Clinical Measurement and Physics Flashcards

An invasive arterial line monitoring system has a measured natural frequency of 8 Hz and a damping coefficient of 0.2. Which of the following best describes the expected effect on the displayed arterial waveform?

Answer: Systolic pressure overshoot with exaggerated systolic peak and a sub-dicrotic undershoot artefact

A natural frequency of 8 Hz is dangerously low — harmonics of a heart rate at 120 bpm extend to ~12 Hz, meaning resonance with the system occurs at physiological frequencies. A damping coefficient of 0.2 is severely underdamped (optimal is 0.64–0.7). The combined effect is resonance amplification of pressure oscillations: systolic pressure is artificially elevated (overshoot) and is followed by a rebound undershoot below the dicrotic notch. This pattern leads to clinically significant overestimation of systolic and underestimation of diastolic pressure.

A patient presents with suspected smoke inhalation. Pulse oximetry reads SpO2 = 85% consistently, regardless of supplemental oxygen therapy. Co-oximetry reveals carboxyhaemoglobin (COHb) of 8% and methaemoglobin (MetHb) of 35%. Which phenomenon best explains the fixed SpO2 reading of approximately 85%?

Answer: MetHb absorbs light equally at both 660 nm and 940 nm wavelengths, producing an R-ratio of approximately 1.0, which corresponds to ~85% SpO2 on standard calibration curves

MetHb absorbs red (660 nm) and infrared (940 nm) light in almost equal proportions, giving an absorbance ratio (R) close to 1.0. Standard pulse oximeter calibration curves assign an SpO2 of approximately 85% to an R-value of 1.0. Consequently, as MetHb rises above ~30–35%, the SpO2 reading is 'pulled' toward 85% irrespective of true arterial oxygen saturation — neither increasing FiO2 nor worsening hypoxia will shift it away from this value. COHb, by contrast, absorbs strongly at 660 nm and is misread as oxyhaemoglobin, pushing SpO2 falsely high.

A patient with severe tricuspid regurgitation undergoes thermodilution cardiac output (CO) measurement via a pulmonary artery catheter. Compared to the true cardiac output, the thermodilution result is most likely to be:

Answer: Overestimated, because a portion of the cold injectate recirculates into the right atrium and is rewarmed before reaching the pulmonary artery thermistor

By the Stewart-Hamilton equation, CO is inversely proportional to the area under the thermodilution temperature-time curve. In severe TR, a fraction of the cold indicator is regurgitated back into the right atrium during RV systole, where it is partially rewarmed by incoming blood before re-entering the right ventricle. The thermistor consequently detects a smaller temperature change than was actually injected, yielding a smaller area under the curve. Since CO ∝ 1/area, this produces overestimation of the true cardiac output — a clinically important error in the haemodynamic monitoring of TR patients.

An ultrasound beam travelling through soft tissue (acoustic impedance Z₁ = 1.63 MRayl) encounters a cortical bone interface (Z₂ = 7.8 MRayl). Using the intensity reflection coefficient formula R = [(Z₂ − Z₁)/(Z₂ + Z₁)]², what percentage of the incident ultrasound intensity is reflected at this interface?

Answer: Approximately 43%

Applying the formula: R = [(7.8 − 1.63) / (7.8 + 1.63)]² = [6.17 / 9.43]² = (0.654)² ≈ 0.428, or approximately 43% of incident intensity reflected. This explains why cortical bone is nearly impenetrable by diagnostic ultrasound — less than 60% of energy is transmitted forward, and that transmitted fraction is then attenuated further by the dense bone matrix. This is clinically relevant when attempting ultrasound-guided regional anaesthesia near bony structures (e.g. paravertebral or sacral nerve blocks).

Entonox cylinders (50% N₂O / 50% O₂) stored in cold environments are at risk of delivering a dangerously hypoxic gas mixture. This occurs when the cylinder cools below the pseudocritical temperature of the mixture, causing N₂O to liquefy and separate from the gas phase. Below approximately what temperature does this occur?

Answer: −6°C

The pseudocritical temperature of the Entonox mixture (50:50 N₂O/O₂) is approximately −5.5°C to −6°C. Below this temperature, N₂O liquefies and settles to the bottom of the cylinder. When the cylinder is subsequently opened, the gas phase (now enriched in O₂) is delivered first; as the liquid N₂O evaporates later, the mixture becomes N₂O-rich and potentially hypoxic. To prevent this, cylinders should be stored above 10°C and, if frozen, re-warmed and inverted several times before use to re-homogenise the mixture. The pure critical temperature of N₂O alone is 36.4°C — the mixture's pseudocritical value is far lower due to the presence of O₂.

During transthoracic echocardiography, continuous-wave Doppler measures an aortic jet velocity of 2.0 m/s. Post-acquisition review reveals that the Doppler beam was aligned at 30° to the true direction of blood flow rather than the assumed 0°. Using the Doppler equation (V_measured = V_true × cos θ), what is the true aortic jet velocity?

Answer: 2.31 m/s

The Doppler equation states V_measured = V_true × cos θ. Rearranging: V_true = V_measured / cos θ = 2.0 / cos 30° = 2.0 / 0.866 ≈ 2.31 m/s. Beam misalignment always causes underestimation of true velocity (since cos θ 0°). At 30°, the underestimation is ~13%. At 45°, it exceeds 29%. This has direct clinical consequences: an aortic jet velocity of 2.31 m/s yields a peak gradient of 4 × 2.31² ≈ 21 mmHg (modified Bernoulli), compared to only 16 mmHg if the 2.0 m/s value is used uncorrected — a difference that may affect grading of aortic stenosis severity. Guidelines recommend keeping the Doppler angle < 20° (cos 20° = 0.94, error < 6%).