Minimum amount of cardio for basic health?

[Fast Twitch Fiber]

What's the least amount of cardio you can do to simply keep your heart in shape? I'm not as concerned about burning calories right now, I just don't want to have a heart attack. Right now i'm doing 20 minutes on the stairmaster 3 times per week and one hour of swimming (not constant swimming of course). Is this enough?

 

[makedah]

Here's an article:

http://www.health.com/health/fitness/article/0,15669,436342,00.html