There is a great deal of other activity in the 2.4Ghz band, if high reliability or lack of interferance is a requirement then the 2.4Ghz band may not be a good choice.
Same is true for most ISM bands.
You need to provide a good description of what you are actualy trying to do, the environment, the distances etc.
Are you a professional electronics designer, lots of potential safety issues with an 'ECG sensor' ?