Capnography

Capnography - Breath by Breath

• Definition: Continuous, non-invasive measurement of CO2 concentration in respiratory gases, primarily end-tidal CO2 (EtCO2).
• Physiological Basis: Utilizes infrared spectroscopy; CO2 absorbs infrared light proportionally to its concentration.
• Normal Values:
  • EtCO2: 35-45 mmHg.
  • $P_aCO_2 - EtCO_2$ gradient: 2-5 mmHg (arterial CO2 is typically higher).
• Clinical Significance of Gradient:
  • Reflects alveolar dead space (ventilation without perfusion).
  • Increased gradient (>5 mmHg): Suggests V/Q mismatch (e.g., Pulmonary Embolism (PE), emphysema, ↓ cardiac output, shock), faulty equipment.

⭐ Capnography provides immediate breath-to-breath feedback on ventilation, perfusion, and metabolism; essential for confirming Endotracheal Tube (ETT) placement and monitoring its position continuously during anesthesia and critical care.

Capnography - Shape Shifter Signs

• Normal Waveform:
  • Phase 0: Inspiration (CO2 ~0 mmHg).
  • Phase I: Dead space.
  • Phase II: Expiratory upstroke.
  • Phase III: Alveolar plateau (P_ETCO₂).
  • Alpha angle (V/Q): 100-110°. Beta angle (rebreathing): 90°.
• Key Abnormal Shapes:
  • Shark Fin: Bronchospasm (asthma, COPD), ETT kink/obstruction. ↑ Alpha angle.
  • Curare Cleft: Spontaneous breath during MV. Dip in plateau.
  • Elevated Baseline: CO2 rebreathing (exhausted soda lime, valve fault). ↓ Beta angle.
  • Staircase Plateau: Uneven alveolar emptying (partial obstruction).
  • Sudden Loss of ETCO2 (to ~0): Esophageal intubation, disconnect, cardiac arrest.
  • Cardiac Oscillations: Ripples on plateau; normal in children, prominent in low flow.

⭐ Sudden loss of waveform (P_ETCO₂ ~0) is a critical sign: check for esophageal intubation or circuit disconnect immediately.

Capnography - Capno Catcher

• ETT Confirmation: Gold standard for ETT. Persistent square waveform after 5-6 breaths.
  • Absent/minimal CO2: Esophageal intubation or severe hypoperfusion (e.g., cardiac arrest).
• CPR & ROSC: Monitor CPR quality: Aim PETCO2 > 10-15 mmHg.

  • ⭐ Sudden significant ↑ETCO2 indicates ROSC!

• Malignant Hyperthermia (MH): Sensitive early sign: Unexplained, progressive, sustained ↑ETCO2 despite constant ventilation.
• Pulmonary Embolism (PE): Sudden ↓ETCO2 with stable ventilation & clear lungs (↑ alveolar dead space).
• Airway & Circuit Integrity:
  • Bronchospasm/Obstruction (kinked ETT): "Shark-fin" appearance (sloping Phase III, prolonged expiratory upstroke).
  • Rebreathing (exhausted absorber, valve issue): Elevated baseline (Phase 0 not zero), ETCO2 may gradually ↑.
  • Disconnect/Leak/ETT displacement: Sudden loss or marked ↓ETCO2 waveform.
• Ventilation Status:
  • Hypoventilation: Gradual ↑PETCO2.
  • Hyperventilation: Gradual ↓PETCO2.

Capnography - Glitch Guardian

• Factors ↑ EtCO₂:
  • Physiological: Hypoventilation, ↑metabolism (malignant hyperthermia, sepsis), rebreathing.
  • Equipment: Exhausted soda lime, faulty unidirectional valves.
• Factors ↓ EtCO₂:
  • Physiological: Hyperventilation, ↓cardiac output, pulmonary embolism, cardiac arrest.
  • Equipment: Circuit leak/disconnection, esophageal intubation, sample line issue.
• Glitches & Caveats:
  • Sample line (kinks, water), calibration errors.
  • P(a-ET)CO₂ gradient: Normally 2-5 mmHg; ↑ in V/Q mismatch.

⭐ Sudden ↓ EtCO₂ to near zero strongly suggests: Esophageal intubation, circuit disconnect, ventilator failure, or total circulatory collapse (cardiac arrest).

High‑Yield Points - ⚡ Biggest Takeaways

• Capnography measures end-tidal CO2 (ETCO2), reflecting ventilation and pulmonary perfusion.
• Normal ETCO2: 35-45 mmHg; sudden ↓ indicates esophageal intubation, apnea, or circuit disconnection.
• Gradual ↑ ETCO2 suggests hypoventilation, malignant hyperthermia, or CO2 rebreathing.
• "Shark fin" appearance (sloping Phase III) indicates bronchospasm or COPD.
• Failure to return to zero baseline (Phase 0) signifies CO2 rebreathing.
• Sudden loss of waveform can mean cardiac arrest, pulmonary embolism, or total airway obstruction.