PEC EPE Biomedical cheat sheet

Biopotential electrodes obey the half-cell potential rule: a Ag/AgCl electrode has a stable offset near +0.2 V versus hydrogen; mismatched electrode pairs create a DC offset that must be removed by high-pass filtering before amplification.

The Nernst equation E = E0 - (RT/zF)·ln(Q) governs membrane equilibrium potentials; at 37 °C, the factor RT/F ≈ 26.7 mV, so a tenfold ion gradient yields roughly 61 mV per monovalent ion.

Goldman-Hodgkin-Katz equation sets resting membrane potential from relative permeabilities: Vm = (RT/F)·ln[(PK[K]o + PNa[Na]o + PCl[Cl]i)/(PK[K]i + PNa[Na]i + PCl[Cl]o)].

The Hodgkin-Huxley model represents membrane current as I = Cm·dVm/dt + INa + IK + IL, with voltage-gated conductances gNa and gK described by gating variables m, h, and n.

Instrumentation amplifier CMRR must exceed about 80-100 dB for ECG/EEG because the desired differential signal is millivolts while common-mode interference (50/60 Hz mains) can be volts; the driven-right-leg circuit further reduces common-mode voltage.

Nyquist criterion: sample at more than twice the highest signal frequency (ECG diagnostic bandwidth 0.05-150 Hz, so ≥300 Hz sampling; EEG up to 70 Hz, so ≥200 Hz) and use anti-aliasing filters before the ADC.

Safety current limits (IEC 60601-1): patient leakage current ≤100 µA normal condition for type BF/CF applied parts, ≤10 µA for type CF; the macroshock threshold for ventricular fibrillation is roughly 100 mA externally, but microshock via intracardiac catheters can fibrillate at ~100 µA.

Bioheat equation (Pennes): ρc·∂T/∂t = ∇·(k∇T) + ρb·cb·ωb·(Tb - T) + Qm, where ωb is blood perfusion and Qm metabolic heat; used to model RF ablation and hyperthermia dosing.

Biomechanics fundamentals: stress σ = F/A, strain ε = ΔL/L0, Young's modulus E = σ/ε in the linear elastic region; Hooke's law applies below the yield point, and viscoelastic tissues require creep/relaxation or Kelvin-Voigt/Maxwell models.

MRI safety zones: the static field B0 causes projectile risk; SAR limits (whole-body ~4 W/kg over 6 min) govern RF heating; gradient coils can induce peripheral nerve stimulation and acoustic noise above ~80 dB, so hearing protection is mandatory.

Ultrasound axial resolution equals half the spatial pulse length (SPL = number of cycles × wavelength); higher frequency improves resolution but reduces penetration, so 2-5 MHz suits deep abdominal imaging and 7-15 MHz suits superficial/vascular imaging.

CT dose metrics: CTDIvol (mGy) measures scanner output per rotation, DLP = CTDIvol × scan length (mGy·cm), and effective dose ≈ DLP × k (k ≈ 0.015 mSv/mGy·cm for abdomen). ALARA and justification drive optimization.

X-ray attenuation follows I = I0·e^(-µx); the half-value layer (HVL = ln2/µ) characterizes beam quality, and photoelectric absorption dominates at low energy while Compton scatter dominates at diagnostic energies in soft tissue.

Pacemaker timing: the base rate sets the lower rate limit (LRL), escape interval = 60/LRL seconds; a sensed intrinsic event resets the timing cycle (inhibit mode) and the AV delay governs atrial-to-ventricular pacing in dual-chamber devices.

Defibrillation energy: biphasic waveforms typically deliver 120-200 J (adult) versus 200-360 J monophasic; impedance-compensated waveforms adjust pulse width to deliver consistent current, and transthoracic impedance is roughly 50-100 Ω.

Sterilization decision rules: steam autoclave 121 °C/15 min or 134 °C/3-4 min for heat-stable metal instruments; ethylene oxide or gamma/electron-beam irradiation for heat-sensitive polymers; aseptic processing for biologics.

Regulatory pathway (US): Class I low risk mostly exempt/510(k); Class II requires 510(k) with substantial equivalence to a predicate; Class III (life-sustaining, high risk) requires PMA with clinical evidence; EU MDR uses Class I/IIa/IIb/III with a Notified Body for most classes.

ISO 10993 biocompatibility: tests are selected by contact category (surface device, external communicating, implant) and duration (≤24 h, 24 h-30 d, >30 d); cytotoxicity, sensitization, and irritation are the first-tier screens.