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Continuous-waveform capnography: A crucial tool for ED clinicians

Many emergency department (ED) patients are at risk for complications associated with airway management and ventilator problems. Continuous-waveform capnography (CWC) is a critical method clinicians can use to monitor patients’ respiratory function. By means of a specialized nasal cannula or attachment for an endotracheal or tracheostomy tube, this noninvasive technique measures end-tidal carbon dioxide (ETco2) over time and displays it as a CO2 waveform. Despite its important benefits, though, CWC is underused.

ETco2 is the percentage concentration of CO2 at the end of exhalation. It reflects CO2 production and elimination as well as ventilatory function and pulmonary perfusion.ETco2 monitoring helps ensure correct endotracheal tube placement during intubation and helps evaluate respiratory and ventilatory status during procedural sedation or mechanical ventilation. It’s also valuable in monitoring the effectiveness of cardiopulmonary resuscitation (CPR) and determining return of spontaneous circulation (ROSC) after cardiac arrest. (See Capnography basics.)

This article discusses CWC use during procedural sedation, intubation and mechanical ventilation, and CPR. It also describes troubleshooting and compares the benefits and drawbacks of capnography and pulse oximetry.

Procedural sedation

Medications commonly given during procedural sedation, like sedatives and analgesics, can cause respiratory depression leading to hypoventilation, apnea, hypoxia, or a combination. CWC can be used before the sedation procedure to identify baseline respiratory function, and during the procedure to identify airway complications and hypoventilation. Obese patients and those with sleep apnea are especially prone to these complications and should be monitored appropriately.

Intubation and mechanical ventilation

In newly intubated patients, ED clinicians can use capnography, capnometry (CO2 measurement alone without a continuous written record or waveform), or both to verify correct endotracheal tube placement. Colorimetric capnometry, a qualitative method, identifies correct tube placement initially by confirming ETco2 presence with a pH-sensitive filter. (See How a colorimetric capnometer works.)

The American Heart Association’s Advanced Cardiac Life Support (ACLS) and Pediatric Advanced Life Support guidelines recommend using CWC during patient transport and procedures involving patient movement. CWC significantly improves prompt identification of endotracheal tube dislodgment over pulse oximetry alone. A study of patients with such dislodgments found approximately 48% of those monitored by capnography had improper tube  placement corrected before pulse oximetry levels declined, compared to just 12% of patients without CWC monitoring.


CWC also can be used to monitor the ventilator status of mechanically ventilated patients. ETcoreadings alert clinicians to the need to adjust ventilator settings, including tidal volume and respiratory rate. Capnography has become the standard for confirming ventilation after intubation and is recommended for all ventilator-dependent patients.

Cardiac resuscitation

For patients in cardiac arrest, highquality CPR is essential for survival. Because CWC measures pulmonary blood flow during cardiac arrest, it indicates the quality of chest compressions. A lowETco2 level indicates poor CPR quality and is linked to a reduced chance of ROSC. Studies show ETco2 levels below 10 mm Hg after 20 minutes of CPR portend poor survival odds. Conversely, levels above 25mm Hg at 5 to 10 minutes after CPR initiation indicate a greater chance of ROSC. (See Benefits of CWC during resuscitation.)

Also, CWC use during cardiac resuscitation helps clinicians recognize ROSC without having to interrupt CPR to check for a pulse. Minimizing such interruptions is crucial for maintaining adequate perfusion and improving survival odds. An abrupt ETco2 rise indicates ROSC from a sudden increase in pulmonary perfusion. (See Using CWC to identify ROSC.)

Troubleshooting

CWC is simple to use—but it’s not perfect. Condensation can obstruct the sampling line; if this happens, the line may need to be disconnected and flushed with an airfilled syringe until clear. If this fails to fix the problem, the line may need to be replaced. Know that an obstructed or disconnected sampling line may cause a flat tracing instead of a square waveform on the capnogram. This can also occur if the patient isn’t breathing, the ventilator becomes disconnected, or the endotracheal or tracheostomy tube becomes occluded or dislodged.

Also, the waveform may not return to zero during inspiration. This may indicate the patient is rebreathing CO2, which may signal ventilator malfunction or inadequate patient oxygenation.

Capnography vs. pulse oximetry

Pulse oximetry is widely used in EDs, both for one-time oxygenation evaluation and continuous monitoring in more critical patients. Pulse oximetry detects hypoxia—but hypoxia can be a late sign of inadequate ventilation.

In contrast, CWC detects early signs of respiratory depression and ventilatory complications and is more sensitive in detecting respiratory compromise in patients undergoing procedural sedation. What’s more, it identifies endotracheal tube dislodgment faster than pulse oximetry, allowing more timely tube correction. Additionally, in patients receiving supplemental oxygen, pulse oximetry may appear normal despite obvious respiratory impairment or hypoventilation. (See Comparing CWC with pulse oximetry.)

Nursing implications

CWC is the standard of care for monitoring pulmonary function in ED patients. Besides yielding insight into the patient’s metabolic, circulatory, and respiratory status, it identifies respiratory complications faster than pulse oximetry. All nurses should be able to use this simple but effective tool to provide the highest-quality care.

When Lauren E. Haines wrote this article, she was a critical care clinical nurse education specialist at Eastern Connecticut Health Network in Manchester.

Selected references

Einav S, Bromiker R, Weiniger CF, Matot I. Mathematical modeling for prediction of survival from resuscitation based on computerized continuous capnography: proof of concept. Acad Emerg Med. 2011;18(5):468-75.

Langhan ML, Ching K, Northrup V, et al. A randomized controlled trial of capnography in the correction of simulated endotracheal tube dislodgement. Acad Emerg Med. 2011; 18(6):590-6.

Nassar BS, Schmidt GA. Capnography during critical illness. Chest. 2016;149(2):576-85. Nolan JP, Kelly FE. Airway challenges in critical care. Anaesthesia. 2011;66(suppl 2):81-92.

Ortega R, Connor C, Kim S, Djang R, Patel K. Monitoring ventilation with capnography. New Engl J Med. 2012;367(19):e27.

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