Physiologically based adjustment of stimulation parameters to an implantable electronic stimulator to reduce data transmission rate

[0002] The present application claims the benefit of U.S. Provisional plication Ser. No. 60/200,181, filed Apr. 27, 2000, which application is incorporated herein by reference.

[0003] The present invention relates to Functional Electro Stimulation FES) systems and more particularly to an improvement to known methods for providing control signals to an FES system. Such control signals are required at a high update rate to provide effective stable control of a patient's movement. The improvement provided by the present invention allows for a high effective update rate while reducing the overall data transmission rate.

[0004] Paraplegics and quadriplegics often have muscles capable of functioning, but are paralyzed due to damage to nerves that carry impulses to the muscles. Functional Electro Stimulation provides paraplegics and quadriplegics with use of their muscles by providing artificial stimulation pulses to the patient's muscles, which stimulation results in a desired movement.

[0005] Prosthetic devices have been used for some time to provide electrical stimulation to excite muscle, nerve or other cells. Some of these devices have been large bulky systems providing electrical pulses through conductors extending through the skin. Disadvantageously, complications, including the possibility of infection, arise in the use of stimulators which have conductors extending through the skin.

[0006] Other smaller stimulators are implanted which are controlled through high-frequency, modulated RF, telemetry signals. An FES system using telemetry signals is set forth in U.S. Pat. No. 4,524,774, issued Jun. 25, 1985 for “Apparatus and Method for the Stimulation of a Human Muscle.” The '774 patent teaches a source of electrical energy, modulated by desired control information, to selectively control and drive numerous, small stimulators disposed at various locations within the body. Thus, for example, a desired, progressive muscular stimulation may be achieved through the successive or simultaneous stimulation of numerous stimulators, directed by a single source of information and energy outside the body.

[0007] Many difficulties arise in designing RF powered implanted stimulators which are small in size, and in passing sufficient energy and control information to the stimulators to satisfactorily operate them without direct connection. A design of a suitable, small stimulator, a microstimulator, which can be easily implanted, such as by expulsion through a hypodermic needle, is taught is U.S. Pat. No. 5,324,316 issued Jun. 28, 1994 for “Implantable Microstimulator.” The '316 patent teaches all the elements required for successful construction and operation of a microstimulator. The microstimulator is capable of receiving and storing sufficient energy to provide the desired stimulating pulses, and also, is able to respond to received control information as to pulse duration, current amplitude and shape. Further, the stimulator of the '316 patent achieves a “charge balancing”, that is, a balancing of current flow through the body tissue in both directions to prevent damage to the tissue which results from continued, preponderance of current flow in one direction. The '316 patent in incorporated herein by reference.

[0008] The microstimulator of the '316 patent requires a control signal comprising the amplitude, width, and frequency of the desired stimulation pulse. In order to achieve effective control of muscles, to move the patient's body in a desired manner, a pulse parameters update rate of approximately 100 Hz is required. Additionally, a plurality of microstimulators may be required to stimulate difference nerves or muscles in a coordinated manner to achieve the desired motion. The requirements for both transmission of the signals by a master control, and receipt of the signals by a multiplicity of microstimulators results in increased power consumption and potentially high error rates. What is therefore needed is a method for reducing the data rate of data that must be transmitted to achieve effective functional control of the patient's muscles.

[0009] The present invention addresses the above and other needs by providing a Functional Electro Stimulation (FES) system utilizing an initial control message, which initial control message provides a complete set of stimulation parameters for a microstimulator, followed by a sequence of high data rate control messages, which high update rate control messages provide a single stimulation parameter for the microstimulator. As a result, effective stimulation is achieved at a much lower data transmission rate than when a complete set of stimulation parameters is transmitted at the high update rate.

