ZA200104816B - Apparatus for electrically stimulating the body. - Google Patents

Description

APPARATUS FOR ELECTRICALLY STIMULATING THE BODY

BACKGROUND OF THE INVENTION

THIS invention relates to a method and apparatus for electrically stimulating the body of a subject.

Various devices are known for applying electrical waveforms to the body of a subject for the treatment of pain. However, a danger exists of damaging the tissue at the site of treatment as the control of the apparatus is often left in the hands of the patient, who has insufficient medical background to effect a safe treatment. In many cases, the patient causes damage to the tissue by applying an electrical waveform with a direct current content which is too high.

This will cause the destruction of the cell walls at the point of current penetration. Alternatively. an over cautious patient will not obtain sufficient pain relief if they keep the current too low.

It is an object of the invention to provide an improved method and apparatus for electrically stimulating the body of a subject.

CONFIRMATION COPY

SUMMARY OF THE INVENTION

According to the invention there is provided apparatus for electrically stimulating the body of a subject, the apparatus comprising: measuring means for measuring the tissue resistance of the body at a treatment site; a waveform generator arranged to generate an electrical waveform to be applied to the treatment site of the body: control means for controlling the waveform generator to adjust at least one characteristic of the electrical waveform depending on the measured resistance of the body at the treatment site; and output means for applying the adjusted electrical waveform to the body at the treatment site.

The sampling means preferably includes at least comparator means for comparing the level of a voltage signal which has been applied to the body at the treatment site with a reference voltage signal.

The control means may be a microprocessor which is adapted to control the waveform generator to select one of a plurality of predetermined waveforms, depending on the sampled resistance. These waveforms may vary in one or more of the following parameters, including frequency duration, DC offset, the voltage level or magnitude and the output waveform shape.

The measuring means is preferably adapted to repeatedly measure the tissue resistance of the body at the treatment site during a treatment cycle, and the contro! means is adapted to control the waveform generator to adjust at least ee . JO ee R — . I —F - (F'Y : -3- one characteristic of the electrical waveform depending on each of the measured resistances during the treatment cycle.

More preferably, the measuring means is adapted to initially measure the tissue resistance of the body at the treatment site and to pass the initial measured resistance to the control means, and the control means is adapted to select a waveform depending on the initial measured resistance.

The control means then controls the waveform generator to adjust the amplitude of the initially selected waveform depending on each of the measured resistances during the treatment cycle.

According to the present invention there is further provided a method of electrically stimulating the body of a subject comprising the steps of: measuring the tissue resistance of the body at a treatment site; adjusting at least one characteristic of an electrical waveform depending on the measured resistance of the body at the treatment site; and applying the adjusted electrical waveform to the body at the treatment site.

One or more of the parameters of the waveform which are adjusted may include frequency, duration, DC offset, the voltage level or magnitude and the output waveform shape.

The method may further include the steps of measuring an initial resistance of the body at the treatment site and selecting a waveform depending on the initial measured resistance.

B WO 01/19452 PCT/1B00/01292

Preferably, after the initial resistance has been measured, the resistance of the body at the treatment site is repeatedly measured and the initially selected waveform is amplitude modulated depending on these measurements.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 is a schematic block diagram of a first embodiment of an apparatus for electrically stimulating the body of a subject according to the invention;

Figure 2a to 2c is a circuit diagram of the apparatus of Figure 1;

Figure 3a to 3c illustrate the output waveforms of the apparatus;

Figure 4a to 4b is a schematic block diagram of a second embodiment of an apparatus for electrically stimulating the body of a subject according to the invention;

Figure 5a to 5b is a circuit diagram of the battery and charging stage of

Figure 4;

Figure 6 is a circuit diagram of the RS232 stage of Figure 4;

Figure 7ato 7b is a circuit diagram of the patient output stage of Figure 4;

