Lead off detection is something that is present in even the most rudimentary ECG systems. A basic implementation of this is to use a simple pull up resistor to VCC and a comparator which indicates that if the lead is removed the signal that is being measured by the output is not some sort of arrhythmia, it is a lead off. The main purpose of it is to tell cardiologists and those who are monitoring patients that what is being seen is not an arrhythmia, it is actually a lead that is disconnected or eroded as a result of electrode gel drying out over time.