disclosed is methodology and apparatus for producing an asymmetrical filter for use with implantable medical devices , and in other input filtering environments . differing forward and reverse characteristic responses are provided by inserting a low value resistor in series with heart connecting leads so that emi input protection may be provided without significantly reducing energy transfer from the protected device . improved protection against voltage transients is provided with present arrangements of differentiated series impedance . higher frequency energy is allowed out of a subject device than is allowed into such device , which allows for attenuation of undesired frequency ranges entering the filter while allowing output pulses to exit without distortion .