Surgery: light – thermal – and electrical application – Light – thermal – and electrical application – Electrical therapeutic systems
Reexamination Certificate
1999-12-03
2001-10-09
Getzow, Scott M. (Department: 3762)
Surgery: light, thermal, and electrical application
Light, thermal, and electrical application
Electrical therapeutic systems
C607S057000
Reexamination Certificate
active
06301505
ABSTRACT:
FIELD OF THE INVENTION
This invention pertains to an electrical tissue stimulating device incorporating a safety control circuit. Such devices include cochlear implant prostheses, brain stem implants and functional electrical stimulators used for stimulating muscle tissue. In particular the present invention relates to a circuit which monitors voltage levels on tissue stimulating electrodes and takes action to reduce the presence of potentially dangerous voltage levels.
DESCRIPTION OF THE PRIOR ART
A problem with prior art electrical tissue stimulator systems is that a residual voltage may build up between stimulation electrodes and tissue due to charge imbalance in the applied stimulation pulses. Such residual voltages can lead to undesirable phenomenon occurring such as electrolytic reactions between tissue and electrodes.
Several schemes have been proposed in the past for dealing with this problem in the context of cochlear implant prostheses. For example, one standard scheme, used in a cochlear prosthesis and described in commonly owned U.S. Pat. No. 4,408,608 involves shorting the electrodes subsequent to the application of a stimulation pulse. The shorting step dissipates charge build up and so reduces the residual electrode voltage.
More recently in U.S. Pat. No. 5,674,264 also owned by the present applicant, there is described a scheme in which electrode voltages are monitored. In the event that a residual voltage is detected then the shape of the next biphasic stimulation pulse is modulated in order to reduce that residual voltage. The modulation involves altering the amplitude or duration of one of the phases of subsequent stimulations in order to drive the residual voltage towards zero.
Both of the above described solutions to the problem of reducing residual electrode voltages have their limitations.
While electrode shorting is highly effective at present, there is a trend in tissue stimulator design towards higher stimulation rates. At higher stimulation rates the time available for shorting is reduced and may be insufficient to permit the residual voltage to fall to a desirably low level. With respect to the second approach there are limitations as to the degree of compensation which may be effected by modulating a single biphasic stimulation pulse so that several stimulation cycles may be required before the residual voltage falls to a desirably low level. A further problem with the system of U.S. Pat. No. 5,674,264 is that it is reasonably complex to implement which is in conflict with design aims for miniaturisation, cost effectiveness, reliability and simplicity.
OBJECTIVES AND SUMMARY OF THE INVENTION
In view of the above limitations of the prior art it is an objective of the present invention to provide a tissue stimulating device including a means for reducing potentially unsafe residual electrode voltage levels.
According to the present invention there is provided an electrical tissue stimulator including:
a) overvoltage monitoring means arranged to monitor at least one electrode of a plurality of electrodes subsequent to delivery of an electrical stimulation signal by said at least one electrode and to generate a suitability-for-stimulation indication or an overvoltage indication in respect of said at least one electrode; and
b) stimulation means coupled to said plurality of electrodes and arranged to apply said stimulation signals, said stimulation means being responsive to said overvoltage monitoring means, said stimulation means applying a stimulation by means of said at least one electrode only in the presence of a corresponding suitability-for-stimulation indication.
Preferably said stimulation means comprises a first portion arranged to generate stimulation commands specifying said stimulation signals and a second portion responsive to said first portion and arranged to generate said stimulation signals in accordance with said commands, said second portion being further responsive to said overvoltage monitoring means whereby said second portion generates a stimulation signal for delivery by a first electrode in accordance with a stimulation command only in the presence of a suitability-for-stimulation indication corresponding to said electrode.
Said electrical tissue stimulator may be arranged as a cochlear implant prosthesis.
Alternatively in the presence of a command to stimulate by means of said first electrode and in the presence of an overvoltage indication, said second portion is arranged to short-circuit said first electrode.
Preferably in the presence of a command to stimulate by means of a first electrode and in the presence of an overvoltage indication in respect of said first electrode said second portion is arranged to open-circuit said first electrode. Open-circuiting is preferable in the case of tissue stimulators which make use of capacitor-coupled stimulation electrodes.
Preferably said overvoltage monitoring means is arranged to measure a voltage difference between a first intra-cochlear electrode and an extra-cochlear electrode and on the basis of said voltage difference generate either said overvoltage indication or said suitability-for-stimulation indication.
In a further embodiment said overvoltage monitoring means is arranged to measure a voltage difference between a first intra-cochlear electrode and a second intra-cochlear electrode and on the basis of said voltage difference generate either said overvoltage indication or said suitability-for-stimulation indication.
Alternatively said overvoltage monitoring means may be arranged to measure a voltage difference between a first subset of electrodes and a second subset of electrodes and on the basis of said voltage difference generate either said overvoltage indication or said suitability-for-stimulation indication in respect of each electrode of said first subset.
Preferably said overvoltage monitoring means detects the presence of overvoltage conditions 1 to 50 micro-seconds prior to the time for commencement of a stimulation specified by a stimulation command.
In one embodiment said overvoltage monitoring means includes multiplexing means arranged to selectively couple electrodes of said plurality of electrodes to a comparator means; said comparator means arranged to generate a suitability-for-stimulation indication if a voltage associated with a selectively coupled electrode falls within a predetermined range and to generate an overvoltage indication if said voltage falls outside said range.
REFERENCES:
patent: 5609611 (1997-03-01), Bolz et al.
patent: 5674264 (1997-10-01), Carter et al.
patent: A-32492/84 (1983-09-01), None
WO 97/21324; International Publ. Date Jun. 12, 1997; International Appln. No. PCT/U95/00805; International Filing Date Dec. 1, 1995; Inventor: Carter, Paul Michael et al; Title: A Feedback System to Control Electrode Voltages in a Cochlear Stimulator and the Like.
Cochlear Limited
Getzow Scott M.
Gottieb Rackman & Reisman, P.C.
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