Abstract
The R-on-T phenomenon was observed in two patients with narrow QRS complexes and repetitive supraventricular tachyarrhythmias. Case 1 had a typical Lown-Ganong-Levine syndrome in which the shortening of the P-R interval was due to a reduction of the A-H interval. In Case 2 the P-R interval (140 msec) was not short but the A-H interval was at the lower limits of normal (50 msec) and the P-A interval at the upper limits of normal (40 msec). Both patients showed the R-on-T phenomenon during periods of 1:1 A-V conduction occurring during atrial stimulation at a rate of 200/min (Case 1) and atrial flutter with a rate of 300/min (Case 2). Although total A-V conduction time was prolonged, the magnitude of this prolongation appeared to have been less than that occurring in normal subjects. This was attributed to the presence of an accessory pathway bypassing the area where the most significant delay occurred at the A-V node. The findings in Case 2 suggest, but do not prove, that a similar bypass might be operative in patients with atrial flutter with periods of 1:1 conduction, regardless as to whether the R-on-T phenomenon is, or is not, present.