Labile variations of intraventricular conduction unrelated to rate changes.

An example of unstable intraventricular conduction is presented. Several unusual features of this type of conduction are described. These are: (1) variable intraventricular conduction which was sensitive to multiple stimuli but not clearly related to changes in heart rate, (2) mid and terminal QRS changes reflected only in the left lateral chest leads and in the mid and terminal QRS portions of the vectorcardiogram, (3) constant association of prolonged P-R interval suggesting that this is a variety of bilateral bundlebranch block, and (4) apparent persistence of the peculiar intraventricular conduction for at least 4 years. Multiple involvement of the conduction system is probably present. and functional variations in distal portions of the left ventricular conduction system account for these labile changes. Predictable QRS patterns can be produced by various stimuli which alter intraventricular conduction.