Diagnostic Electrophysiology Studies and Intracardiac Electrograms
Baseline intervals and what they mean
At the start of a study, measure PA, AH, and HV. Normal AH is roughly 60–125 ms and reflects AV nodal conduction; normal HV is roughly 35–55 ms and reflects His-Purkinje conduction. The His potential sits between the atrial and ventricular electrograms on the His catheter. A prolonged HV (especially >100 ms) signals diffuse His-Purkinje disease and a high risk of progression to complete heart block - a common exam stem.
Programmed stimulation
Burst pacing and extrastimulus testing (S1S2 with decrementing S2) define refractoriness. Wenckebach cycle length marks the AV node's block point. Sinus node recovery time assesses SA node function. For tachycardias, the key discriminator is the response to entrainment: the post-pacing interval minus the tachycardia cycle length (PPI − TCL) is short (<30 ms) when you are pacing inside the circuit, and long when you are not. Resetting with a return cycle equal to the TCL suggests a fully excitable gap.
Traps
Do not confuse AH prolongation (AV nodal) with HV prolongation (infranodal). A short HV with a wide QRS suggests a bystander accessory pathway or ventricular origin. His-refractory premature beats that terminate a tachycardia without depolarizing the atrium point to an atrial-independent mechanism. Always verify catheter position by electrogram, not by fluoroscopy alone.
Worked example
Tachycardia CL 300 ms; pacing from the RV apex gives a PPI of 380 ms. PPI − TCL = 80 ms, far above 30 ms, so the RV apex is outside the circuit - argue against typical AVNRT or orthodromic AVRT and consider atrial tachycardia or a different site.