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

Bhavani Shankar Kodali MD

Updated September 2022

details 1


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.


hyper metabolic status


expiratory valves.

Expiratory Unidirectional valve


 Expiratory valve - Normal


  Incompetent expiratory valve


The capnogram shows prolonged phase II as well as slanting of descending limb of inspiratory segement. The expiratory unidirectional valve was noted to be fixed in the open position and thus allowing previously exhaled gases, high in CO2, to be rebreathed with each inspiration. The CO2 waveform returned to normal soon after the valve was removed, cleaned, dried, and replaced.


Pyles, ST, Berman LS, Modell JH. Expiratory valve dysfunction in a semiclosed circle anesthesia circuit-verification by analysis of carbon dioxide waveform. Anesthesia Analgesia 1984;63:536-7.

contaminated monitors.


Contamination of CO2 sensor

This is a trend capnogram showing sudden increase in the base line as well as height of the waveform and indicates contamination of the carbon dioxide analyzer.



Inspiratory Unidirectional Valve

Displaced Inspiratory valve
Close up view
openvalve openvalve1
 Normal Inspiratory valve function      =CO2
 Inspiratory valve Incompetent
 Plateau is prolonged due to rebreathing
Plateau is prolonged due to rebreathing
An incompetent inspiratory valve will allow exhaled CO2- containing gas to enter the inspiratory limb of the circuit during expiration. During the next inspiration, the CO2-containing gas in the inspiratory limb enters the patient, extending the expiratory alveolar plateau (phase III) of the time capnogram (Fig. above). A decrease in CO2 follows the extended alveolar plateau and represents the appearance of the CO2-free gas from the machine end of the inspiratory limb. During this latter phase of inhalation, CO2 concentration may reach zero. The capnogram thus created may be indistinguishable from the normal capnogram, at least during the initial phase of rebreathing. The time capnograph is unable to reveal rebreathing because it is unaware of the beginning of actual inspiration.1


Kumar et al. Canadian Journal of Anaesthesia 1992;39:997-9.