Charge density and charge per phase as cofactors in neural injury induced by electrical stimulation

Stimulating electrodes of various sizes were used to investigate the interactions of two stimulus parameters, charge density and charge per phase, in determining the threshold of neural injury induced by electrical stimulation. Platinum electrodes ranging in size from 0.002 to 0.5 cm ² were implanted over the parietal cortex of adult cats. Ten days after implantation, the electrodes were pulsed continuously for 7 h using charge-balanced, current-regulated, symmetric pulse pairs 400 mu s per phase in duration and at a repetition rate of 50 Hz. The results show that charge density (as measured at the surface of the stimulating electrode) and charge per phase interact in a synergistic manner to determine the threshold of stimulation-induced neural injury. This interaction occurs over a wide range of both parameters: for charge density from at least 10 to 800 mu C/cm ², and for charge per phase from at least 0.05 to 5.0 mu C per phase. The significance of these findings in elucidating the mechanisms underlying stimulation-induced injury is discussed.