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Designed, Produced, and maintained (Edition 12, 2022) by 

Bhavani Shankar Kodali MD

Updated September 2022


Clinical Aspects

Bhavani Shankar Kodali MD


Various factors result in either increased or decreased/absent PETCO2.

PETCO2 increased

 CO2 output Pulmonary perfusion
 Alveolar Ventilation Technical errorsMachine faults
Malignant hyperpyrexia
Sodium bicarbonate
Tourniquet release
Venous CO2 embolism
 Increased cardiac output

Increased blood pressure
Bronchial intubation
Partial airway obstruction
Exhausted CO2 absorber
Inadequate fresh gas flows
Leaks in breathing system
Faulty ventilator
Faulty valves

PETCO2 decreased

 CO2 output
 Pulmonary perfusion Alveolar VentilationTechnical errors Machine faults
Reduced cardiac output
Pulmonary embolism
Cardiac arrest

Total airway obstruction
Partial airway obstruction
Accidental tracheal extubation

Circuit disconnection
Sampling tube leak
Malfunction of ventilator

The table above shows various factors that influence PETCO2 measurements at the lips. However, it is strongly recommended that capnometry be used always in conjunction with capnography as the latter, besides furnishing  diagnostic and therapeutic information, is the only way to confirm the adequacy of CO2 sampling. Inadequate CO2 sampling leads to falsely low PETCO2 values which may be erroneously interpreted.


1. Bhavani Shankar K, Moseley H, Kumar AY, Delph Y. Capnometry and anaesthesia. Review article. Canadian J Anaesth 1992;39:6:617-32.

Dual Plateau capnogram caused by cracked sample filter

Dual Plateau capnogram caused by cracked sample filter


A dual plateau capnogram was observed by Body  et al during anesthesia and IPPV. They found that the cause of the abnormality was due a microscopic crack in the filter membrane separator  cartridge of the Ohmeda RGM 5250 gas analyzer. The authors hypothesized that, during the initial phase of expiration, aspiration of the gas sample by the analyzer produced a negative pressure at the cracked sample filter that aspirated room air.  With the commencement of inspiration, positive pressure in the anesthesia circuit reduced the negative pressure at the sample filter and eliminated or reduced the aspiration of the ambient air, thus allowing, a second plateau more accurately reflected alveolar PCO2.


(Provided by Simon Body and James Philips)
Body et al. Dual Plateau capnogram caused by cracked sample filter. Anesth Analg 2000;Jan:233-34 (withpermission).
Zuspan J, Martin M, Benumof JL. End-tidal CO2 excretion waveform and error with gas sampling line leak. Anesth Analg 1988;67:81.

Tips on using capnography- Abnormal values and shapes


Capnography tips

Bhavani Shankar Kodali MD

Tips on using capnography- Abnormal values and shapes

The following guidelines can be used to assess abnormal capnograms. However, a thorough understanding of underlying principles of capnography will to help you to maximize the benefit of capnography in rectifying a clinical abnormality during anesthesia.

1. Rule out technical causes of abnormal capnograms. The guidelines listed in the previous two pages should help in minimizing abnormal waveforms as a result of technical errors. The author is privileged to have been called when abnormal capnograms were observed in the operating rooms. The most frequent abnormal capnograms which were as a result of technical problems included a loose connection between the sampling tube and the monitor, cracked connection at the monitor end of sampling tube, and sticking unidirectional valves of the closed circuit. The abnormal capnograms observed are as follows.

Sticking Inspiratory unidirectional valve Loose connection between the sampling tube and the monitor
inspflip leakjim
Details Details


2. Look at the PETCO2 value: Determine if low, normal or high. Follow the differential diagnosis scheme as outlined in the table (Clinical aspects).


3. A low PETCO2 is commonly due to hyperventilation and progressively decreasing metabolism during anesthesia. A normal shape of capnogram may generally rule out sampling errors. A progressive decease in PETCO2 for a given ventilation may suggest low cardiac output and therefore the attention should be directed to cardiovascular system. The author (after reviewing several papers on cardiac output and PETCO2) believes that a significant decrease in PETCO2 for given ventilation is most likely due to a reduction in cardiac output and uses this concept in day to day practice during anesthesia. A decreased blood pressure but unchanged PETCO2 generally suggests that cardiac output is probably still maintained in the light of decreased SVR a common scenario during anesthesia. However, If PETCO2 also decreases with decreasing blood pressure, then it probably suggests reduced pulmonary blood flow consequent to a low cardiac output. Of course, one has to keep in mind rarer causes of such a scenario which include, air embolism or thormboembolism.


