Method of Improving Sensory Tolerance With Modulated Nerve Stimulator

This invention relates to the field of medical electronics, and more particularly to a method and apparatus having pulse generating circuits which generate a train of electrical pulses having coordinated pulse width, amplitude, and pulse rate for controlling the average energy output of the circuits for stimulation of deep afferent nerves to cause the release of endogenous opiates, which suppress patient pain.

It is well known that pain can be alleviated by electrical pulses applied to the surface of the body or to electrodes implanted within the body. Initially, this electrical stimulation was applied in such a manner that the energy was only sufficient to stimulate sensory nerves. Efforts were made to avoid stimulating muscle nerves which produce fasciculations. Subsequently it was found that electrical stimulation at sufficiently high levels to elicit muscle contractions, which are an indication the deep afferent sensory pathways are activated, results in greatly improved long term analgesia. Pain patients are in general unable to tolerate the unpleasant sensations which accompany high intensity stimulation.

To avoid this it has been proposed to stimulate deep sensory nerves which short trains to monophasic pulses, the pulse trains being at a low frequency. By this procedure, the current necessary to stimulate deep sensory nerves can be reduced by one-third to one-half. Conventional transcutaneous electric nerve stimulation usually consists of a continuous train of pulses with three variable parameters. The rate may vary between 1 to 100 pulses per second, output between 0 to 70 milliampheres peak-to-peak, and pulse width between 0 to 400 micro-seconds. High rate transcutaneous electrical nerve stimulation usually refers to rates greater than 50 pulses per second. At these higher rates, if the interval is increased to a level which produces muscle contractions and attended deep sensory pathway activation, few patients can tolerate the resulting sensations. By interrupting the pulse train periodically at a low rate, that is, by cycling the stimulation on and off, the sensations can be reduced to a degree, but generally patients cannot tolerate even interrupted pulses at the levels necessary for deep afferent nerve stimulation.

The present invention is directed to a method of producing electrical pulses at levels sufficient to stimulate deep afferent nerves and cause the release of opiates to suppress pain in a patient. The method maintains the average energy in a train of electrical pulses within a predetermined range by coordinating the changes of electrical pulse rates and width and amplitude of the pulses. The method comprises the supply of a train of electrical pulses. The repetition rate of the electrical pulses is selectively increased and decreased. When the repetition rate of the pulses is increased, there is an increase in output energy. The output energy is counteracted by simultaneously decreasing the width and amplitude of the electrical pulses to maintain the average energy of the train of electrical pulses within selected values. When there is a decrease in the repetition rate of the electrical pulses, there is a decrease in the average energy. This is counteracted by simultaneously increasing the width and amplitude of the electrical pulses to maintain the average energy of the train of the electrical pulses within the selected energy range. The electrical pulses are applied to the patient at levels sufficient to produce stimulation of deep afferent nerves so as to cause the release of endogenous opitates, such as endorphin, which suppress patient pain.

According to the preferred embodiment of the method of the invention, the train of electrical pulses has a repetitive rate that is selectively increased and decreased. The average energy in the train is controlled by varying the width and amplitude of the electrical pulses over a predetermined range simultaneously with an increase or decrease in the repetitive rate of the electrical pulses. The width and amplitude of the electrical pulses is increased when the repetitive rate of the electrical pulses is decreased and, inversely, when the width and amplitude of the electrical pulses is decreased, the repetitive rate of the electrical pulses is increased. The limits of the range of the pulse width and amplitude are cyclically altered in conjunction with cyclically altering the repetitive rate of the electrical pulses between first and second values. The electrical pulses so produced are applied to the patient to stimulate deep afferent nerves and cause the release of opiates, which suppress pain.

The method of the invention is embodied in a transcutaneous electrical nerve stimulation apparatus in which the stimulus pulses are modulated in both time and intensity in a prescribed manner, the pulse amplitude and width decreasing, while the pulse repetition rate increases, an vice versa. The result of this complex variation in the electrical stimulus is to produce a comfortable and pleasant sensation at levels sufficient to produce muscle contractions and stimulation of deep afferent nerves and so cause the release of endogenous opiates, such as endorphin, which suppress pain. The sensory tolerance is highly individualized, analogous to individual preferences in music or art.

Various advantages and features of novelty which characterize the invention are pointed out with particularity in the Claims annexed hereto and forming a part hereof. However, for a better understanding of the invention, its advantages, and objects obtained by its use, reference should be had to the drawing which forms a further part hereof, and to the accompanying descriptive matter, in which there is illustrated and described a preferred embodiment of the invention.

In the drawings, in which like reference numerals indicate corresponding parts throughout the several views,

FIG. 1 is a diagram of strength-duration curves for various classes of nerve fibers;

FIG. 2 shows the relation between the pulse modulations of FIG. 1 and the subjective perception of intensity by the patient;

FIG. 3 is a block diagram of electrical stimulation apparatus according to the invention; and

FIG. 4 is a circuit diagram of the device of FIG. 2.

FIG. 1 is a diagram of how strength-duration curves would appear for a different class of nerves if all were plotted on the same coordinates. Here stimulus intensity increases along the vertical axis and pulse width increases along the horizontal axis. Curve A shows the path of modulation in which the pulse width is varied, the amplitude remaining constant. Curve B shows the path of modulation in which amplitude increases with pulse width. This is the preferred modulation to maximize the recruitment with time of various classes of nerves, thereby increasing sensory input to the biological system. Curve C shows the path of modulation in which amplitude decreases with pulse width.

Turning now to FIG. 2, the curves are plotted which show the variation of the perception or subjective awareness of stimulation intensity on the vertical axis with rate decrease and pulse amplitude and pulse width on the horizontal axis increase. Curve D corresponds to Curve B of FIG. 1, and shows that as the pulse amplitude and pulse width increase, the intensity perceived by the patient also increases. Curve E shows that if the pulse rate changes, amplitude and pulse width remaining the same, the patient perceives a decrease in stimulus intensity. Curve F shows that when the rate decreases, as intensity and pulse width increase, no great change in stimulus is sensed by the patient. The rate changes are encoded at the synaptic level as the integral, or time average, of the number of pulses per second. Thus, an increase in pulses per second is experienced as an increase in intensity. The subjective intensity versus rate function is a non-linear S-shaped curve. Stimulus intensity changes are encoded as changes in recruitment of the number of nerve fibers. This is a complex function approximately describing an S-shaped curve. In view of the above, if we increase rate, and also increase pulse width and amplitude simultaneously, we will have a double increase in input energy. This results in an intolerable sensation. This perceived increase in time average intensity is undesirable. Instantaneous sensation of changes in energy is highly desirable, if there is an avoidance of accommodation and adaption. Accommodation is the adjustment of nerves to a constant stimulus such that the stimulus is no longer perceived by the organism. The accommodation to a belt, wrist watch, or glasses, is a common example of this phenomenon. Nerve action potentials are no longer propagated. Adaption is an adjustment at the perception levels to a stimulus, even though accommodation has not taken place. An example of adaption is the distraction phenomenon, whereby an individual fails to notice pain from an injury due to a stronger sensory input from another source.

It is important to avoid both accommodation and adaption to the stimulus if one is to override pain. The stimulator device and method of the present invention accomplishes this in the following manner. The method includes the varying of the input at the synaptic level via rate modulation to give instantaneous variation in information to the central nervous system, thus, avoiding adaption. The variation of the input of the pulse width changes the class of nerve fiber and number recruitment. This avoids accommodation. The variation in intensity changes the number of fibers and class A, B, and C recruitment. This results in the avoiding of adaption and accommodation.

The invention accomplishes all of the above in a simple, straight forward, economical manner. The method and apparatus accomplishes the improved control of pain, while increasing the tolerance to high levels of stimulation, by affecting the changes in perceived intensity of stimulus in a manner such that no unpleasant increases in time average energy are perceived.

