Quantitative End-Tidal CO2 – the New Scoop

Quantitative End-Tidal CO2 (ETCO2) was in vogue for a while, and seemingly some centres adopted it as standard of care, while others found that it did not make a significant clinical impact so it fell out of style and left to ‘practitioner preference’. With increasing experience and use, we have mounting evidence to do a deeper dive into its use, particularly in the Emergency Department. 

 

Bottom Lines

• Quantitative ETCO2 is the only widely accessible method of monitoring the quality of ventilation on a breath-to-breath basis.
• Quantitative ETCO2 should be used for all airway management situations and should be applied early on in the resuscitation.
  • It is standard of care for confirming endotracheal tube (ETT) placement.
  • It has several other uses in airway management outside of confirming ETT placement.
• Quantitative ETCO2 should be applied routinely for all cardiac arrests and has several uses:
  • Quantitative ETCO2 >10 (ideally >20) is a marker of good quality chest compressions.
  • A sudden increase in quantitative ETCO2 is an early indicator of ROSC.
  • A persistently low quantitative ETCO2 despite prolonged resuscitation is an indicator of low likelihood of ROSC.

 

What IS Quantitative ETCO2?

The term “quantitative ETCO2” generally encompasses 2 components:

Capnography: the real time measurement of the partial pressure of CO2 (pCO2) throughout each respiratory cycle. Presented as a visual tracing as below.

• Phases:
  • Phase 1: initial expiration of gas from anatomical dead space that is essentially unchanged from inspired gas = flat line.
  • Phase II: initial expiration of alveolar gas where gas exchange did occur ALONG with residual anatomical dead space gas = sharp increase in pCO2.
  • Phase III: expiration of pure alveolar gas  relative plateau (slight increase as the alveolar gas towards the end of expiration had more time for gas exchange compared to the beginning)
  • Phase 0: inspiration which is not detected by the ETCO2 sensor = sharp drop to baseline.
• The shape of the waveform will change with different lung pathologies, however this is beyond the scope of this post.
  • For all the uses of quantitative ETCO2 listed below, the only relevant waveform is the normal one above.

 

Capnometry: the numerical pCO2 value at the end of expiration (the peak value in a typical waveform). Normal value 35-45mmHg.

 

• Capnometry is a real time composite of ventilation quality, metabolic production of CO2, and pulmonary perfusion.
  • Factors that affect any or all of these 3 components will alter the ETCO2 value in a real time basis.

 

 

Quantitative ETCO2 in Airway Management

Quantitative ETCO2 has several important uses in airway management, in both the pre/peri-intubation and post-intubation phases.

Peri-Intubation

Early apnea detection:

• In the critically ill patient, the impact of the apneic period during RSI is often underestimated.
• Waveform ETCO2 allows for identification of apnea the instant it starts (represented as a flat line) and may facilitate earlier laryngoscopy and earlier intubation to minimize the apneic time.

 

Ventilation efficacy:

• Particularly when an intubation attempt fails and the patient requires re-oxygenation prior to a subsequent attempt, rapid detection of adequate ventilation is essential.
• Oxygen saturation (SpO2) can lag for up to 2 minutes which is a long time to wait when having to make critical airway decisions
• Quantitative ETCO2 provides immediate feedback of ventilation – if there is an appropriate waveform and a reasonable capnometric ETCO2 value, you can be confident you are ventilating the patient adequately even if SpO2 has not yet caught up.

 

Confirming ETT Position

Correct ETT position is confirmed by quantitative ETCO2 when there is a SUSTAINED appropriate waveform AND a detectable capnometric ETCO value AFTER providing 6 tidal breaths.

The evidence:

• There are a plethora of studies showing how effective quantitative ETCO2 is at confirming tube placement, with sensitivities and specificities consistently >90% and often approaching 100%.
• Clinical methods of confirming ETT position (tube misting, chest rise, lung auscultation, etc.) are unreliable in isolation, and in combination with each other.
• Head-to-head studies of ETCO2 versus clinical examination for ETT tube placement confirmation consistently show superiority of ETCO2.
• Multiple international expert guidelines including the Difficult Airway Society and the Project for Universal Management of Airways (PUMA) group recommend quantitative ETCO2 monitoring over colorimetric ETCO2 devices in all intubations to confirm ETT placement, where available.

 

Quantitative ETCO2 in Cardiac Arrest

 

Measurement of CPR Quality

Good quality chest compressions will return a minimum sustained ETCO2 value >10mmHg, and ideally >20mmHg.

• If ETCO2 is trending downward, and particularly if it drops <10mmhg, check chest compression quality and consider swapping out providers.
• This is supported by the most recent AHA and ILCOR guidelines for CPR.

 

Early ROSC Detection

A sudden increase in ETCO2 value suggests ROSC, even with ongoing chest compressions.

• There is no specific ETCO2 value or cutoff – a sudden increase is all you’re looking for.
• ETCO2 rise does NOT replace usual practice for confirming ROSC (manual pulse checks +/- point-of-care ultrasound).
  • The purpose of using ETCO2 in this way is to prepare for the next pulse check and prepare to initiate ROSC protocols in a fairly reliable manner.
• The above approach is also supported by the most recent ILCOR guidelines for CPR.

 

Termination of Resuscitation

A persistently low ETCO2 after prolonged resuscitation is a predictor of a low likelihood of achieving ROSC.

• Current evidence has NOT reached consensus on a particular ETCO2 value at a particular time during the resuscitation to use as a threshold.
  • A fairly conservative approach appears to be ETCO2 <10mmHg after 20 minutes of good quality CPR (associated with <0.5% likelihood of ROSC based on most recent literature).
• ETCO2 value should NOT be used alone in the decision to terminate resuscitation, but it can be taken into consideration with all the other factors and clinical gestalt we use when making this decision in usual practice.
  • This is also supported by the most recent AHA and ILCOR guidelines for CPR.