Don't know much about defillibrators but would imagine that they work on DC/few Hz at most and the current essentially loops through the chest.
The key problem with EMC (ElectroMagnetic Compatibility) is usually coupling and this gets worse as the frequency increases. HF as previously mentioned can be a problem due to the power ivolved but generally problems occur when the wavelength is similar to the cavity/wire length.
The power pack to the defillibrator (presumably a switchmode at ~ few tens of KHz) could be a problem eg to an ADF but they would be well shielded, just think of the requirements for the hospital environment.