Abstract:
In the case of current stimulation apparatus, in particular, protection installations are necessary which, in every instance, disconnect the patient from the operating voltage of the apparatus when maximally permissible patient currents are exceeded. For this purpose, for example, safety fuses or also electromagnetic disconnection devices (relays) are known. In accordance with the disclosure, a safety circuit for disconnecting the patient output from the operating voltage is employed as the protection installation which also responds when the patient current deviates from a preselected desired value adjusted on an intensity adjustment member. For this purpose, an output stage as current waveform generator for constant-current stimulation current is connected with the safety circuit.

Description:
BACKGROUND OF THE INVENTION 
     The invention relates to an electromedical apparatus, in particular, a current stimulation apparatus, comprising an output stage as the current waveform generator for constant-current stimulation current and comprising a protection installation, associated with the output stage, which disconnects the patient output from the operating voltage when a specifiable maximum value of the patient current is exceeded. 
     Known current stimulation apparatus have output stages for constant-current supply of patient current which operate with a relatively high operating voltage (up to several hundred volts). It is thus to be guaranteed that, even in the case of extremely high patient resistances, the currents with a correspondingly high operating voltage requirement, which are necessary for therapy or diagnosis, respectively, can still be applied. 
     Due to the high operating voltages connected to the patient, the danger could exist that, in the case of malfunction, the current flowing through the patient may assume dangerously high values. According to VDE-regulations, therefore, special protection installations are prescribed for all current stimulation apparatus and diagnosis apparatus which limit (or restrict) the patient current, in every possible instance, to I Patient  &lt;80 mA eff . 
     From the German OS No. 2,749,792 (U.S. Pat. No. 4,200,108), a stimulation current apparatus is also known in which the protection installation is specially designed such that, in the case of increases in the patient current, brought about through rapid resistance changes, the operating voltage control unit of the apparatus is correspondingly downwardly controlled. This protection installation thus offers protection, in particular, for such an instance of malfunction in which the current supply electrodes become detached (or separated) from the application location, whereby the current transfer surface is reduced, so that the current density on the skin at the point of application can assume very high value and can thereby lead to unpleasant irritations. This protection installation is also effective against excessively rapid intensity changes resulting from too rapid operation of the intensity adjustment element. By contrast, this known circuit does not become engaged when the actually flowing patient current assumes an excessively high value due to apparatus errors, or the like. Current stimulation apparatus of the state of the art exhibits, for this purpose, so-called quick-acting safety fuses, for example, in the patient current circuit as a protection installation, which fuses are destroyed by correspondingly high currents and thus interrupt the current flow. Safety fuses of this type are, however, as is known, imprecise and also unsafe. Also, electromagnetic switching installations (relays) here bring about no fundamental improvements. Although they switch more rapidly (in the millisecond range), they are able to provide only protection against a specified excess current. 
     SUMMARY OF THE INVENTION 
     Therefore, it is the object of the invention to disclose an improved protection installation in the case of electromedical apparatus. 
     The object is solved in accordance with the invention in that, as the protection installation, a safety circuit for disconnecting the patient output from the operating voltage responds even when the patient current differs from the value adjusted at an intensity adjustment member, for which purpose the output stage is connected with the safety circuit. 
     Preferably, the safety circuit comprises a bistable switching stage, for example formed of two transistors in a push-pull circuit, which connects the patient output to the operating voltage in the one state, and which disconnects it from the operating voltage in the other state. However, the switching stage is here not only controlled solely by an excess current signal, but additionally by a signal obtained through comparison of a selected desired value of the patient current with the actually flowing current. The deviation of the patient current from the preselected desired or nominal value thus likewise effects a disconnection of the safety installation. Such a deviation, which can also consist in an undercurrent (insufficient current), is an indication of an apparatus malfunction, so that the operating voltage is also interrupted when the selected maximum value of the patient current for disconnection has not yet been attained. 
     In the case of a preferred realization of the safety circuit with the bistable transistor switching stage, through feedback of the collector path of the second transistor of the bistable switching stage to the emitter path of the first transistor of the bistable switching stage, the flipping over or switching off of the switching stage will occur already in the microsecond time range (&lt;10 μs). This switched-off state will be maintained for as long as the operating voltage is connected. Only through disconnection and subsequent reconnection of the operating voltage does the patient current circuit with the first transistor become conductive again. The speed of the electronic safety device produced in this manner is thus more rapid by powers of ten as compared with the known safety fuses and switches. 
     Further advantages and details of the invention are apparent from the following Figure description of an exemplary embodiment on the basis of the drawing in conjunction with the remaining subclaims; and other objects, and advantages will be apparent from this detailed disclosure and from the appended claims. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 illustrates a current stimulation apparatus with an output stage and a safety circuit in the form of the block circuit diagram; and 
     FIG. 2 illustrates an embodiment of the inventive safety circuit. 
    
    
     DETAILED DESCRIPTION 
     In FIGS. 1 and 2, 1 denotes a terminal for receiving a control voltage U ST  of a current stimulation apparatus, and 2 denotes a terminal for receiving an operating voltage source. The operating voltage U B  amounts, for example, to +200 V. 
     A control voltage, U ST  which is proportional to the desired patient current, is fed in by an intensity adjustment member (not illustrated) via the control input 1. Patient currents of desired waveform can be preselected. Between the voltage terminals 1 and 2 and patient outputs 4 and 7, a safety circuit 3, as protection installation, is connected, on the one hand; and an output stage 5, as current waveform generator, is connected, on the other hand. Corresponding currents are generated from the control voltages by the output stage 5; the output stage 5 is connected to ground potential via a defined resistance 6. The current flowing in the resistance 6 corresponds to the patient current I Patient . 
