How unusual is cardiac collateral circulation?

It's usual. It is very usual. In fact, it develops when main arterial flow is compromised by the developing clolesterol plaques.

Related Questions

How unusual is cardiac collateral circulation that supplies the heart?

Quite common. Collateral circulation is quite common in people who have progressive development of coronary artery narrowing. Several examples of collateral circulation are normal consequences of the native coronary circulation. Read more...