Abstract:
An apparatus for testing the impedance of a medical lead connecting an implantable stimulation device to a nerve or a muscle. The implantable device is of the type comprising a capacitor for stimulating the nerve or the muscle. The system of the invention comprises a current generator for generating a testing current “I” during a calibrated testing pulse and a power circuit coupled to the capacitor and to the current generator for determining if the capacitor is charged by the testing current during the calibrated testing pulse.

Description:
BACKGROUND OF INVENTION 
     1. Technical Field 
     The present invention relates to medical implantable devices and more particularly to the test of medical lead impedance of implantable neurostimulators used for paraplegic people. 
     2. Background Art 
     It is a reality that either for natural reasons or consecutively to a malady or a traumatism, nerves or muscles of the human body may be affected and suffer from lack of stimulation or at least from a deficient stimulation. Electronic solutions to replace such deficiencies use Functional Electrical Stimulation devices (FES). State of the art of such devices mainly address the crucial problem of the heart attack with the well-known pacemakers. However, another important concern in which research of implantable solution is very active is the paraplegia domain in which the muscles are inert due to spinal injury. 
     Contrary to some devices like pacemakers which run continuously and need real time measurements, the implantable devices for paraplegic alternate active and inactive time periods which offer room for different measurements. Furthermore, as the generation of human walking is a complex algorithm, a great number of medical leads are used at the same time to activate the muscles of the inactivated legs (typically 16 or more leads are used as compared to two leads for cardiac pacemaker). In case some of the leads break, the device may be still active, and it is less vital that a real time integrity measurement be made for paraplegia devices than for pacemakers. Therefore, implantable devices for paraplegic do not need to be tested in real time, and checking can be done either at the system setup or during an active stimulation. 
     Generally, an implantable device performs two main functions: stimulation and telemetry. The stimulation is obtained by the generation of electrical pulses in order to deliver a current through a network of electrodes and medical leads in contact with the part of the body to be stimulated (muscle or nerve). The telemetry function is a feedback operation which allows to get information onto the integrity of the implantable device, such as measuring the level of power supply of the component or testing the value of the lead impedance formed by the association of the stimulated tissue plus the electrode in contact with it plus the wire connecting the electrode to the implantable device. 
     A first problem arises when the network of electrodes is placed within the body of a patient because several damages may affect the integrity of the leads such as partial or a total fracture in the electrical wires. However, the leads may be tested during the surgical operation which is not the purpose of the present invention. 
     Moreover, such kind of injury and others such as fibrosis may also appear during the time period the patient is using the stimulation system. It is therefore mandatory that these elements be tested regularly to be sure that stimulation pulses are sent with efficiency. Because of the non-accessibility of the F.E.S. device which is embedded within the patient body, the testing of the components is not easy. However, the testing of lead impedance has been addressed in many patents and only a few are described immediately hereinafter. 
     U.S. Pat. No. 4,949,720 discloses an apparatus for measuring the lead impedance in a pacemaker. The invention includes a large number of FET transistors operated in parallel to discharge a capacitor though the heart tissue. Pacer lead current is monitored by measuring the current through a small number of these transistors. The current monitoring function is performed by a current-to-voltage converter coupled to an analog-to-digital converter which may make one or more voltage measurements during the output pulse. 
     U.S. Pat. No. 4,140,131 disclose an apparatus for stimulating body tissue and in particular the heart, as including a device or circuit responsive to the initiation of stimulation and/or to the failure or pending failure of a component of the stimulating apparatus to provide the patient with a perceivable stimulation to a second, different portion of body tissue. There is disclosed an impedance level detector for sensing the impedance presented between the outputs of the stimulation apparatus to provide a warning signal indicating that the output impedance falls outside a predetermined range. In particular, the impedance level detector output is sensed by stimulation control logic to apply a first train of pulses at a first rate to an auxiliary electrode for stimulating the second portion of tissue. Further, there is included a voltage level detector for sensing when the power source voltage depletes below a predetermined level, to actuate the stimulation control logic to provide a second train of warning pulses to the auxiliary electrode, at a second, different rate than that of the first train. In this fashion, the patient not only is warned as to the pending failure or failure of a component of his pacemaker, but also is able to identify the failing component. 
     With these solutions, either the voltage (V) or the current (I) are measured across the lead impedance (R) and the final value of the impedance is obtained by computing the well-known Ohm&#39;s equation: V=R×I. 
