Capnometry and the arterial-expired carbon dioxide gradient

The measurement of end-tidal CO2 is a favourite topic of the examiners, eg. in  Question 6 from the second paper of 2005, "Critically evaluate  the use and limitations  of End-Tidal Carbon Dioxide measurement in Intensive Care practice".
Capnography is discussed in greater detail elsewhere:

There is also an excellent site by Prasanna Tilakaratna which explains infra-red absorption spectrophotometry using vividly colourful diagrams.

Use of capnography in the ICU

  • Confirmation of ETT placement
  • Airway disconnection alarm
  • Monitoring during transport
  • During CPR to assess adequacy of cardiac compression
  • Recognition of spontaneous breath during apnoea test
  • Neurosurgical patient to provide protection against unexpected hypercapnia
  • Quick bedside assessment of bronchospasm
  • Alert of sudden changes in pulmonary perfusion (eg. PE)
  • Early alert of PEA in the absence of continuous BP monitoring
  • More accurate monitoring of respiratory rate

Principles of capnometry

  • The sensor is an infrared absorption spectrophotometer.
  • CO2 absorbs infra-red radiation (it being a greenhouse gas and all) and so expired air enriched with CO2 will absorb more infra-red light than CO2-poor air.
  • There are two sensor types: mainstream and sidestream.
  • In sidestream sensors, the respiratory gases are aspirated via a thin samling line, creating a 200ml/minute leak. The infrared sensor is at the end of that line. The disadvantage of this method, apart from the leak, is the delay in gas transport - up to 4 seconds might pass before you notice that the patient has stopped producing CO2.
  • In mainstream sensors, the sensor is positioned across the airway. The advantage of this is an immediate measurement, without delay. The disadvantage is increased dead space. Furthermore, humidity rain-out tends to confuse the sensor.

Advantages of end-tidal CO2 monitoring

  • Continuous monitoring
  • Immediate feedback regarding cardiac output and ETT position
  • Waveform analysis is possible
  • Cheap
  • Increased safety; decreased risk of undetected airway circuit disconnection

Disadvantages of end-tidal CO2 monitoring

  • Produces vigilance-impairing false alarms
  • EtCOvalues may not correlate with PaCO2 values and the two may be substantially different
  • The monitor in-line connector creates a small amount of apparatus dead space
  • The adaptor fitted to the end of the ETT may be heavy, and may increase the risk of accidental extubation, particularly in children and neonates
  • The gas sampling models of EtCO2 monitors can diminish the delivered minute volume, as they access the circuit gas at a rate of about 200ml/min.
  • Nitrous oxide can confuse some capnometers (i.e. be mistaken for CO2)
  • The presence of helium can cause the EtCO2 measurement to be incorrectly elevated in some capnometers (i.e. those which use a reporting algorithm that assumes that the only gases present in the sample are those that the device is capable of measuring)

Evidence and Guidelines

  • EtCO2 rapidly detects lifethreatening complications in transported patients.
  • American Heart Association Guidelines for Cardiopulmonary Resuscitation make the following recommendations
    • Use EtCO2 to assess ETT position
    • Use EtCO2 to assess efficacy of CPR
    • Use EtCO2 to confirm the return of spontaneous circulation

Conditions which cause a persistent flat capnograph trace

  • 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
  • Cardiac / respiratory arrest
  • Apnoea test in a brain dead patient

Conditions which increase the gradient between end-tidal and arterial PCO2

This specific issue was asked about in Question 9.2 from the second paper of 2008.

  • Pulmonary perfusion
    • Pulmonary embolism
    • Fat embolism
    • Air embolism
    • Cardiac failure (RHF)
    • Cardiac arrest
  • Ventilation
    • Increased V/Q mismatch due to high PEEP
    • Increased alveolar dead space
    • High FiO2 (causing shunt into poorly ventilated alveoli)
  • Artifact
    • The presence of helium can cause the EtCO2 measurement to be incorrectly elevated in some capnometers (i.e. those which use a reporting algorithm that assumes that the only gases present in the sample are those that the device is capable of measuring)
    • The presence of nitrous oxide can confuse some capnograph devices, and the NO2may be misinterpreted as CO2
    • The use of an inline HME filter can reduce the end-tidal CO2 concentration.



The best, most detailed review:

Walsh, Brian K., David N. Crotwell, and Ruben D. Restrepo. "Capnography/Capnometry during mechanical ventilation: 2011." Respiratory care 56.4 (2011): 503-509.

Whitaker, D. K. "Time for capnography–everywhere." Anaesthesia 66.7 (2011): 544-549.

Kodali, Bhavani Shankar. "Capnography outside the operating rooms." Anesthesiology 118.1 (2013): 192-201.

Yamauchi, H., et al. "Dependence of the gradient between arterial and end-tidal PCO2 on the fraction of inspired oxygen." British journal of anaesthesia (2011): aer171.

Razi, Ebrahim, et al. "Correlation of End-Tidal Carbon Dioxide with Arterial Carbon Dioxide in Mechanically Ventilated Patients." Archives of trauma research 1.2 (2012): 58.

Ahrens, Tom, Helen Wijeweera, and Shawn Ray. "Capnography. A key underutilized technology." Critical care nursing clinics of North America 11.1 (1999): 49-62.

Kingston, E. V., and N. H. Loh. "Use of capnography may cause airway complications in intensive care." British journal of anaesthesia 112.2 (2014): 388-389.

Ortega, Rafael, et al. "Monitoring ventilation with capnography." New England Journal of Medicine 367.19 (2012).

Rückoldt, H., et al. "[Pulse oximetry and capnography in intensive care transportation: combined use reduces transportation risks]." Anasthesiologie, Intensivmedizin, Notfallmedizin, Schmerztherapie: AINS 33.1 (1998): 32-36.

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, has a series of excellent diagrams and is otherwise an indispenasable resource for this topic.