CO2 Measurement

Infrared absorption
Gases with 2 or more different atoms absorb infrared radiation.
Each gas absorbs IR at characteristic wavelength.
System comprises:

  • IR source
  • Filter – only desired wavelength transmitted
  • Crystal window (sapphire) as glass absorbs IR
  • Sample chamber
  • Photodetector
  • Fraction of radiation absorbed compared with reference gas
  • Most capnographs sidestream
  • 150mls aspirated, filtered and dried → measuring chamber → back to circuit all in 1s
  • Zeros automatically by entraining room air
Other
Mass spectrometry
Accurate, rapid, multiple gases, bulky and expensive.
Colorimetric
CO2 and water form carbonic acid which will change a pH sensitive colour indicator.
Raman effect
Radiation absorbed by molecules and then re-emitted at different characteristic wavelength.
Needs a laser.

Info from a CO2 trace

Respiratory rate

No CO2
  • Trachea not intubated, disconnection, ventilator not turned on
High CO2
  • Inadequate ventilation
  • Soda lime exhaustion
  • MH
  • Rebreathing
Low CO2
  • Overventilation
  • Low CO, embolus
Waveforms
  • Obstruction
  • NMBD wearing off
  • Rebreathing