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A Comprehensive Educational Website (established 1998)

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

Bhavani Shankar Kodali MD

Updated September 2022

From Numbers- PETCO2 values- Capnometry

Clinical uses of capnography

Bhavani Shankar Kodali MD

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

PETCO2 increased

output Pulmonary perfusion Alveolar Ventilation Technical errors

Machine 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 Ventilation Technical errors

Machine faults
Hypothermia 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.

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