EDAIC Clinical Measurement and Physics 5 — Questions and Answers
Question 1: What is the significance of the 'isobestic point' at approximately 805 nm in oximetry?
- It is the wavelength at which all hemoglobin species scatter light equally
- It is the wavelength at which oxyhemoglobin and deoxyhemoglobin absorb light equally, used as a reference (Correct answer)
- It is the optimal wavelength for detecting carboxyhemoglobin
- It marks the boundary between red and infrared light
Correct answer: It is the wavelength at which oxyhemoglobin and deoxyhemoglobin absorb light equally, used as a reference
At the isobestic point (~805 nm), HbO₂ and Hb have identical molar extinction coefficients, so absorbance is independent of saturation and can serve as a reference for total hemoglobin.
Question 2: The Joule-Thomson effect describes the temperature change when a gas expands through a valve. Clinically, this is most relevant when:
- Oxygen from a high-pressure cylinder cools as it passes through the reducing valve (Correct answer)
- Nitrous oxide heats rapidly on emerging from a cylinder
- Soda lime heats up during CO₂ absorption
- Vaporizers warm as carrier gas flows through them
Correct answer: Oxygen from a high-pressure cylinder cools as it passes through the reducing valve
Real gases (including oxygen) cool on isenthalpic expansion through a regulator; this Joule-Thomson cooling can cause frost on cylinder valves during rapid discharge.
Question 3: Which type of spirometry measurement is most useful for detecting early small airways disease in an EDAIC candidate?
- Forced vital capacity (FVC)
- FEV₁/FVC ratio
- Forced expiratory flow between 25% and 75% of FVC (FEF 25–75%) (Correct answer)
- Peak expiratory flow rate (PEFR)
Correct answer: Forced expiratory flow between 25% and 75% of FVC (FEF 25–75%)
FEF 25–75% reflects mid-expiratory flow predominantly from small airways and is the most sensitive spirometric index for early obstructive disease in peripheral airways.
Question 4: In neuromuscular monitoring, post-tetanic count (PTC) is used when:
- TOF ratio exceeds 0.9 and full recovery is confirmed
- Deep block is present and TOF gives zero twitches, allowing estimation of depth via post-tetanic facilitation (Correct answer)
- Single-twitch mode shows 100% response
- Double-burst stimulation reveals residual block
Correct answer: Deep block is present and TOF gives zero twitches, allowing estimation of depth via post-tetanic facilitation
PTC uses 5-second 50 Hz tetanus followed by 1 Hz single twitches; counting facilitated post-tetanic twitches quantifies the degree of intense neuromuscular block when TOF = 0.
Question 5: Infrared absorption spectroscopy in anaesthetic gas analysers is based on the principle that:
- All gases absorb infrared radiation equally and are distinguished by refractive index
- Polyatomic molecules absorb infrared radiation at characteristic wavelengths corresponding to vibrational modes (Correct answer)
- Paramagnetic molecules absorb infrared at wavelengths proportional to their magnetic susceptibility
- Monatomic gases like xenon have the highest infrared absorption
Correct answer: Polyatomic molecules absorb infrared radiation at characteristic wavelengths corresponding to vibrational modes
Heteronuclear polyatomic molecules (CO₂, N₂O, volatile agents) have dipole moment changes during vibration, allowing them to absorb IR at specific wavelengths used for identification and quantification.
Question 6: A fuel cell oxygen analyser generates its own current without an external power source. What electrochemical reaction drives this?
- Oxidation of a gold cathode by oxygen producing electrons measured by a galvanometer
- Oxygen is reduced at a gold cathode in alkaline electrolyte; lead anode is oxidized, generating a current proportional to PO₂ (Correct answer)
- Oxygen oxidizes a platinum membrane, releasing protons detected at an anode
- Hydrogen fuel reacts with oxygen to produce a voltage proportional to temperature
Correct answer: Oxygen is reduced at a gold cathode in alkaline electrolyte; lead anode is oxidized, generating a current proportional to PO₂
In the galvanic (fuel cell) analyser, O₂ is reduced at the gold cathode (O₂ + 2H₂O + 4e⁻ → 4OH⁻) while the lead anode is oxidized, producing a current directly proportional to oxygen partial pressure.
Question 7: When using a manometer to set PEEP during mechanical ventilation, a reading of 5 cmH₂O corresponds to approximately how many mmHg?
- 5 mmHg
- 3.7 mmHg (Correct answer)
- 7.4 mmHg
- 10 mmHg
Correct answer: 3.7 mmHg
1 cmH₂O ≈ 0.735 mmHg; therefore 5 cmH₂O × 0.735 ≈ 3.7 mmHg, reflecting the difference in density between water and mercury.
EDAIC Clinical Measurement and Physics 5
What is the significance of the 'isobestic point' at approximately 805 nm in oximetry?