When is Capnography Useful in the ED? Part I

Authors: Brit Long, MD (@long_brit, EM Attending Physician, SAUSHEC) and Michael Vivirito, RN, CEN (Joint Base Elmendorf Richardson Medical Center, Emergency Medicine) // Edited by: Alex Koyfman, MD (@EMHighAK)

The EMS phone rings, with a crew bringing in a 62-year-old male in cardiac arrest. Initial rhythm was ventricular fibrillation, and they have provided two shocks. They arrive, your team transfers him over, compressions continue, and you check access and monitors. Of all the monitors you have, what is something that can not only prognosticate, but assess compression quality and suggest ROSC?

Capnography! This post will evaluate the literature behind several common uses of capnography in the ED: cardiac arrest, endotracheal tube (ETT) placement, and procedural sedation. Part II will evaluate several other uses of capnography in critical illness: trauma, sepsis, DKA, pulmonary pathology, and fluid responsiveness.

A Little Background

Capnography includes the noninvasive measurement of CO₂ partial pressure during respiration (primarily exhalation). This can be displayed as a color (colorimetric or qualitative), number (quantitative), and as a function of time with waveform.^1-8  A normal waveform displays several phases, and interpretation of this waveform can provide valuable information based on several features.^1-9  More specifically, the quantitative monitor provides a numeric value which is the end-tidal carbon dioxide (EtCO₂) plateau in phase III.

1) Verification and Monitoring of ETT

Capnography shines in verifying and monitoring ETT placement and position. In fact, it may be one of the most accurate means of confirming correct placement, other than direct visualization of the tube between the chords.^7-9 Fogging and condensation of the ETT can occur in over 80% of esophageal intubations.¹⁰ Chest wall movement is not reliable, and neither are breath sounds (16% of esophageal intubations).^10-12 Qualitative markers will turn yellow with tracheal placement and remain purple (except for several circumstances such as recent carbonated beverage, gastric fluids on the marker, etc.).^1-4,9,13 Sensitivity approaches 100% for qualitative markers,^14-19 and quantitative markers offer even more important information with four phases as described above.^9,20-22  Quantitative capnography will still display the same findings with right mainstem bronchus intubation, as well as if the ETT is placed just above the hypopharynx (it will become erratic over time due to malpositioning). Flat waveforms are commonly seen in ETT obstruction, technical malfunction of the monitor or tubing, prolonged cardiac arrest, and complete airway obstruction distal to the ETT. ^1-9,19 Sensitivity in cardiac arrest is 60-100%, which varies due to cause and duration of arrest.^9,14-19

Monitoring ETT location during patient movement is vital to ensuring the ETT remains in the appropriate position. Dislodgement or misplacement may occur in up to 23% of transports not undergoing continuous capnography monitoring.^23,24 Capnography should be used during transport of intubated patients to monitor for displacement.^9,25,26

2) Cardiac Arrest

Capnography has been used since 1989 for cardiac arrest. The specific EtCO₂ level is a reflection of cardiopulmonary blood flow, and the value correlates with cardiac output from chest compressions.^{9,19-23,27,28} The 2015 ACLS guidelines state quantitative waveform is recommended to monitor compressions and for ROSC.⁹ Depth and quality of compressions affect EtCO_2. Compressions should target values greater than 10 mm Hg, with 20 mm Hg optimal. Decreases in EtCO₂ suggest poor compression quality or another etiology that has not been treated (hemorrhage, pneumothorax, tamponade, myocardial infarction, among others).^9,29,30

A rapid and abrupt increase in EtCO₂ levels suggests ROSC, specifically levels 30-40 mm Hg or an increase of at least 10 mm Hg.³¹ A specific level is not recommended, but rather the sudden increase. With ROSC and a perfusing rhythm, cardiac output increases, resulting in EtCO₂ increase. This could potentially be used to monitor for ROSC without pulse checks. If sudden increase in EtCO₂ is observed, ^{9,19-23,30,31} rhythm assessment can be conducted,^29,31-33 but chest compressions should be continued otherwise.