     Another approach to determine the integrity of the leads consists in measuring a stimulation capacitor voltage and to deduce the impedance lead value from a well-known general equation: 
     
       
           V=V 0 ×e   (−T/RC) , 
       
     
     wherein V0 is the initial voltage of the stimulation capacitor and (1/RC) is the time constant. Such method is illustrated in the two following patents: In U.S. Pat. No. 5,891,179 from Er et al., a real-time impedance monitoring system is provided for use with an implantable medical device having an implantable electrical lead. The impedance monitoring system includes components for determining the electrical impedance of the lead as a function of time, with the determination being made substantially in real-time, and components for graphically displaying the electrical impedance of the lead as a function of time, with the display also being generated substantially in real-time. 
     In U.S. Pat. No. 5,201,865 from Kuehn, a method and apparatus for measuring lead impedance during pre selected test mode operation of an implantable body tissue stimulator is presented. Analysis circuitry is periodically triggered into operation, such as on each reprogramming by the physician or periodically as a function of elapsed time or number of stimulation events counted from the preceding measurement. The actual lead impedance, measured from the output circuit from the body tissue-stimulator pulse generator, and taking into account impedance of the interconnection between the lead connector pin and the pulse generator connector block, the lead electrical conductor and its connections with the electrode and the connector pin and the electrode-tissue interface, is calculated as a function of the ratio of the elapsed times that it takes to discharge a capacitor from a first reference voltage to a second reference voltage through a precision resistor and through the lead impedance itself. The calculated lead impedance may be stored in memory with a suitable time tag, employed to automatically effect a change in operating modes or change a lead and electrode selection, if measured lead impedance falls outside normal high and low impedance boundary values. In the pacing context, calculated lead impedance may be employed to adjust sense amplifier sensitivity and pacing output pulse parameters. The method and apparatus may also be employed to calculate cardioversion/defibrillation lead impedance through selective partial discharge of high voltage output capacitors. 
     The &#39;179 solution implies that the voltage measurement be picked at the end of the impedance path and thus that an extra-wire from the implant is required. Then, in case of paraplegia where a great number of electrodes are required, such solution is not acceptable due to the number of extra-wires that would be required. 
     Furthermore, in paraplegic application, the leads are not necessarily proximate to the FES device and the wiring may be long and difficult to access, thereby rejecting a solution as the aforementioned one. 
     In the &#39;865 patent, the reader may assume that all the circuitry is powered with a unique common voltage VDD. In the case a high power voltage (i.e. 40 volts) is used for the stimulation capacitor and a lower voltage (5 volts) is used for the rest of the circuitry, this prior art solution is not acceptable as such because the circuitry must be adapted. 
     An important constraint of paraplegia is the need of interconnection of low powered components (the implant is powered less than 5 v) with high power output stages (high voltage over 40V and high current over 20 mA ) which are needed for the stimulations. 
     Therefore, even if the cited pacemakers solutions are convenient for the measurement of lead impedance in the cardiology context, none of the aforementioned patents offers a complete solution for testing the functionality of leads impedance in paraplegic implantable device. 
     Thus, there is still a need for a simple, short time functional testing solution for paraplegic implantable stimulator. 
     SUMMARY OF INVENTION 
     It is therefore a feature of the present invention to provide an apparatus for testing the impedance of a medical lead connecting an implantable stimulation device to a nerve or a muscle. The implantable device is of the type comprising a capacitor for stimulating the nerve or the muscle. The system of the invention comprises a current generator for generating a testing current “I” during a calibrated testing pulse and a power circuit coupled to the capacitor and to the current generator for determining if the capacitor is charged by the testing current during the calibrated testing pulse. 
     Various other objects, features, and attendant advantages of the present invention will become more fully appreciated as the same becomes better understood when considered in conjunction with the accompanying drawings, in which like reference characters designate the same or similar parts throughout the several views. 
    
    
     BRIEF DESCRIPTION OF DRAWINGS 
     FIG. 1 shows an implantable FES using the circuit of the present invention. 
     FIG. 2 shows a general block diagram of the relevant circuits of the present invention. 
     FIG. 3 shows a preferred implementation of control circuit of the present invention. 
     FIG. 4 is a timing diagram representing the main signals flowing in the circuits of FIG.  2 . 
    
    
     DETAILED DESCRIPTION 
     In a preferred embodiment, the current generator comprises a pulse generator for generating the calibrated testing pulse. According to the type of test, i.e. the test of a nerve or of a muscle, a respective resistive path allows to maintain an adapted voltage at the output of the pulse generator. 
