End-tidal capnometry waveform interpretation

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.

oesophageal intubation capnometer waveform

A flat or nearly flat trace

  • The patient is dead
  • Cardiac / respiratory arrest
  • Apnoea test in a brain dead patient
  • Oesophageal intubation has occurred
  • Ventilator disconnection
  • Airway obstruction (eg. patient suddenly bit down on the tube)
  • ETT perforation (the end tidal gas is escaping via the hole before it gets to the capnograph)
  • Capnograph disconnection or obstruction
  • Water droplet contamination of capnography module
capnography waveform in endobronchial intubation

Endobronchial intubation

  • incomplete obstruction: if the left lung was completely isolated (i.e. you have the cuff inflated in such a way as to block it totally) you would not see this waveform.
capnometry waveform in bronchospasm Bronchospasm
capnometry waveform in mechanical airway obstruction

Mechanical airway obstruction

  • the airway obstruction is an obstacle to both inspiratory and expiratory flow
reversal of alveolar capnometry slope in emphysema

Reversal of alveolar slope in emphysema

Also seen in several other situations:

  • anaesthetic machines with rapid gas flow, where the ventilator contributes fresh gas to the tubing next to the capnometer.
  • Pneumothorax with massive air leak - the air leak sucks CO2-rich air out of the capnometer, tracking fresh gas back through it.
cardiac oscillations of the capnometer curve

Cardiac oscillations

  • may be a feature of cardiomegaly.
The Curare Cleft

The "Curare Cleft"

  • The muscle relaxant is wearing off in a patient on a mandatory mode of ventilation
  • Most frequently seen in anaesthetic machines where there is a constant flow of fresh gas across the circuit.
rebreathing of Co2

Recirculated CO2 due to a saturated CO2 absorber

  • If the CO2 absorbing lime bucket is saturated, the circuit becomes inundated with expired CO2 and the baseline gradually increases.
end tidal co2 in hypothermia

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 :

  • Decreased respiratory rate
  •  Decreased tidal volume
  •  Increasing metabolic rate (fever, shivering, etc)
high peak of alveolar plateau in poorly compliant lungs 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.