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

Bhavani Shankar Kodali MD

Updated September 2022

Analysis of CO2 waveforms: 2

Clinical Aspects

Bhavani Shankar Kodali MD

Analysis of CO2 waveforms:  

Prolongation phase II and III:             
Prolongation or slanting of the expiratory upstroke phase II occurs when there is obstruction to expiratory gas flow (e.g., asthma, bronchospasm, obstructive pulmonary disease, and kinked endotracheal tube,1-9 or in the presence of leaks in the breathing system.10   A sidetream capnograph may allow gas mixing within the sampling tube (dispersion) if sampling rate is slow (50 ml.min-l) or if the tubing is too long or has too wide a bore, or both. Such dispersion of gases can also result in prolongation of the expiratory upstroke.11-13  The slope of the expiratory plateau (phase III) can be increased during pregnancy as a normal physiological variation.5,14  Besides, it can also result from factors that produce obstruction to expiratory gas flow which may also result in a prolonged phaseII.1-9  

 Non Pregnant  Capnogram during cesarean section
(The slope the expiratory plateau is increased as a normal physiological variation in pregnancy)


 Normal  Airway obstruction (eg., bronchospasm). Phase II and phase III are prolonged and alpha angle (angle between phase II and phase III) is increased   


 Normal  Capnograms recorded with prolonged response time (Base line is elevated, prolongation of phase II and III, prolongation of inspiratory descending limb)

Curare Cleft: 
A dip in the plateau (curare cleft) indicates a spontaneous respiratory effort during mechanical ventilation.5,8,10

 Curare cleft

A dip in the plateau indicates spontaneous respiratory effort

It can also result from surgical manipulations in abdomen

Terminal dip of alveolar plateau:  Dilution of PETCO2 by fresh gas flow (FGF)  in circuits and ventilators using a continuous flow may result in the dilution of expired gases by the FGF’s producing a terminal dip in alveolar plateau. This results in falsely low PETCO2 values.

 Normal  Dilution of PETCO2 by fresh gas flow (use of PEEP or CPAP in IMV bird ventilators, can also occur as a result of dilution of expired gases by FGF in rebreathing circuits)


1.    Weingarten M. Anesthetic and ventilator mishaps: prevention and detection. Crit Care Med 1986;14:1084-6.

2.    Paloheimo M, Valli M, Ahjopalo H. A guide to CO2 monitoring. Finland: Datex Instrumentarium, 1988.

3.    Van Ganderingen HR, Gravenstein N, Van der Aa JJ, Gravenstein JS.  Computer-assisted capnogram analysis.  J Clin Monit 1987;3:194-200.

4.    Weingarten M. Respiratory monitoring of carbon dioxide and oxygen:  a ten-year perspective. J Clin Monit 1990;6:217-25.

5.    Bhavani-Shankar K, Moseley H, Kumar AY, Delph Y. Anaesthesia and capnometry. (Review article).  Can J Anaesth 1992;39:617-32.

6.     Smallhout B, Kalenda Z. An Atlas of Capnography. 2nd e. Utrecht:Kerckebosch-Zeist, 1981.

7.    Nuzzo PF.  Capnography in infants and children.  Pediatric Nursing 1978;May-June:30-8.

8.   Cote CJ, Liu LMP, Szyfelbein SK, et al.  Intraoperative events diagnosed by expired carbon dioxide monitoring in children.  Can Aaesth Soc J 1986;33:315-20.

 9.   Leigh MD, Jones JC, Mottley HL.  The expired carbon dioxide as continuous guide of the pulmonary and circulatory systems during anaesthesia and surgery.  J Thoracic and Cardiovasc Surg 1961;41:597-610.

10.    Adams AP. Capnography and pulse oximetery. In: Atkins RS, Adams AP (Eds.). Recent Advances in Anaesthesia and Analgesia.  London: Churchill Livingston, 1989;155-75.

11.    Pascucci RC, Schena JA, Thompson JE.  Comparison of a sidestream and mainstream capnometers in infants. Crit Care Med 1989;17:560-2

12      Badgwell JM, Kleinman SE, Heavner JE.  Respiratory frequency and artifact affect the capnographic baseline in infants.  Anesth Analg 1993;77:708-11

13.    Schena J, Thompson J, Crone RK.  Mechanical influences on the capnogram.  Crit Care Med 1984;12:672-4.

14.    Shankar KB, Moseley H, Kumar Y, Vemula V, Krishnan A. The arterial to end-tidal carbon dioxide tension difference during anaesthesia for tubal ligations. Anaesthesia 1987;42:482-6.

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