Left bundle branch block (LBBB) preclude the detection of STEMI because the ST segment is usually deviated in all leads in the presence of LBBB. Some valuable criteria help diagnose true STEMI in patients with LBBB and symptoms suggestive of AMI.
- ST elevation ≥0.1 mV in the same direction as the QRS, in the leads where the QRS complex is positive (5 points)
- ST depression ≥0.1 mV in the same direction as the QRS in V1 to V3, where the QRS complex is negative (3 points)
- ST elevation ≥0.5 mV in the opposite direction as the QRS (2 points)
- ST elevation or depression in the opposite direction as the QRS with a ST/S ratio of −0.25 or less
(ie that the ST deviation is bigger than 25% of the S wave amplitude)
- – ≥0.1 mV in the same direction as the QRS or
- – ST/S ratio ≥ 25%
A score ≥ 3 is very specific for AMI (98%) but not very sensitive (20%).
Smith-modified 3rd criterion
A new criterion from Smith et al. significantly increases the sensitivity (up to 90%)
All those criteria can be put together like this: