Hi Patrick, I hate to be the dissenting opinion here, but I think it would be best to use a subdomain or something connected to the main brand domain.
For example, I feel heart.nebraskamed.com (or even building up nebraskamed.com/heart, but giving it a distinct brand and look) would get off to a better start by leveraging the existing DA, while also being separate enough to have an associated brand. Branding is super tricky, but I know it's also a marathon to launch a new website from scratch and compete for new business. Is there a lot of cardiology competition? Are you trying to compete locally or regionally, or both? Plus, wouldn't you want to leverage the strength of the local brand, for something as serious as cardiology where the best-of-the-best expertise is sought?
Just a few thoughts. Of course if your organizational compass points to a completely separate domain, I would establish a linking strategy and go from there.
Best of luck!
Kristine