Figure 8a to 8b is a circuit diagram of the voltage control stage of Figure

I ER a I PU _- - 3 i

Figure 9a to 9b is a circuit diagram of the 52 volt regulator stage of

Figure 4;

Figure 10a to 10c is a circuit diagram of the microcontroller stage of Figure 4,

Figure 11a to 11d is a flow diagram illustrating the steps carried out by the apparatus of Figures 4 to 10;

Figure 12 is a schematic biock diagram of a third embodiment of an apparatus for electrically stimulating the body of a subject according to the invention;

Figure 13a to 13b is a circuit diagram of the TRS power supply module of

Figure 12; and

Figure 14a to 14b is a circuit diagram of the TRS control module of Figure 12.

DESCRIPTION OF EMBODIMENTS

The present invention is an apparatus for electrically stimulating the body of a subject. The apparatus is a monocycle apparatus, which means that the electrical treatment waveform is polarised and does not contain positive and negative cycles. Such monocycle apparatus is known to have the drawback of causing tissue damage when the treatment current is not carefully regulated.

However, the benefit of a monocycle apparatus is better penetration of the treatment current into the underlying tissue.

The present invention addresses the problem of causing tissue damage when using a monocycle apparatus by measuring the resistance of the tissue to which the treatment electrical waveform is to be applied, and adjusting the waveform accordingly. The tissue of a body is able to tell a great deal about the underlying conditions in the body. When an injury occurs, there is swelling of the tissue which causes poorer than normal biood circulation within the tissue. This in turn causes a rise in the resistance of the tissue to an electrical current.

Referring to Figure 1, in a first embodiment of the present invention, a power supply 10 is connected to an output voltage generation block 12 which in turn is connected to an output stage block 14. Positive and negative output contacts 16 and 18 respectively are connected to the output stage 14. These output contacts 16 and 18 are in the form of replaceable self-adhesive pads which are applied to the body of the subject.

A 41.2 volt reference signal is applied to the body of the subject at the treatment site, through contact 16. A resultant voltage is received back through contact 18 and is passed to a comparator 20. The comparator 20 compares the received voltage with the 5 volt reference voltage and determines at least one characteristic of a treatment electrical waveform to be applied to the body of the subject.

It will be appreciated that another method of determining the resistance of the tissue. such as measuring the amount of current transmitted through the body. could be used.

The comparator 20 is connected to control means in the form of a microprocessor 22 which is in turn connected to and controls the output stage 14. The microprocessor 22 also controls a communication link 24 which allows the apparatus to be connected to the communications port of a personal computer via a 9 pin D connector and screened wire 26.

Referring now to Figure 2, power for the apparatus is supplied by a rechargeable battery 28. The battery 28 can be connected to a mains supply for recharging through contacts 30, fuse 32, step-down transformer 34 and a bridge rectifier 36.

A first LED 38 indicates the presence of mains power for the purpose of charging the battery 28. Relay 40 disconnects the rest of the unit from the power supply when the battery is charging. This is a safety requirement to prevent lethal mains current from being administered to the patient.

A voltage regulator comprising of a regulator 42, a 100 uH inductor 44 and a diode 46 generate a treatment voltage of 52 volts. The regulator 42 switches the inductor 44, and the energy discharged from the inductor 44 is rectified by diode 46. The resultant voltage is stored in capacitor 48 which acts as a reservoir. The 52 volts output is limited by resistor 50 and two zener diodes 52 which have a combined threshold of 57 volts. This voltage is then applied to the base of transistor 54 which is a bipolar power transistor. Resistor 56 fine tunes and drops the 52 volts to 40 volts which is termed the treatment voltage.

The emitter of transistor 54 is connected to the positive output 16 of the device.

In series with the negative output 18 of the device is a current limiting resistor 58 and a current bypass resistor 60. These resistors limit the direct current component of the treatment current to a maximum of 1.2 mA through the human body. In parallel to resistor 58 is a bypass capacitor 62 of 0.22 uf.

