At Persyst we use neural networks for EEG interpretation. Our latest version has human-level performance for epileptogenic spike detection. We are now working on bringing the seizure detection algorithm to human-level performance.
Funny you should ask, detecting QRS complexes has been my first project since starting here. I know of a few papers where the authors have applied neural networks to EKGs, but the applications have been purely academic. I'm not aware of any other companies that use NNs in practice. (There may well be some, but they tend to be secretive about how their algorithms work.) At any rate, the false positive rate of our software is now about an order of magnitude lower than anything else on the market.
