End Tidal Capnography in the Emergency Department

With quantitative capnography, obtaining the typical box-waveform on the capnogram reflects endotracheal intubation. In comparison, a flat capnogram is more indicative of an esophageal intubation (Figure 3).¹⁰ While other things may cause this waveform, such as technical malfunction or complete airway obstruction distal to the tube, tube placement confirmation to rule out esophageal intubation would be the first step to troubleshooting this waveform. In addition, if the ETT is placed in the hypopharynx above the vocal cords, the waveform may initially appear appropriate but will likely become erratic appearing over time.¹⁰

Quantitative capnography does have some limitations. For example, a main-stem bronchus intubation would still likely demonstrate normal-appearing capnography, so secondary strategies and a confirmatory chest x-ray are still indicated. False-negative ETCO₂ readings can occur in low CO₂ elimination states, such as cardiac arrest, pulmonary embolus, or pulmonary edema, while false-positives can theoretically occur after ingestion of large amounts of carbonated liquids or contamination of the sensor with stomach contents or acidic drugs.¹⁰ However, many of these misleading results can be caught by simply checking for an appropriate waveform.

Cardiac Arrest

Capnography has numerous uses in the monitoring, management, and prognostication of intubated patients in cardiac arrest.^1,3,4,10,14 Under normal conditions, EtCO₂ is 35-40 mm Hg. While the body still makes CO₂ during cardiac arrest, it will not reach the alveoli without circulating blood.¹⁰ Without CPR, CO₂ accumulates peripherally and won’t reach the lungs, causing EtCO₂ to approach zero. This means that EtCO₂ correlates directly with cardiac output during CPR, as long as ventilation remains constant.

This means the effectiveness of cardiac chest compression can be assessed in intubated patients using EtCO₂, with higher values during CPR correlated with increased return of spontaneous circulation (ROSC) and survival.^14-18 Using EtCO₂ monitoring during cardiac arrest may improve outcomes,¹⁹ and the American Heart Association (AHA) recommends monitoring capnography during cardiac arrest to assess compression efficacy.^10,20 EtCO₂ >20 mm Hg is considered optimal, while EtCO₂ <10-15 mm Hg is considered suboptimal.^4,10,16 In a recent meta-analysis, the average EtCO₂ was 13.1 mm Hg in those who did not obtain ROSC, compared to 25.8 mm Hg in those who did.²¹ As such, goal EtCO₂ for effective compressions may be even higher in future recommendations. If EtCO₂ is low, either compression technique should be improved or a different operator should do compressions. Every 1 cm increase in depth will increase EtCO₂ by approximately 1.4 mm Hg.16 Interestingly, compression rate is not a significant predictor of EtCO₂ over the dynamic range of chest compression delivery.¹⁶

An abrupt increase in EtCO₂ is an early indicator of ROSC.^{10,14-16,22,23} A return of a perfusing rhythm will increase cardiac output. This allows for accumulated peripheral CO₂ to reach the lungs, subsequently causing a rapid rise in EtCO₂.²⁴ It is important to note that when it comes to evaluating for ROSC, the actual numbers are less important than the change from pre- to post-ROSC. Providers should look for a jump of at least 10 mm Hg on capnometry.4 Nevertheless, an abrupt rise in EtCO₂ is a non-sensitive marker for ROSC (33%, 95% CI 22-47% in one multicenter cross-sectional study), meaning that the lack of an abrupt rise of EtCO₂ may not necessarily mean a lack of ROSC.²³