EDAIC Clinical Measurement and Physics 3 — Questions and Answers
Question 1: Which principle underlies the function of a vaporizer using the 'plenum' design (e.g., TEC series)?
- Measured-flow injection of liquid agent into the fresh gas stream
- Temperature-compensated splitting of gas flow between a bypass and a vaporizing chamber (Correct answer)
- Desflurane heated to above its boiling point and pressure-regulated
- Direct liquid injection controlled by a microprocessor
Correct answer: Temperature-compensated splitting of gas flow between a bypass and a vaporizing chamber
Variable-bypass vaporizers split fresh gas flow; a thermostat-controlled valve adjusts the fraction passing through the vaporizing chamber to maintain accurate output across temperatures.
Question 2: When using a Wright respirometer to measure tidal volume, which error is most likely?
- Overreading at low flow rates due to inertia
- Underreading at high flow rates due to over-spinning of the vane (Correct answer)
- Accurate measurement regardless of flow rate
- Significant underreading caused by gas humidity
Correct answer: Underreading at high flow rates due to over-spinning of the vane
The Wright respirometer underestimates at high flows because the lightweight vane cannot keep pace, and it overestimates at very low flows due to vane inertia.
Question 3: Paramagnetic oxygen analyzers exploit which property of oxygen?
- O₂ is diamagnetic and repelled by a magnetic field
- O₂ is paramagnetic due to unpaired electrons and is attracted to a magnetic field (Correct answer)
- O₂ absorbs infrared radiation at a specific wavelength
- O₂ has a unique refractive index measurable by interferometry
Correct answer: O₂ is paramagnetic due to unpaired electrons and is attracted to a magnetic field
Oxygen has two unpaired electrons making it paramagnetic; it is drawn into a magnetic field, allowing concentration measurement by displacement of a nitrogen-filled dumbbell.
Question 4: A damped arterial line waveform (overdamped system) will produce which measurement error?
- Falsely high systolic and falsely low diastolic pressure
- Falsely low systolic and falsely high diastolic pressure, with accurate mean (Correct answer)
- Falsely elevated mean arterial pressure
- No error in systolic but significant error in diastolic pressure
Correct answer: Falsely low systolic and falsely high diastolic pressure, with accurate mean
Overdamping attenuates the waveform peaks, causing underestimation of systolic and overestimation of diastolic pressure, while mean arterial pressure remains relatively accurate.
Question 5: Which gas law explains why a patient's tidal volume may decrease when measured at body temperature compared to the ventilator setting calibrated at room temperature?
- Boyle's law — pressure-volume relationship at constant temperature
- Charles's law — volume increases proportionally with absolute temperature (Correct answer)
- Dalton's law — partial pressures are additive
- Henry's law — gas solubility changes with pressure
Correct answer: Charles's law — volume increases proportionally with absolute temperature
Charles's law (V∝T at constant P) means that gas warmed from ~20°C to 37°C expands, so the delivered volume at body temperature is larger than at room temperature, not smaller — measuring at body temp vs. BTPS vs. ATPS must be reconciled.
Question 6: In the context of electrical safety in the operating room, 'microshock' refers to:
- High-voltage electrostatic discharge from surgical drapes
- Ventricular fibrillation induced by as little as 10–50 µA applied directly to the myocardium (Correct answer)
- Burns caused by faulty diathermy grounding pads
- Patient movement artifact on the ECG monitor
Correct answer: Ventricular fibrillation induced by as little as 10–50 µA applied directly to the myocardium
Microshock (10–50 µA) applied directly to the heart (e.g., via a central line or pacing wire) can cause VF, whereas currents >10 mA are required for macroshock via intact skin.
Question 7: What does a resonant frequency of 12 Hz in an invasive pressure monitoring system indicate?
- The system is critically damped and will accurately reproduce all physiological waveforms
- The system may resonate and distort frequencies approaching 12 Hz, risking inaccuracy at high heart rates (Correct answer)
- The transducer requires recalibration at that frequency
- The system is safe because physiological signals rarely exceed 12 Hz
Correct answer: The system may resonate and distort frequencies approaching 12 Hz, risking inaccuracy at high heart rates
Physiological waveforms contain harmonics up to ~10 Hz; a natural frequency of 12 Hz means resonance could amplify or distort these harmonics, causing inaccurate pressure readings.
EDAIC Clinical Measurement and Physics 3
Which principle underlies the function of a vaporizer using the 'plenum' design (e.g., TEC series)?