Electrophysiology Study Basics
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Direct answer
Electrode catheters at the high right atrium, the His position, the right ventricular apex and inside the coronary sinus turn the ECG into an intracardiac map with millisecond resolution. The AH interval (about 55-125 ms) measures atrium-to-His travel across the AV node; the HV interval (about 35-55 ms) measures His-to-ventricular travel down the Purkinje skeleton — a long HV marks infra-nodal disease that predicts high-grade block. Programmed stimulation then stress-tests the circuitry: drive trains (S1) with progressively earlier extrastimuli (S2, S3) to refractoriness, hunting the discontinuous conduction curve of dual AV nodal physiology (an AH jump of 50 ms or more per 10 ms prematurity) or the retrograde ventriculo-atrial conduction of an accessory pathway, and inducing the clinical tachycardia under control with defibrillator pads on.
What you must remember
- Catheter positions and their jobs: high right atrium (pacing and recording), His bundle catheter at the tricuspid septal leaflet (the conduction-axis ruler), right ventricular apex (pacing and inducibility testing), coronary sinus (the left atrium's only practical access).
- Interval grammar: PA 25-45 ms (intra-atrial conduction), AH 55-125 ms (AV node — the physiologically delayable segment), HV 35-55 ms (His-Purkinje — the fixed segment; a prolonged HV above about 55-60 ms signals infra-nodal disease and, with symptoms, pacing).
- Programmed stimulation protocol: drive S1 at cycle lengths such as 600 and 400 ms, then single, double, triple extrastimuli decremented in 10 ms steps to local refractoriness; endpoints — tachycardia induced, refractoriness reached, or protocol exhausted.
- Dual AV nodal physiology: an AH jump of 50 ms or more after a 10 ms shortening of the coupling interval proves slow and fast pathways — the substrate of AVNRT; the echo beat that follows clinches it.
- Accessory pathway evidence: pre-excited baseline ECG (short PR, delta wave) or, with concealed pathways, eccentric retrograde atrial activation during ventricular pacing — the earliest atrial signal hunts the pathway's location.
- Safety frame: defibrillator pads on before induction, sedation light (deep sedation suppresses arrhythmias), anticoagulation managed for left-sided access, staff ready for immediate cardioversion.
- What the study decides: mechanism of unexplained tachycardia, site of AV block (nodal versus infra-nodal), inducibility of ventricular tachycardia, and mapping before ablation — the diagnostic first half of every ablation.
One study, from baseline to induction
Follow a 26-year-old with recurrent regular palpitations and a normal resting ECG: baseline AH 90 ms, HV 45 ms — the axis is normal. Programmed atrial extrastimuli step S2 coupling down from 400 ms in 10 ms decrements; the AH creeps up a few milliseconds each step until, at 310 ms, it leaps from 120 to 210 ms — a 90 ms jump, far past the 50 ms threshold: dual AV nodal physiology. A further extrastimulus produces a ventricular echo beat, then the tachycardia starts — narrow complex, cycle length 380 ms, His preceding every QRS, retrograde atrial activation earliest at the His catheter: typical slow-fast AVNRT. Pads were on from the start; the tachycardia is pace-terminated; the map is complete — the jump, the echo beat and the earliest retrograde site become the ablation roadmap, and the patient is potentially cured the same afternoon.
Where students slip
The interval boundaries get garbled: AH is nodal time and HV infra-nodal, and candidates who swap them swap the clinical meaning — a long HV with syncope implies His-Purkinje disease and a pacemaker, a long AH is usually nodal and physiological. The AH jump criterion is quoted as 30 ms in older texts; examiners expect 50 ms or more for a 10 ms decrement, and the jump plus an echo beat is the two-mark answer. Induction is treated as a complication rather than the goal — the study deliberately provokes the arrhythmia under monitoring, which is why pads precede induction and sedation stays light. The coronary sinus catheter's role is underestimated: without it the left atrium is electrically invisible and eccentric retrograde activation — the accessory-pathway signature — cannot be localised. Finally, every intracardiac event has a surface footprint; a candidate who cannot align the His deflection with the PR segment has learned the numbers without the geometry.
Frequently asked questions
What are the normal AH and HV intervals and what do they represent?
AH (atrium to His bundle) is about 55-125 ms and represents AV nodal conduction; HV (His bundle to ventricle) is about 35-55 ms and represents His-Purkinje conduction. A prolonged HV indicates infra-nodal disease with pacing implications.
What is dual AV nodal physiology on an electrophysiology study?
Demonstration of a sudden AH jump of 50 ms or more when atrial extrastimulus coupling shortens by just 10 ms — evidence of slow and fast pathway conduction, the substrate for AV nodal re-entrant tachycardia.
What is programmed stimulation in an EP study?
Fixed drive trains (S1) with progressively earlier extrastimuli (S2, S3, S4) delivered to atrium or ventricle map conduction, assess refractoriness and attempt induction of clinical tachycardias under monitored conditions.
Why is the coronary sinus catheter essential in an EP study?
It is the only practical route to record and pace the left atrium; without it, left atrial activation and retrograde pathway conduction cannot be localised, crippling diagnosis and ablation mapping.
What safety preparations precede tachycardia induction?
Pads applied and tested, sedation light, pacing and termination protocols ready, staff positioned for cardioversion — induction is the objective, so resuscitation readiness is the prerequisite.