     From FIG. 1 it is apparent that the apparatus is so designed that the safety circuit lies in the operating voltage-conducting branch, on the one hand, but that the safety circuit also receives inputs from terminal 1 and from defined resistance 6 for representing the preselected and actual patient currents, on the other hand. The more detailed function of this inventive safety circuit can be seen in detail from FIG. 2. 
     In the operating voltage branch with terminals 2 and 4, a first transistor 31 is arranged in emitter-connection, whose collector path is connected to the output 4. A second transistor 32 in emitter-connection is connected in parallel to terminal 2, the voltage at the emitter of the first transistor 31, which voltage is decreased as a function of current in a resistance 33, being connected, via a resistance 34, as the control signal, to the base of the second transistor 32. The second transistor 32, in turn, is connected on the collector-side, as control line, to the base of the first transistor 31. Thus, a bistable flip-flop is formed by the transistors 31, 32, in push-pull connection. This flip-flop stage connects the patient output 4, in the one state, to the operating voltage U B , and in the other state, disconnects it from the operating voltage U B . The control signal for the switching stage thus formed is derived from the value of the patient current itself, on the one hand, and from a comparison of the adjusted patient current U ST  with the actually flowing current, on the other hand. On the collector side, there is additionally parallel-connected to the second transistor 32 an RC-network with resistance 35 and capacitor 38, whereas the collector side of the first transistor 31 is fed back to the emitter path of the first transistor via a blocking diode 39 and a voltage divider with resistances 36 and 37. 
     The control line for the second transistor 32 is connected to the output of a comparator 41 via a third switching transistor 40. There is transmitted to the inputs of the comparator 41, the control voltage U ST , selected at the intensity adjustment member, and a voltage corresponding to the actually flowing patient current I Patient . For this purpose, the control voltage U ST  is connected, in a low-resistance fashion, from the control input 1 via an operational amplifier 42 to the inverting input of the comparator 41. The operational amplifier 42 has a feedback connection from its output to its inverting input. Connected to the other input of comparator 41 is the voltage developed across the defined resistance 6, which voltage is proportional to the patient current. Via a voltage divider with resistances 43 and 44, this voltage is reduced by a specifiable value, for example, 20%. The input lines of the comparator 41 are coupled together via a capacitor 45. By means of the capacitor 45, upon selection of an intensity value as the command (or desired) value, the control voltage U ST  is initially overcoupled to the second line. The consequence of this is that, at the comparator 41, the effectively present current intensities are actually compared, and that an output signal is generated not solely on the basis of phase shifts in the case of changing current waveforms. 
     During operation of the current stimulation apparatus with the described safety circuit, in the normal case, the first transistor 31 is conductive, so that the operating voltage is connected to the patient output 4 via the emitter-collector-path. The preselected current is generated via the output stage 5 and connected to the other patient output 7. If the current now increases beyond the safety-determined (or stipulated) limit value of 80 mA, the voltage drop at the resistance 33 increases correspondingly. If this voltage drop is greater than the threshold base-emitter voltage of the transistor 32, transistor 32 passes over into the conductive state, whereby the base of the first transistor 31 is activated by the collector path. Transistor 31 is thereby blocked; the cutoff protection has been activated (or triggered). 
     Due to the feedback via the resistance 36, the flipping-over of the bistable switching stage, formed by the transistors 31 and 32, and the blocking of the first transistor 31, proceeds very rapidly. Values of less than ten microseconds (&lt;10 μs) are obtained. The blocking of transistor 31 is maintained for as long as the operating voltage U B  is connected. After disconnection of the operating voltage U B , the collector path of the transistor 32 is discharged via the RC-network 35, 38. With reconnection of the operating voltage, the switching stage is then always in the state in which the first transistor 31 is conducting; i.e., in which the cutoff protection is in its reset condition permitting normal operation. 
     With the comparator 41, during operation of the electromedical apparatus, the voltage drop at the resistance 6, which is reduced according to a preselected quantity, is constantly compared with the control voltage U ST . Thus, if the patient current exceeds the value preselected at the intensity adjustment member connected with control input 1, for example by 20%, the comparator 41 switches over and the output signal of the comparator 41 is added to the control signal for the second transistor 32. In this manner, the second transistor is rendered conductive and, accordingly, the first transistor 31 is blocked. 
     In the exemplary embodiment according to FIG. 2, an operational amplifier as comparator 41 is specifically so connected that solely patient current values which are greater than the selected command (or desired) value lead to the additional activation of the safety circuit. Through connection of an additional operational amplifier with reverse polarity; or utilization of a window discriminator instead of comparator 41, all deviations (deviations of either polarity) can be detected and utilized as control signal for the bistable switching stage with transistors 31 and 32. 
     The cutoff protection, formed by the described circuit, thus becomes operative (or effective) in every instance even when the patient current, indeed, still lies below the allowable maximum current, but differs from the value preselected at the intensity adjustment member connected to control input 1. A particularly effective protection of the patient against malfunctions of the current stimulation apparatus is thus provided. 
     It will be apparent that many modifications and variations may be effected without departing from the scope of the novel concepts and teachings of the present invention. 
     A showing of the intensity adjustment member which supplies the preselected control value U ST  at input terminal 1 is found in the first figure of German application No. P 29 39 234.5 filed Sept. 27, 1979, and in the corresponding U.S. application in the names of Kurt Weigert and Karl Hudek, U.S. Ser. No. 182,466 filed Aug. 28, 1980.