The atrioventricular (AV) node is the specialized conduction system, which provides electrical coupling between the upper and lower chambers of the heart. Thanks to its peculiar anatomical and functional properties, the AV node plays a key role in both physiological and pathological conditions. In particular, at high atrial rates, it protects the heart from life-threatening ventricular arrhythmias. Despite the impressive amount of information concerning AV nodal conduction, the actual role played by AV node functional properties (as nodal recovery (1;2) and concealed conduction (3)) in the generation of ventricular rhythm during spontaneous atrial tachyarrhythmias, as atrial flutter (AFL) and fibrillation (AF), is still incompletely understood. To clarify these aspects, AV synchrograms (4), displaying the time course of the phase of atrial activation in ventricular cycle, were constructed and quantified in 12 patients (62.7+/-16.4 years) with AFL and AF. Atrial electrograms and ECGs were recorded during an electrophysiological study after light sedation with diazepam (10 mg i.m.). The occurrence (i.e. percentage of synchronized beats, SB) and order (i.e. locking ratio of ventricular beats over atrial beats, LR) of synchronization between atrial and ventricular electrical activities were analyzed in relation with atrial rate changes and arrhythmia irregularity. Synchronized epochs were identified in all patients, demonstrating the non-randomness of ventricular rhythm in AF. The occurrence of synchronization decreased at decreasing atrial cycle length, ACL, (SB=71.2+/-28.0% at ACL=228.6+/-33.0ms to SB=47.1+/-24.2% at ACL=174.7+/-16.4 ms, p<0.05) and at increasing atrial cycle length variability (SB=70.1+/-31.0% at stdACL=4.5+/-1.7ms in AFL vs SB=48.2+/-21.4% at stdACL=24.4+/-12.9ms in AF, p<0.05). The decrease in atrial cycle length involved a global decrease in locking ratio from LR=0.36+/-0.11 to LR=0.28+/-0.09 (p<0.005). More specifically, AV synchrograms showed that locking orders organized according to a Farey sequence at increasing atrial frequency (5). The decrease of synchronization and concurrent decrease in locking ratio (corresponding to an increase in the number of blocked atrial beats) at increasing atrial frequencies is consistent with an increased contribution of concealed conduction effects (3), while the well-defined sequence of locking orders at changing atrial frequency is a clear marker of the nonlinear recovery of excitability in the nodal tissue (2;5). Thus these results provide evidence for a crucial role of AV node functional properties, as nodal recovery and concealed conduction, in the determination of ventricular rhythm during atrial tachyarrhythmias. Since both properties progressively hinder AV conduction in presence of high atrial rates, they contribute to protect the heart from dangerous high frequency ventricular rhythms.