The capacitor 62 provides an alternating current bypass to ground through the patient. This gives a treatment current ratio of 3.7. That is 7 parts of alternating current to 3 parts of direct current.

The DC (direct current) is used for offset purposes, which means that the treatment pulse or waveform floats on a direct current component. The direct current component gives greater penetration properties to the AC component into the tissues than an AC component alone.

However, caution needs to be taken to ensure that the DC component does not cause tissue damage which occurs if the DC component is too large. In practice the DC component is limited to 1 mA. It must be stressed that the AC and DC current components are combined at the point of penetration. If the two values combined exceeds approximately 2.7 mA then the unit will cause some form of tissue damage. Tissue damage is also largely dependent on the tissue resistance, which is determined by several factors i.e. gender, temperature of the tissues, ambient temperature, activity of the tissues and skin sensitivity.

It is therefore preferable to increase the AC component to be more than the

DC component. The reason for this is that the AC component is taken at a root mean square value, so the current transfer is interrupted and not constant, as in the case of DC.

The direct current blocking capacitor 62 was added to the circuit to stop the transfer of high direct currents to the tissue. The current limiting resistor 58 was also added, to maintain the DC current component at less than 1 mA.

The base connection of transistor 54 is tied to four driver transistors 64, 66, 68 and 70. These are all small signal transistors each having a different load effect on the base of transistor 54. This in turn varies the output of the emitter of transistor 54 and therefore will determine the different characteristics of the treatment electrical waveform.

Transistor 64 determines the direct current offset of the treatment waveform.

Transistors 66, 68 and 70 act to pull the base of transistor 54 low, causing a step in the waveform, as illustrated in Figure 3.

- In order to determine the resistance of the tissue at the treatment site, four h operational amplifiers 20 are used. These are configured to act as comparators, comparing the voltage received back from the body to a 5 volt reference voltage. The non-inverting inputs of the operational amplifiers have been tied together and in turn tied to a resistor 72 which is connected to the upper half of the current limiting resistor 58.

Before treatment commences, the positive and negative contacts 16 and 18 respectively are connected to the body via self-adhesive pads and the 40 volts floating on the positive output of the device is conducted through the body.

The resultant voltage is received back at the comparators 20 and is compared k to the 5 volt reference voltage. If the received voltage is less than or equal to oo the 5 volt reference voltage, the comparator will not switch on.

The outputs of the comparators 20 are connected to the microprocessor 22.

The comparators will therefore give a one or a zero indication to the microprocessor 22. When the connective tissue is of a low resistance, all of the comparators will be on and the microprocessor will know that there is a low tissue resistance and will switch the transistors 64 to 70 to produce a low treatment pulse. W only one comparator is on, the microprocessor will know that a high resistance has been sampled in the treatment tissue, and a high treatment electrical waveform will be produced. The resistors connected to the reference inputs of the comparators reduce the effective voltage so that each comparator will have a different reference voltage, for example, 5 volts, 4 volts, 3 volts etc. The more comparators, the greater the resolution achievable.

The low, medium and high treatment waveforms are illustrated in Figures 3a to 3c. These pulse waveforms were selected to effect maximum current transfer at the designated frequency. The low treatment waveform is a half wave with a 7 volt DC offset. The waveform is a continuous pulse burst of 166 Hz.

The medium treatment level pulse is a square wave continuous pulse with a stepped decay on a 7 volt DC offset. The waveform has a frequency of 152

Hz.

The high treatment pulse is a square wave with a stepped decay and an 8 volt offset. The waveform has a frequency of 156 Hz.

A higher treatment is needed when the resistance of the connective tissue is high to effect better penetration. This is because the treatment current is lessened by the high tissue resistance.

The output from the apparatus can be displayed on a personal computer via microprocessor 24, which is an RS232 interface which converts the signals generated by the microprocessor 22 to a standard which is able to be monitored on a personal computer. The microprocessor 24 transmits the signals via a 9 pin D connector and screened wire 26 to the communications port of a personal computer. The standard software used on the PC will process the signals to values and graphs to be displayed on the screen.

