04-Bio-A8 · May 2014
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
Paper format: National Exams, May 2014 — 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.
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.
The single-electrode demand/inhibit pacemaker — the VVI mode in the standard pacemaker code (Ventricle paced, Ventricle sensed, Inhibited response) — uses one lead placed in the right ventricle that both senses the patient's intrinsic ventricular depolarization (the R-wave) and delivers the pacing stimulus through the same electrode. It is "synchronous" in the sense that it synchronizes its behaviour to the patient's own rhythm: on every cycle the device first listens for an intrinsic R-wave; if one is detected within the programmed escape interval, the pacemaker inhibits (withholds) its own output and simply restarts its timing window from that beat. Only if no intrinsic beat occurs before the escape interval expires does the device fire a pacing pulse. The device therefore paces on demand — only when the heart fails to beat on its own — rather than pacing at a fixed rate regardless of the patient's underlying rhythm (the older, now obsolete, fixed-rate/asynchronous mode).
The demand/inhibit design is the most widely used pacemaker mode because it is both the simplest implantable configuration (a single lead, a single chamber) and the safest: an asynchronous, fixed-rate pacemaker can deliver a pacing pulse during the vulnerable period of a spontaneous beat's T-wave (the "R-on-T" phenomenon), which can trigger ventricular fibrillation, whereas the demand/inhibit device avoids competitive pacing entirely because it never fires when the patient's own conduction is adequate. It is also economical to implant and program, imposes the lowest surgical and lead-related risk of any pacing configuration, and is fully adequate for the majority of patients whose indication is intermittent or complete heart block rather than a need for atrioventricular synchrony, which is why VVI (and its atrial equivalent, AAI) remain the workhorse single-chamber modes despite the availability of more complex dual-chamber devices.
The sense amplifier is a high-gain, band-limited differential amplifier that picks up the intracardiac electrogram from the lead tip, filtered to pass the R-wave's characteristic frequency content (roughly 10–30 Hz) while rejecting the much larger, slower T-wave and any afterpotential left by a preceding pacing pulse. The R-wave detector / refractory-logic block compares the amplified signal to a programmable sensing threshold and declares an intrinsic beat only once that threshold is crossed outside a blanking/refractory window that follows each sensed or paced event, which prevents the device from re-triggering on the tail of the same beat or on a paced afterpotential. The timing and inhibit/demand logic is the state machine at the heart of the device: it runs an escape-interval timer (the reciprocal of the programmed base pacing rate) that is reset to zero every time a genuine intrinsic beat is sensed; if the timer reaches its limit with no sensed beat, it issues a pace command. The pulse generator produces a constant-current (or constant-voltage) stimulus of programmed amplitude and pulse width, energized from the device's lithium battery, sized to reliably capture the myocardium with a safety margin above the measured pacing threshold while conserving battery life. Finally, the output circuit and lead/electrode interface couples that pulse to the myocardium through the same lead used for sensing, via a blocking capacitor that prevents any net DC current (which would cause electrode corrosion and tissue damage) from flowing into the heart.
Once implanted, the pacemaker's parameters are not fixed: a programmer console communicates with the device non-invasively by radio-frequency (or, in older devices, inductive) telemetry through the skin, reading the device's diagnostic data and writing new settings into its memory. Parameters routinely adjusted this way include the base (escape-interval) pacing rate, the pacing pulse amplitude and pulse width (adjusted just above the measured capture threshold, with a safety margin, to conserve battery life), the sensing threshold and sensitivity (to reliably detect true R-waves while rejecting noise or muscle artifact), the refractory/blanking period duration, and, in rate-responsive devices, the activity-sensor response curve. The clinician follows up periodically, measuring pacing and sensing thresholds and lead impedance non-invasively through the programmer, and re-optimizes these parameters as the patient's physiology, lead maturation (fibrosis raises the capture threshold over the first weeks after implant), or clinical needs change — all without any further surgery.
A patient with intermittent complete heart block receives a single right-ventricular VVI lead; at implant the surgeon measures a 0.5 V capture threshold and programs the output to 2.5 V/0.4 ms for a 5× safety margin, then at three-month follow-up the threshold has risen to 0.8 V (lead maturation), so the programmer is used non-invasively to confirm continued adequate margin without reoperating.
