End-Tidal CO2 Monitoring: Capnography in Pre-Hospital Care

By Jointra Editorial Team, Certified EMT

What Is Capnography?

Capnography is the continuous measurement and graphical display of carbon dioxide concentration in exhaled breath over time. End-tidal CO2 (EtCO2) is the concentration of CO2 at the end of a normal exhalation — the point that best approximates alveolar (and therefore arterial) CO2 levels.

Normal EtCO2: 35–45 mmHg (corresponds to PaCO2 of approximately 40 mmHg, with a typical gradient of 2–5 mmHg).

The Capnography Waveform

A normal capnogram has a characteristic shark-fin shape:

• Phase I (baseline): Inspiration and early expiration — CO2 near zero (dead space gas)
• Phase II (upstroke): Transition from dead space to alveolar gas — CO2 rising
• Phase III (plateau/alveolar plateau): Alveolar gas dominates — CO2 remains elevated; the peak is the EtCO2 value
• Phase 0 (downstroke): Inspiration begins — CO2 returns to baseline

Loss of the normal waveform shape is as diagnostically important as the numeric value.

Primary Use: Confirming ETT Placement

Continuous waveform capnography is the gold standard for confirming and monitoring endotracheal tube placement. A sustained waveform through at least 6 breaths after intubation confirms tracheal placement with near-100% specificity.

Esophageal intubation: CO2 may be transiently detected from carbonated beverages or air swallowed during BVM ventilation — but the waveform will quickly disappear (typically within 5–6 breaths). A sustained waveform does not occur with esophageal placement.

NAEMSP, AHA, and virtually all major EMS organizations mandate continuous waveform capnography for all intubated patients. Auscultation alone is insufficient.

Monitoring Ventilation Quality

Hyperventilation: EtCO2 <35 mmHg

• Causes cerebral vasoconstriction
• Dramatically worsens outcomes in TBI (target 35–40 mmHg)
• Common error in post-arrest resuscitation and ROSC management
• Monitor rate and EtCO2 — not just chest rise

Hypoventilation: EtCO2 >45 mmHg

• Causes cerebral vasodilation, rising ICP
• Indicates inadequate ventilatory support

CPR quality monitoring: During cardiac arrest, EtCO2 reflects pulmonary blood flow (and therefore cardiac output). A low EtCO2 (<10 mmHg) during CPR indicates poor chest compression quality or inadequate rate/depth. Rising EtCO2 during CPR is a real-time indicator of improving perfusion.

ROSC detection: A sudden spike in EtCO2 (often to >35–40 mmHg) during CPR is a reliable early indicator of ROSC — often detected before a palpable pulse.

Capnography in Non-Intubated Patients

Nasal cannula-style EtCO2 sampling devices allow continuous monitoring in spontaneously breathing patients.

Uses:

• Monitoring sedation (procedural sedation)
• Detecting respiratory depression in opioid overdose or after naloxone
• Monitoring asthma and COPD patients in respiratory distress
• Detecting hypoventilation in AMS patients

Interpreting Waveform Changes

Shark-fin pattern → Slanted plateau (upsloping Phase III): Indicates obstructive airway disease (asthma, COPD). Uneven emptying of lung units with different time constants creates a sloped plateau.

Sudden loss of waveform:

• Tube dislodgement (most common)
• Cardiac arrest (check pulse immediately)
• Equipment failure or disconnection

Gradual decrease in EtCO2:

• Hyperventilation
• Decreasing cardiac output
• Worsening pulmonary emboli

Gradual increase in EtCO2:

• Hypoventilation
• Increasing metabolism (seizure, fever, shivering)
• Rising cardiac output after ROSC

The Bottom Line

If your intubated patient doesn't have waveform capnography, you do not know where the tube is. No other monitoring modality replaces it.