The apparatus also has LED 74 which indicates that the unit is on. LED 76 indicates that the apparatus is in a “sense” mode and is turned on when the unit is sampling the tissue resistance. LED 78 is a status indicator showing conditions such as timeout, standby and treatment progress. LED 80 is a charge full indicator which is on when the battery has been fully charged.

A block diagram of a second embodiment of the invention is illustrated in

Figure 4, with like parts being indicated by like reference numerals.

In this embodiment, the comparator stage 20 of the first embodiment has been replaced by an analogue to digital convertor. A voltage control stage 82

EE EN a EE ] BE. A adjusts the treatment intensity. In the previous embodiment, the intensity was regulated by the change in pulse shape and width. The present embodiment now controls the amplitude of the treatment pulse, as will be explained in more detail below.

Figure 5 shows a circuit diagram of the battery and charging stage 10 of Figure 4. A rechargeable battery 28 is charged through a diode 84. An LED 86 gives a visual indication of the charged status of the battery and is red when the battery is low, orange when the battery is half-full and green when the battery 1s fully charged.

The battery 28 is connected to a mains power supply for recharging through a bridge rectifier 88.

A relay 90 isolates the battery and charging circuits during charging. Diodes 92 and 94 isolate the remainder of the circuit from the charging circuit.

A potential divider consisting of resistors 96, 98 and 100 scale the 12 volts to 5 volts for an analog to digital converter sensing the battery condition (see Figure 10).

Figure 6 illustrates the RS232 stage of the apparatus of Figure 4. The RS232 interface converts the signals generated by the microprocessor 22 from TTL levels to EIA232 levels which can be input to a personal computer. The signals are transmitted via a 9 pin connector 102 to the personal computer. As mentioned above, standard software is used on the PC to process the signals to values and graphs which can be displayed on the PC screen.

Figure 7 is a circuit diagram of the patient output stage 14. Transistors 104, 106, 108 and 110 are used for wave shaping, and receive their wave inputs from the micro-controller 22. These inputs marked “Wave in 1” to "Wave in 4”

are logic level signals (0 to 5 volt) applied to the transistors 104 to 110 at different timing intervals and which turn these transistors on. When each of the transistors are on, resistors 222 to 228 have different values (as indicated on the drawing), and therefore when transistors 104 to 110 are triggered, they have a different biasing effect on transistor 112. This in turn has a different effect on the 41.2 volt wave shape.

For example, for a level of 41.2 volts for 1.2 ms, none of the transistors 104 to 110 are triggered. For a wave level at 28 volt for 0.8ms, transistors 108 and 110 are triggered for 0.8ms. Finally, for a 4 volt signal, transistor 104 is triggered and transistor 106 to 110 are switched off. By repeating these values, the wave cycle is repeated.

Referring again to Figure 7, transistor 112 outputs the wave into block 114 where the electrodes to the patient are connected.

Resistors 114 to 120 are used as a potential divider so that the treatment voltage can be fed back to the micro-controller 22 to be measured, as will be : described in more detail below.

Resistors 122 to 128 are used as a potential divider to pass to the micro- controller 22 what the feed-back voltage through the patient is. As will be explained below, the micro-controller 22 uses the treatment voltage and the feed back voltage to calculate the true tissue resistance.

Resistors 130 and 132 are direct current by-pass resistors, limiting the current to safe levels at maximum treatment voltage, as has been described previously.

Referring now to Figure 8, the voltage control stage 82 receives pre-selected voltage inputs from the micro-controller through data input lines 134 to 138.

These three data lines 134 to 138 allow a binary value of 0 to 7, which value is applied to a 4051 CMOS analogue multiplexer device 140.

