PEC EPE Biomedical common mistakes
PEC EPE Biomedical · Understand the traps and how to avoid them.
Treating the whole body as a single homogeneous conductor when computing current density or impedance, so that the calculated value ignores the different conductivities of skin, fat, muscle, and bone in series or parallel.
Candidates learn Ohm's law in a single-resistor model and carry it over unchanged; the layered anatomy is not visualized, and the exam's phrasing ('a current path through the arm') invites a single-value shortcut.
What to do instead
Before computing, sketch the path and label each tissue layer with its approximate conductivity, then decide whether the layers are in series (current passes through each) or in parallel (current splits). State the assumed model explicitly in the answer; examiners award the reasoning even when the numeric result differs slightly.
Confusing the sensing and reference electrodes of a biopotential amplifier, or omitting the reference electrode entirely when drawing a measurement circuit.
Textbook diagrams are often simplified to two terminals, and the role of the reference (often a driven-right-leg or common reference) is glossed over during revision.
What to do instead
For every biopotential (ECG, EEG, EMG), draw three electrodes and label signal, reference, and ground, then explain what the reference does for common-mode rejection. Practice redrawing the full instrumentation-amplifier front end from memory until the reference path is automatic.
Applying the standard electrical-safety current thresholds (e.g., let-go current, ventricular-fibrillation threshold) to a patient-contact (type CF/BF) scenario without adjusting for the low-impedance intracardiac or direct-contact path.
Candidates memorize a single set of macroshock numbers and do not distinguish macroshock from microshock, so the much lower microshock threshold is missed.
What to do instead
Maintain a two-column table: macroshock (external skin contact) versus microshock (intracardiac or catheter contact), with the relevant current limits and the reason for the difference. When a question mentions a catheter, pacemaker lead, or conductive fluid path to the heart, default to the microshock column.
Describing a medical device standard by name (e.g., 'it complies with IEC 60601') without stating which collateral or particular standard applies and what specific test or clause is involved.
Standards are studied as a list of numbers to memorize rather than as a hierarchy of general, collateral, and particular requirements, so answers stay at the label level.
What to do instead
Build a hierarchy map: the general standard, the collateral standards for particular hazards (e.g., electromagnetic compatibility, usability), and the device-specific particular standard. For each, note one concrete test it mandates (e.g., leakage-current measurement, immunity level). Answer questions at the level of the specific clause and test, not the family name.
Computing signal-to-noise ratio or amplifier gain without accounting for the electrode–skin half-cell potential and its drift, then concluding the design meets specification.
Half-cell offset is treated as a minor detail, and candidates focus on the desired biopotential amplitude alone; the DC offset saturating the amplifier is overlooked.
What to do instead
In every amplifier design question, first list all DC and low-frequency sources (half-cell potential, polarization, motion artifact), then check that the gain and input range keep the amplifier out of saturation, and specify the high-pass corner needed. Show the offset budget as a short table of contributions.
Treating the sampling rate as sufficient because it exceeds twice the highest frequency of interest, while ignoring the higher-frequency interference and the need for an anti-aliasing filter ahead of the ADC.
The Nyquist rule is memorized as a single inequality; the practical requirement for an analog low-pass filter before sampling is forgotten, and interference above the band is assumed absent.
What to do instead
For any digitization question, state three things: the band of interest, the chosen sampling rate, and the anti-aliasing filter cutoff and order placed before the ADC. Explain what alias would appear if the filter were removed, using a concrete frequency example.
Mixing up the hemodynamic quantities in the Windkessel or compliance model, for example substituting arterial compliance for peripheral resistance or confusing flow with velocity when applying the continuity equation.
The variables share units and symbols across formulas, and candidates memorize equations without tracking what each symbol physically represents in the vascular bed.
What to do instead
Annotate each equation with units and a one-line physical meaning for every symbol, then work one numeric example where a change in compliance versus resistance produces different pressure-wave shapes. Check dimensional consistency before submitting any calculation.
Answering a biomaterials question about biocompatibility by listing material properties (strength, corrosion resistance) instead of addressing the host response: protein adsorption, inflammation, fibrous encapsulation, and degradation products.
Candidates transfer mechanical-engineering habits into the biological domain and equate 'suitable material' with 'strong and inert', missing the biological performance criteria.
What to do instead
For every implant-material question, structure the answer in three stages: initial protein and cell interaction, acute and chronic inflammatory response, and long-term fate (encapsulation or degradation). Then mention mechanical properties only as a secondary constraint. This structure also earns partial credit when the specific material is unknown.
Describing an imaging modality's resolution or contrast by a single number without specifying which resolution (spatial, temporal, contrast) and under what conditions it was measured.
Revision summaries compress modality comparisons into one 'best resolution' figure, and the exam's comparative phrasing rewards a precise, multi-axis comparison that candidates have not practiced.
What to do instead
Prepare a comparison grid with rows for spatial resolution, temporal resolution, contrast mechanism, and ionizing-radiation dose, and columns for each modality (ultrasound, MRI, CT, X-ray, nuclear). When answering, name the axis being compared and the trade-off involved rather than quoting one figure.
Presenting a device design or clinical-engineering solution without stating the hazard analysis, the applicable risk-control hierarchy, and the verification method, so the answer reads as a description rather than an engineering justification.
Candidates answer the 'what does it do' part of the question and skip the 'how do you know it is safe and effective' part, because risk management is studied as a separate topic rather than as the frame for every design answer.
What to do instead
Adopt a fixed answer skeleton for design questions: intended use, identified hazards, risk controls in order (inherent safety, protective measures, information for use), and verification/validation evidence. Practise condensing this skeleton into three or four sentences so it can be appended to any design answer within the time limit.