ECG: Left Bundle Branch Block

Key points

• Mnemonic WiLLiaM MaRRoW: LBBB = 'W' in V1, 'M' in V6; RBBB = 'M' in V1, 'W' in V6.
• Causes of LBBB: ischaemic heart disease · hypertension · aortic stenosis · cardiomyopathy; rare — idiopathic fibrosis, digoxin toxicity, hyperkalaemia.
• New LBBB is always pathological and may signify MI.
• Diagnosing MI with existing LBBB is difficult — use Sgarbossa criteria.

Decision points in this pathway

• New LBBB?

The full step-by-step algorithm, including what to do at each branch, opens in the app for approved members.