Since you are designing a medical device there will be a very specific procedure for design approval. Be sure you understand the EMC (Electromagnetic Compatibility) certification and testing requirements. For military spec products it's MIL STD 461, most likely there is one similar for medical electronics. There are two components to any electromagnetic wave, an electric component and a magnetic component. At low frequencies the shielding requirement is usually a magnetic problem. Shielding is designed using high permeability metals. One product is mu-metal. See:http://en.wikipedia.org/wiki/Mu-metal
Frequently the electronics is housed in a mu-metal box. The magnetic component is effectively shunted through the mu-metal.
Be sure you understand the physics of an electromagnetic wave and how different techniques are needed for shielding the magnetic and electric components of the wave. Also understand the difference between conducted and radiated interference. As Apple has found out with it's phone antenna problem, it's not a simple design problem. Sometimes is just more cost effective to hire an expert consultant.