The "R-on-T" phenomenon: an update and critical review.

The "R-on-T phenomenon" is the superimposition of an ectopic beat on the T wave of a preceding beat. Early observations suggested that R-on-T was likely to initiate sustained ventricular tachyarrhythmias. More recent experimental and clinical observations suggest that R-on-T is not a critical determinant of primary ventricular fibrillation in acute myocardial infarction; represents few of the initiating beats of paroxysmal ventricular tachycardia; and represents at worst only a small risk in terms of sudden death. Apparently when the capacity for sustained repetitive beating has not been clinically obvious, R-on-T is quite unlikely to result in ventricular tachyarrhythmias, even in the presence of coronary heart disease. However, in the setting of acute myocardial infarction, inability to always identify the precursors of tachyarrhythmias strengthens the argument for prophylactic treatment of patients.