Left bundle-branch block is rarely an isolated disorder of conduction, additional disorders being found in 29 of 30 patients studied by intracardiac stimulation techniques. These included disorders of sinus node function (prolonged maximum sinus node recovery time (corrected) in 23%, prolonged sinuatrial conduction time in three of eight patients), atrioventricular node function (prolonged AH interval in 33%, prolonged effective and functional refractory periods in 37% and 74%, respectively), "His bundle to right bundle branch" conduction (prolonged HV interval in 53%), and ventriculoatrial conduction (absent in 62%). It is postulated that at least half of the cases of left bundle-branch block were incomplete, even though the duration of the QRS complex exceeded 120 ms, because of (further) leftward deviation of the mean frontal QRS axis with sufficiently premature atrial extrastimuli. Block may be complete or incomplete in left bundle-branch block with left axis deviation of -30 degrees or more on the standard electrocardiogram.
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