I do not know if this is what you want to know, but there are implantable electronic devices the size of a pendrive, which can be inserted into the subcutaneous tissue of the left pectoral region and can remain in our body for up to 3 years. Examples of these are the devices from biotronik, medtronic and st. jude medical.
Marcio is referring to loop recorders. These are similar to the idea behind the Holter monitor, but rather than storing continous multi-lead heart rhythm data, loop recorders only store brief heart "episodes", not long term data streams. When downloaded, you just get short data files showing what the device identified as abnormal (usually, a few seconds in length).