Intraventricular conduction defects are common following repair of various forms of congenital heart disease. Such defects may affect adversely the long-term prognosis of patients in whom cardiac hemodynamics were adequately restored. Review of previously published studies suggests that the site of the conduction defect may be the reason for the different prognoses reported for patients from different institutions. The so-called "trifascicular block" pattern which sometimes occurs following open heart surgery is probably due to a more extensive lesion to the branching and penetrating parts of the His bundle rather than additional injury to the posterior left bundle branch fibers. Transient complete heart block in the immediate postoperative period seems to be a predictor for late development of complete heart block or sudden death at least as powerful as right bundle branch block and left anterior hemiblock.

Specialized fiber electrograms were recorded in 64 patients during open-heart surgery. Intervals were measured from the proximal His bundle, distal His bundle, and right and left bundle branches to the earliest QRS deflection in the limb lead ECG (pH-Q, dH-Q, RB-Q, and LB-Q interval, respectively). There was an increase of pH-Q intervals with increasing age such that at age 3 months the normal range of pH-Q interval was 13-27 msec, while at 14 years of age it was 32-54 msec. For patients 15 years of age and older, the normal range of pH-Q interval was 35-54 msec, similar to that reported by several investigators using catheter technic and suggesting that most His bundle electrograms recorded by catheter technic in fact record from the proximal His bundle. The dH-Q interval varied from 18 to 35 msec, the RB-Q interval from 18 to 30 msec, and the LB-Q interval from 20 to 39 msec. These normal ranges of RB-Q and LB-Q intervals are greater than previously reported. Overlaps existed between the normal range of pH-O and dH-Q or LB-Q intervals, dH-Q and RB-Q or LB-Q intervals and, at least in children, the pH-Q and RIB-Q intervals. Thus the timing of a specialized fiber electrogram does not necessarily reflect its anatomic location. AV nodal electrograms were not recorded although the recording sites included the known anatomic location of the AV node. Conduction velocity in the His bundle was determined and averaged 1.5 m/sec (range 1.3-1.7 m/sec).

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