Resp – Capnography

Master answer

Principles

• Beer Lambert law: At a given wavelength, the amount of infrared radiation absorbed by gas is proportional to the concentration of gas present

• CO₂ is a heteronucleic molecule, and so absorbs infra-red light

• Infrared light shone across a sample of gas
  • Narrow band light emitted from infrared source
    • Band frequency chosen which fits the peak absorption frequency of CO₂
    • _{4.23micrometres}
  • Shone across gas, and absorbed at a detector
  • Detector emits signal to analyser -> screen
  • CO₂ reading outputted is inversely proportional to CO₂ present in sample, as per Beer-Lambert Law

• Gas can be sampled at the patient within the ventilation circuit (in-line), or in a sample of gas diverted away to a separate analysing chamber (sidestream)

Sources of error and limitations

• Sampling
  • Entrainment of atmospheric gas if leak in sidestream line
  • Occlusion of sidestream line causes loss of gas sampling
  • Water condensation absorbs IR light -> erroneously high ETCO₂
  • Modern capnography includes a water trap and heater (to reduce condensation)
• Calibration
  • Incorrect calibration of analyser
• Interference
  • Other gases (notably N₂O) have a similar absorption spectrum
  • Presence may falsely elevate measured ETCO₂ (esp. if infrared frequency band too broad)
  • Presence of other gases causes ‘collision broadening’
  • Absorption spectrum of CO₂ is broadened
• Pressure
  • Partial pressure, rather than percentage composition, is measured
  • If pressure ↓ (e.g. by suction drawing gas into sampling chamber), erroneously low measured ETCO₂
• Sampling chamber
  • If too large, mixing of gas between respiratory cycles -> compression of waveform and erroneously low measured ETCO₂­­­

Mooney 2016

Answer by Mooney