04-Bio-A8 · May 2015
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
Paper format: National Exams, May 2015 — 04-Bio-A8 Biophysical Measurements. Three hours, open book, any non-communicating calculator. Seven questions of equal value (20 marks each); five constitute a complete paper and only the first five appearing in the answer book are marked. All seven are solved here, because this set is a study resource rather than an examination script. Every question is qualitative/descriptive — there is no numerical data to compute — so each answer follows flowing prose with instrumentation block diagrams where the question explicitly asks for one. Questions 2 and 4 cover nerve-fibre stimulate/record instrumentation and the Einthoven-triangle/Lead I ECG.
Reference texts (the books an open-book candidate should have on the desk 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.
Einthoven's triangle models the heart as a single, fixed equivalent current dipole located at the geometric centre of an (approximately) equilateral triangle whose three vertices are the right arm (RA), left arm (LA) and left leg (LL) electrode sites. The model rests on three simplifying assumptions: the torso is a homogeneous (uniform-resistivity) volume conductor with the heart at its electrical centre, so the potential at any surface point depends only on the instantaneous cardiac dipole and its distance/orientation, not on local tissue inhomogeneities; the limbs act as passive conducting extensions that carry the shoulder/hip surface potentials out to the standard wrist/ankle electrode sites without measurable further attenuation or distortion; and the three triangle sides are equal in length, so each of the three limb leads is an equally-weighted projection of the same instantaneous cardiac vector onto a different frontal-plane axis — only their orientation differs.
Lead I is defined as the potential at LA minus the potential at RA, i.e. a bipolar recording along the horizontal top edge of the triangle with LA as the positive terminal; it therefore records the component of the instantaneous cardiac dipole vector $M(t)$ projected onto this horizontal (0°) axis. Relating the trace to the cardiac cycle: the small initial P wave is atrial depolarization, whose net vector during normal sinus rhythm points generally down and to the patient's left, giving a small positive deflection on Lead I. The QRS complex is ventricular depolarization: an initial small deflection can occur as the interventricular septum depolarizes first (its vector is comparatively small and variably oriented, so it may appear as a modest Q or be absent on Lead I), followed by the large, dominant R wave as the much larger left-ventricular free-wall depolarization wavefront sweeps down and to the left — strongly aligned with, and therefore projecting heavily onto, the positive (LA) end of the Lead I axis — and a small terminal S wave from the last-depolarizing basal/outflow-tract regions, whose vector points away from LA. Finally the T wave reflects ventricular repolarization; although the ventricular repolarization wave travels in the opposite direction to depolarization (epicardium-to-endocardium, because epicardial action potentials are shorter), a repolarization wavefront carries the opposite polarity of charge, so its net dipole points the same way as the depolarization vector and the T wave is normally concordant with (same polarity as, i.e. upright like) the QRS complex on Lead I.
Ag/AgCl surface electrodes with conductive gel are placed at the RA, LA and LL sites (a fourth, right-leg electrode is normally added purely as a driven reference/ground to further suppress common-mode interference, though it carries no signal of its own). The lead selector is simply a differencing network that forms whichever bipolar combination is required — Lead I as LA minus RA, in this case — from the three raw electrode potentials. Because the ECG is only about 1 mV in amplitude while 60 Hz mains and motion-artifact interference can be far larger, the instrumentation amplifier must combine high input impedance (so as not to load the skin–electrode interface) with a very high common-mode rejection ratio, amplifying the small differential (LA−RA) signal while rejecting whatever near-identical interference both electrodes pick up together. An isolation amplifier follows, galvanically separating the patient-connected front end from the rest of the instrument's mains-powered circuitry — a mandatory patient-safety measure (IEC 60601) that limits any fault leakage current that could otherwise flow through the electrodes into the patient. A bandpass filter with a 60 Hz notch then restricts the signal to the diagnostic ECG bandwidth (roughly 0.05–100/150 Hz, wide enough to preserve the ST-segment baseline and the QRS's fast slew without clinically significant distortion) and removes any residual mains pickup. Finally an ADC digitizes the filtered lead signal for a display/recorder — a real-time trace, a printed strip, or digital storage for later review.
A bedside cardiac monitor continuously displaying Lead I (or, more often, several simultaneously derived limb and augmented leads from the same three or four electrodes) lets clinical staff watch heart rate and rhythm in real time, while the isolation amplifier stage is exactly what makes it safe to also connect the same patient to other mains-powered equipment (an infusion pump, a defibrillator) without creating a shock hazard through the ECG leads.
