A functional physiological block protects the ventricles from the risks of excessively rapid atrial rhythms. Pathological AVB is classified in three degrees according to whether the ventricular excitation is delayed, intermittent or absent. The site of AVB can be accurately determined by endocavitary electrophysiological studies but can also be estimated from the surface electrocardiographic recordings. fundamental research has questioned the sle responsibility of conduction defects in Wenckebach and paroxysmal blocks: they suggest the presence of abnormalities of excitability in pathological zones.