     A determination device preferably in the form of a power transistor is base-connected to the output of the pulse generator. A measure of the voltage resulting at the emitter point of the power transistor is made to determine if the testing current is provided by the collector path of the transistor or by the base path. If the tested channel is connected, the testing current flows from the high voltage of the muscle or nerve to the current generator, thereby charging the stimulation capacitor. In case the tested channel is disconnected, the current sunk by the current generator must be provided by one of the resistive path, thereby generating a characteristic voltage drop at the output of the pulse generator. The chosen voltage drops are such that no stimulation may be applied to the muscle or nerve. 
     Referring to FIG. 1, an exemplary implantable device  100  to be implanted in a paraplegic patient is described. To the extent that certain components of device  100  are conventional for stimulation application in their design and operation, such components (a conventional programmer for example) will not be described here and only the circuits operating in conjunction with the present invention are now described. The implantable device  100  comprises an Application Specific Integrated Circuit (generally ASIC)  102  powered at a VDD voltage. The ASIC to be detailed later receives input data on a “DATA” line from external circuitry to be operated either for stimulation or for measurements. The measured data are outputted on a “MEASURE” line. A controllable switch component having a command line  104  allows to discharge a stimulation capacitor  106  after each stimulation operation. The stimulation capacitor  106  is connected between an electrode  108  and a power transistor  110  in which flows a stimulation current I. The electrode  108  is connected to a muscle  112 . It is to be understood that the electrode could be connected to a nerve for neural stimulation. As already explained, muscle and neural stimulation for paraplegia require a high power voltage VPP due to the intrinsic impedance of the muscle or the nerve. 
     The details of the relevant circuits which make up the testing device of the present invention are shown in block circuit form in FIG. 2 with reference numerals being the same for circuits identical to FIG.  1 . For sake of clarity, only two electrodes are shown on the figure but one of ordinary skill could easily extend the concept of the invention to a plurality of electrodes either connected to muscle or nerve. 
     A first Digital to Analog current converter  200  provides a first programmable calibrated stimulation current “I” to feed epymisial channels, depending on the value of intensity weights on the D/A converter input. A second Digital to Analog current converter (not shown for clarity reason) provides a second programmable calibrated stimulation current to feed neural channels. Each DAC may be conventional n-bits converter. Both DACs may be respectively activated by a command signal “CO” which enables the stimulation current “I” to be sunk only when the input data (the binary intensity weights) have reached stable values. In the preferred embodiment, the first DAC  200  is a 8-bits converter for muscle stimulation while the second DAC is a 6-bits converter for neural stimulation, and the stimulation current I 1  is in the range of 0 to 25 mA for the epymisial case while the stimulation current I 2  is in the range of 0 to 3 mA for the neural case. 
     A control circuit  202  inputting epymisial and neural selection signals “EPY” and “NEU” is coupled to power transistor  110 . Preferably, one control circuit is associated to each electrode  108  and one circuit is active at a time. The selection of the active control circuit may be realized by common address decoding circuits (illustrated as address bits A 0 -An on FIG.  2 ). In response to a pulsed input signal “PW”, control circuit  202  generates a calibrated command stimulation signal “STI” on base of power transistor  110 . 
     One preferred implementation of control circuit  202  is shown on FIG. 3, with circuit  300  being composed of five transistors of FET type. However, the person skilled in the art will easily devise other circuit design required by other technology such as bipolar transistors for example. Transistor T 1  receives on its gate the “EPY” signal and having its drain connected to power voltage VDD. Similarly transistor T 3  receives on its gate the “NEU” signal and having its drain connected to power voltage VDD. The source of T 1  is connected to the drain of transistor T 2 . The source of T 3  is connected to the drain of transistor T 4 . Transistors T 2  and T 4  are gate connected to receive the pulsed command signal “PW”. The sources of T 2  and T 4  are connected to the drain of transistor T 5  which also inputs on its gate pulsed command signal “PW”. The source of T 5  is connected to low voltage (ground voltage VG). The output “STI” of control circuit  300  is available on the output line  302 . 