The present invention provides stimuli having variations in the pulse amplitude, pulse width, and pulse rate necessary to accomplish the result of Curve F. As shown in FIG. 3, a pair of patient treatment units 20 and 21 are shown as energizing pairs of electrodes 22 and 23. Each unit comprises a pulse amplifier and pulse-width-to-amplitude converter. The units are energized through conductors 24 and 25 from monostable pulse generators 26 and 27, respectively, having individual manual amplitude controls 30 and 31 for adjustment by the patient. Generators 26 and 27 are triggered through conductors 32 and 33 from an astable pulse generator 34 having a manual rate control 35 for adjustment by the patient. Generators 26 and 27 are controlled through conductors 36 and 37 by a modulation pulse generator and control 40, which also controls generator 34 through a conductor 41. Latching of control 40 is accomplished by a switch 42 actuated with control 35 by a mechanical connection 43.

Turning now to FIG. 4, astable pulse generator 34 is shown to comprise a regenerative comparator, further referred to as a Schmitt trigger 50, a capacitor 51, and feed back resistors 52 and 53, the former being variable by an adjusting knob 54 and comprising manual adjustment 35 of FIG. 4.

Monostable pulse generator 26 is shown to comprise an inverter 60, a capacitor 61, and a variable resistor 62 connected through a switch 63 to a positive bus 64 energized from the positive terminal of a battery 65 having its negative terminal grounded. A filter capacitor 66 is provided. Switch 63 and resistor 62 are actuated by a common knob 67 and constitute adjustment 30 of FIG. 3.

Pulse amplifier 20 comprises a capacitor 70, a MOSFET (field effect transistor) 71, and a transformer 72 having a primary winding 73 shunted by a capacitor 74 and connected to bus 64, and a secondary winding 75 connected to electrodes 22.

Monostable pulse generator 27 is shown to comprise a Schmitt trigger 80, a capacitor 81, and a variable resistor 82 connected through a switch 83 to positive bus 64. Switch 83 and resistor 82 are actuated by a common knob 87 and constitute adjuster 31 of FIG. 3.

Pulse amplifier 21 comprises a capacitor 90, a MOSFET 91, and a transformer 92 having a primary winding 93 shunted by a capacitor 94 and connected to bus 64, and a secondary winding 95 connected to electrodes 23.

Modulation generator 40 comprises a Schmitt trigger 100, a capacitor 101 connected to bus 64 through switches 63 and 83 in parallel, and a feed back resistor 102. It is connected to generator 26 through a resistor 103, a diode 104, and conductor 36, and to generator 27 through a resistor 105, a diode 106, and conductor 37. Modulation generator 40 also comprises a diode 110, a pair of Schmitt triggers 111 and 112, a pair of MOSFETS 113 and 114, a coupling resistor 115, a coupling capacitor 116, and a programmable unijunction transistor 117 powered through a pair of resistors 120 and 121. A modulation mode selector switch 122 is associated with transistor 117, and is actuated with knob 54 by a mechanical connection 123 as a part of manual adjustment 35.

A visual monitoring circuit 130 is shown to comprise a light emitting diode 131 energized from battery 65 through switches 63 and 83 in parallel under the control of a MOSFET 132 an a current limiting resistor 133. Input circuitry for MOSFET 132 has a common resistor 134 and includes a diode 135 connected to Schmitt trigger 60 and a diode 136 connected to Schmitt trigger 80.

In use, switch 122 is closed to inhibit modulation as will be described below, the circuit is energized to provide continuous stimulation to assist in placing one or both pairs of electrodes 22 and 23 on the patient's body in optimum positions, one or both of switches 63 and 83 being closed for this purpose, and monitoring circuit 130 giving indication of correct operation of the stimulator. After the electrodes are positioned, switch 122 is opened and the character of the stimuli is adjusted at will by operating controls 30, 31, and 35.

Schmitt trigger 50 with capacitor 51 and resistors 52 and 53 comprises astable pulse generator 34 and sets the basic pulse rate by the setting of resistor 52. The output rectangular pulses from generator 34 are supplied to capacitors 61 and 81, which, along with variable resistors 62 and 82, respectively, produce differentiated pulses in the form of positive and negative spikes. Negative going spikes are supplied to the inputs of Schmitt triggers 60 and 80. Schmitt triggers are known to exhibit a hysteresis phenomenon, which enables electric circuits comprising Schmitt trigger 60, variable resistor 62, and capacitor 61 to form a monostable pulse generator 26. Generator 26 has an output rectangular pulse having its width determined by variable resistor 62 and capacitor 61. In the same way, Schmitt trigger 80, with capacitor 81 and variable resistor 82, form another monostable generator 27.

