What Happens to EtCO₂ When a Patient Becomes Apneic
When a patient becomes apneic, end-tidal CO₂ (EtCO₂) drops to zero or shows an absent waveform because no CO₂ is being exhaled from the lungs. 1
The Physiologic Mechanism
EtCO₂ monitoring measures the CO₂ concentration in exhaled breath. The signal depends entirely on the highest value of PCO₂ in the exhaled breath—not on tidal volume or flow magnitude 1. During apnea:
- No gas exchange occurs → no CO₂ is delivered to the measurement device
- The capnography waveform flatlines → absence of signal deflections indicates no CO₂ is being exhaled
- This creates a characteristic pattern that has been used clinically to score and detect apneic events 1
Clinical Context: Different Scenarios
During Sleep Apnea Monitoring
The absence of signal deflections (no CO₂ exhaled) is the defining feature used to score apneas in polysomnography 1. This is particularly useful in pediatric patients where EtCO₂ monitoring is listed as an acceptable alternative sensor for apnea detection 1.
During Cardiac Arrest
In untreated cardiac arrest, EtCO₂ approaches zero with continued ventilation because CO₂ continues to be produced in the body but there is no CO₂ delivery to the lungs without circulation 2. Normal EtCO₂ is 35-40 mm Hg, but during cardiac arrest without CPR, it drops toward zero 2.
During Procedural Sedation
Abnormal EtCO₂ findings (including absent waveform) occur before oxygen desaturation in 70% of patients with acute respiratory events 3. The absent waveform or severely reduced EtCO₂ signals hypoventilation or apnea earlier than pulse oximetry changes 4.
Important Caveats
Technical limitations can create false readings:
- Mouth breathing can impair detection if using nasal cannula sampling 1
- Nasal cannula occlusion prevents accurate measurement 1
- Signal excursions can persist during inspiratory apnea if small expiratory puffs with high PCO₂ are present 1
The capnography signal typically lags behind the actual airflow signal 1, so there may be a brief delay between cessation of breathing and the flatline appearing on the monitor.
Clinical Significance
The absent EtCO₂ waveform is highly specific for apnea and serves as an early warning system that precedes hypoxemia 3. In procedural sedation, an absent waveform was one of three identifiers (along with EtCO₂ >50 mm Hg or absolute change >10 mm Hg) that captured all patients with respiratory depression 4.
During resuscitation, an abrupt sustained increase in EtCO₂ to normal values (35-40 mm Hg) indicates return of spontaneous circulation 2, 5, making the transition from absent/low to normal EtCO₂ a critical prognostic marker.