With regards to finding a device that can continuously detect heart rate for long periods at a time, the following website has a potential for detecting heart rate wireless continuously.
http://www.technologyreview.com/communications/19825/?a=f.
Progress of the device can be found at: http://www.technologyreview.com/communications/19825/page2/.

There is also a paper available which may be used as part of the investigation as well:
Although the paper was written in 2006, there would have been advances on the technology since then. -

Title: A wearable device for continuous monitoring of heart mechanical function based on Impedance CardioGraphy
Proceedings of the 28th IEEE EMBS Annual International Conference
New York City, USA, Aug 30-Sept 3, 2006
by: G. Panfili, L. Piccini, Member, IEEE, L. Maggi, S. Parini and G. Andreoni
Bioengineering Department
Politecnico di Milano, Italy
Email: guido.panfili@gmail.com
Can be located at: http://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=4463167&isnumber=4461641

It is difficult to find a heart monitoring device that athletes use to measure heart beats continuously. There appears to be a lapse in time for most devices, and finding a device that would be continuously monitoring even at rest was difficult to find. There is possibility to use Timex Heart Rate monitors watches and Polar Electro watches, as they are used in Ironman (Triathlon), which last normally more than 8 hours, but during this period athletes are continuously moving. The website to Timex watch with some detail: http://www.sport-tiedje.co.uk/timex/ironman_t5f001.htm. Polar Electro website is with some detail can be found at: http://www.polar.fi/multisport_campaign/ .