College Answer

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.

Discussion

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:

Rationale

• CO₂ elimination is an important component of gas echange
• This can be assessed indirectly by serial measurements of arterial PaCO₂; however ideally the measurement should be performed continuously.
• Trends in gas exchange are an important parameter to observe in patients whose respiratory function is compromised
• CO₂ monitoring is also of critical importance in patients with increased intracranial pressure

Applications in 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

Advantages

• 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

• Produces vigilance-impairing false alarms
• EtCO₂ values 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

Capnography is discussed in greater detail elsewhere:

• The normal capnometry waveform
• The relatioship of abnormal capnograph waveforms to lung pathology

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