METHOD AND DEVICE FOR FEEDBACK CONTROL OF NEURAL STIMULATION
A method of controlling a neural stimulus by use of feedback. The neural stimulus is applied to a neural pathway in order to give rise to an evoked action potential on the neural pathway. The stimulus is defined by at least one stimulus parameter. A neural compound action potential response evoked by the stimulus is measured. From the measured evoked response a feedback variable is derived. A feedback loop is completed by using the feedback variable to control the at least one stimulus parameter value. The feedback loop adaptively compensates for changes in a gain of the feedback loop caused by electrode movement relative to the neural pathway.
1 . A neurostimulation system comprising:
an implantable device for suppressing pain by applying neural stimulation, the implantable device comprising:
a stimulus source configured to provide stimuli to be delivered to a tissue in the dorsal column of a patient, each stimulus defined by a stimulus intensity parameter;
measurement circuitry for measuring a neural response evoked in response to the stimulus; and
a control unit configured to:
control the stimulus source to provide a stimulus in accordance with the stimulus intensity parameter;
measure via the measurement circuitry a neural response evoked by the stimulus;
determine from the measured evoked neural response a feedback variable; and
complete a feedback control loop by applying an adjustment to the stimulus intensity parameter based on the feedback variable, and
a processor configured to set a speed of the feedback control loop based on a characteristic of the measured evoked neural response.
2 . The neurostimulation system of claim 1 , wherein the control unit is configured to complete the feedback control loop by determining a proportional adjustment to the stimulus intensity parameter, wherein the proportional adjustment is determined by multiplying an error between the feedback variable and a target intensity by a gain.
3 . The neurostimulation system of claim 2 , wherein the processor is configured to set the speed of the feedback control loop by setting a value of the gain.
4 . The neurostimulation system of claim 3 , wherein the processor is configured to set the value of the gain by:
comparing each of a plurality of measured neural responses to a corresponding stimulus intensity parameter to determine the characteristic of a neural response growth curve, wherein the neural response growth curve is a relation of a neural response intensity to the stimulus intensity parameter; and
setting the gain based on the determined characteristic of the neural response growth curve.
5 . The neurostimulation system of claim 4 , wherein the characteristic is a slope of the neural response growth curve.
6 . The neurostimulation system of claim 5 , wherein the processor is configured to set the gain such that the feedback control loop has a predetermined corner frequency.
7 . The neurostimulation system of claim 6 , wherein the processor is configured to set the gain to be the predetermined corner frequency divided by the slope.
8 . The neurostimulation system of claim 6 , wherein the predetermined corner frequency is between a heartbeat frequency and a sample rate of the feedback control loop.
9 . The neurostimulation system of claim 1 , wherein the processor is part of an external computing device in communication with the implantable device.
10 . The neurostimulation system of claim 9 , wherein the setting a value forms part of a fitting procedure carried out by the external computing device for the implantable device.
11 . The neurostimulation system of claim 1 , wherein the processor is configured to set the speed when the patient is in a most sensitive posture.
12 . A method of suppressing pain by applying neural stimulation, the method comprising:
delivering stimuli to a tissue in the dorsal column of a patient, each stimulus defined by a stimulus intensity parameter;
measuring a neural response evoked by the stimulus;
determining from the measured evoked neural response a feedback variable;
completing a feedback control loop by applying an adjustment to the stimulus intensity parameter, based on the feedback variable; and
setting a speed of the feedback control loop based on a characteristic of the measured evoked neural response.
13 . The method of claim 12 , wherein completing the feedback control loop comprises determining a proportional adjustment to the stimulus intensity parameter by multiplying an error between the feedback variable and a target intensity by a gain.
14 . The method of claim 13 , wherein setting the speed of the feedback control loop comprises setting a value of the gain.
15 . The method of claim 14 , wherein setting the value of the gain comprises:
comparing each of a plurality of measured neural responses to a corresponding stimulus intensity parameter to determine the characteristic of a neural response growth curve, wherein the neural response growth curve is a relation of a neural response intensity to the stimulus intensity parameter; and
setting the gain based on the determined characteristic of the neural response growth curve.
16 . The method of claim 15 , wherein the characteristic is a slope of the neural response growth curve.
17 . The method of claim 16 , wherein setting the value of the gain comprises setting the gain such that the feedback control loop has a predetermined corner frequency.
18 . The method of claim 17 , wherein setting the value of the gain comprises setting the value of the gain to be the predetermined corner frequency divided by the slope.
19 . The method of claim 17 , wherein the predetermined corner frequency is between a heartbeat frequency and a sample rate of the feedback control loop.
20 . The method of claim 12 , wherein the speed is set when the patient is in a most sensitive posture.