Critically evaluate the use and limitations of End-Tidal Carbon Dioxide measurement in Intensive Care practice.
Measurement of ETCO2 implies the use of a quantitative device, and usually this is one which allows assessment of waveform morphology (ETCO2 vs time). Specific roles include: confirmation of tracheal placement of artificial airway, pattern recognition of ETCO2 waveform, use of value of ETCO2 during cardiac arrest or hypotensive states, prediction of arterial PaCO2.
Confirmation of tracheal placement is highly sensitive and specific in the presence of pulmonary blood flow. False negative values may occur with minimal pulmonary blood flow, but should not usually occur with adequate CPR. False positives are very uncommon and short lived (eg. CO2 in stomach).
Waveform pattern can assist in the diagnosis in particular of expiratory flow obstruction (and gas trapping) and attempts at spontaneous breathing particularly during apnoea testing.
During cardiac arrest, the absolute level of ETCO2 is proportional to pulmonary blood flow (and hence cardiac output). It may be used to guide cardiac compression, but apart from this it adds little to prognostication (ie. confirms patient that patient likely to die is likely to die). Sudden decreases
in ETCO2 may be indicative of the decrease in pulmonary blood flow associated with pulmonary emboli.
Prediction of PaCO2 from ETCO2 is fraught with difficulty. Very few candidates demonstrated an understanding of this area. The major limiting factors are pulmonary blood flow and V/Q balance. Unless these factors are constant, even the trending of the relationship of between PaCO2 and ETCO2 unreliable. Unfortunately if the PaCO2 is important (eg. major head injuries), it must be measured.
Utility in neonates and children may be impaired by small tidal volumes.
Though EtCO2 has been discussed in Question 9.2 from the second paper of 2008, it was not a "critically evaluate" style of question.
A systematic "critical evaluation" should resemble the following:
Applications in ICU
Evidence and Guidelines
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.
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.