[0010] In the natural use of human muscles, physiological stimulation is achieved by a change of frequency of the pulse sent to a muscle. In FES systems, stimulation levels may be controlled by changing pulse magnitude, pulse width, or pulse frequency. In a preferred embodiment, the pulse magnitude, pulse width, and pulse frequency are initially set to neutral values, which neutral values may subsequently be adjusted when needed, but changes to the level of stimulation to obtain muscle contraction is achieved by controlling pulse frequency only.

[0011] In accordance with one aspect of the invention, the control of pulse frequency is achieved by sending frequency change commands at a 100 Hz transmission rate. A typical range of pulse frequencies is from 10 to 200 Hz. The preferred step size of frequency changes is ±8 Hz or ±16 Hz. If a frequency change larger than the step size is desired, the change may be achieved over several commands. Advantageously, limiting the size of the command to ±16 results in a short message length and thus facilitates high command transmission rates.

[0012] It is a further aspect of the invention to provide feedback of stimulation parameters from the microstimulators. The feedback is used to verify that the previous control messages have been correctly received. If an error is identified, the stimulation parameters may be re-initialized, or alternatively, the high data rate control message may be used to correct the error.

[0013] It is a feature of the invention to provide a Mode in the control message, which Mode indicates whether the message is the initial control message or the high data rate control message. Based on the value of Mode, the microstimulator either processes the message as the initial control message or as the high data rate control message.

[0014] It is an additional feature of the invention to provide an ID in the control message, which ID indicates which of a multiplicity of microstimulators the control message is directed to. Based on the ID contained in the message, each microstimulator either processes the message or ignores the message.

[0015] The above and other aspects, features and advantages of the present invention will be more apparent from the following more particular description thereof, presented in conjunction with the following drawings wherein:

[0016] FIG. 1 depicts a master controller adjacent to a patient's leg, and a leg muscle to be stimulated;

[0017] FIG. 2 shows a multiplicity of miscrostimulators within the leg muscle separated from a master controller by the patient's skin; and

[0018] FIG. 3 depicts a functional flow diagram for a microstimulator controlled in accordance with the present invention.

[0019] Corresponding reference characters indicate corresponding components throughout the several views of the drawings.

[0020] The following description is of the best mode presently contemplated for carrying out the invention. This description is not to be taken in a limiting sense, but is made merely for the purpose of describing the general principles of the invention. The scope of the invention should be determined with reference to the claims.

[0021] A Functional Electro Stimulation (FES) system generates electrical signals to stimulate nerves and muscles to provide movement for paraplegics and quadriplegics. FIG. 1 shows a master controller 14 of an FES system located proximal to a patent 10. The master controller 14 transmits control signals to a multiplicity of stimulators implanted in the various muscle groups including lungs 12a,12b,heart 12C, arms 12d-12g,and legs 12h-12k.

[0022] A view of microstimulators 16a-16iimplanted in the leg muscle 12jis shown in FIG. 2. The master controller 14 is separated from the microstimulators 16a-16iby the patient's skin 18. U.S. Pat. No. 5,324,316 issued Jun. 28, 1994 for “Implantable Microstimulator” teaches the elements required for successful construction and operation of a microstimulator. Such microstimulator advantageously may be implanted through a large gauge needle and provides a minimally invasive implant procedure. However, the small size of the microstimulators dictates requirements that both the antenna for receiving the command signals, and the internal circuits for processing command signals, be as small and as low power as possible. Such size and power limitations conflict with functional requirements for high update rates for stimulation parameters to obtain stable and effective artificial control of muscles.

[0023] FIG. 3 shows a functional diagram of a preferred embodiment of the FES system of the present invention. To accommodate both small size and high update rates, the FES system of the present invention provides an initial values control message which sets stimulation parameters to initial values, and the FES system subsequently provides a high update rate control message to update a single stimulation parameter, to efficiently control the movement of the muscles. The human body physiologically controls a muscle by changing the frequency of the pulses carried by the nerve fibers to the muscle. A preferred embodiment of the present invention similarly uses nominal values of pulse amplitude and width, which values remain constant, while the pulse frequency is adjusted to control the stimulation level.

