20-Bio-B6 Analytical Biochemistry · December 2019
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
Paper format: National Exams, December 2019 — 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).
Q3(iv)'s marks belong to Q4(i)'s 12-mark opening sub-part, not to Q3; Q5(ii) and (iii) each carry their own 5 marks rather than a combined total; Q6(ii) covers the instrumentation for the whole ICU bedside monitor, not the pulse oximeter alone.
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.
Heart sounds (S1: mitral/tricuspid valve closure at the start of systole; S2: aortic/pulmonic valve closure at the start of diastole, plus any murmurs from turbulent flow) are captured with a phonocardiogram (PCG) sensor — a piezoelectric or electret contact microphone placed at a standard auscultation site (apex, or the tricuspid/pulmonic/aortic areas) — over a broadband 20 Hz-2 kHz range. Amplitude is measured by envelope-detecting the rectified, low-pass-filtered PCG signal; duration of each S1/S2 complex is measured by threshold-crossing the envelope (time above a fraction of its peak). A simultaneous ECG (lead II) gives the electrical reference: the R-wave marks the start of ventricular depolarisation, so the R-wave-to-S1 interval is the isovolumic contraction time and S2's timing marks end-systole, letting cardiac sounds be correlated directly against the electrical cycle. Arterial blood pressure is estimated non-invasively from PPG using pulse transit time (PTT): with a PPG sensor placed peripherally (e.g. finger or toe) and using the ECG R-wave as the proximal timing reference, the pulse arrival time (PAT) — the interval from the R-wave to a defined point on the PPG pulse foot or maximum upslope — is measured on every beat. By the Moens-Korteweg relation for wave speed in an elastic tube, $PWV=\sqrt{Eh/(\rho d)}$, a rise in arterial pressure stiffens the vessel wall (increases the effective elastic modulus $E$), increasing pulse wave velocity and therefore shortening the transit time; a subject-specific calibration against a reference cuff measurement converts PAT into a continuous, beat-by-beat systolic/diastolic blood-pressure estimate. Because this is an indirect surrogate (not a direct pressure reading, unlike a catheter transducer), the design includes periodic cuff recalibration to correct for drift in arterial compliance over time.
The PCG contact microphone, ECG electrodes and two PPG probes (a proximal site near the heart, e.g. ear or finger, and optionally a distal site, e.g. toe, for a longer, more sensitive path length) each feed a dedicated front-end amplifier with a bandwidth matched to its signal (20 Hz-2 kHz for PCG; 0.5-150 Hz for ECG; 0.5-5 Hz AC-coupled for the PPG pulse waveform, per Question 6). All channels are digitised on a common, synchronously-clocked multi-channel ADC (at least 4 kHz for the PCG channel to resolve S1/S2 timing to better than 1 ms; 500 Hz-1 kHz for ECG and PPG) so that timing differences between channels are measurement, not sampling-clock, artifacts. A digital signal processor then: envelope-detects and thresholds the PCG channel for S1/S2 amplitude and duration; matched-filters the ECG for QRS/R-wave detection; computes PAT as the R-wave-to-PPG-foot interval and converts it to a blood-pressure estimate via the calibrated PTT relation; and finally drives a display that overlays the PCG envelope, the ECG waveform (with R-wave markers) and the beat-by-beat blood-pressure trend on a common time axis, so a clinician can see cardiac-sound timing, electrical timing and pressure together.
(1) Ambient acoustic/respiratory noise on the PCG channel — breath sounds and room noise overlap the low end of the heart-sound spectrum; a bandpass filter tuned to 20-200 Hz (where S1/S2 energy is concentrated) rejects most of it, and the recording protocol asks the patient to breathe normally rather than deeply (per the question's own "during normal breathing"), with an optional brief breath-hold for the cleanest reference recording. (2) Motion artifact on the PPG channels — limb or finger movement changes the optical coupling and mimics or swamps the genuine pulsatile signal (the same mechanism as pulse-oximetry motion artifact); an accelerometer-based artifact flag and a signal-quality index reject corrupted beats from the PTT calculation. (3) Muscle (EMG)/movement artifact on the ECG — suppressed by the standard ECG bandpass/notch filtering and by excluding any beat whose R-wave detection coincides with a flagged movement event on either the ECG or PPG channel, so a single contaminated beat cannot corrupt the PAT-to-blood-pressure estimate.
Every patient-connected lead (PCG contact sensor, ECG electrodes, PPG probes) must be treated as at minimum a type-BF applied part and driven through an isolated front end (Question 5), since the ECG connection in particular offers a direct path toward the heart. The PPG LED optical power must be kept within the photobiological safety limits of IEC 60601-2 for skin/tissue heating and eye safety. Because the PTT-based blood-pressure estimate is an indirect surrogate, a clinical-safety issue exists distinct from the electrical one: without periodic cuff recalibration the displayed pressure can drift and mislead clinical decisions, so the design must flag a stale calibration rather than silently continuing to display an uncalibrated trend. Finally, sensor attachment must be secure and comfortable for continuous ICU/ward use (low-irritation adhesives for the PPG probe, strain relief on all cables) to avoid skin injury during prolonged monitoring.
