Capnography super simplified : The basics

is synonymous with patient safety during anesthesia and sedation, and a boon 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.
WHY CAPNO? IN PICU Beware the falsely reassuring statement " He must be breating, sats are ok " Use Etco2 to guage ventilation.

Picture1The patient admitted in ED and critical care are too sick unlike the OR where the patients are relatively stable and screened for fitness of procedure or surgery.
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.
1. Capnography helps in the 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 (cote et al).
What is Capnography  
= Measurement of co2 during expiration

Infrared absorption of CO2 as a principle of operation..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 detectors 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 og co2 molecule, the measured absorbence is compared with standard absorbence and the co2 at et end is calculated.

Main-stream capnographs
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.
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 easy to connect, do not require sterilization as they are disposable, and can be used in awake patients.

Image source: Reference 2
Best thing about sidestream capnography is it can be used in spontaneuosly breathing patients, Specialised nasal cannula are available with device itself or simple modification of regular nasal cannula like this can be done to use sidestream capnography device.
(Image source: The ICU book, Paul Marino)


Real time main stream capnogarphy vs sidestream with delayed reflection of respiratory cycle.

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
Image source: Reference 2
with detection  of CO2. The outer perimeter of the device contains colour coded section indicating the concentration of exhaled CO2 associated with each colour change

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 no to very low co2 this air is then mixed with the air from conducting zone and the 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 is further divided into three phases.

phase I (Anatomical and apparatus dead space gases)

phase II Rapid rise (Mixture of Anatomical dead space and Physiological dead space gases)

phase III alveolar Plateau (Co2 rich gas from alveoli and reflects alveolar evolution of CO and always has a mild positive upslope. This positive up slope is not well appreciated in a time capnogram. In Volume capnogram, the positive slope is prominent as CO concentration is plotted against evolving expiratory volume. 

Phase 0 (Fresh Co2 free gas is inhaled and CO2 conc falls rapidly to zero)

Alpha angle (Angle bewteen phase 2 and phase 3. Generally reflect V/Q status of lung )
Beta angle (Angle between phase three and descending limb. Generally its is 90 degree)
Volume capnography is much more beneficial in the PICU setting. 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.
Magnified Capnogarph of a mechanical breath showing various phases mentioned above where ETCO2 is plotted against time.

Capnograph where ETCO2 is plotted against Tidal volume.

a-ETCO2 GRADIENT Index of alveolar dead space:
Normally , the PETCO2 < PaCO2 (average of all alveoli) by 2-5 mmhg as a result of temporal, spatial, and alveolar mixing defects.
“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”.

                                      ETCO2 CLOSELY RESEMBLES PACO2
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”

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 chronic obstructive airway disease, the slope of phase III is increased together with  an increase in the alpha angle.  The morphology of capnogram can offer tremendous information about underlying V/Q abnormality.
Figure showing virtual spaces occupied  by   verious volumes when capnograph is plotted.
   X is  total exhaled volume.
   Y is alveolar dead space and reflects   quantitative V/Q missmatch.
    Z is anatomical dead space.

Gas exchange abnormality
Increased anatomical dead space
Open vent circuit
Shallow breathing
Increased alveolar dead space
Obstructive lung disease
Excessive lung inflation
Low cardiac output
Pulmonary embolism

While interpreting abnormal capnogarph these should be taken in consideration first
sampling error
calibration error
leaks or occulsion in sampling lines
difficulty in obtaining a true end-tidal CO2 

2.The ICU book by Paul Marino
3.Kodali, Bhavani Shankar Anesthesiology. 118(1):192-201, January 2013.
4.National audit project
5.Quick guide to capnography philips
6.Capnography in pediatric Intensive care medicine, Ajay Desai, Great Ormond Street Hospital, London
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