Critically evaluate  the use and  limitations  of End-Tidal Carbon Dioxide measurement in Intensive Care practice.

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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.

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 that patient 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.   The major limiting factors are pulmonary blood flow and V/Q balance.  Unless these factors are unchanging, 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.

Discussion

This question is identical to Question 6 from the second paper of 2005.