EDAIC Clinical Measurement and Physics 2 — Questions and Answers
Question 1: Which property of a gas is described by the Beer-Lambert law as it applies to pulse oximetry?
- Light absorption is proportional to concentration and path length (Correct answer)
- Light scattering increases with particle size
- Refractive index changes with temperature
- Gas viscosity decreases at higher altitudes
Correct answer: Light absorption is proportional to concentration and path length
Beer-Lambert law states that absorbance is proportional to the product of molar absorptivity, concentration, and path length, forming the basis of oximetry.
Question 2: In a rotameter (variable-area flowmeter), what keeps the bobbin floating at a fixed height for a given flow rate?
- Magnetic levitation by the metal tube
- Balance between gravity and the upward drag force of gas flow (Correct answer)
- Bernoulli effect creating downward suction
- Spring tension calibrated to gas density
Correct answer: Balance between gravity and the upward drag force of gas flow
The bobbin reaches equilibrium when the upward viscous drag and pressure forces from gas flow exactly balance the bobbin's weight.
Question 3: What is the clinical implication of the Hagen-Poiseuille equation when a tracheal tube is halved in internal diameter?
- Resistance increases 16-fold (Correct answer)
- Resistance doubles
- Flow rate doubles
- Resistance decreases by half
Correct answer: Resistance increases 16-fold
Hagen-Poiseuille states resistance is proportional to 1/r⁴; halving the radius (r→r/2) increases resistance by 2⁴ = 16 times.
Question 4: A pressure transducer for invasive blood pressure monitoring is calibrated at heart level. Raising the transducer 10 cm above heart level will:
- Falsely elevate the displayed pressure by ~7.5 mmHg
- Falsely lower the displayed pressure by ~7.5 mmHg (Correct answer)
- Have no effect because the transducer is zeroed
- Double the systolic reading
Correct answer: Falsely lower the displayed pressure by ~7.5 mmHg
Every 13.6 cm of height difference corresponds to ~10 mmHg; raising the transducer 10 cm reduces the hydrostatic column and causes a false low reading of ~7.5 mmHg.
Question 5: Which electromagnetic frequency range is used by a standard nerve stimulator to produce a 'train-of-four' stimulus?
- 2 Hz square-wave pulses delivered four times (Correct answer)
- 50 Hz continuous sine wave
- 0.1 Hz single-twitch mode
- 4 Hz tetanic stimulation
Correct answer: 2 Hz square-wave pulses delivered four times
Train-of-four uses four supramaximal square-wave pulses at 2 Hz (0.5-second intervals), allowing neuromuscular block assessment via T4:T1 ratio.
Question 6: In capnography, a sustained elevated baseline (above zero) between breaths most likely indicates:
- Esophageal intubation
- Rebreathing of CO₂ due to exhausted soda lime or defective expiratory valve (Correct answer)
- Bronchospasm causing gas trapping
- Pulmonary embolism
Correct answer: Rebreathing of CO₂ due to exhausted soda lime or defective expiratory valve
An elevated baseline (>0 mmHg) between inspirations signifies inspired CO₂, classically caused by exhausted absorber or a stuck expiratory valve.
Question 7: The critical temperature of nitrous oxide is 36.5°C. What does this mean for a full cylinder at room temperature?
- The cylinder contains only gas at high pressure
- The contents are in equilibrium between liquid and vapor phases (Correct answer)
- The gas is at risk of combustion above 36.5°C
- The cylinder pressure equals atmospheric pressure
Correct answer: The contents are in equilibrium between liquid and vapor phases
Below its critical temperature, N₂O exists as a liquid-vapor mixture; the cylinder pressure (approximately 50 bar) reflects the saturated vapor pressure, not the amount remaining.
