2016A01 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 4.28μm

Beer-Lambert law

· I = I₀e^-ax

· Absorption ∝ pCO₂ x distance travelled

Side stream analyser:

Sources of error:

Processor

Confusion with other gases:

· N₂O 4.5μm, CO 4.7μM (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 V_T

· 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 response time

· Prone to obstruction

· Prone to leaks