Conclusions
Concealed NV fibers can mediate orthodromic SVT or be a bystander of AVNRT. V-A dissociation usually occur during such SVTs. An NV fiber not only expresses the characteristics of an AP, but also the characteristics of the AV node. Multiple QRS fusion morphologies during ventricular entrainments or His bundle refractory period ventricular stimuli at a fixed position can discriminate NV fibers from NF fibers.