End-tidal capnography has appeared multiple times in the CICM exams. Whereas the Part I questions are typically concerned with how it is measured, in Part II the candidates are expected to interpret the waveforms and comment on the utility of the practice. This chapter is more concerned with EtCO2 waveform interpretation.
Historically, Question 13.3 from the second paper of 2013 and Question 30 from the first paper of 2011 asked the candidates to explain why an EtCO2 waveform might be of a particular shape, Specifically, the three shapes which have come up have been a totally flat waveform, a sawtoothed bronchospastic wavefrom, and an EtCO2 which is gradually rising over a few breaths (suggesting worsening hypercapnea due to inadequate CO2 clearance). In contrast, Question 1 from the first paper of 2018 asked the candidates why the trace is no longer detectable, suggesting that equipment issues are also possible answers. Of course, there are many other possible waveforms and reasons for absent trace, and those might one day come up in the SAQs. This chapter is a brief summary of the more common patterns. A not-so-brief series of chapters on the normal capnograph waveform and the interpretation of abnormal bnormal capnography waveforms also exists, but is excessive for the purposes of last-minute revision.
![]() |
A flat or nearly flat trace
|
![]() |
Endobronchial intubation
|
![]() |
Bronchospasm |
![]() |
Mechanical airway obstruction
|
![]() |
Reversal of alveolar slope in emphysema Also seen in several other situations:
|
![]() |
Cardiac oscillations
|
![]() |
The "Curare Cleft"
|
![]() |
Recirculated CO2 due to a saturated CO2 absorber
|
![]() |
A low end-tidal CO2 in hypothermia The patient is simply not producing enough CO2! |
|
A high, and gradually rising end-tidal CO2 The patient is producing too much CO2. The tidal volume is inadequate. This could be the result of :
|
![]() |
A high peak of the alveolar phase in poorly compliant lungs This pattern is called a "pigtail" capnogram. Causes (apart from ARDS) include obesity and pregnancy. |
Most of this information comes from only two textbooks. With "Basic Assessment and Support in Intensive Care" by Gomersall et al (as well as whatever I picked up during the BASIC course) as a foundation, I built using the humongous and canonical "Principles and Practice of Mechanical Ventilation" by Tobins et al – the 1442 page 2nd edition.
Thompson, John E., and Michael B. Jaffe. "Capnographic waveforms in the mechanically ventilated patient." Respiratory care 50.1 (2005): 100-109.
Babik, Barna, et al. "Effects of respiratory mechanics on the capnogram phases: importance of dynamic compliance of the respiratory system." Crit Care 16 (2012): R177.
Additonally, capnography.com has a series of excellent diagrams and is otherwise an indispenasable resource for this topic.