Capnography basics super simplified


is synonymous with patient safety during anesthesia and sedation, and a must during CPR. Since the first infrared CO2 measuring and recording apparatus by Luft in 1943, capnography has evolved into an essential component of standard anesthesia monitoring armamentarium. 

In 1978, Holland was the first country to adopt capnography as a standard of monitoring during anesthesia.
Beware the falsely reassuring statement " He must be breating, sats are ok " 


The risk of encountering major airway complication in ICU,s is about 66 times more often than OT/OR because of lack of continuous capnogarphy monitoring. Pules ox can identify the oxygenation failure but not the ventilatory failure, this is specially important during sedation procedures.
  1. Differential diagnosis of hypoxia to enable remedial measures to be taken before hypoxia results in an irreversible brain damage.
  2. Provides information about Co2 production, pulmonary perfusion, alveolar ventilation, respiratory patterns, and elimination of Co2 from the anesthesia circuit and ventilator.
  3. Effective in the early detection of adverse respiratory events.
  4. Capnography and pulse oximetry together could have helped in the prevention of 93% of avoidable anesthesia mishaps according to ASA closed claim study.
  5. Better detection of potentially life-threatening problems than clinical judgment alone.
What is Capnography  = Measurement of Co2 during expiration


Infrared absorption of CO2 is a principle of operation. It uses Beer-Lambert law. A known concentration of infrared light is traverses through the exhaled gases. Carbon dioxide, being a poly atomic gas, absorbs infrared light. The remaining beam of light is detected by sensors and exhaled CO2 values are computed.

Carbon dioxide selectively absorbs specific wavelength of infrared light 4.3 micrometer. The ammount of light is proportional to ammount of Co2 molecule, the measured absorbence is compared with standard absorbence and the Co2 at ET is calculated.


A) Main-stream capnography
  1. A sample cell or cuvette, airway adapter, is inserted directly in the airway.
  2. A lightweight infrared sensor, emitted light is detected by a photo detector located on the opposite side of the airway.
  3. Produces waveforms that reflect real-time CO measurements during a respiratory cycle without a delay.
B) Side-stream capnography
  1. Sensor located in the main unit , away from the patient, and a pump aspirates gas samples from the patient’s airway into the main processing unit.
  2. The capnographs will have a delay in displaying co2 concentration
  3. A main problem encountered in the ICU setting is the blockage of the sampling tubes

Advantage of side-stream capnography is that expiratory gases can be obtained from the nasal cavity using nasal adaptors or with a simple modification of the standard nasal cannula.

These devices are
  1. Easy to connect,  
  2. No sterilization as they are disposable
  3. Can also be used in spont breathing patients.

Image source: Reference 2
Real time main stream capnogarphy vs sidestream with delayed reflection of respiratory cycle

C) Qualitative CO2 measurement

Quick and simple method of determining if an endotracheal tube has been placed properly. Devise contains filter paper that is impregnated with a PH-sensitive indicator that changes colour on exposure to CO2. The outer perimeter of the device contains colour coded section indicating the concentration of exhaled CO2 associated with each colour change
Image source: Reference 2


1. Capnometry:

The measurement and display of CO2 on a digitial or analogue monitor. Maximum inspiratory and expiratory CO2concentrations during a respiratory cycle are displayed.

2. Capnography:

A graphic display of instantaneous CO2 concentration during a respiratory cycle (CO2 waveform or capnogram)

3. Capnograms: Time and Volume

Can be of two types: ETCO2 can be plotted against expired volume or against time (time capnogram) during a respiratory cycle.
4. PETCO2:Partial pressure of CO2 at the end of expiration

5. (a-ET)PCO2:Arterial to end-tidal CO2 tension/pressure difference or gradient

Waveform phases

Little bit of lung physiology will help understanding typical waveform, so during expiration the first air the sensor breaths is from deadspace which contains very low co2. This air is then mixed with the air from conducting zone and then Co2 content increases rapidly, lastly followed by air in alveoli with highest concentration of co2.
Image Source: Reference 6
A time capnogram can be divided into inspiratory and expiratory segments. The inspiratory segment (phase 0) is further divided into three phases

phase I : Sensor starts detecting the Co2 from anatomical dead space gases

phase II : Rapid rise, Mixture of Anatomical dead space and Physiological dead space Co2

Alpha angle : Angle bewteen phase 2 and phase 3

phase III : Alveolar Plateau The sensor detects Co2 rich gas from alveoli and has a mild positive upslope. This positive up slope is not clearly seen in a time capnogram. In Volume capnogram, the positive slope is prominent as CO concentration is plotted against evolving expiratory volume.

Beta angle : Angle between phase three and descending limb

Phase 0 : Fresh Co2 free gas is inhaled and CO2 concentration falls rapidly to zero giving rise to 90 degree angle between phase III and Phase 0

Magnified Capnograph of a mechanical breath showing various phases mentioned above where ETCO2 is plotted against time.


Alpha angle
Increases in alpha angle (angle between phase II and phase III) and the slope of phase III are a good refection of V/Q perfusion status of the lung.  

In asthma or any obstructive airway, the slope of phase III is increases together with  an increase in the alpha angle.  

Beta angle
Its a 90 degrees angle between phase III and the descending limb of capnogram. Increases in case of re-breathing in circuit due to failure of exhalation valve.


The volume capnogram can be related to components of tidal volume; physiological dead space and alveolar ventilation .

A noninvasive estimate of physiological dead space can be obtained from volume capnography

Capnograph where ETCO2 is plotted against Tidal volume
Image shows virtual spaces occupied by various volumes when volume capnograph is plotted.
X is  total exhaled volume
Y is alveolar dead space and reflects quantitative V/Q mismatch
Z is anatomical dead space


The gas exchange in lungs is not a perfect model and there are always mixing defect of gases wiht the blood in capillaries, as a result the PaCo2 is always higher than the Normally ETCO2 by 2-5. The changed in this can be used to assess the alveloar dead space. The more the difference more is the dead space. 

On mechanical ventilator worsening of this gradient can be taken as increasing dead space while decrease from previous values can be taken as improvement of disease or response to the therapy like bronchodilators in asthma 

IIn   In Children, the (a-et)pco2 gradient is Smaller (0.65-3 mm hg) than adults. This is due to a better V/Q matching, and hence a lower alveolar dead space in children. Therefore, changes in alveolar dead space correlate well With changes in (a-et)pco2  “Hence (a-et)pco2 is an Indirect estimate of V/Q mismatching of the lung.

Increased anatomical dead space

Open vent circuit
Shallow breathing

Increased alveolar dead space

Obstructive lung disease

Excessive lung inflation
Low cardiac output
Pulmonary embolism


Before interpreting abnromal capnograph and EtCo2, first think about
  1. Sampling error
  2. Calibration error
  3. Leaks or occlusion in sampling lines
  4. Difficulty in obtaining a true end-tidal CO2
1.The ICU book by Paul Marino
2.Kodali, Bhavani Shankar Anesthesiology. 118(1):192-201, January 2013.
3.National audit project
4.Quick guide to capnography philips
5.Capnography in pediatric Intensive care medicine, Ajay Desai, Great Ormond Street Hospital, London

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