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.