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
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
Others
4 phases of capnograph
- Inspiratory - baseline
- Expiratory upstroke - consists of dead space gas and early CO2 output
* slope is an indication of dead space volume
- Alveolar plateau - exhalation of alveolar gas
- 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]