2016A Question 01
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.
Examiner Report
62% of candidates achieved a pass in this question.
The main points expected for a pass included a discussion of the principles of infrared analysis including details of molecular structure, the Beer-Lambert Law and how this pertains to the clinical circumstances in the question. A description of the equipment required for sidestream analysis was also required, and how each component works. Finally, sources of error were required in relation to the functioning of this equipment. In summary, this was an equipment question.
Most answers included a reasonable discussion of the principles of infrared analysis, and could describe most of the equipment. However, outlines of how the equipment might not function were less well handled. Many answers contained headlines such as “blocked capnography tubing” or “collision-broadening” but did not go on to describe how that will influence the clinical interpretation of the capnography.
This was an equipment question but many answers contained physiological discussions about the production and excretion of CO2 and how these affect the capnography trace. These discussions did not score marks.
Model Answer
Structure:
- Principles: Infrared, Beer-Lambert
- Diagram
- Sources of error
- Comparison to mainstream analysers
Principles
| Principle | Description |
|---|---|
| Infrared absorption | - Gases with ≥2 types of atoms can absorb infrared radiation - CO2 max absorption at wavelength 4.28μm |
| Beer-Lambert law | - - |
Side Stream Analyser
Sources of Error
| Source | Description |
|---|---|
| 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 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
| Category | Description |
|---|---|
| Pros | - No increase in dead space - Lighter - Remote monitoring easier |
| Cons | - Longer transit time → longer response time - Prone to obstruction - Prone to leaks |