Resp – Capnography

Master answer

PaCO₂

• Factors affecting PaCO₂
  • Rate of metabolism and CO₂ production
  • FiCO₂ (usually negligable)
  • Alveolar ventilation
    • Increased alveolar ventilation decreased PaCO₂

End-Tidal CO₂

• CO₂ contained in gas at the end of tidal expiration
• Contains:
  • Alveolar CO₂
    • High CO2 diffusion – PACO₂ = PaCO₂
    • Only 0.7mmHg Alveolar-arterial CO₂ gradient for 10% shunt
  • Alveolar Dead Space CO₂
    • Anatomical – conducting airways
    • Physiological – West zone 1 / Apex
    • Varies with posture and pathology
• Measured by capnometer
  • In-line or side-stream
  • Used IR absorbance at 4.23 μm
  • Side-stream: lag in CO₂ detection due to increased dead space between respiratory tract and analyzer

End-tidal to Arterial PaCO₂ difference

• Normal ETCO₂ – PaCO₂ ~5mmHg
• Can be due to Artificially low ETCO₂ or Artificially High PaCO₂

Artificially low ETCO2

1. Dead space (VD)
   • Assumed PCO2 however usually low pCO2 but non 0 pCO2
   • Contributes to ETCO2-PaCO2 ~5mmHg
   • Pathological ↑VD
     • ↑alveolar dead space (normal individuals = 0)
       • Mixing of gas between perfused and non-perfused alveoli → ETCO2 < PaCO2
       • Equivalent to ↑ West Zone 1
         • → PE, hypotension, excess PEEP/IPPV
         • COAD – poor perfusion of hyper-expanded alveoli
2. Closing capacity (CC): 
   • Usually 40 CC may exceed FRC
     • ↑ Airways collapse/gas trapping → airways closure → ↑alveolar pCO2 within alveoli not exhaling
     • Exacerbated by
       • low volume ventilation close to FRC
       • High FiO2 for extended periods → denitrogenation → loss of airways splinting = CC closer to FRC
3. Sampling line problems:
   • Too far from trachea (ie ↑ mechanical dead space)
   • Too long – unable to adequately sample as gas is trapped in line
   • Air leakage/line blockage leading to measurement error with entraining of room air, loss of expired air
4. Machine:
   • Loss of calibration of sampling unit
   • Excess H2O in water trap interfering with measurement
   • Interference from other gases (N2O) artificially raises ETCO2 due to interference with IR absorber “Collision broadening”
5. Incorrect timing of measurement
   • Inadequate expiration time → failing to reach plateau = incomplete alveolar expiration = falsely low ETCO2

Artificially high PaCO2

• Should be measured from arterial sample
• Venous sampling will artificially ↑pCO2
• Loss of calibration of machine

Gladwin / Sakurai 2016

Answer by Gladwin, Sakurai