Capnographs

Technology: How It Works

• Morgan and Mikhail's Clinical Anesthesiology, 7e, p. 226-227
• Barash Clinical Anesthesia, 9e, p. 2492

The Normal Capnogram Waveform

• W - Inspiratory baseline (zero CO2 during inspiration)
• V - Expiratory upstroke (initial rise as dead-space gas mixes with alveolar gas)
• X - Expiratory plateau (alveolar gas; near-flat segment)
• Y - Peak EtCO2 (highest point at end-exhalation)
• Z - Inspiratory downstroke (rapid fall as fresh gas enters)
• Tintinalli's Emergency Medicine, p. 122 (Fig. 16-2)

Abnormal Waveform Patterns

• Morgan and Mikhail's Clinical Anesthesiology, 7e, p. 227
• Scott-Brown's Otorhinolaryngology, Vol 1, p. 390

EtCO2 During Cardiac Arrest

• At cardiac arrest onset, PETCO2 falls abruptly
• During effective CPR, PETCO2 rises and correlates with cardiac output, coronary perfusion pressure, and compression efficacy
• A sudden rise in EtCO2 during CPR signals return of spontaneous circulation (ROSC) before pulse can be palpated
• Persistently very low PETCO2 (<10 mmHg) after 20 minutes of adequate CPR may support a decision to terminate resuscitation
• The 2015 resuscitation guidelines recommend continuous waveform capnography for all intubated patients during resuscitation
• Roberts and Hedges' Clinical Procedures in Emergency Medicine, p. 415

Clinical Applications

1. Confirm Endotracheal Tube Placement

• Most reliable method for confirming tracheal vs. esophageal intubation
• Tracheal placement: sustained CO2 waveform over multiple breaths
• Esophageal placement: brief, declining CO2 signal that disappears within ~6 breaths
• Does not reliably detect mainstem bronchial intubation (both lungs must be assessed separately)

2. Monitor Ventilation Adequacy

• Continuous breath-by-breath respiratory rate (more accurate than impedance-based monitoring, which can misread chest wall movement without airflow)
• Detects hypoventilation, apnea, and airway obstruction earlier than pulse oximetry, especially when supplemental O2 is being given
• In one study, respiratory depression was 17.6 times more likely to be detected with capnography than standard monitoring alone

3. Procedural Sedation and Analgesia (PSA)

• Capnography detects earliest respiratory compromise during sedation
• Hypoxemia (SpO2 <90%) occurred an average of 45.6 seconds after the onset of apnea/disordered breathing detected by capnography
• All episodes of apnea >20 seconds in one MAC study were missed by the anesthesia provider but caught by capnography
• ACEP gives capnography a Level B recommendation for PSA in the ED

4. Pulmonary Embolism / Decreased Cardiac Output

• Pulmonary embolism increases alveolar dead space → widens PaCO2-EtCO2 gradient
• A sudden fall in EtCO2 with no change in ventilation = suspect PE, air embolism, or abrupt drop in cardiac output
• Very sensitive for venous air embolism during neurosurgery (sitting position)

5. Malignant Hyperthermia

• Rapidly rising EtCO2 with fever/rigidity is an early metabolic warning sign

6. COPD Management

• In COPD, the absence of an expiratory plateau makes EtCO2 an unreliable surrogate for PaCO2; transcutaneous CO2 (tcCO2) monitoring is preferred in these patients

Limitations

• EtCO2 is always slightly lower than PaCO2 (normal gradient 2-5 mmHg)
• In critically ill patients with V/Q mismatch, the gradient can be much wider and EtCO2 should not be used to estimate PaCO2
• Mouth breathing entrains room air and lowers readings
• Sampling line obstruction or water precipitation causes false low readings
• High aspiration rates in sidestream devices may dilute readings in small pediatric patients

Summary Table