f2016A01 Discuss the physical principles and equipment used for measurement of
exhaled carbon dioxide by infrared analysis using sidestream sampling.
Outline the potential sources of error.

 

List:

·      Principles: infrared, Beer-Lambert

·      Diagramme

·      Sources of error

·      Comparison to mainstream analysers

 

Principles:

Infrared absorption

·   Gases with ≥2 types of atoms can absorb infrared radiation

·   CO2 max absorption at wavelength 4.28nm

Beer-Lambert law

·   I = I0e-ax

·   Absorption pCO2 x distance travelled

 

Side stream analyser:

 

Sources of error:

Processor

Confusion with other gases:

·   N2O 4.5nm, CO 4.7nM (causes falsely high pCO2)

·   H2O vapour absorbs widely (causes falsely high pCO2)

·   Bronchodilator propellant and halothane

·   FIX: reference chamber AND water trap

Collision broadening:

·   Widening of the absorption peak for a gas when in the presence of another

·   Due to collision between molecules raising their energy level

·   50% N2O widens CO2’s absorption peak by 10%

·   FIX: presence of reference chamber

Ram-gas effect:

·   Pressure-drop across the sampling line

·   ↓ total pressure inside sample chamber -> ↓ CO2 partial pressure

·   Failure of calibration

·   FIX: regular three point calibration

Sampling

Dilution by fresh gas

·   Sampling of fresh gas may occur in expiration if high RR and low VT

·   e.g. neonates and young children

Sampling line disconnection

·   Complete: no signal

·   Partial: entrainment of room air

Blockage by water condensation

·   FIX: water trap

Patient

·   Cardiogenic oscillation (if apnoeic)

·   Underreads if plateau not reached e.g .severe asthma

 

Compared to mainstream analyser:

Pros

·   No increase in dead space

·   Lighter

·   Remote monitoring easier

Cons

·   Longer transit time, longer rise time? Response time?

·   Loss of 150mL/min fresh gas flow rate

·   Prone to obstruction

·   Prone to leaks

 

 

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