How does the vagus nerve relate to heart disease?

Brake on the heart: The vagus nerve (cranial nerve 10) is the "brake" on the heart. When it is activated it slows down the heart. In heart disease, it can be either helpful or part of the problem, depending on the particular disease process at work. If it's over-active, it can force the heart to beat to slowly. If it isn't working, it can let the heart beat to fast.

Slows the heart: In general the vagus nerve is like a shock absorber on a speed bump or pothole. When large swings in the heart rate occur due to various factors, the vagus nerve modulates this. The vagus nerve does not relate to heart disease directly but can have deleterious effects if it is stimulated at a time when higher heart rates are required. It is integral in patients with syncope (passing out).