A 76-year-old man attends the Emergency Department following a near-collapse episode while standing. He describes light-headedness with transient visual dimming but no loss of consciousness. Similar episodes have occurred twice over the preceding month.

He has a background of ischaemic heart disease and hypertension. Examination is unremarkable, and he is haemodynamically stable.

An ECG shows right bundle branch block with left anterior fascicular block. The PR interval is prolonged. There are no acute ischaemic changes. He is admitted for cardiac monitoring.

What is the approximate annual risk of progression to complete heart block in symptomatic patients with this ECG pattern?