Cabrera’s sign is used to diagnose an acute myocardial infarction in the setting of a left bundle branch block and consists of notching at 40 milliseconds in the upslope of the S wave in lead V3 and V4. This has a poor sensitivity of 27% for myocardial infarction.

Determining the presence of an acute myocardial infarction when a patient has a baseline left bundle branch block present is difficult. Besides the Cabrera’s sign, the Sgarbossa criteria and the Chapman sign can be helpful.