Problems in the recognition of conduction disturbances in the His bundle.

Two cases with conduction disturbances in the His bundle are described. In each it was not possible to obtain both components, H and H', of the "split" His bundle potential at one time from a single bipolar electrode catheter with an interelectrode distance of 10 mm. Initial failure to record the proximal His component (H) let to the incorrect diagnosis of block located within the atrioventricular (A-V) node, of third degree, in patient A, and of first to second degree in patient B. However, careful withdrawal of the catheter resulted in the appearance of proximal components (H), accompanied by disappearance of the distal components (H') of the His bundle potential. Apparently the lesion in the His bundle and the recording electrodes were spatially related in such a way that the bipolar electrode could not override the lesion. In view of these findings it is advocated that in patients the A-V conduction disturbances the His bundle catheter should be withdrawn carefully, if the initial recording reveals His bundle potentials bearing a time relation to the ventricular complexes, or advanced carefully, if these His potentials are related to the atrial activations, in order to demonstrate the (co)existence of a lesion in the His bundle. Search for a proximal His potential by withdrawal of the catheter should be done, if possible, while the atrium is paced at a rate in excess of the spontaneous sinus rate since the H potential may be obscured by the atrial complex if the A-V nodal transmission time is short at the spontaneous rate, as was the case with patient A. In patient B the not yet described phenomenon of bradycardia-dependent block within the His bundle was observed.