of neuromuscular blocking agents, spontaneous respiration ceases further illustrating the need for continuous monitoring of ventilation.

Differences in ventilation

Differences in ventilation

Perioperative care

Clinician working with screen in the Perioperative care unit
  • Mostly planned operations
  • Patient generally has healthy lungs
  • Patients are anesthetized and often paralyzed
  • No work of breathing concerns
  • Short-term monitoring


Intensive care

Patient in the Intensive Care unit
  • Very complex & started at sub-opportune times
  • Many different pathologies & traumas
  • Try to maintain spontaneous ventilation
  • Try to keep the work of breathing to a minimum

Normal ventilation/perfusion ratio

No significant aACO2 difference

In a healthy person with optimal ventilation-perfusion ratio aACO2 difference is 1-4 mmHg. The arterial-to-alveolar difference is due to a mismatch of ventilation and perfusion of the alveoli in the lungs. Even in healthy patients, there are some parts of the lungs which are not perfused as well as they are ventilated or vice versa.

Normal ventilation and perfusion ratio

Ventilation-perfusion mismatch

Increased aACO2 difference

Capnography provides continuous information on EtCO2, and when you compare the changes to PaCO2 (BGA) you will follow the degree of ventilation-perfusion mismach and possibly reduce frequency of blood gas analyses.

Perfusion mismatch

Parts of the lungs may also be well-perfused but poorly ventilated. Main reasons for shunt perfusion:

  • Obstructed bronchi (mucus, foreign bodies, infection)
  • Unilateral endobronchial intubation
  • Increased closing volume (diaphragm pushing against the lungs)

Main reasons for dead space ventilation are:

  • Pulmonary thromboembolism
  • Patient position (side lying)
  • Pulmonary hypoperfusion

Normal capnogram

Normal capnogram

The CO2 waveform is referred to as the capnogram and it reflects the different  stages in breathing. The capnogram is an important diagnostic tool because its shape is virtually identical in all basically healthy patients. Any change in shape should be investigated.

EtCO2 represents the CO2 value at the end of expiration and this is typically displayed on the monitor screen.



Normal capnogram

Normocapnia    EtCO2    4.0-6.0 vol %               35-45 mmHg
Hypocapnia       EtCO2    less than 4.0 vol %     35 mmHg
Hypercapnia     EtCO2    more than 6.0 vol %   45 mmHg

Common terms in CO2 monitoring:

  • ”capno” means CO2 level in breath
  •  ”hyper” means too much
  •  ”hypo” means too little
  •  ”carbia” refers to CO2 level in the blood

Units for measurement

  • vol %        volume percentage
  • mmHg     partial pressure
  • kPa           kilopascal (partial pressure)  
                      100 kPa=750 mmHg
                      1 vol % = 1 kPa
                      1 kPa = 7.5 mmHg
                      1 mmHg = 0.13 vol% (or kPa)

Alveolar minute ventilation is usually adjusted to achieve normocapnia, where EtCO2 is in the range 4.0-6. 0% (35-45 mmHg)


Benefits of normoventilation

  • Maintains normal physiology
  • Maintains normal cerebral perfusion1
  • Spontaneous breathing reappears easily
    ➔ short uneventful recovery

With normoventilation, disturbances in ventilation, circulation and metabolism can be more easily recognized and monitoring enables to maintain the physiologically optimal minute ventilation. Spontaneous breathing can be instituted more easily by allowing the EtCO2 rise at the end of the anesthesia and patient usually experiences a more rapid recovery.

During lengthy anesthesia, there is typically a very gradual decrease in EtCO2 due to the depressant effect of anesthetic agents and the effect of hypothermia slowing the patient’s metabolism.

1 Z. Kalenda, Mastering Infrared Capnography, p48, 1989



Hypocapnia/Hyperventilation capnogram
An ETCO₂ level lower than normal, less than 4.0 % (34 mmHg), is called hypocapnia.

Possible reasons:

  • Hyperventilation
  • Decreased cardiac output
  • Decreased metabolism
  • Hypothermia

In anesthesia the common cause of hypocapnia is too high alveolar ventilation (hyperventilation)

In recovery or ICU hypocapnia in the spontaneously breathing patient may be result of hyperventilation due to fear, pain, failed weaning attempt or shock (circulatory failure)

Disadvantages of prolonged hyperventilation:

  • Vasoconstriction leading to decreased coronary and cerebral blood flow
  • Unnecessary respiratory alkalosis
  • Depression of respiratory center

In specific cases there is an advantage in hyperventilating the patient. For example, in neuroanesthesia hypocarbia is used to deliberately reduce cerebral perfusion and intraacranial pressure.



Hypercapnia/Hypoventilation capnogram
An ETCO₂ level higher than normal, more than 6.0 % (45 mmHg), is called hypercapnia.

Possible reasons:


A. Spontaneous respiration

  • Depression of respiratory center
  • Mechanical disturbances (muscle paresis, hemo/pneumothorax, airway obstruction

B. Artificial ventilation

  • Airway obstruction
  • Pneumothorax
  • Leak
  • Disconnection
  • Over-feeding (excess calorie intake)
  • Ventilator dysfunction
  • Mismanagement of artificial ventilation
  • Rebreathing
  • Permissive hypercapnia
  • Circulatory insufficiency
  • Altered V/Q ratio
  • Increased metabolism (e.g. burn patients)
  • Bicarbonate infusion
  • Compensated metabolic alkalosis depression of respiratory center



Rebreathing capnogram

Failure of the capnogram to return to the baseline may indicate rebreathing of the exhaled CO2. This can be an inherent feature of the anesthesia delivery system (for example with Mapleson type of breathing systems) or it may be due to an exhausted CO2 absorber in a circle system. Finally, it may also be due to inadequate fresh gas flow given to the patient as set in the ventilator.

