capture detect window are described in u s pat no 4 795 366 issued to callaghan et al and in the above cited u s pat no 4 305 396 issued to wittkampf both of which are incorporated herein by reference in their entireties fig3 discloses a preferred sense amplifier for use in conjunction with the electronic capture detection system this form of sense amplifier is more fully described in co pending u s patent application ser no 566 636 for a 34 field density clamp for sensing cardiac depolarizations 34 filed oct 8 1990 by hudrlik which is incorporated by reference herein in its entirety the active circuitry of the sense amplifier 26 attempts to maintain an equilibrium condition between the sensing electrodes the field perturbation caused by the passing wavefront is nulled out by the active circuitry which attempts to maintain a fixed relationship between the potentials at the electrodes current supplied to the electrodes in the attempt to maintain an electrode electrolyte equilibrium condition is passed through a virtual load the current delivered through the virtual load is monitored and forms the basis for the detection of the passing depolarization wavefront it is preferred to also monitor the voltage across the virtual load and multiply it with the current measurement to characterize the power delivered.