20-Bio-B6 Analytical Biochemistry · May 2014
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
Paper format: National Exams, May 2014 — 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.
Light at two wavelengths — red (~660 nm) and infrared (~940 nm) — is transmitted through a vascular tissue bed (finger, earlobe). By the Beer-Lambert law, transmitted intensity is $I = I_0 e^{-\varepsilon c L}$, where the absorption coefficient $\varepsilon$ depends on wavelength and on which haemoglobin species (oxy- or deoxyhaemoglobin) is present. Deoxyhaemoglobin (Hb) absorbs red light substantially more strongly than oxyhaemoglobin (HbO₂) does — this is exactly why deoxygenated blood looks darker/bluer — while at the infrared wavelength the two species' absorption coefficients are much closer (HbO₂ absorbing slightly more). Because arterial blood pulses with each heartbeat, the light path length through blood (and hence transmitted intensity) has a small pulsatile (AC) component riding on a much larger, steady (DC) component from non-pulsatile tissue, venous blood, bone and skin. Taking the ratio of AC to DC at each wavelength isolates the arterial-blood-only contribution and cancels out the (unknown, wavelength-independent) fixed path length and non-arterial absorbers — the principle of photoplethysmography. The normalised ratio $R = \dfrac{AC_{red}/DC_{red}}{AC_{IR}/DC_{IR}}$ is then compared against an empirically-derived calibration curve (built from co-oximeter reference measurements on volunteers) relating $R$ to arterial oxygen saturation, SpO₂.
An LED driver time-multiplexes the red and infrared LEDs (each on briefly, several hundred Hz to ~1 kHz per wavelength) so a single photodiode detector can be shared between both wavelengths. The photodiode's photocurrent is converted to a voltage by a transimpedance amplifier, then a synchronous demultiplexer (locked to the LED drive timing) splits the signal back into separate red and infrared channels. Each channel is further split into an AC path — a bandpass filter around 0.5-5 Hz matching the pulse-rate range (30-240 bpm) — and a DC path (low-pass filter with automatic gain control, AGC, to normalise the baseline level). All four resulting values ($AC_{red}$, $DC_{red}$, $AC_{IR}$, $DC_{IR}$) are digitised and passed to a microprocessor, which computes the ratio $R$, looks up SpO₂ from the stored calibration curve, derives pulse rate from the periodicity of the AC waveform, and drives a display showing SpO₂, pulse rate, and typically the plethysmographic waveform itself, with configurable low-saturation alarms.
Melanin (in darker skin) and greater tissue thickness both increase the fixed (DC) light absorption without adding useful signal, which reduces the pulsatile AC component's signal-to-noise ratio relative to the baseline. This is accommodated by (1) automatic gain control that increases LED drive current/intensity until the DC signal reaches an adequate level at the photodiode, keeping the AC component within a usable measurement range without saturating the detector; (2) increasing the AC-channel amplifier gain once the baseline has been normalised; and (3) in some designs, using reflectance-mode sensors on more highly perfused, thinner tissue (e.g. the forehead) as an alternative measurement site when transmission-mode signal quality is marginal.
(1) Motion artifact — patient movement changes the optical coupling and effective path length through the tissue, generating spurious signal components that fall in the same low-frequency band as the genuine pulsatile signal and can swamp or mimic it. (2) Ambient light interference — external light sources (particularly other pulsed or flickering sources such as fluorescent or certain surgical lighting) leak into the photodetector and can beat against the LED multiplexing frequency, producing spurious apparent pulsation; this is mitigated by shielding the sensor from ambient light and by synchronous detection referenced precisely to the LED drive timing.