The multiplexer 140 is connected to a resistance ladder 230. The different voltage tappings derived from the resistance ladder 230 are selected through multiplexer 140 to be fed to the 52 volt regulator stage 12. The resultant voltage is thus output from pin 3 on multiplexer 140 and is passed to the 52 volt regulator stage illustrated in Figure 9. This process thus also serves as a digital to analogue convertor.

Capacitor 142 is used to give a gradual change to the control voltage so that the patient does not feel a jolt every time the voltage changes.

Referring to Figure 9, the 52 volt regulator is switched on and off by the micro- controller 22 through a relay 144.

At the end of the treatment cycle the relay 144 is switched off to remove the - treatment voltage from the patient. This essentially protects the device from short circuits when disconnecting the electrodes.

A microprocessor 146 acts as a switching regulator that generates 52 volts from a 12 volt input. Diodes 148 and 150 together with resistor 152 protect the output voltage from exceeding 57 volts. A variable resistor 152 is used to trim the voltage to the patient output stage to 41.2 volts.

This is done because it is better to generate a higher voltage than to work on the analogue to digital convertor voltage level of 41.2 volt, due to the fact that the regulation of the voltage levels may vary, and this has a direct bearing on the accuracy of the measurements. Thus, trimming 52 volts to 41.2 volis makes the readings more accurate.

It will thus be appreciated that the 52 volt output to the patient output stage is not higher than 41.2 volts under normal conditions.

The line marked “voltage control in” from the voltage control stage 82 is thus used to amplitude modulate the treatment voltage based on the sample tissue resistance.

The output of the regulator 146 is adjusted by the feed back value from the multiplexer 140 illustrated in Figure 8. The more the value selected on the multiplexer 140, the lower the output will be from the regulator 146. For example, if the micro-controller 156 in Figure 10 senses a resistance during the treatment and decides the amplitude of the treatment voltage is to high (judging by the calculated tissue resistance), it then writes a value to the : multiplexer 140 to decrease the amplitude. The value written is 0 for a high amplitude and 7 for a very low amplitude with anything in between for a medium amplitude.

Figure 10 illustrates the micro-controller 22 which is the heart of the circuitry of the present invention. Microprocessor 156 is an 8 bit microprocessor. Inputs

RAO, RA1 and RA3 are configured as 10 bit analogue to digital converters.

These are indicated as “patient sense in", “voltage level sense” and "battery sense in’.

LED 158 indicates whether the power is on or off LED 160 indicates the initial tissue resistance by its rate of flashing, as will be explained in more detail below.

LED 162 indicates low battery and patient sensed conditions. The LED 162 thus provides a double function. If the microprocessor detects that the battery has reached a low level then all functions are stopped and the LED 162 glows to indicate “low battery”.

During a patient sense condition, both LEDs 160 and 162 flicker, indicating that the patient's resistance was sensed and a treatment parameter selected.

A buzzer 164 provides an audible indication of the above conditions. The buzzer buzzes once the patient is sensed to alert him or her to press the start button to commence the treatment.

The start button is a tactile switch 166, while tactile switch 168 is used to temporarily suspend treatment. Finally, tactile switch 170 is a security switch mounted on the enclosure to protect against unauthorised opening.

A flow diagram illustrated in Figure 11 shows the steps carried out by the microprocessor 154 when the apparatus is in operation.

Referring to Figure 11, the apparatus is switched on and the programme started 172. The microprocessor 154 conducts an apparatus check 174. This apparatus check 174 is a security check to see if the enclosure was opened. If the enclosure has been opened then the machine shuts down completely 176, and no functions are available. The user is then requested to enter a security code 178. This is done via the RS232 link to the user's PC.

If both the apparatus check 174 and the security code entry 178 are positive, the apparatus goes into standby mode. The apparatus checks for a patient 180 by checking for any resistance on the “patient sense” input. If the measured resistance reading remains infinite, then the machine remains in the standby condition. The above is re-checked every 0.5 seconds until a patient is detected.

