What are the clinical applications and limitations of capnography (carbon dioxide monitoring) in patients undergoing surgery or who are critically ill, particularly those with underlying respiratory or cardiac conditions, such as chronic obstructive pulmonary disease (COPD) or heart failure?

Clinical Applications and Limitations of End-Tidal Capnography

Primary Clinical Applications

Confirmation of Endotracheal Tube Placement

Waveform capnography is the gold standard for confirming correct tracheal intubation and must be used routinely for all intubation attempts. 1 The Association of Anaesthetists guidelines establish this as mandatory practice with both high sensitivity and specificity for detecting correct tube placement. 1

• A flat capnogram after attempted intubation indicates oesophageal intubation until proven otherwise, even during cardiac arrest where an attenuated but recognizable waveform will be present if the tube is correctly placed. 1
• During cardiac arrest with effective CPR, waveform capnography demonstrates 100% sensitivity and 100% specificity for confirming tracheal tube position. 1
• The American Heart Association recommends exhaled CO2 detection for confirming tube position in neonates, infants, and children with perfusing rhythms in all settings (Class I, LOE C). 1
• Failure to use or correctly interpret capnography for detecting oesophageal intubation has been classified as a Never Event in the UK. 1

Continuous Airway Monitoring

Uninterrupted capnography monitoring must occur from induction through emergence, including during all patient transfers, and should continue until the tracheal tube or supraglottic airway is removed. 1

• Capnography is essential at all times in patients with tracheal tubes, supraglottic airway devices, and those sedated without verbal response. 1
• The British Journal of Anaesthesia guidelines state that failure to use capnography in ventilated ICU patients probably contributes to >70% of ICU airway-related deaths. 1
• During sedation, capnography should be used whenever verbal contact with the patient is lost, and is advised even for lighter sedation levels to monitor airway patency and respiratory pattern. 1

Detection of Tube Displacement or Complications

If an intubated patient's condition deteriorates, capnography changes should prompt immediate evaluation using the DOPE mnemonic: Displacement, Obstruction, Pneumothorax, Equipment failure. 1

• The depth of tracheal tube insertion should be documented and checked each shift, with capnography monitoring maintained continuously. 1
• Apparent cuff leak or changes in capnography waveform should be assumed to indicate partial extubation until proven otherwise. 1

Additional Monitoring Roles

• Capnography detects bronchospasm, lung pathology, rebreathing of carbon dioxide, and metabolic alterations such as malignant hyperthermia. 1
• In cardiac arrest, capnography provides information on ventilation, perfusion, and metabolism, with values reflecting pulmonary blood flow during resuscitation. 2, 3

Critical Limitations

Low Perfusion States

The most significant limitation is that absent or low end-tidal CO2 may reflect inadequate pulmonary blood flow rather than tube misplacement, particularly during cardiac arrest or severe shock. 1

• During cardiac arrest, if exhaled CO2 is not detected, confirm tube position with direct laryngoscopy (Class IIa, LOE C), as absence of CO2 may indicate very low pulmonary blood flow. 1
• An intravenous bolus of epinephrine may transiently reduce pulmonary blood flow and exhaled CO2 below detection limits. 1
• Pulmonary embolism reduces pulmonary blood flow and CO2 delivery to the lungs, causing false-negative readings. 1

Severe Respiratory Disease

Capnography may be least useful in the sickest patients with severe respiratory failure due to increased V/Q mismatch and widened P(a-ET) gradient. 4

• Severe airway obstruction (status asthmaticus) and pulmonary edema may impair CO2 elimination below detection limits. 1
• In patients with severe respiratory failure, increased V/Q mismatch consistent with widened P(a-ET) gradient can lead to erroneous PETCO2 values. 4
• The least reliable application is reflecting alveolar ventilation (PaCO2) in patients with lung disease. 4

Equipment and Technical Issues

• Contamination with gastric contents or acidic drugs (endotracheally administered epinephrine) may cause a consistent color rather than breath-to-breath color change in colorimetric devices. 1
• A large glottic air leak may reduce exhaled tidal volume through the tube and dilute CO2 concentration. 1
• Rarely, absent capnography waveform may result from tube obstruction (severe pulmonary edema, severe bronchospasm, blood), secretions, or water in the capnograph circuit, but tube misplacement should always be initially assumed. 1

Special Populations

• Safe use and interpretation of waveform capnography in very small babies (<1 kg) requires specialist expertise. 1
• In nonintubated patients, end-tidal CO2 averages 3.6 mm Hg lower than PaCO2, with dead space being the primary determinant of the P(a-et)CO2 gradient. 5
• In bariatric patients, transcutaneous CO2 values averaged 4.5 ± 5.5 mm Hg higher than corresponding end-tidal measurements and were slow to change. 6

Essential Clinical Caveats

Capnography must always be used in conjunction with clinical assessment and should never be the sole method of evaluation. 2, 4

• Alternative confirmation methods (palpation, auscultation) lack sensitivity and specificity compared to capnography. 1
• Auscultation and observation of chest wall movement are unreliable signs of correct tube placement, particularly in the critically ill. 1
• Pulse oximetry may remain normal for up to 3 minutes following hyperoxygenation even without effective ventilation, making it inadequate for immediate tube placement confirmation. 1
• Clinicians managing airways independently must be skilled at recognizing normal and abnormal capnogram patterns. 1