Cardiac Electrophysiology
Cardiac Electrophysiology
The Cardiac Electrophysiology domain tests how you turn intracardiac recordings and pacing maneuvers into a mechanism and a clinical decision. Expect questions on conduction intervals, refractoriness, dual AV nodal physiology, entrainment, and the differential diagnosis of narrow- and wide-complex tachycardia.
Core rules
- Measure the AH interval from the first atrial deflection on the His catheter to the first His deflection (normal 50–120 ms); HV from the first His to the earliest ventricular activation (normal 35–55 ms). A prolonged HV (>55 ms) localizes disease to the His-Purkinje system, not the AV node.
- AV nodal conduction is decremental: extrastimuli prolong AH. His-Purkinje conduction is not. A 10 ms shortening of the coupling interval that produces a ≥50 ms AH jump defines dual AV nodal physiology.
- Refractoriness is rate- and drive-train-dependent. Typical effective refractory periods: atrium 200–300 ms, AV node 250–350 ms, ventricle 200–250 ms, His-Purkinje 300–450 ms. Do not interpret a single ERP in isolation.
- Entrainment: the post-pacing interval minus the tachycardia cycle length estimates distance from the pacing site to the circuit. A difference ≤30 ms is consistent with pacing from within the circuit.
Worked example
A 45-year-old has a narrow-complex tachycardia at 180 bpm. Atrial extrastimulus testing from 400/280 ms produces an AH jump from 90 to 180 ms, followed by tachycardia with a short RP interval and a septal VA of 40 ms. The mechanism is AVNRT: the jump shows slow-pathway conduction, and the short septal VA places atrial activation near the AV node. The correct target is the slow pathway, not the fast pathway, because slow-pathway ablation preserves normal AV conduction.
Traps
- Calling a long HV AV nodal disease. AV nodal block prolongs AH or produces Wenckebach; infranodal block produces a long HV or Mobitz II.
- Missing catheter-induced ectopy that initiates a nonclinical arrhythmia.
- Measuring intervals during atrial pacing or after isoproterenol without noting the changed autonomic state.
- Forgetting that a His potential can be obscured by atrial or ventricular electrograms; verify with pacing maneuvers.