Clinical Measurement and Physics Flashcards

Which property of a gas is described by the Beer-Lambert law as it applies to pulse oximetry?

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.

In a rotameter (variable-area flowmeter), what keeps the bobbin floating at a fixed height for a given flow rate?

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.

What is the clinical implication of the Hagen-Poiseuille equation when a tracheal tube is halved in internal diameter?

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.

A pressure transducer for invasive blood pressure monitoring is calibrated at heart level. Raising the transducer 10 cm above heart level will:

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.

Which electromagnetic frequency range is used by a standard nerve stimulator to produce a 'train-of-four' stimulus?

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.

In capnography, a sustained elevated baseline (above zero) between breaths most likely indicates:

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.

The critical temperature of nitrous oxide is 36.5°C. What does this mean for a full cylinder at room temperature?

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.