EDAIC Clinical Measurement and Physics 4 — Questions and Answers
Question 1: Fick's law of diffusion states that the rate of gas transfer across a membrane is directly proportional to:
- Membrane thickness and gas solubility only
- Surface area, partial pressure gradient, and solubility; and inversely proportional to membrane thickness and square root of molecular weight (Correct answer)
- Molecular weight and temperature only
- Viscosity of the gas and partial pressure
Correct answer: Surface area, partial pressure gradient, and solubility; and inversely proportional to membrane thickness and square root of molecular weight
Fick's law: flux = (A × D × ΔP) / T, where D ∝ solubility/√MW; thicker membranes and higher molecular weight reduce transfer, while larger area and pressure gradient increase it.
Question 2: In pulse oximetry, which two wavelengths of light are used to distinguish oxyhemoglobin from deoxyhemoglobin?
- 590 nm (yellow) and 700 nm (red)
- 660 nm (red) and 940 nm (infrared) (Correct answer)
- 500 nm (green) and 800 nm (near-infrared)
- 760 nm (red) and 1000 nm (infrared)
Correct answer: 660 nm (red) and 940 nm (infrared)
Pulse oximeters use 660 nm red light (absorbed more by deoxyhemoglobin) and 940 nm infrared light (absorbed more by oxyhemoglobin) to calculate the ratio R.
Question 3: A soda lime canister turns violet/purple. This colour change indicates:
- The canister is at optimal capacity and functioning well
- The pH indicator dye has responded to CO₂ absorption capacity exhaustion (Correct answer)
- Water condensation inside the canister
- Contamination with nitrous oxide
Correct answer: The pH indicator dye has responded to CO₂ absorption capacity exhaustion
Ethyl violet indicator in soda lime turns purple when the granules are exhausted and the pH drops, signaling that CO₂ absorption is impaired.
Question 4: When calibrating a Wheatstone bridge circuit used in a pressure transducer, a known resistance is substituted to simulate a known pressure. This technique is called:
- Zero-point calibration
- Electronic calibration (cal factor substitution) (Correct answer)
- Gain normalization
- Span adjustment via mercury manometer
Correct answer: Electronic calibration (cal factor substitution)
Electronic calibration substitutes a precision resistor into the bridge to simulate a defined pressure, allowing the displayed value to be verified without a fluid column.
Question 5: The Severinghaus electrode measures which blood gas parameter?
- PO₂ via a platinum cathode covered by a gas-permeable membrane
- PCO₂ via a pH electrode surrounded by a bicarbonate solution and a CO₂-permeable membrane (Correct answer)
- pH via a glass electrode
- Lactate concentration via an enzymatic reaction
Correct answer: PCO₂ via a pH electrode surrounded by a bicarbonate solution and a CO₂-permeable membrane
The Severinghaus electrode uses a CO₂-permeable membrane; dissolved CO₂ equilibrates with bicarbonate solution whose pH change is sensed by an inner glass pH electrode.
Question 6: Adiabatic compression of oxygen in a cylinder regulator can cause ignition of lubricants. This phenomenon is governed by which principle?
- Boyle's law — volume decreases with pressure at constant temperature
- The adiabatic heating law — rapid compression without heat loss raises gas temperature dramatically (Correct answer)
- Joule-Thomson effect — gas cools on expansion
- Dalton's law — partial pressure of oxygen rises
Correct answer: The adiabatic heating law — rapid compression without heat loss raises gas temperature dramatically
When oxygen is rapidly compressed adiabatically, all the work done on the gas converts to internal energy (heat), potentially raising temperature high enough to ignite contaminants.
Question 7: In a circle breathing system, the purpose of the unidirectional (one-way) valves is to:
- Reduce resistance to breathing
- Ensure gas flows in one direction, separating fresh gas delivery from exhaled gas (Correct answer)
- Control the concentration of volatile agent
- Prevent back-pressure in the flowmeters
Correct answer: Ensure gas flows in one direction, separating fresh gas delivery from exhaled gas
Unidirectional valves create a one-way circuit so exhaled gas passes through the CO₂ absorber before returning to the patient, preventing rebreathing of CO₂.
EDAIC Clinical Measurement and Physics 4
Fick's law of diffusion states that the rate of gas transfer across a membrane is directly proportional to: