Patterned Electrical Tissue Stimulator

The invention pertains to the field of nerve stimulators used in the field of medicine for the relief of pain. More particularly, the invention relates to an automatically controlled electrical tissue stimulator for use with a multiple electrode system for generating spatially and temporally patterned electrical nerve stimulation.

Electrical nerve stimulators have become widely used in recent years in the field of medicine for the treatment of chronic intractable pain. Such devices include circuitry for generating electrical pulses, and electrode leads for delivering the pulses to the site of pain within the body. The electrical stimulating pulses produce the effect of masking the sensation of pain, and this method is preferable to drug therapy for many types of pain, because it avoids subjecting the patient to possible dangerous side effects. In the control of chronic pain by a nerve stimulator, there are generally provided adjustments or controls so that the stimulation delivered by the device can be adjusted or controlled according to the needs of the patient, which sometimes vary from day to day, or even minute to minute. Ideally, the stimulus repetition rate, the stimulus intensity, and the stimulus situs should be controllable to provide maximum flexibility in meeting the patient's needs. Transcutaneous stimulators are worn or carried outside the body and have electrodes secured to the skin over the affected area to apply the electrical stimulation thereto. Generally, transcutaneous stimulators comprise a set of electrodes or an electrode pair with leads connected to a portable controller, with adjustments for stimulus frequency and intensity. Electrical stimulation is generally provided with pulses that may be adjusted for varying frequency, width, or amplitude. In the prior art, monophasic and bi-phasic type pulses are generally used.

Monophasic type pulses induce current in the body tissue that flows in only one direction, the electrodes having a fixed polarity. It is generally known in the art that monophasic pulses of this type produce a "stinging" sensation or pain for the patient. The bi-phasic pulse is comprised of two sequential monophasic pulses of alternating polarity. In this manner, electrical nerve stimulation may be produced without a net DC current, thus avoiding the "stinging" pain associated with it.

While the most common method of controlling stimulation intensity is varying the pulse width or amplitude, the technique of temporal integration or temporal summation is also known in the art. To achieve varying degrees of stimulation intensity using temporal summation, two or more threshold or sub-threshold stimulation pulses are delivered to the body tissue in a sufficiently short period of time so that they are summed or integrated to produce a discernible stimulus. Hereinafter these "pulse trains" will be referred to as pulse bursts. The technique is advantageous in that a series of relatively low amplitude pulses (approximately 5 milliamps or less) may be used to produce stimulation requiring 80 milliamps or more using the single pulse stimulation method. Specifically, the advantage comes from being able to use a standard FET current source as opposed to the fly back transformer or pulse transformer current source most commonly used in the single pulse method and from the reduced likelihood of the patient's experiencing "stinging pain".

In a conventional electrical nerve stimulation system, stimulation pulses are delivered to body tissue through one pair of electrodes. Generally, each electrode has an area on the order of 4.5 square inches. While this method is advantageous because of its simplicity, it is hindered by its poor ability to distribute current uniformly throughout the volume of tissue beneath and between electrode pads. The problem is generally attributed to the varying conductivity of the tissue and the tissueelectrode interface. Usually, local tissue breakdown results in a single isolated current path from one electrode to the other. Through the use of a multiple electrode system where each electrode has an area on the order of 10 square millimeters, this problem has been alleviated. However, to operate a multiple electrode system using the conventional single pulse method, it is necessary that each electrode pair have an independent current source. This is generally impractical in a portable system because of space and weight limitations. In addition to providing for better current distribution in the stimulated tissue, the multiple electrode system may be used to generate spatial patterns of stimulation. Such a prior art device exists, using switches to manually select spatially separated electrode pairs.

While pulse width and pulse amplitude provide control over perceived stimulus intensity, it is well known that controlling the pulse rate may also improve the efficacy of stimulation. Many prior art devices provide adjustable pulse rate with a manual control. While such manual control provides the patient with a means for varying the stimulus perception when any particular pulse rate becomes painful, annoying, or imperceptible, the method usually requires that the patient adjust the pulse rate regularly. This is sometimes impracticable, i.e., if the patient is sleeping or his hands are immobile. Much of the difficulty associated with manual adjustment of pulse rate has been overcome through the use of automatic controls that vary the pulse rate randomly or rhythmically, such as the 1/f fluctuation technique. Findings have shown that such methods are also more effective in alleviating pain. See Pain Control by Transcutaneous Electrical Nerve Stimulating Using Irregular Pulse of 1/f Fluctuation, Kintomo Takakura, Keiji Sano, Yukio Kosugi & Jyun Ikebe, Applied Neurophysiology, D. 42, 1979, page 314. The efficacy of electrical nerve stimulation is also improved by modulating the frequency of the pulse rate variation.

Although the automatic control of pulse rate, pulse width, and pulse amplitude has been provided for in prior art systems, an automatic system combining these features with spatial variation of the stimulus situs has not. However, due to the wide variety of patient needs, a demand exists for such a system.

The present invention is an apparatus and method for producing patterned electrical tissue stimulation comprising a current generation means connected through a multiplexing means to a plurality of electrodes, the multiplexing means and the current means being connected to a control means, whereby electrical tissue stimulation is produced.

In the drawing:

FIG. 1 is a block diagram of a Patterned Electrical Nerve Stimulator according to the present invention.

FIG. 2 is an illustration of possible electrode placement on a patient; and

FIGS. 3, 3a, 3b, and 3c are a flow chart of the software according to the present invention.

The invention as embodied in the block diagram of FIG. 1 comprises seven principal components: user interface 150, microprocessor 100, data latch 105, analog multiplexers 110 and 115, analog switch 120, and constant current source 125. In the embodiment, microcomputer 100 functions as the controller for elements 110, 115, and 120. While the embodiment shown in FIG. 1 utilizes an 1802 type microcomputer, any microcomputer or custom designed controller chip capable of performing the necessary functions may be substituted. User interface 150, which is connected to microprocessor 100 through multi-line connector 155, provides for on/off and stimulus intensity control.

Data latch 105 is connected to microcomputer 100 through data bus 103. It serves to store and synchronize output data from microcomputer 100 for control of analog multiplexers 110 and 115. Data buses 107 and 108 carry control data out of data latch 105 into analog multiplexers 110 and 115, respectively. Buses 107 and 108 each comprise four individual data lines which deliver multiplexing information to analog multiplexers 110 and 115.

Analog multiplexers 110 and 115 provide for a 1 to 16 multiplex. The parallel four bit codes applied through data buses 107 and 108 to the multiplexers 110 and 115 respectively, determine the multiplexing connections. Output lines 112 and 117 are connected through a suitable conductor arrangement 170 so that either input lines 111 or 116 to the respective analog multiplexers 110 and 115 may be connected to any of the 16 electrodes 200 (only eight are depicted) as shown in FIG. 2. While the embodiment shown in FIG. 1 features a dual 1 to 16 multiplexing system, it will be seen that minor modifications would provide for a lesser or greater multiplexing capability.

The inputs to analog multiplexers 110 and 115 are connected through conductors 113 and 118 to the respective outputs 114 and 119 of analog switch 120. Microprocessor Q line output 144 controls analog switch 120 so that outputs 114 and 119 may be connected to inputs 132 and 130, respectively, or 130 and 132, respectively. On/off control of analog switch 120 is provided for by line 148 to microprocessor 100. Inputs 132 and 130 are connected through conductors 140 and 138 to respective outputs 134 and 136 of constant current source 125. As illustrated, outputs 134 and 136 of constant current source 125 have the respective polarities of positive and negative. In the described configuration it will be seen that the polarity of outputs 114 and 119 toggle or alternate in response to signals input from Q line 144 to analog switch 120.

In the preferred embodiment, electrode array 200 comprises 16 independent transcutaneous electrodes each with an area on the order of 10 square millimeters. However, it will be seen that electrode array 200 may be comprised of subcutaneous electrodes. The electrodes comprising array 200 may be of any type generally known in the art, only reduced in size. Each electrode in the array has two connections, one to a corresponding output 112 of multiplexer 110 and the other to the corresponding output 117 of multiplexer 115. For example, electrode 235 would be connected to analog multiplexers 110 and 115 at each respective "1" output. This allows for any two electrodes in array 200 to be selected as a stimulus pair. The electrodes comprising electrode array 200, as shown in FIG. 2, may be secured to (or in) the patient's body to form any desirable arrangement. For example, the electrodes may be arranged in groups of four (210) or in a line (215). In any event, it is generally known in the art that a minimum spacing of approximately one to two inches is necessary in order for the patient to perceive a spatial separation of stimulus origin.

It is also generally known in the art that patients are unable to temporally distinguish between stimulation pulses effectuated within about 4 milliseconds of each other. In the preferred embodiment, this physiological limitation allows a single perceptible stimulation pulse to be produced with a sequential pulsing of two or more electrode pairs. The operation of the circuitry shown in FIG. 1 to create such an effect will be described in the following paragraphs.

For the purpose of illustration, imagine that the tissue below the perimeter defined by electrodes 235, 220, 225, and 230 (as shown in FIG. 2) is sought to be electrically stimulated. As will be explained later (in the software discussion), this task may be performed in the system's "immediate" mode. The four electrodes (i.e., 235, 220, 225, and 230) may be paired to form up to six independent combinations. As stated above, stimulation pulses delivered to any of the six possible pairs within a 4 millisecond period will be temporally indistinguishable to the patient. Due to circuit and technique limitations in the particular embodiment shown as hereinafter described, a maximum of four pairs of electrodes may be pulsed within any given 4 millisecond period. For this example, assume the four electrode pairs 235 and 230, 220 and 225, 220 and 230, and 235 and 225 will be sequentially pulsed so as to blend the four pulse bursts into a single perceived stimulation event.

Referring to FIG. 1, it will be seen that the first step in accomplishing the desired stimulation is connecting constant current source 125 outputs 134 and 136 to the first pair of electrodes 235 and 230, respectively. To do so, microprocessor 100 loads the proper multiplexing data into data latch 105 through data bus 103. This multiplex code is then conducted to multiplexers 110 and 115 through data buses 107 and 108, respectively, resulting in the connection of electrodes 235 and 230 to inputs 111 and 116. In the preferred embodiment, this sequence of events takes about 50 microseconds. Once the electrode pair has been connected, electrical tissue stimulation is produced by constant current source 125 and analog switch 120. Constant current source 125 produces a constant current of up to 5 milliamps between its outputs 134 and 136 given any reactance less than 2.5 kilohms. After enabling analog switch 120 through line 148, microprocessor Q line output 144 is used to toggle analog switch 120 so that bi-phasic pulses with a current amplitude of approximately 2-4 milliamps and a period of approximately 80 microseconds are delivered to the selected electrode pair. Although each individual bi-phasic pulse is marginally perceptible, a series of two or more produce an easily discernible effect. In the present example, approximately ten such pulses could be sequentially delivered to the electrode pair 235 and 213, consuming a total time interval of approximately 800 microseconds. It should be seen that while the toggling of electrode polarities could be accomplished by reversing the multiplexing connections, the additional time required to do so as compared with toggling analog switch 120 with Q line 144 makes it less desirable. While Q line 144 may be toggled with only a few instructions in the microprocessor, the reversal of multiplexing connections requires additional instructions and the further delay of data latch 105.

After delivering up to ten bi-phasic pulses to the first selected electrode pair, microprocessor 100 automatically connects the next pair. It will be seen that the total elapsed time between the connection of the first electrode pair, the subsequent pulsing, and the connection to the second electrode pair will be no more than 1 millisecond. In this manner, four distinct stimuli (pulse bursts) may be used to produce what is perceived by the patient as a single stimulation event or pulse. This technique provides a major advantage over the conventional two electrode system in that it can greatly enhance tissue current distribution without altering the patient's stimulus perception.

The microprocessor software most pertinent to the key operations of the present invention is flowcharted in FIG. 3. Initialize block 310 represents the functions performed by the microprocessor at the outset of each use. Initialize functions include, but are not limited to, such things as clearing data latches and presetting program counters. Flow chart box 320 represents the software code which retrieves stimulation instructions from memory chips such as EPROMS or ROMS. In the present embodiment these instructions are stored in two eight-bit words, one word containing the electrode pair choice and the other word frequency-duratin code. The frequency code controls the time between stimulation bursts (stimulation frequency), while the duration code controls the length of time between changes in the stimulation frequency. The stimulation frequency and its duration are both programmable. The former from 2 to 500 Hz and the latter from 500 milliseconds to 10 seconds. The software which decodes these eight-bit words into usable form is represented by box 325. Using a standard 4k ROM, it will be seen that up to 10 hours of continuously varying stimulation can be accomplished.

As previously described, user interface 150 includes two buttons for control of stimulus intensity. During the initialization process, microprocessor 100 presets the intensity control value to its minimum setting. Flow chart block 330 corresponds to the software which retrieves this intensity value from memory for use during the stimulation process. In the present embodiment, the intensity value controls the quantity of sequential bi-phasic pulses delivered during each pulse burst. Block 335 represents the software which places the intensity value retrieved from the memory into an intensity pulse counter. The compliment of this value is then loaded into the intensity pulse compensation counter (block 340). After these counters have been loaded, data latch 105 is loaded with the eight bits corresponding to the two selected electrodes (block 345). The software (indicated by block 350) then initiates the electrical nerve stimulation process by enabling analog switch 120.

Flow chart boxes 355-390 represent the software which produces bursts of bi-phasic pulses and the corresponding delays thereafter. Basically, a series of bi-phasic pulses is produced and then followed by a complementary delay period so as to maintain a uniform operational delay for all possible stimulation intensity levels. After each such burst is complete, the decoded frequency (decoded in box 325) is retrieved from the memory and analyzed by the software corresponding to box 400. This analysis reveals either a programmed delay or an "immediate" mode command. The "immediate" mode "if" statement in box 400 provides the means whereby the system can switch immediately to another electrode pair without a delay (e.g., as described in the illustrative example). If in the "immediate" mode the software relating to block 400 directs the microprocessor 100 to block 320, where the next segment of stimulation data is read from the memory.

When not in the "immediate" mode, the program advances to flow chart block 405, where the software represented thereby converts the frequency to an appropriate delay count. The software corresponding to block 410 interrogates the duration counter for decision in block 415. If the duration counter is equal to zero the program advances to block 420 where the duration code is converted to appropriate duration count. If the duration counter is not at zero, the program advances to software block 425, which comprises a software delay designed to simulate the delay introduced by block 420. From both blocks 420 and 425 the program advances to the software represented by blocks 430 through 450. The software represented by these blocks comprises the microprocessor 100 user interface 150 interrogation routine. This routine involves sensing the stimulation buttons on user interface 150. If either the increment or decrement button is currently depressed, the software will adjust the intensity value accordingly providing there have been no changes within the last three seconds (block 440). If neither button is depressed, the program will jump from block 430 to block 445. Program flow chart boxes 445 allow the pattern source to be altered during operation. For example, a patient may have several alternative patterns to choose from. These patterns may be in one master ROM or several smaller ones. Next, the program begins to loop through blocks 455 to 470. This routine provides for the delay from each stimulation burst to the next and for control of the duration of the stimulation frequency In the case where the duration period is incomplete, box 475 directs the program flow back to box 330 to reinitiate the process which begins therefrom. If the duration count is complete, box 475 directs the program flow to box 320 where new instructions are read from the memory and the stimulation process is reinitiated. While the foregoing program flow chart description describes in adequate detail the interaction and operation of the software of the present invention, it will be observed that other programs which adequately perform these and other tasks may be substituted.

It is readily seen that spatially modulated or patterned stimulation can be accomplished by changing the stimulating electrode pair or pairs every so often as necessary to produce a traveling stimulus sensation to the patient. In a similar manner, the frequency of stimulation may be varied automatically (by varying the duration code) so as to provide any known type of temporal variations (e.g., 1/f, rhythmic, random, or progressive). These spatial and temporal patterns are easily programmed into the memory media directly (e.g., using a PROM burner) or through any higher level software systems adapted for such use.

In the present invention, the capability to spatially and temporally modulate the electrical stimulus provides the means to overcome the problems normally associated with electrical nerve stimulation. For example, the habituation effects caused by excessive stimulation to any particular tissue situs or that induced by monotony of stimulation frequency and intensity. Furthermore, the present invention provides the means for ready adaptation of electrical stimulus technique so that a wide variety of patient needs may be satisfied. ##SPC1##