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As far as I know, the Lown classification is out of date. In my opinion, R-on-T phenomena refer to the observation that external electrical cardioversion in the vulnerable phase leads to torsades or ventricular fibrillation. - Iblue ( discussion ) 13:25, Nov. 27, 2019 (CET)
The R-on-T phenomenon describes the observation in the ECG that a ventricular extrasystole is very early, i.e. H. occurs even during the repolarization of the previous heart action. This is shown in the EKG by the fact that the R-wave of the extrasystole falls into the T-wave of the previous heart action.
Because at this early point in time part of the heart muscle is not yet excitable (and therefore refractory), a premature chamber action can trigger life-threatening cardiac arrhythmias (e.g. ventricular fibrillation ).