Schmitt trigger 100 with capacitor 101 and resistor 102 comprises a second astable pulse generator for setting the basic rate of change of modulation for the stimulator. Transistor 117 is switched to latch in the modulation mode via diode 110, once it is selected by switch 122. When the switch is momentarily actuated, transistor 117 turns on and latches until power is removed, reverse biasing diode 110 and allowing capacitor 101 to charge. This is periodically switched by Schmitt trigger 100 to produce a second train of pulses which are communicated through resistor 103 and diode 104 to Schmitt trigger 60, and through resistor 105 and diode 106 to Schmitt trigger 80.

When the output pulse of Schmitt trigger 100 is at a high level, resistor 103 is in effect placed in parallel with resistor 62. The reduced resistance of the parallel combination causes capacitor 61 to charge more rapidly, resulting in switching of Schmitt trigger 60 sooner so that the width of the pulses from Schmitt trigger 60 is reduced. When the output of the Schmitt trigger 100 is at a low level, diode 104 is reverse biased, since its anode is now at ground and its cathode is at supply voltage through resistor 62. This results in the output pulses of Schmitt trigger 60 increasing in width to a preset value determined by the setting of resistor 62. Thus, as Schmitt trigger 100 switches between high and low outputs, the pulse width alternates between wider and narrower.

This variation in pulse width is amplified by transistor 71 and transformer 72. The parameters of transformer 72 are such that as the output pulse width varies from wide to narrow, the output pulse amplitude varies from high to low, so that the stimulus to the patient fluctuates periodically in intensity and pulse width.

An action similar to that just described for Schmitt trigger 60 also occurs for Schmitt trigger 80 and affects the output of patient unit 21. A second sequence of events occurs to vary the pulse rate of generator 34. When the output of Schmitt trigger 100 goes to a high level, the signal is conveyed to Schmitt trigger 112 whose output goes to a low level. If transistor 113 is turned on, which is the case when transistor 117 is on and a low signal is supplied to Schmitt trigger 111, then the output pulse of Schmitt trigger 112 can be communicated through transistor 113 to turn transistor 114 off.

When Schmitt trigger 100 goes to a low level, Schmitt trigger 112 has a low input and a high output. Transistor 113 is on transmitting the high level to transistor 114 to turn it on, thus connecting capacitor 116 in parallel with capacitor 51 of generator 34 and suddenly lowering the rate of pulses from Schmitt trigger 50. When Schmitt trigger 100 returns to a high level, capacitor 116 is disconnected from Schmitt trigger 50 which resumes its former rate of pulses. As pointed out above, when the output of Schmitt trigger 100 is high, the pulse width is decreased and the amplitude is reduced. This occurs simultaneously with the increase in the pulse repetition rate.

In one embodiment of the invention the output voltage varied between 0 and 60 volts, the pulse width range was 0 to 400 micro-seconds, the pulsing rate was from 1 to 100 pulses per second, the modulation rate was 1 cycle per second, and the changes during modulation were 0 to 40 percent amplitude, 0 to 60% pulse width, and 10 to 40% phase rate change.

From the above it will be evident that the invention comprises a stimulator in which pulse repetition rate varies between two levels, the variation being accompanied by changes in the pulse width and amplitude, so that an increase in the average pulse energy due to increase in repetition rate is counteracted by a simultaneous decrease due to individual pulses being reduced in width and amplitude.

Numerous characteristics and advantages of the invention have been set forth in the foregoing description, together with details and structure of the function of the invention, and the novel features thereof are pointed out in the appended claims. The disclosure, however, is illustrative only, and changes may be made in detail, especially in matters of shape, size, and arrangement of parts, within the principle of the invention, to the full extent indicated by the broad general meaning of the terms in which the appended claims are expressed.