Analysis of prognostic significance of ventricular arrhythmias after myocardial infarction. Shortcomings of Lown grading system.

The Lown grading system for ventricular arrhythmias has been used in observational and experimental studies of ischaemic heart disease. This grading system uses three levels of ventricular premature depolarisation frequency and four complex features to assign patients to one of seven grades. We tested several of the major assumptions of the Lown grading system in a group of 400 patients who had recently experienced acute myocardial infarction. The Lown grading system assumes that the frequency of ventricular extrasystoles exerts a negligible risk force in patients who have complex ventricular extrasystoles. We found, however, that the frequency of ventricular extrasystoles contributed significant additional risk for cardiac death even in the three highest Lown grades, 4A, 4B, and 5. The Lown grading system assumes that, of the four complex features used, R on T ventricular extrasystoles have the greatest risk for subsequent cardiac death. We found that paired ventricular extrasystoles and ventricular tachycardia had more prognostic significance than R on T ventricular extrasystoles. It is important for prognostic stratification that subgroups which are merged into a given Lown grade should be relatively homogeneous with respect to outcome. We found a lack of homogeneity in the three highest Lown grades. Grade 5 contained 16 subgroups with a mortality risk which ranged from 0 to 75 per cent; statistically significant differences in subsequent mortality were found among these subgroups. Most of the shortcomings of the Lown grading system in our acute myocardial infarction population resulted from failure to give sufficient weight to ventricular extrasystoles frequency and to repetitive ventricular extrasystoles.