20-Bio-B6 Analytical Biochemistry · May 2013
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
Paper format: National Exams, May 2013 — 04-Bio-B6 Bioinstrumentation. Three hours, open book, non-communicating calculator permitted. Six questions of equal value (25 marks each); four constitute a complete paper and only the first four appearing in the answer book are marked. All six are solved here as a complete study resource. Every question is a design/essay question (block-diagram instrumentation-system design, or descriptive explanation).
Reference texts (the books a candidate should have reviewed for this subject):
Question text not reproduced: the examination questions are © Engineers and Geoscientists BC. Open the official past paper (linked at the top of this page) to read the question, then follow the worked solution below.
Phonocardiogram channel. A piezoelectric or electret-condenser contact transducer on the chest wall (standard auscultation sites) converts heart-sound vibration to an electrical signal. A preamplifier followed by a 20 Hz-1 kHz bandpass filter captures the full physiological range (S1/S2 concentrate energy around 20-150 Hz; murmurs and higher-frequency components extend toward 1 kHz), then an anti-alias filter and ADC sampling at ≥2 kHz (Nyquist for the 1 kHz upper edge) digitise both the amplitude envelope and precise onset/offset timing needed to measure sound duration.
ECG channel. Standard limb/chest electrodes feed a high-CMRR instrumentation amplifier (gain ~1000, bandpass 0.05-100 Hz) with a driven-right-leg circuit for mains rejection; an ADC sampling at ~1 kHz preserves the QRS complex's fast slew for precise timing of electrical events (e.g. the R-wave) against which the heart sounds are correlated.
Arterial blood-pressure channel. The pressure transducer (a Wheatstone-bridge strain gauge) coupled through the fluid-filled catheter/needle assembly to the artery converts arterial pressure into a bridge imbalance; a bridge amplifier and a 0-50 Hz low-pass filter (ample for the arterial waveform's harmonic content, which is concentrated well below 30 Hz) condition the signal before a modest-rate ADC (~200 Hz) digitises it.
Synchronisation and display. All three ADCs share a common sample clock/trigger so the digital processor can measure, for example, the interval from the ECG R-wave to the first heart sound (an isovolumetric-contraction-time-type measurement) and relate the systolic upstroke of the pressure waveform to S1/S2 timing. A multi-trace scrolling display shows all three waveforms on one common time axis, with the measured sound amplitude/duration annotated, and the data are stored for later review.
Mains 60 Hz pickup affects the ECG and, to a lesser extent, the bridge-amplifier outputs; it is removed with the same notch-filter/high-CMRR/driven-right-leg approach discussed in Question 2. Respiration causes a slow baseline modulation of both the phonocardiogram (chest-wall movement changes transducer coupling) and the arterial pressure trace; a high-pass filter and, where necessary, asking the patient to briefly hold their breath during acquisition reduce this. Electrode motion artifact on the ECG is minimised with well-prepared skin and adhesive Ag/AgCl electrodes as in Question 2. Turbulent flow noise or catheter "whip" artifact in the pressure line (fluid-filled catheters can resonate) is controlled by keeping tubing short and stiff and by the 0-50 Hz filter, which is below most resonance-induced ringing frequencies of a properly primed catheter system.
The arterial catheter/needle is an invasive, direct-blood-contact connection, so strict aseptic technique is required, and — critically — because this pathway offers a low-impedance route toward the central circulation, every patient-connected lead (ECG, phono, and especially the pressure transducer's electrical interface) must be electrically isolated from the mains-powered instrument (isolation amplifiers/optical isolation, per Question 5) to guard against microshock. The applied parts are classified as type CF (cardiac floating), which carries the strictest leakage-current limit under IEC 60601; continuous monitoring with alarm limits guards against catheter dislodgement or line disconnection.
