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.
Macroshock is current entering the body externally, through intact skin, from contact with an energised conductor. Skin's relatively high impedance limits the current that reaches internal tissue, so macroshock hazard thresholds are in the milliampere range: roughly 1 mA for perception, tens of mA for "let-go" and muscular tetanus, and above about 100 mA current through the chest can induce ventricular fibrillation. Microshock is current delivered directly to, or very close to, the heart via a low-impedance conductive path that bypasses skin resistance entirely — a saline-filled catheter, an intracardiac pacing wire, or any electrode with direct or near-direct electrical contact with cardiac tissue. Because the protective skin impedance is bypassed and the current density at the myocardium is far higher for the same total current, currents as small as 10-20 µA applied directly to the heart can induce fibrillation — roughly three to four orders of magnitude smaller than the macroshock fibrillation threshold.
Hospitalised or instrumented subjects frequently have multiple invasive or semi-invasive connections — ECG electrodes, indwelling catheters, pacing wires, IV lines with conductive fluid — that create low-impedance electrical pathways toward or into the heart, removing the skin's normal protective resistance and converting what would be a harmless macroshock-level leakage current elsewhere on the body into a microshock hazard at the heart. Patients may also be anesthetised, sedated, unconscious, or otherwise unable to sense or react to (withdraw from) a shock before injury occurs, and may be physiologically compromised (e.g. cardiac patients) and thus more vulnerable at lower current levels. Finally, multiple pieces of electrical equipment are often connected to the same patient simultaneously, multiplying the number of possible leakage-current or ground-fault paths that could find a route to the heart.
The core reference is the IEC 60601 series (Medical Electrical Equipment — General Requirements for Basic Safety and Essential Performance), adopted in Canada as CSA C22.2 No. 601 and its collateral/particular standards (e.g. 60601-1-2 for electromagnetic compatibility); in the US, equipment is also assessed against AAMI/ANSI and UL 60601-1 requirements. These standards define maximum allowable leakage currents according to the equipment's applied-part classification — type B (body, no direct cardiac contact), type BF (body floating, isolated patient connection), and type CF (cardiac floating) — with CF applied parts (direct cardiac contact) subject to the strictest limits, typically on the order of 10 µA, consistent with the microshock threshold in (i).
Three complementary isolation approaches are used, each providing a barrier that prevents a direct galvanic (conductive) path between the patient-connected circuit and mains-referenced circuitry:
| Method | Principle | Advantage | Disadvantage |
|---|---|---|---|
| Transformer isolation | Magnetic coupling of the power supply across a galvanic break; no conductive path from mains to the patient-side supply | Well-established, can pass moderate power, simple | Bulky/heavy, limited bandwidth — suited to isolating supply power, not high-fidelity analog signals |
| Optical isolation (optocouplers) | Signal is converted to light by an LED and recovered by a phototransistor/photodiode across a non-conductive gap | Compact, very high isolation (breakdown) voltage, good bandwidth for digital or PWM-encoded signals | Direct analog optocoupling is nonlinear and drifts with LED aging/temperature (mitigated by converting the signal to digital/PWM before crossing the barrier, or using a linearised optocoupler pair) |
| Isolation amplifiers | The patient-side analog signal modulates a carrier (amplitude or pulse-width modulation), which crosses the barrier via a small transformer or capacitor and is demodulated on the far side | Preserves analog signal fidelity while providing a true galvanic isolation barrier — the standard building block at the very front end of ECG/EEG/EMG amplifiers | More complex circuitry; the modulation/demodulation process adds some noise and nonlinearity, and cost is higher than a simple optocoupler |
