End-tidal CO2 (EtCO2) monitoring is a measure of metabolism, perfusion, and ventilation. In the ED, we typically think of a EtCO2 as a marker of perfusion and ventilation. However, EtCO2 is an extremely powerful surrogate for endotracheal tube (ETT) Position, CPR Quality, Return of spontaneous circulation (ROSC), Strategies for treatment, and Termination (of CPR). Do these letters look familiar? They should! In this post we take a deep dive into each of these potential uses of EtCO2 in the ED.

Background

The PQRST mnemonic comes from a team in Norway who in 2014 published on how it can help providers optimize all the ways that EtCO2 can help with managing patients.¹ As a reminder, EtCO2 represents the amount of carbon dioxide at the end of exhalation. Capnography, however, reflects both a number (EtCO2 in mm Hg) and a waveform. Figure 1 reviews the meaning of each phase of the waveform.² It’s useful to keep these terms separate in our minds when reviewing the PQRST mnemonic.

Position of the ETT (P)

ACEP’s policy on verification of ETT placement reminds us that physical examination and fogging in the tube are not reliable to confirm placement. Providers should use EtCO2 detectors (e.g. waveform capnography or colorimetry) to confirm ETT position. Ultrasound in the hands of an experienced provider is also recommended. ³

Waveform EtCO2 (versus colorimetric) can be particularly helpful because you may get color change with the first few breaths of an esophageal intubation. This can be misleading and potentially detrimental to the patient. But the waveform does not lie. ²

Quality of CPR (Q)

EtCO2 is essentially to ensuring quality CPR. We typically assess quality of CPR by palpable pulses, but this can be challenging and even unreliable. Arterial diastolic pressure >25 mm Hg may be useful, but not all patient scenarios will be amenable to placement of an arterial line. However, EtCO2 provides the same information and is more reliable. Teams should aim for EtCO2 at least >10 mm Hg and ideally >20 mm Hg.⁴

Where do these numbers come from? These values are approximately 1/4 the normal EtCO2 (35-45 mm Hg), and ideal CPR will provide at least 1/4 of cardiac output.

Return of Spontaneous Circulation (R)

With the use of EtCO2, teams may recognize ROSC before a scheduled pulse check. An increase of EtCO2 to normal values (35-45 mm Hg) or an increase by 10 mm Hg is about 97% specific. One caveat is that sodium bicarbonate may transiently raise the EtCO2, but this does not reflect ROSC.²

Strategies for Treatment (S)

The waveform morphology during CPR can help us decide what our next intervention should be. Examples include ETT dislodgment, cuff leak, and bronchospasm.

Termination (T)

EtCO2 may provide an additional data point when deciding whether it is appropriate to terminate CPR. Approximately 20 minutes into a resuscitation, the likelihood of ROSC is significantly higher if EtCO2 >20 mm Hg and much less if EtCO2 <10 mm Hg.⁴

Caveats

The PQRST mnemonic should be applied to patients with an ETT. The utility of EtCO2 in patients who are receiving bag mask ventilation (BVM) or have a laryngeal mask airway (LMA) is less understood and researched. The correlation between serum pCO2 and EtCO2 is also less understood; animal studies do not show much correlation, and as you would expect with CPR, there will likely be some dead space and/or V/Q mismatch.

Take-Home Points

There is incredible utility in using EtCO2 in CPR. Examples include:

1. Evaluating the position of the ETT
2. Evaluating the quality of CPR
3. Timely recognition of ROSC
4. Identifying best treatment strategies
5. Contributing to the decision to terminate resuscitation

Keep these points in mind the next time you receive an alert that a patient is en route with CPR in progress! Check out the video below for an example of how to set up EtCO2 on an Alaris IV pump!

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