Capnograph

... graphical measurement of CO2 versus time

Measurement options

• Mass spectrometry
• Photoacoustic infrared
• Raman scattering

Most common: infrared absorption spectroscopy

Disadvantage

• Interference from other gases
• Response time of the machine

Interference

• Water vapour is strong absorber of IR. All samples need to be dried prior to analysis.
• CO2 absorbs IR at 4300nm, which is close to peak absorption of N2O and CO (around 4500nm)
• Collision broadening

Response time

Consists of transit time and rise time

Transit time (lag time)

... is time for gas to reach sensor

• Can be in seconds

Depends on

• Sampling flow rate
• Diameter of sampling tube
• Length of sampling tube
• Presence of filter
• Respiratory rate

Rise time

... time for analyzer to respond to an input signal

• Normally in milliseconds

Others

4 phases of capnograph

1. Inspiratory - baseline
2. Expiratory upstroke - consists of dead space gas and early CO2 output
   * slope is an indication of dead space volume
3. Alveolar plateau - exhalation of alveolar gas
4. Inspiratory downslope - beginning of next inspiration

End-tidal CO2

• End-tidal pCO2 is normally 2-5 mmHg lower than PaCO2.
• The size of the difference is an index of the amount of alveolar dead space present
  [Ref: KB2:p270]