If a patient is detected, the first tissue resistance check 182 is conducted. A feed back voltage is generated on the “patient sense” lines. This voltage is compared against an internal 5 volt reference i.e. 41.2 volt will give a 5 volt, full scale deflection. This measured voltage is then used to calculate the initial tissue resistance and an initial treatment parameter is selected (low, medium or high — as illustrated in Figure 3).

If the tissue resistance is less than 40 kQ then the low treatment setting is selected, as illustrated in Figure 3A.

If the resistance is between 40 kQ2 and 65 k(Q2, the medium treatment setting is selected, as illustrated in Figure 3B.

Finally, if the resistance is between 65 kQ2 and 500 kQ, the high treatment setting is selected as illustrated in Figure 3C.

The first treatment wave is the waveform illustrated in one of the Figure 3a to 3c, depending on the level of treatment selected.

The waveform starts at 14.7 volts peak to peak and after 8 seconds adjusts to a higher level depending on the measured tissue resistance

The microprocessor 54 measures the time elapsed during the treatment, and after every 8 seconds the microprocessor 154 again checks the tissue resistance 190. This is done by the microprocessor 156 comparing the present treatment voltage which is fed back to it via the “Voltage level out” line In

Figure 7 and the voltage measured after the treatment waveform has passed through the treatment site. This is measured using the line “Patient sense out”, in Figure 7.

This tissue resistance check is carried out using the following formula:

Vdpt = patient voltage drop.

Rs = series resistor value. icp = Patient circuit current.

Vs = Voltage source.

Rt = Total Resistance.

Rpt = Patient’s tissue resistance.

Vdpt / Rs = lcp

Vs /lcp = Rt

Rt - Rs = Rpt

Once the patient resistance is calculated, the treatment voltage is then amplitude modulated according to the following table:

Resistance Treatment voltage kQ to 50 kQ 14.7 volts peak to peak 500 to 120 kQ 17 volts peak to peak © 120 kQ to 199 KO 22 volis peak to peak 200 kQ to 299 kQ 36 volts peak to peak 300 and 850 kQ 41.2 volts peak to peak

Thus, after the first reading, the wave shape is selected and this wave shape is amplitude modulate depending on further resistance measurements. This effectively makes the treatment two dimensional.

It should be noted that the peak to peak voltage levels listed in the above table are slightly different to the peak to peak voltage levels illustrated in Figure 3.

The prototype of the present embodiment was implemented using the same waveform shapes as those illustrated in Figure 3, but with the peak to peak voltage levels listed above.

Figure 12 is a schematic block diagram of an alternative embodiment of the voltage control stage 82 and 52 volt regulators stage 12 in analogue form. A tissue resistant sampling (TRS) control block 192 is connected to a tissue resistant sampling (TRS) power supply module 194.

Figure 13 is a more detailed circuit diagram of the TRS power supply module 194 of Figure 12.

Microprocessor 196 is a universal switching regulator which generates 52 volts from a 9 volt supply. This is supplied from a 9 volt battery which is connected to the circuit at connector 232 in Figure 14. The regulator 196 is biased through resistors 198 and 200.

The 52 volts is used to supply the treatment voltage and current. Capacitor 202 is used to filter the 52 volts.

Resistor 204 is a current sensing resistor. As the tissue resistance in the subject drops. more current is drawn through the apparatus. This causes a voltage drop across resistor 204 which is sensed by transistor 206 and 208. A feed-back network is established adjusting the regulator 196 through resistor 210 to supply less voltage as the tissue resistance decreases. Variable resistor 212 is used to set the maximum treatment voltage to 41.2 volts.

Capacitors 214 and 216 are used to stabilise and filter the feed-back loop.

The treatment voltage is output at the line labeled “treatment” which is passed to the processor and wave form generator. In this embodiment, the wave form generator works in the same manner as for the digital embodiment illustrated in

Figures 4 to 12. The pulse shapes are therefore the same with the only difference being the analogue method of amplitude modulation based on the tissue resistance.

