Capnography beyond ROSC

What is Capnography?

It is a non-invasive technique where CO2 is analysed continuously, in the gases entering and leaving the lungs. Look at it like a non-invasive way of estimating the PaCO2. Normal ETCO2 ranges between 35-45mmHg.

Different ways of estimating ETCO2:

1. Waveform capnography (Quantitative): When ETCO2 is graphically displayed and analysed quantitatively (real time quantitative waveform as depicted below). Capnograph is the device that measures and displays the waveform. It gives us a number i.e very close to the PaCO2 (typically underestimates PaCO2 by 5mmHg in healthy adults). Therefore, a high ETCO2 is almost always associated with hypercapnia. 

Normal Capnograph (Adults)

2. Capnometry (Quantitative): When only the numerical value is displayed without the waveform.

                         

3. Qualitative: Does not tell us one fixed number instead gives a range of ETCO2 using the calorimetric device. Ex. 21-30mmHg. Hence, utility is limited to ET tube placement verification.

Calorimeter device

How is it different from SpO2?

SpO2 measures O2 saturation in the blood whereas ETCO2 measures CO2 in the airway. ETCO2 reflects the elimination of CO2 by the lungs and thus paints an immediate picture of patient's condition on the monitor, in contrast to SpO2 that remains normal for several minutes (even when you hold your breath).

SpO2 and ETCO2                             

When interpreting ETCO2, look at both, the numeric value as well as the waveform. If you look only at the numeric value, this is like only looking at the heart rate without the ECG wave pattern!

Normal ETCO2 waveform:

Phase I aka Dead space ventilation/ Inspiratory baseline: Beginning of exhalation, no gas exchange here. There is almost no CO2 in airway during inspiration, so baseline is normally zero.

Phase II aka Ascending/Expiratory upstroke/ Early exhalation phase: CO2 from alveoli reaches the upper airway and causes a rapid rise in the amount of CO2 that is detected on the monitor.

Phase III aka Expiratory plateau phase: CO2 rich air is exhaled out

ETCO2 i.e. end of exhalation contains the highest concentration of CO2 (normal value is about 35-45 mmHg).

Phase IV aka Descending/ Expiratory downstroke: Inspiration begins and CO2 in the airways drops down to zero.

Normal capnograph wave

Note: The baseline represents the inspiratory phase (not expiratory)

Normal capnograph

Increase in ETCO2 (See the waveform below)

Hypoventilation

Increased muscular activity

Malignant hyperthermia

Increased Cardiac Output or Bicarbonate infusion

Relief from bronchospasm

Increasing ETCO2 (Ex. Hypoventilation during procedural sedation)

Decrease in ETCO2 (See the waveform below)

Hyperventilation

Muscle relaxants and hypothermia

Decrease cardiac output

Pulmonary embolism 

Bronchospasm

Decreasing ETCO2

Clinical Applications:

1. Airway management: To confirm the ET tube placement, also for continuous confirmation of the airway
2. Monitoring CPR efficacy: Target ETCO2 >20 during CPR
3. Estimates prognosis during CPR: An end-tidal CO2 value < 10 mm Hg after 20 min of resuscitation has been shown to be very accurate in predicting death
4. Confirming ROSC without stopping chest compressions: With ROSC ETCO2 shows as a sudden increase
5. Monitoring PaCo2 in a Traumatic Brain Injury/ Post Cardiac Arrest care
6. Guide to ventilation during procedural sedation: Shows hypoventilation i.e increasing ETCO2 at least 60 seconds prior to hypoxia 
7. Fluid responsiveness (5% or greater increase in ETCO2 following a passive leg raise is a non invasive way of predicting fluid responsiveness)
8. Diagnosing other pathologies (PE, DKA)

ETCO2 waveform in other pathologies:

Airway obstruction pattern (Bronchospasm)

Rebreathing

Esophageal intubation

On Muscle relaxants

Take Home

• ETCO2 is a surrogate marker for arterial CO2. If increased, it confirms hypercapnia.
• ETCO2 changes immediately with changes in the amount of CO2 in the airway (unlike SpO2 that shows a lag)
• When using waveform capnography, look at the ETCO2 numeric value as well as the waveform.
• Eliminate pulse checks during CPR, use ETCO2 instead.

References:

1. Heradstveit BE, Heltne JK. PQRST - A unique aide-memoire for capnography interpretation during cardiac arrest. Resuscitation 2014; 85:1619-20.
2. Monnet, X. et al. ETCO2 is better than arterial pressure for predicting volume responsiveness by PLRT. Intensive Care Med. 2013 Jan;39 (1): 93-100
3. http://www.medscape.com/viewarticle/812011
4. http://edtech2.boisestate.edu/meganjacobson/502/capno.html
5. http://www.carefusion.com/pdf/Center_for_Safety/Documents/RC1706-L3017-Capnography-Handbook.final.pdf