During CPR, EtCO₂ can be used as a prognostic factor, though times vary. Literature has evaluated levels from 3 minutes onward, with most at 20 minutes. A value greater than 20 mm Hg is correlated with improved chance of ROSC and levels less than 10 mm Hg associated with decreased survival.^28,29,34-42 One meta-analysis states levels of 25 mm Hg are strongly associated with ROSC.⁴² Increase in capnography in inpatient PEA arrest demonstrates OR 4.8 for ROSC.⁴³  Another study found EtCO₂ to have sensitivity and specificity to be 88% and 77%, respectively, for ROSC at 5 and 10 minutes.⁴¹ Currently, capnography should not be used in isolation to determine cause of arrest, prognosticate, or evaluate for ROSC. It can be used in association with other measures. One great review uses the mnemonic PQRST for capnography use in arrest.⁴⁴

3) Procedural Sedation

A common ED use for capnography is procedural sedation, as it can detect hypoventilation earlier than pulse oximetry, especially with supplemental oxygen use. It can also monitor for upper airway obstruction, laryngospasm, and bronchospasm.^45-54 Complications during sedation are rare, with one meta-analysis finding 5 out of 1000 patients requiring airway assistance.⁵⁵ If respiratory depression or apnea occurs, EtCO₂ will demonstrate elevated plateau levels with decreased breaths.^45-54 ACEP has a Level B recommendation stating “Capnography may be used as an adjunct to pulse oximetry and clinical assessment to detect hypoventilation and apnea earlier than pulse oximetry and/or clinical assessment alone in patients undergoing procedural sedation and analgesia in the ED.”⁴⁵

The literature varies on the advantages of capnography.^56-60 A Cochrane review evaluating 1272 patients found capnography did not reduce clinically significant events, despite significant heterogeneity and bias.⁵⁸ Airway repositioning did occur more frequently in capnography. Sivilotti et al. found no difference in capnography versus pulse oximetry for hypoxia detection.

Many studies support the utility of capnography in monitoring. Langhan et al. in 2015 found decreased hypoventilation and desaturation in patients undergoing capnography.⁶¹ Miner et al. in 2002 found capnography was able to detect all respiratory depressions, with pulse oximetry detecting one third.⁵⁰ Burton et al. found capnography detected respiratory changes in 70% of cases before pulse oximetry,⁵¹ with Anderson et al. finding capnography diagnosed 100% before pulse oximetry.⁵⁴  Vargo et al. found 100% sensitivity for capnography versus 50% for oximetry in detecting hypoventilation.⁵³ A meta-analysis found capnography was over 17 times as likely to detect hypoventilation,⁴⁹ and other studies suggesting it can decrease risk of hypoxia. One systematic review suggests capnography decreases hypoxemic episodes, with relative risk 0.71 (95% CI 0.56-0.91).⁶²

Capnography allows emergency physicians to closely evaluate ventilation at the bedside based on the waveform and value, especially when supplemental oxygen is used.^45,62,63 In settings where one provider is completing the procedure and sedation, capnography provides safety, as pulse oximetry alone demonstrates a response delay in tissue hypoxemia.^45,63

Where does this leave us?

Waveform capnography is helpful in ETT verification and detection of displacement, cardiac arrest, and procedural sedation. Evaluation of the absolute value of end tidal levels and the waveform provides valuable information. Part II will evaluate other potential uses of waveform capnography including obstructive airway diseases, PE, trauma, sepsis, seizures, and fluid responsiveness. Stay tuned for more!

Key Points:

– Capnography involves the noninvasive measurement of CO₂ partial pressure during respiration, displayed as qualitative or quantitative forms.

– A waveform depicts the respiratory cycle based on CO₂ production, with end tidal CO₂ the plateau in phase III.

– Waveform capnography possesses several key uses in the ED.

– Capnography is one of the most reliable means of assessing ETT placement and position. Other measures such as tube condensation, chest movement, and breath sounds are not reliable.

– Transport of the intubated patient should include waveform capnography to monitor ETT placement.

– Waveform capnography is useful to evaluate compression quality during CPR, monitor for ROSC, and as a tool for prognostication.

– Use of capnography in procedural sedation can assist in detecting hypoventilation when compared to pulse oximetry alone.

Acknowledgements: Thank you to Bhavani-Shankar Kodali, MD, of http://www.capnography.com/new/, for providing the figures for this post.

References/Further Reading

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