Medical Knowledge: Mechanisms, Tracings, and Device Physiology
Core concepts
This is the mechanistic heart of the exam. You must interpret the surface ECG and intracardiac electrograms, understand reentry versus automaticity versus triggered activity, and localize accessory pathways and focal arrhythmias. Know the pharmacology of antiarrhythmics by Vaughan-Williams class and by their distinctive toxicities, and know device physiology: sensing, pacing, refractory periods, mode switching, tachyarrhythmia detection and discrimination algorithms, and cardiac resynchronization principles. Inherited channelopathies and cardiomyopathies (long QT, Brugada, catecholaminergic polymorphic ventricular tachycardia, arrhythmogenic cardiomyopathy, hypertrophic cardiomyopathy) appear as risk-stratification questions.
Traps
Beware the distractor that treats a wide-complex tachycardia as ventricular tachycardia only when it is "obviously" so; when in doubt, treat as ventricular tachycardia. Questions exploit the difference between antegrade and retrograde pathway conduction, the meaning of a short versus long RP tachycardia, and the pseudoinfarct or pseudoaneurysm pattern that misleads. Drug questions often hinge on a single discriminating toxicity or contraindication rather than the primary mechanism.
Worked example
A regular narrow-complex tachycardia with a short RP interval and a retrograde P wave buried in or just after the QRS suggests atrioventricular nodal reentrant tachycardia; a long RP tachycardia with a P wave well before the next QRS suggests atypical nodal reentry, a permanent form of junctional reciprocating tachycardia, or an atrial tachycardia. Use the RP relationship and the response to adenosine or to pacing maneuvers as your decision tree, not the rate alone.
Rule of thumb
For any wide-complex tachycardia, apply the principle that ventricular tachycardia is the default diagnosis until proven otherwise, and look for atrioventricular dissociation, capture or fusion beats, and extreme axis as supportive features.