EMI detection in an implantable pacemaker and the like

An implantable pacemaker receives electrical signals from a patient's heart through an electrode, the signals being either intrinsic cardiac signals or repetitive noise signals, due for instance to electromagnetic inference. The pacemaker includes circuitry for determining if the signals from the electrode are cardiac in origin or not by extracting certain signal characteristics from the signals. For example, the noise signals produced by EMI are repetitive, i.e., they have a fairly constant amplitude except for the initial peak. On the other hand cardiac signals have at most three peaks of decreasing amplitudes. These characteristics are used by the determined circuitry to differentiate between cardiac and noise signals. While this determination is taking place, the received signals are stored or delayed. Signals identified as cardiac signals are processed. The processed signals are compensated for the delay caused by the noise detection circuitry.