Monitoring FiO2 is the best method to determine the optimal fresh gas flow. Ref. Text book: Understanding Anesthesia equipment. 5th edition. 2008 by Lippincott Williams & Wilkins

If expiratory time is too short for CO2 curve to represent the EtCO2 value, the next inspiration will contain CO2 which can be seen as a rise in the baseline.



Esophageal intubation2

Esophageal intubation capnogram

If intubation is difficult i.e. vocal cords are not visible, it may happen that the ET-tube is inserted accidentally into the esophagus. If this is not detected immediately the patient will die. The best indicator for early detection is capnometry.

Some CO2 may be detected due to gas entering the stomach during manual ventilation. After removal of the endotracheal tube and successful re-intubation, high EtCO2 values are recorded because of CO2 accumulation during apnea.

American Heart Association (AHA) recommends to use capnography to confirm the correct positioning of the ET-tube as well as in confirmation of the effectiveness of chest compressions during CPR.


Cardiac arrest and resuscitation, sudden fall to zero2

Cardiac arrest and resuscitation capnogram

EtCO2 monitoring provides a continuous information in detecting return of spontaneous circulation during cardiac resuscitation.
American Heart Association (AHA) recommends to use capnography to confirm the correct positioning of the ET-tube as well as in confirmation of the effectiveness of chest compressions during CPR.


Vasopressors, bicarbonate, tourniquet2

Vasopressors, bicarbonate, ortourniquet capnogram

Sudden but transient rise in ETCO2 while inspired CO2 remains zero may be due to the release of a tourniquet or infusion of bicarbonate.

Some orthopedic operations are performed using tourniquet in order to get the surgical field bloodless. The use of tourniquet will lead to ischemic reaction in the limb and after the release of tourniquet the acidotic substance will be released into the circulation. This effect can be seen as transient rise in ETCO2.



Efforts of spontaneous breaths2

Efforts of spontaneous breaths capnogram
Efforts of spontaneous breaths can be seen as a “camel” capnogram

Insufficient muscular relaxation and/or inadequate depth of anesthesia are allowing the patient ”fight” against the mechanical ventilation. These shallow, efforts of spontaneous breaths can be seen as clefts on the capnogram plateau.


Partially obstructed airway2

Partially obstructed airway capnogram
A deformed capnogram, with a slowly rising leading edge, may indicate a partial obstruction of the airway.

Possible causes:

  • Secretions
  • Bronchospasm
  • Kink of ET-tube



Sudden disappearance of CO2 waveform2

Capnogram showing a sudden disappearance of CO2 waveform
If the CO₂ waveform is normal but then drops to zero from one breath to the next.

Possible reasons:

  • Ventilator disconnection
  • Totally obstructed ET-tube
  • Gas sampling line disconnected
  • Cardiac arrest


Sudden decrease of ETCO2*

Capnogram showing a sudden decrease of ETCO2

A rapid fall in ETCO2 over a period of several breaths may indicate:

  • Pulmonary embolism
  • Cardiac arrest
  • Severe hypotension (massive bleeding)
  • Leak in sampling system


Cardiogenic oscillations*

Capnogram showing cardiogenic oscillations
Cardiogenic oscillations in the expiratory CO₂ wave are caused by thoracic gas movements resulting from cardiac contractions during side stream gas sampling.

Possible reasons:

  • Continuous gas flow fed directly into the inspiratory limb
  • Side stream gas sampling taken at the Y-peace
  • Slow respiration rate
  • Long expiration time


Sample flow rate effect

  • Table presents different sampling volumes depending on respiration rate
  • The I:E ratio is assumed to be 1:2
22 Sample flow rate effect

The effect of the sample flow rate by the side stream analyzer is seen in the table. The amount of gas drawn by the monitor is dependent on the respiratory rate. When the respiratory rate increases the sample gas will decrease by breath. Also the total volume sampled is devided between the inspiration and expiration.

Example: with a respiration of 30, which may be common for small babies, the total volume sampled is 4 ml only, where 1,3 ml is drawn during the inspiration. This makes the 120 ml flow rate suitable for small babies as well.

Clinical benefits

Patient safety

  • Confirms the correct place of ET-tube
  • Continuous measurement to detect hyperventilation or hypoventilation quickly
  • Estimation of increased/decreased shunt
  • ETCO2 trend vs PaCO2

Optimal minute ventilation=normoventilation

Suggested reading

J. S. Gravenstein, M. B. Jaffe, N. Gravenstein, D. A. Paulus: Capnography. Text book

Capnography, Second Edition, Anesthesiology 3 2012, Vol.116, 744-745.

Rationales and Applications for Capnography, Monitoring During Sedation, Devin Carr, MSN, MS, RN, RRT, ACNS-BC, NEA-BC, CPPS and Anna Cartwright, BSN;

End-Tidal Capnography
Updated: Jan 17, 2020.
Author: Robert Thomas Arrigo, MD, MS; Chief Editor: Zab Mosenifar, MD, FACP, FCCP

How to Read and Interpret End-Tidal Capnography Waveforms

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