Clinical Measurement and Physics Flashcards

A patient with severe methemoglobinemia has an arterial blood gas confirming SaO₂ of 58%. What SpO₂ value would a conventional 2-wavelength pulse oximeter most likely display?

Answer: Approximately 85%, irrespective of the true SaO₂

Methemoglobin absorbs nearly equally at both 660 nm (red) and 940 nm (infrared) wavelengths, yielding an absorbance ratio (R) of approximately 1.0. Standard pulse oximeter algorithms interpret R ≈ 1.0 as SpO₂ ≈ 85%. As the MetHb fraction rises, SpO₂ readings converge toward this value regardless of the true SaO₂ — overestimating saturation when it is critically low, and underestimating it when it is near normal. This is fundamentally different from carboxyhemoglobin, which causes false elevation toward normoxic values.

A nitrous oxide cylinder at 22°C reads 52 bar on the pressure gauge. After verified clinical use equivalent to approximately half the cylinder's content by weight, what pressure would the gauge most likely display?

Answer: Approximately 52 bar, essentially unchanged from the initial reading

At 22°C, nitrous oxide is below its critical temperature of 36.5°C, so the cylinder contains a liquid–vapour equilibrium. The gauge pressure reflects the saturated vapour pressure of N₂O (~52 bar at room temperature), which remains constant as long as any liquid phase persists. Pressure only begins to fall when the liquid is nearly exhausted (roughly the last 25% of content). Consequently, the pressure gauge cannot be used to estimate remaining N₂O content — the cylinder must be weighed. This contrasts with oxygen cylinders, where all contents are gaseous at room temperature and pressure is directly proportional to remaining volume.

A fast-flush test of an arterial line transducer system reveals two oscillations before returning to baseline. Knowing the natural frequency is adequate, which damping coefficient (ζ) best characterises the optimal system response for arterial waveform accuracy, and why is it preferred over critical damping (ζ = 1.0)?

Answer: ζ = 0.64, because it yields a maximally flat frequency response with less than 5% overshoot

A damping coefficient of 0.64 (the Butterworth or maximally flat condition) is optimal for arterial pressure transducer systems. It provides a flat frequency response up to the system's natural frequency, limiting overshoot to 1.0) smooths the waveform and narrows pulse pressure. Severe underdamping (ζ < 0.4) causes resonance, exaggerating systolic peaks and producing artefactually wide pulse pressure.

Thermodilution cardiac output measurements are performed via a pulmonary artery catheter in a patient with severe tricuspid regurgitation (regurgitant fraction ~55%). Compared with the true forward cardiac output, thermodilution will most likely:

Answer: Underestimate cardiac output because recirculating cold injectate prolongs the temperature–time curve

The Stewart-Hamilton equation calculates cardiac output as inversely proportional to the area under the blood-temperature–time curve (∫ΔTB dt). With severe tricuspid regurgitation, cold injectate regurgitates back into the right atrium and recirculates, prolonging the thermal deflection at the pulmonary artery thermistor. This enlarges the integral, and because CO is inversely related to this area, the calculated CO is artifactually low — an underestimation of true forward output. The degree of error correlates with regurgitant fraction severity. Tricuspid regurgitation, intracardiac shunts, and respiratory variation are all recognised sources of thermodilution inaccuracy.

In a patient anaesthetised with 68% nitrous oxide, an infrared sidestream capnograph displays ETCO₂ of 6.8 kPa while arterial blood gas confirms PaCO₂ of 5.4 kPa. Beyond the normal a-ETCO₂ gradient, which physical mechanism most specifically accounts for the capnograph's overestimation of CO₂?

Answer: Collision (pressure) broadening of the CO₂ infrared absorption band caused by N₂O molecules

Collision broadening (pressure broadening) occurs when N₂O molecules collide with CO₂ molecules, widening their infrared absorption band and increasing apparent absorbance at the 4.26 µm CO₂ detection wavelength. High N₂O concentrations therefore cause the capnograph to overread ETCO₂. Modern capnographs incorporate correction algorithms when N₂O is detected simultaneously; uncorrected or legacy devices may exhibit clinically significant error. Doppler broadening (caused by thermal molecular motion) is a separate phenomenon not specific to N₂O–CO₂ mixtures. Raman-scatter-based devices handle gas analysis differently and are not susceptible to this particular error.

A patient with a temporary transvenous pacing lead is connected to monitoring equipment in the ICU. Regarding electrical safety, what is the approximate threshold for ventricular fibrillation via microshock — 50/60 Hz alternating current applied directly to the ventricular myocardium?

Answer: 100–180 µA (microamperes), approximately 1000-fold lower than the macroshock threshold

Microshock describes current delivered directly to or near the myocardium, circumventing the skin's protective resistance (typically 1–100 kΩ). As little as 100–180 µA of 50/60 Hz AC current applied directly to the heart can trigger ventricular fibrillation — roughly three orders of magnitude less than the macroshock fibrillation threshold of ~100 mA through intact skin. Patients with intracardiac catheters, transvenous pacing wires, or pulmonary artery catheters are at microshock risk. All equipment in direct cardiac contact must meet IEC Class CF (cardiac float) standards, with leakage current limited to <10 µA. Even touching a poorly earthed piece of equipment while inadvertently contacting an exposed pacing wire terminal can be fatal.