4. If PETCO2 is high, determine to see if the base line of capnogram is at zero or elevated. If at zero, hypoventilation is suggested. If PETCO2 increases despite progressively increasing ventilation , a hypermetabolic state should be kept in mind (rare but can happen such as malignant hyperpyrexia or thyrotoxic storm).

5. Baseline elevation in the presence of increased, normal, or low PETCO2 suggests rebreathing. A rebreathing can be normal in rebreathing circuits, but definitely abnormal in a closed circuit anesthesia and suggests exhaustion of soda lime or defective unidirectional inspiratory or expiratory valves.

6. A sudden elevation of base line and PETCO2 usually suggests contaminated monitors.

7. If capnogram is abnormal in shape, proceed to analyze starting from base line (phase I), upstroke (phase II), alveolar plateau (phase III) and descending limb (phase 0).Commonly seen abnormal capnograms

Phase I: Elevation: Rebreathing
Phase II: Prolongation: Airway obstruction
Phase III: Increased slope: Airway obstruction; Cleft in phase III: Spontaneous effort during controlled ventilation

Phase 0: Descending limb usually makes a nearly 90 angle with alveolar plateau. An increase in this angle (a sloping descending limb) usually suggests rebreathing.For an atlas of abnormal capnogram details: see Capno-encyclopedia.A sample list of abnormal capnograms are as follows:

Curare cleft
bronch curare
Ripple effect Contamination
ripple contamination
Sampling tube leak Hypoventilation
Bain circuit Apnea
Inspiratory unidirectional valve defect Spontaneously breathing capnograms in children

Sticking Inspiratory unidirectional valve Details


There is downward flip on the plateau. This capnogram was recorded when the inspiratory valve of circle system was not falling back completely into the seat, thereby resulting in partial rebreathing. Red indicates the inspiratory portion of the capnogram.

Minimize technical problems in CO2 sampling

Capnography tips


Bhavani Shankar Kodali MD

Minimize technical problems in CO2 sampling

FGF can dilute PETCO2 measurements. A Right angle connection between the breathing system and the sampling tube increases the accuracy of PETCO2 measurements as shown below

Right angle connection between the breathing system and the sampling tube increases the accuracy of PETCO2 measurements FGF can dilute PETCO2 measurements

Falsely low PETCO2 values frequently arise due to errors in sampling of the end-tidal gas. Admixture of end-tidal gas with fresh gas flow can occur during the use of T-piece breathing systems on the Ayre principle (eg., Jackson Rees, Bain anesthetic system) or during the use of certain lung ventilators, which produce constant flow. The problem of admixture of the end-tidal gas by the fresh gas flow from the breathing circuits can be minimized by interposing a right angle adaptor between the breathing system and the endotracheal tube, and interposing the sampling tube on the patient's side of the angle piece.1

Admixture of end-tidal gas with air can result from leaks in the sampling system, poor anesthetic face mask fit and during sampling of end-tidal gas from the nasal cavity in mouth breathing patients

Tachypnea/ low tidal volumes

Caution is to be exercised while monitoring PETCO2 during low tidal volume frequent breathing. Errors can result for two reasons.

(a) With frequent low tidal volume breathing, exhalation may not be complete and alveolar gases may not have fully migrated to the patient's airway. Therefore the end-tidal gas is not good representative of the alveolar gas.

(b) Analyzers may underestimate PETCO2 when breathing frequency is greater than 30 breaths.min-1, particularly if the response time of the analyzer is greater than the respiratory cycle time of the patient.

Under these circumstances of low tidal volume and frequent breathing during anesthesia the accuracy of PETCO2 can be improved by an assisted single large tidal volume. This is rewarded with adequate alveolar gas and measured PETCO2 (squeeze) is more representative sample of alveolar gas than the passive PETCO2. Therefore "squeeze PETCO2" could be used as the basis of assessing PaCO2 and, thus, alveolar ventilation.2,3


1. Gravenstein N, Lamptang S, Beneken JEW. Factors influencing capnography in the Bain circuit. J Clin Monit 1985;1:6-10.

2. Sweadlow DB, Irving SM. Monitoring and patient safety. In Blitt CD (Ed.). Monitoring in Anesthesia and Critical Care Medicine. 2nd ed.,New York: Churchill Livingstone, 1990:33-64.

3. Bhavani Shankar K, Steinbrook R, Mushlin PS, Freiberger D. Transcutaneous carbon dioxide monitoring during laparoscopic surgery in pregnancy. Canadian J Anaesth 1998;45:164-9.


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