Fainting can be due to fall in blood sugar levels, heat exhaustion and unusually strenuous exercise, but in a significant number of cases the blackouts are mainly because of an underlying heart problem that increases the risk of a heart attack or stroke. In most cases fainting is caused by a sudden fall in blood pressure after jumping up from a lying position. Doctors are hopeful that a device called Sleuth, collects evidence on how the heart is performing and transmits it to a computer could help them the reason for frequent fainting.

This implant that records every single heartbeat for up to two years has the potential to save the lives of patients whose fainting episodes are due to hidden heart defects that are potentially fatal.
Source