A simple digital filter to remove line-frequency noise in implantable pulse generators

This study investigated computer simulations and animal studies of a simple digital notch filter to remove 50-and 60-Hz line-frequency-noise interference. The digital notch filter was achieved by computing running subtraction of the intracardiac electrogram and the electrogram recorded one notch-sample period previously. Simultaneous rejection of the two worldwide line frequencies was obtained by computation of the minimum of two separate notch-filter outputs. Power consumption of this numerical algorithm was reduced for applications in implantable devices by operating the notch filters only from 20 msec to 50 msec after a sense event. A sense event was classified as a noise sense if the sum of notch-filter output during this window was less than a preset threshold of the sum of the raw data during the same time window. Otherwise, the sense event was classified as a true sense. The computer simulations determined an optimum threshold value of 33%. The filter was tested in five animal studies using a signal generator to inject additive noise interference. The results indicated that this simple filter could be implemented in an implantable pulse generator and could effectively exclude incorrect line-frequency-noise senses at the notched frequencies.