A lead for an implantable cardiac prosthesis, having an integrated protection against the effects of magnetic resonance imaging (“MRI”) fields. A protection circuit (26), placed at the distal end of the lead comprises a resistive component (28) interposed between the electrode (E1, E2) and the distal end of the conductor (22, 24) associated with this electrode. A normally-open controlled active switch (34, 36), allows in its closed state to short-circuit the resistive component. A control stage. (32) is coupled to the conductors and detects the voltage of a stimulation pulse applied on the conductor(s), and selectively controls by this voltage the closing of the active switch for a duration at least equal to the duration of detected stimulation pulse.