[0024] Select stimulation 20 of the master controller 14 determines which microstimulator to control and the type of command to be provided. ID is set to the address of the selected microstimulator, and either initial values control 24 or high update rate control 28 is selected. The initial values control message is required when the FES system is turned on, and may subsequently be issued to reinitialize the microstimulator if required. A high update rate control message is required when the muscle tissue that the selected microstimulator is implanted in, is required to contract to achieve a desired body movement. The Mode is set to Init when the initial values control message is required, and Mode is set to Control when the high update rate control message is required.

[0025] Decision box 22 selects the appropriate processing based on the Mode set in the select stimulation 20. If the Mode is Init, the decision box 22 selects initial values control 24. The initial values control 24 processing sets PA to a predetermined initial pulse amplitude, PW to a predetermined pulse width, and PF to a predetermined pulse frequency. The ID, Mode, PA, PW, and PF form the initial values control message.

[0026] If Mode is Control, the decision box 22 selects high update rate control 28. The high update rate control 28 determines a desired stimulation level. A new pulse frequency required for the desired stimulation level is determined. A Delta_F command is computed by subtracting the present pulse frequency from the new pulse frequency. In a preferred embodiment the step size of changes to pulse frequency are limited to ±8 or to ±16 to keep the message size small. If a change in frequency greater than the limit is desired, the change is made over two or more transmissions. The ID, Mode, and Delta_F form the high data rate control message.

[0027] A transmit function 26 transmits messages 30 from the master controller 14 to the microstimulators 16a-16j.The microstimulator with an address corresponding to ID (in FIG. 3, microstimulator N) executes the message 30. If Mode is Init, the microstimulator sets the stimulation Amplitude to PA, Width to PW, and Freq to PF. If the Mode is Control, the microstimulator adds Delta_F to the current value of Freq. When the FES system is actively stimulating the muscles, the high data rate control message is generated and sent at a sufficient update rate to provide smooth stable motion of the body member being moved, preferably at a 100 Hz message rate.

[0028] In order to prevent data transmission errors or processing errors from accumulating, the present values of pulse Amplitude, Width, and Freq are periodically back transmitted using back telemetry 32 from the microstimulators 16a-16ito the master controller 14. These values are then compared to the intended values in the master controller. If only the pulse frequency has drifted, the error may be corrected using the high data rate control message. If other stimulation parameters have developed errors, the microstimulator may be re-initialized by a new initial control message.

[0029] Those skilled in the art will recognize that other combinations of initial and high data rate control messages fall within the scope of the present invention. For example, the pulse width and frequency may be held constant and the pulse amplitude varied to provide functional control over the patient's muscles. Similarly, any combination of two or more of the pulse parameters may be combined. In other embodiments, the electro stimulation may be provided by signals other than electrical pulses. The present invention equally applies to any form of stimulation where initial stimulation parameters may be set, and less than the entire set subsequently controlled at a high data rate. While the embodiment described above sends high data rate commands in the form of changes to existing simulation parameters, the high data rate commands may also specify the desired parameter directly.

[0030] Additionally, while the present invention focuses on Functional Electro Stimulation (i.e., providing artificially induced use of muscles to paraplegics and quadriplegics,) the present invention also applies to stimulation used to prevent muscle atrophy in paraplegics and quadriplegics, and to systems used for general muscle development. Further, the present invention may be applied to the stimulation of heart or lung muscles, or other organ related muscles. In still other applications, the microstimulator may stimulate other body tissue, including nerve tissue. The invention applies to any application of externally controlled stimulation wherein high update rates of stimulation parameters are useful.

[0031] While the invention herein disclosed has been described by means of specific embodiments and applications thereof, numerous modifications and variations could be made thereto by those skilled in the art without departing from the scope of the invention set forth in the claims.