Pressure Monitoring Compliance — FRCA Primary MCQ

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HardPhysics & Clinical MeasurementPressure Monitoring ComplianceFRCA Primary

In a saline-filled catheter–diaphragm transducer pressure-monitoring system, compliance is defined as ΔV/ΔP. Assuming unchanged materials, wall thickness, temperature and absence of gas bubbles, which single modification increases the total compliance of the system?

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Correct answer: B — Increasing the length of otherwise identical connecting tubing

Explanation lettering: B = shown as A · D = shown as B · E = shown as C · C = shown as D · A = shown as E

D is correct. Compliance is the volume change produced per unit pressure change. Lengthening otherwise identical tubing adds both fluid volume and distensible tubing wall, so a given pressure change produces a larger total volume displacement. Increased compliance reduces the natural frequency and therefore impairs faithful transmission of rapidly changing pressure waveforms. Changing transducer height alters the hydrostatic pressure offset but not intrinsic compliance. Narrower tubing contains less fluid and has less distensible wall volume, although it also increases resistance. A stiffer diaphragm undergoes less displacement for a given pressure and therefore has lower compliance; reducing diaphragm diameter has the same general effect. The original assertion that increased compliance necessarily causes greater damping was removed because damping ratio also depends on system resistance and inertance.

Reference: Pettersson GB, Kronogård CO, Rådmark T. Studies of the dynamic response of esophageal manometry systems. Scandinavian Journal of Gastroenterology. 1986;21(3):305–312. https://www.rcoa.ac.uk/media/49221