CAN SIGNAL-AVERAGED ELECTROCARDIOGRAPHY B E INTERPRETED IN CASES OF COMPLETE BUNDLE-BRANCH BLOCK

The aim of this study was to evaluate the results of signal-averaged ( SA) ECC in cases of complete right (RBBB) or left bundle branch block (LBBB). One hundred and seven patients had RBBB; 42 without cardiac di sease (O), 56 with chronic myocardial infarction (MI) and 9 with prima ry cardiomyopathy (CMP). Seventy-four patients had LBBB : 20 without c ardiac disease, 26 with chronic myocardial infarction and 28 with prim ary cardiomyopathy. A SA ECG (Cardionics, Fidelity) was performed with a 40 Hz band pass and compared with the recordings of 72 healthy cont rols without bundle branch block. The duration of the averaged QRS (QR S dur), the voltage of the last 40 milliseconds (RMS40) and duration o f terminal activity < 40 mu V (LAS) were measured. The analysis of res ults showed that QRS dur was significantly longer in subjects with ven tricular tachycardia (VT) (p < 0.05) and in those with advanced cardia c disease (p < 0.05), whatever the type of bundle branch block, and th at only the RMS40 distinguished patients with VT from those without VT , irrespective of the underlying cardiac disease and the type of bundl e branch block. However, the study of the diagnostic value of each par ameter showed very mediocre results : RMS 40 < 20 mu V in myocardial i nfarction and < 17 mu V in cardiomyopathy had sensitivities and specif icities in RBBB of 73% and 50% respectively, incalculable in CMP, in L BBB 70% and 33%, 77% and 60% respectively; the LAS was unusable. The a uthors conclude that it is hazardous to interprete SA ECG in bundle br anch block, especially in advanced cardiac disease where the specifici ty of the criteria becomes very low (< 50%).