Isolated left posterior fascicular block: a reliable marker for inferior myocardial infarction and associated severe coronary artery disease.

The electrocardiographic (ECG) pattern of isolated left posterior fascicular block (LPFB) is a rare condition. It seems that true LPFB is often not recognized, whereas in cases of simple QRS axis of +60 degrees to +100 degrees in the frontal plane, the diagnosis of LPFB is made erroneously. Both facts rely on controversial and partially misleading opinions in the literature. Therefore, a retrospective and prospective study was performed in order to determine the prevalence of LPFB and to correlate its presence to the underlying disease. Retrospective study: Of a cohort of 830 patients referred in 1988 to a cardiologic laboratory for invasive investigation of certain or suspected coronary artery disease (CAD), 163 patients had an old inferior myocardial infarction (IMI). Nine patients (5.5%) showed the typical pattern of LPFB; eight of these had three-vessel disease. The diagnosis of IMI had been made only in one case before entry of the patient into the hospital, since LPFB generally masks IMI. Prospective study: 2502 ECGs were investigated, 1710 from a department of cardiology and 792 from two departments of internal medicine. Six LPFBs were detected (0.24%), all associated with IMI and four of them with three-vessel CAD. It is concluded that LPFB is a rare but clinically important intraventricular conduction disturbance. Its appearance is reliably connected with IMI and generally reflects severe three-vessel CAD, requiring invasive investigation.