Tissue Response: Muscle : Page 4

Charge Densities.
Tissue damage after electrical continuous muscle stimulation for 6 hours as a function of cathodic and anodic charge densities, as reported by Scheiner et al (1990). Empty circles indicate stimulation protocols that did not cause tissue damage greater than a passive implant; filled circles indicate protocols that did cause damage greater than in passive implants. The net dc current, indicative of net unrecoverable charge, is not the only factor responsible for damage. For a given dc value, e.g. 20 uA/mm2, tissue damage is significant when monophasic protocols are used, but not when imbalanced charge protocols with equivalent net dc current is used. This suggests that the net unrecoverable charge can be partitioned into various irreversible processes in different ways, depending on the stimulation protocol used.

Electrode Potential.
Response of a 4 sq. mm stainless steel electrode to various pulsing protocols. When pulsed monophasically, the electrode potential 20 msec after the first pulse is more negative than the potential prior to the first pulse, thus the second pulse begins at a lower potential. This “ratcheting” effect, observed in the first 10 pulses, continues until a potential range is reached where irreversible processes occur at a rate such that all injected charge goes into unrecoverable charge. This is a requirement for steady state to be achieved.
When pulsed with a charge balanced biphasic protocol, the potential at the end of the anodic phase of the first pulse is more positive than the potential prior to the first pulse, and the second pulse begins at a higher potential. The potential at the beginning of each pulse moves positive until either there are no irreversible processes occurring, or the same amount of unrecoverable charge goes into processes cathodically and anodically (in either case, the net unrecoverable charge is zero). Typically, the steady state values are approached within a few pulses.