Sites of congenital and surgical heart block as defined by His bundle electrocardiography.

Catheter recordings of His bundle electrograms were obtained in seven patients with congenital heart block (CHB) and in two with surgical heart block (SHB). In the latter two patients block occurred following total correction of tetralogy of Fallot. In six patients with CHB block occurred proximal to H. In one CHB patient block occurred in the His bundle with "split" H potentials. Intraventricular conduction was normal in five of the patients with CHB who had narrow QRS and H-V intervals ranging from 35 to 45 msec. H-V intervals were short in two CHB patients (25 and 30 msec), one of whom had QRS widening with initial slowing. In the latter patient a functioning infranodal bypass (Mahaim tract) inserting into the right ventricular septum could explain the findings. In the two patients with SHB block was distal to H with P-H intervals of 125 msec and 160 msec, respectively. The degree of bradyeardia and the occurrence of symptoms partially correlated with