Stimulus induced pH changes in cochlear implants: an in vitro and in vivo study

Large pH changes have been shown to be potentially harmful to tissue. The present study was designed to examine stimulus induced changes in pH for a variety of stimulus parameters both in vitro and in vivo, in order to ensure that stimulation strategies for neural prostheses result in minimal pH change. Stimulation using charge balanced biphasic pulses at intensities both within and well above maximum clinical levels for cochlear implants (0.025-0.68 microC per phase), were delivered to platinum electrodes in vitro [saline, phosphate buffered saline (PBS), or saline with human serum albumin (HSA)], and in vivo (scala tympani). Stimulus rates were typically varied from 62.5 to 1000 pulses per second (pps), although rates of up to 14,500 pps were used in some experiments. The pH level was recorded using a pH indicator (Phenol red) or pH microelectrodes. While electrical stimulation at intensities and rates used clinically showed no evidence of a pH shift, intensities significantly above these levels induced pH changes both in vitro and in vivo. The extent of pH change was related to stimulus rate and intensity. In addition, pH change was closely associated with the residual direct current (dc) level. As expected, stimulation with capacitive coupling induced little dc and a minimal pH shift. Moreover, no pH shift was observed using alternating leading phase pulse trains at intensities up to 0.68 microC per phase and 1000 pps. Saline with HSA or buffered solutions dramatically reduced the extent of pH shift observed following stimulation in unbuffered inorganic saline. Reduced pH shift was also observed following in vivo stimulation. These findings provide an insight into mechanisms of safe change injection in neural prostheses.