     During testing operation, a pulse signal “PW” is applied to the gate of transistors (T 2 ,T 4 ) to turn them ON and to the gate of transistor T 5  to turn OFF. According to the active selection signal “EPY” or “NEU”, one of the transistor T 1  or T 3  is ON, which means testing of a nerve channel or a muscle channel. The initial high voltage (VDD) on output line  302  is lowered by the voltage drop due to the resistive path made by either T 1  and T 2  or by T 3  and T 4 . If there is no electrical discontinuity in the tested channel, the voltage on output line  302  (i.e. the base voltage of power transistor  110 ) remains high, and the current sunk in the emitter of power transistor  110  is provided by the collector of this latter. If there is an electrical discontinuity in the tested channel, the voltage on output line  302  decreases significantly as the current sunk in the emitter of transistor  110  is provided by the resistive path of control circuit  300  through the base of power transistor  110 . The value of each resistive path is chosen such that the current for epymisial testing or neural testing allows a significant voltage drop while avoiding a stimulation effect of the nerve or muscle. 
     In the preferred implementation, for a power voltage VDD of 5V, the voltage drop is around 2V with a current of 2 mA for epymisial testing and a current of 200 uA for neural testing. 
     Coming back to FIG. 2, the circuits to measure the voltage on the emitter of power transistor  110  will now be described. 
     A voltage Analog to Digital Converter  204  is connected to the emitter of power transistor  110 . A sampling clock signal “SC” is applied to the input of the converter  204  to sample the voltage of the emitter. The voltage is converted in a well-known manner into parallel bits. The parallel bits are stored in a register circuit  206 . Register  206  is a common shift register which swaps the parallel bits into serial bits. The serial bits are outputted at a predetermined cycle time defined by a register clock “RC”. 
     In an alternate embodiment, a selector circuit may be connected between the emitter of transistor  110  and A/D converter  204  in order to sample other analog data such as battery voltage level, power supplies measurement. A selector signal can be easily designed by a person skilled in the art to determine the selected type of data to be measured. 
     On top of FIG. 2, an electrical representation of the muscle or the nerve is shown. The muscle or nerve is generally represented as a resistor  112 . The electrode plus the lead is schematically illustrated by connection point  108 . Stimulation capacitor  106  is in series between the lead connection point  108  and the collector of power transistor  110 . A switch circuit is connected between the collector of power transistor  110  and the high power voltage VPP. The switch is closed in inactive mode (no stimulation, no test) to allow the stimulation capacitor  106  to be discharged through the impedance path ( 108 , 112 ). 
     The switch is open in stimulation or testing mode, and a controlled current is sunk through the power transistor  110  during a calibrated time window “STI” as previously explained. The capacitor is thus charged at a constant current I. In a preferred embodiment, the high power voltage is 40V in order to get a sufficient current (&gt;20 mA ) to the muscle, which offers a resistive value in the range of 1500 ohms. 
     To restate, the principle of impedance testing is to detect if a current is flowing from the muscle or nerve through the stimulation capacitor Cs. If the electrode is connected to the muscle (or the nerve) there is a current flowing in the muscle (or the nerve), but if no current is flowing it means that the electrode is disconnected from the muscle (or the nerve) or broken. 
     Reference is now made to the timing diagram of FIG.  4 . To operate in test mode the circuit of the invention, a DC current is first determined by the settings of the intensity weights on the D/A converter inputs, which are validated by the “CO” command. 
     Then the type of stimulation is selected by activation of one of the selection signals “EPY” or “NEU”. The stimulation signal “STI” is generated on the output of the control circuit and one power transistor  110  becomes active as already explained. During the duration of the stimulation signal “STI”, the sampling clock “SC” is running in order to convert the emitter voltage of the active power transistor  110  into P-binary data. Finally, the P-binary data are stored into a P-bits register  206 . The stored data are then serially outputted as shown on last line “MEASURE” of FIG. 4 during P clock cycles “RC”. The output pattern is thus representative of the voltage emitter status. 
     One advantage the solution is that the test is completely performed internally to the implantable device. Other advantages include that only a few devices are required, no extra I/Os are required for the ASIC, and no extra feed through wires for the implant are required. Still other advantages include no extra external components are required for the implant, implant is simpler (no extra wiring), the power consumption is optimized (the ASIC is low powered), there is no limitation of the number of lead connections, and the solution offers flexibility on the timings as the impedance test can be performed either during a normal stimulation or during a test mode without any stimulation of the muscle or the nerve. 
     It is to be understood that the provided illustrative examples are by no means exhaustive of the many possible uses for my invention. 
     From the foregoing description, one skilled in the art can easily ascertain the essential characteristics of this invention and, without departing from the spirit and scope thereof, can make various changes and modifications of the invention to adapt it to various usages and conditions. 
     It is to be understood that the present invention is not limited to the sole embodiment described above, but encompasses any and all embodiments within the scope of the following claims: