Indications, equipment, and nursing care

End-tidal carbon dioxide (ETCO₂) monitoring, noninvasive measurement of exhaled CO₂, provides valuable insight into a patient’s metabolism (production of CO₂ as a byproduct of cellular metabolism), perfusion (transport of CO₂ into the lungs), and ventilation (expulsion of CO₂ via the respiratory system). ETCO₂ can be a numerical value (capnometry), or when coupled with a patient’s respiratory rate, a graphical-numerical value (capnography). ETCO₂ represents the partial pressure of CO₂ within arterial and venous blood, which serves as an important marker of alveolar ventilation. As a result, ETCO₂ monitoring can aid early detection of clinical deterioration related to cardiac output and pulmonary blood flow.

The why

Clinical use of capnography began in the 1950s when capnograph monitors became available via modern infrared technology. Advances in ETCO₂ continuous monitoring technology over the past 70 years have made capnography a key vital sign component to ensure patient safety during anesthesia. The American Society of Anesthesiologists endorses capnography as a standard of care for general anesthesia as well as moderate and deep procedural sedation.

By incorporating a patient’s respiratory rate and exhaled CO₂ values, capnography helps clinicians detect early warning signs of ventilatory compromise earlier than pulse oximetry readings (blood oxygen saturation levels). For this reason, other specialties outside of the operating room—critical care, toxicology, emergency medicine—should use ETCO₂ capnography monitoring as a standard of care.

ETCO₂ monitoring provides clinicians with an accurate approximation of the partial pressure of carbon dioxide (paCO₂), which rep­resents alveolar CO₂. Clinicians can monitor ETCO₂ invasively or noninvasively to guide decision making; paCO₂ readings are obtained invasively by collecting arterial blood gas (ABG) samples. In healthy patients with normal lung function, typical capnometry readings range between 35 and 45 mmHg. A normal ETCO₂ reading typically is 2 to 5 mmHg less than the corresponding paCO₂ value. ETCO₂ and paCO₂ correlations increase in variability with age, pulmonary disease, cardiac dysfunction, and other conditions commonly found in patients admitted for hemo­dynamic or respiratory instability.

This variability differs significantly in patients with unhealthy lungs. In fact, when ventilator settings are adjusted or patients are followed over time, changes in ETCO₂ and PaCO₂ are poorly correlated and may even go in opposite directions. Therefore, ABG sampling is the preferred monitoring tool for patients receiving mechanical ventilation. The inherent inaccuracies in ETCO₂ require cautious use as a surrogate for PaCO₂ when caring for patients with unhealthy lungs who require mechanical ventilation.

The when

Many indications exist for ETCO₂ monitoring in various clinical settings. For example, it helps confirm tracheal airway placement and rules out esophageal placement. After artificial airway placement, deliver six breaths via a bag-valve oxygen delivery device and then measure ETCO₂. Taking readings before six breaths could lead to false high readings if the patient consumed antacids, recently drank a carbonated beverage, or had bag or mask ventilation before intubation. Continuous ETCO₂ monitoring of patients with artificial airways can help detect the migration or loss of the airway, especially during transport.

Capnography can serve as an early indicator of airway compromise and as a more accurate indicator of tube dislodgement than auscultation. It also offers insight into respiratory and cardiovascular function by providing information about ventilation and CO₂ removal that continuous pulse oximetry doesn’t. Poor ventilation and rising CO₂ can lead to respiratory acidosis and cardiovascular collapse, which may go undetected with pulse oximetry. Capnography helps identify abnormalities of exhaled air flow, CO₂ elimination, and the ratio of dead-space volume to tidal volume, which optimize mechanical ventilation.

ETCO₂ monitoring has multiple uses during cardiopulmonary resuscitation (CPR), including as an important feedback mechanism for chest compressions. Adequate compressions should result in ETCO₂ readings between 10 and 20 mmHg. Readings of less than 10 mmHg signify slow and shallow compressions, which may indicate performer fatigue. Code leader feedback should include a plan to change chest compression performers to improve compression quality. During CPR, continuous capnography also aids in detecting rapid CO₂ upstroke, which could indicate a return of spontaneous circulation. Evidence also suggests an association between persistently low ETCO₂ readings during CPR and overall poor outcomes.

Patients undergoing anesthesia or procedural sedation should receive continuous ETCO₂ monitoring before, during, and when recovering from sedation administration. Gallagher’s research has shown that patients who receive ETCO₂ monitoring during sedation have decreased rates of hypoxic events. Because patients may have increased ETCO₂ readings while their pulse oximetry remains normal or at baseline, monitoring helps clinicians ensure adequate ventilation during the procedure. Compared to pulse oximetry monitoring alone, ETCO₂ monitoring helps clinicians recognize hypoventilation or apneic events more rapidly.

ETCO₂ monitoring also can aid detection of opioid-induced respiratory depression. Patients who are intoxicated or sedated from accidental or intentional drug overdose may have impaired ventilation. Similar to treating patients undergoing anesthesia or sedation, ETCO₂ monitoring can assist the clinician in differentiating effective vs ineffective ventilation in obtunded or unconscious patients. Patients with an altered mental status, such as those who’ve had a seizure, also benefit from ETCO₂ monitoring as changes in capnography typically precede changes in pulse oximetry readings during hypoxic events.

Research continues to explore the use of ETCO₂ monitoring when predicting fluid responsiveness. Fluid therapy is a fine line to navigate as hypovolemic patients need volume to maintain homeostasis and prevent organ ischemia; however, too much volume can lead to fluid overload. The noninvasive passive leg raise (PLR) test gives patients a reversible bolus of fluid by allowing blood to drain back to the heart. It simulates the effects of a fluid bolus and increases preload. Toupin and colleagues studied the PLR test on patients while continuously monitoring their ETCO₂ and correlating these readings with ongoing hemodynamic monitoring of cardiac output, heart rate, and mean arterial pressure. ETCO₂ changes >2 mmHG immediately after the PLR test predict a patient’s responsiveness to a fluid bolus. This research aims to remove the need for invasive testing, particularly on patients receiving ventilation in intensive care.

The how

Various methods of obtaining ETCO₂ readings exist, but the most common are the sidestream and mainstream methods. With sidestream, small bore tubing carries exhaled gas to a monitor for analysis. This tubing can be in nasal cannula form or attached to an endotracheal tube. Nasal cannulas with extension pieces may be the best option for patients who breath primarily through their mouths.

In the mainstream method, a CO₂ analyzer placed close to the exhaled gas is attached directly to a nasal cannula, mask (most frequently used with pediatric patients), or ventilator circuit. (See Mainstream vs sidestream.)

Monitoring equipment

The monitors analyzing the ETCO₂ samples delivered via sidestream or mainstream methods can operate as a standalone system or by incorporation into the physiologic monitor of the ventilator circuit. For patients who aren’t intubated, supplemental oxygen can be delivered via an ETCO₂ reading–capable nasal cannula or mask simultaneously with monitoring.

Patient-controlled analgesia infusion pumps can integrate ETCO₂ monitoring technology with medication delivery. Using the sidestream method, the ETCO₂ adaptor cable is plugged directly into the monitor module, which analyzes exhaled CO₂, fractional inspired CO₂, and respiratory rate for patients who are intubated and those who aren’t. This monitoring aids in reducing opioid-induced respiratory depression by pausing an opioid infusion when clinical parameters aren’t met and alerts clinicians via an alarm system.

A colorimetric CO₂ detector helps to verify proper endotracheal tube placement during intubation. The detector contains a pH-sensitive chemical indicator that changes color from purple/blue to yellow in the presence of CO₂. Use it only for verifying airway tube placement; it can’t provide a numerical value or graph ETCO₂ waveforms.

Waveform analysis

Successful capnography use requires that healthcare professionals can interpret the waveforms. ETCO₂ monitoring technology consists of light passing from an infrared capnometer through a patient’s exhaled gas and measuring the absorption of light by the gas. As CO₂ increases, more light is absorbed, which results in the capnography waveform.

In typical conditions, four important phases occur in an ETCO₂ capnography waveform. The first phase represents the respiratory baseline, which contains dead space gas typically free of CO₂. The second occurs at the beginning of exhalation represented by a rapid upstroke in the waveform. Next comes the leveling of the upstroke and alveolar plateau—the peak point or end-tidal value in capnometry. In the fourth phase, a rapid downstroke of the waveform represents inhalation, which contains no CO₂.

Several different waveforms can occur depending on a patient’s condition. For example, patients with chronic obstructive pulmonary disease (a chronic inflammatory lung condition that obstructs airflow from the lungs, which results in CO₂ retention) have a unique ETCO₂ waveform that takes on a shark fin appearance with no plateau, which typically occurs in the third phase. This shark fin appearance correlates with the patient’s inability to adequately eliminate CO₂. (See Capnography waveforms.)

Nursing implications

Nurses caring for patients on ETCO₂ monitoring should have awareness of ETCO₂ alarms. In the event a patient’s alarm sounds, first assess the patient for distress, check the airway for occlusion, remove any obstructions, and adjust any twisted tubing. If the patient isn’t in distress and you don’t see a need for immediate intervention, determine why the alarm sounded. Alarms can indicate loose equipment connections, the need for patient repositioning (potentially using airway positioning supports), and a change in waveform quality. If the alarm continues, collaborate with the provider and respiratory therapy team to troubleshoot the issue. If necessary and ordered by the pro­vider, replace the patient’s equipment. (See ETCO₂ monitoring: Nursing steps.)

Patient and family education

You can help patients and their families understand the need for ETCO₂ monitoring. Explain that it can detect early changes in clinical status, which helps prevent harm. To reduce any anxiety and increase adherence with wearing necessary devices, explain the procedure and the purpose of the required equipment. Education is particularly important for patients who aren’t intubated and must use the mainstream capnography device; the equipment can feel heavy when it sits close to the patient’s face. When they understand the importance of monitoring, patients may more likely adhere to wearing the equipment. Placing a protective dressing under the equipment can help reduce surface pressure.

A tool for safety

ETCO₂ monitoring, a tool once used almost exclusively in patients receiving anesthesia, has proven useful in various settings, including the emergency department, medical–surgical units, critical care, ambulatory care, critical care transport, and postoperative suites. Longstanding evidence shows the importance of ETCO₂ monitoring in many clinical situations, including intubation, CPR, and sedation. Current literature aims to link ETCO₂ monitoring with fluid responsiveness predictability, but not enough established evidence exists to develop concrete evidence-based clinical guidelines. However, we do know that continuous ETCO₂ monitoring provides healthcare professionals with real-time data to make clinical decisions that support patient safety and optimal outcomes, while minimizing the need for additional invasive procedures.

The authors work at the Hospital of the University of Pennsylvania in Philadelphia. Sinead Donnelly Hellings is a clinical practice lead, Jessica Fuller is director of nursing practice, and Stephanie Maillie is a clinical nurse specialist for quality.

American Nurse Journal. 2024; 19(9). Doi: 10.51256/ANJ092406

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Key words: capnography, end-tidal carbon dioxide monitoring, capnometry