Why do different Bioamp Patient cables have different levels of resistance?

The MLA2540 Patient cables for the ADInstruments Dual Bio Amps (FE135, ML818 & ML856) come in various varieties (some even include inductors). They may or may not contain a 1k resistor. 

These series (safety) resistors internal to the cable are part of the current limiting defibrillator protection circuitry inside clinical monitors. We obviously do not provide this with our Bio Amps, so newer versions of the cable may be the ones without the 1k resistor. The resistance in the cable will have no effect on the Bio Amp as the input impedance is several factors higher.