Referring to Figure 14, the treatment voltage is passed to the TRS control stage 192 via the line labeled “treatment” to a transistor 216.

Microprocessor 218 is used to switch on and off the regulator 196 to conserve power, extending the battery life. This is done through transistors 234 and 236 in Figure 14.

Thus Figures 13 and 14 provide an analogue alternative method of implementing the apparatus.

The apparatus amplitude modulates the treatment wave based on the current through the current sensing resistor 104.

It will thus be appreciated that in the amplitude modulating embodiments, the treatment wave is adjusted continually throughout the treatments cycle to give an effective treatment. This is because the tissue resistance varies greatly (depending on the degree of injury) during the treatment.

The apparatus automatically maintains the balance between using a higher voltage and current to break down resistance due to inflammation, metabolic waste and poorer than normal blood circulation, and the need to use a lower voltage and current to avoid tissue damage.

It has also been found in trials that treatment is most effective if applied to the point on the body with the maximum electrical resistance. The apparatus of the present invention aided the user in identifying this point on the body as the rate of flashing of the status light indicated to the user the point of highest

' resistance. If a point of low resistance is selected, the user is able to reposition the electrodes accordingly. it has also been found that if the amplitude is not amplitude modulated the excessive voltage and current counter acts the treatment benefit. Thus healing occurs more quickly in the patient using the present invention.

Finally, it has further been found that this point of highest resistance is in fact often not where the actual pain is felt. This obviously greatly aides the user of the machine in giving a safe and effective treatment.

Claims (11)

¥ CLAIMS:

1. Apparatus for electrically stimulating the body of a subject, the apparatus comprising: measuring means for measuring the tissue resistance of the body at a : treatment site, a waveform generator arranged to generate an electrical waveform to be applied to the treatment site of the body; control means for controlling the waveform generator to adjust at least one characteristic of the electrical waveform depending on the measured resistance of the body at the treatment site; and output means for applying the adjusted electrical waveform to the body at the treatment site.

2. Apparatus according to claim 1 wherein the waveform varies in one or more of the parameters including frequency, duration, DC offset, the voltage level or magnitude and the output waveform shape.

3. Apparatus according to claim 1 or claim 2 wherein the measuring means is adapted to repeatedly measure the tissue resistance of the body at the treatment site dunng a treatment cycle. and wherein the control means is adapted to control the waveform generator to adjust at least one characteristic of the electrical waveform depending on each of the measured resistances during the treatment cycle.

4. Apparatus according to claim 3 wherein the measuring means is adapted to initially measure the tissue resistance of the body at the treatment site and to pass the initial measured resistance to the control means, and wherein the control means is adapted to select a waveform depending on the initial measured resistance.

5. Apparatus according to claim 4 wherein the control means is adapted to control the waveform generator to adjust the amplitude of the initially selected waveform depending on each of the measured resistances during the treatment cycle.

6. Apparatus according to any preceding claim wherein the measuring means includes a comparator for comparing the level of a voltage signal which has been applied to the body at the treatment site with a reference voltage signal.

7. Apparatus according to any preceding claim wherein the control means is a microprocessor.

8 A method of electrically stimulating the body of a subject comprising the steps of: measuring the tissue resistance of the body at a treatment site: adjusting at least one characteristic of an electrical waveform depending on the measured resistance of the body at the treatment site; and applying the adjusted electrical waveform to the body at the treatment site.

9. A method according to claim 8 wherein one or more of the parameters of the waveform which are adjusted include frequency, duration, DC offset, the voltage level or magnitude and the output waveform shape.

10. A method according to claim 8 or claim 9 further comprising the steps of measuring an initial resistance of the body at the treatment site and selecting a waveform depending on the initial measured resistance.

11. A method according to claim 10 wherein after the initial resistance has been measured, the resistance of the body at the treatment site is repeatedly measured and the initially selected waveform is amplitude modulated depending on these measurements.