AV-nodal reentrant tachycardia (AVNRT) is a self-sustaining electrical loop: an impulse circles continuously through fast and slow AV-nodal pathways because the circuit's cycle length (CL) is longer than the tissue's effective refractory period (ERP) — the returning wavefront always finds excitable tissue ahead of it.
CL (ms) = 60000 / reentry rate (bpm)
ERP_AV (ms) = ERP_baseline + sensitivity × vagal tone (%)
Reentry persists while: ERP_AV(t) < CL
Circuit terminates the instant: ERP_AV(t) ≥ CL
Carotid sinus massage (CSM) mechanically stretches baroreceptors in the carotid sinus wall. Their afferent signal increases medullary vagal (parasympathetic) outflow, which slows conduction and lengthens the AV node's refractory period — a real, measured effect on AV nodal tissue specifically (not on the fast atrial/ventricular myocardium that forms the rest of the loop).
- Hold to Apply Massage — ramps vagal tone up toward the pressure target while held, then decays back to a resting baseline (~5%) over a few seconds after release, mirroring the transient vagal burst a real manual carotid massage produces.
- Massage pressure — how strongly you press, setting the peak vagal tone the maneuver can reach.
- Carotid sinus sensitivity — patient variability in baroreceptor responsiveness (ms of ERP prolongation per % of vagal tone). Older, hypertensive, or carotid-sinus-hypersensitive patients sit toward the high end.
- Reentry circuit rate — sets the tachycardia's intrinsic cycle length; faster circuits (shorter CL) are mechanistically harder to terminate for the same vagal effect.
- Drag the ring — the reentry-loop view above can be dragged left/right to rotate it, so you can track the wavefront around the far side of the circuit.
When ERPAV reaches the cycle length, the returning wavefront meets AV-nodal tissue that hasn't yet repolarized — conduction blocks, the circuit is extinguished, and the heart's normal pacemaker (the sinoatrial node) resumes control, dropping the rate to a normal sinus rhythm. This is the exact mechanism used at the bedside to terminate paroxysmal SVT before reaching for adenosine. The ECG strip below the ring is a stylized rendering of this: during AVNRT the retrograde P wave is typically buried inside the QRS complex (so only narrow, regular spikes are shown), while after termination normal sinus beats show a separate P wave ahead of each QRS.
Clinical note: real carotid sinus massage is performed only after excluding carotid bruits/stenosis (embolic stroke risk) and with ECG/monitoring in place — this model is for physiological education only.