Abstract:
An active implantable medical device having an RF telemetry circuit. The device is in particular a stimulation, resynchronization, defibrillation and/or cardioversion device. It includes a principal circuit, an RF telemetry auxiliary circuit and a supply battery for the principal and auxiliary circuits. It is envisaged to have between the supply battery and the auxiliary circuit a regulating circuit including an accumulator of electric power coupled with the auxiliary circuit to deliver a current ready to feed the auxiliary circuit, and a load circuit coupled with the supply battery to maintain the accumulator on a predetermined level of load.

Description:
FIELD OF THE INVENTION 
   This invention relates to “active implantable medical devices” as defined by the Jun. 20, 1990 directive 90/385/CEE of the Council of the European Communities. 
   BACKGROUND OF THE INVENTION 
   The above-identified definition includes in particular devices that monitor cardiac activity and generate impulses of stimulation, resynchronization, defibrillation, and/or cardioversion in the event the device detects a disorder in heart rate. It also includes, for example, neurological devices, pumps for distribution of medical substances, cochlear implants, and implanted biological sensors, as well as devices for measurement of pH or bio-impedance (such as trans-pulmonary impedance or intracardiac impedance measurements). 
   With such devices, it is possible to operate a data exchange with a “programmer,” which is an external instrument that can be used to check the parameter settings of the devices, to read information recorded by the devices, to register information with the devices, and to update the internal control software of the devices. This data exchange is carried out by telemetry, i.e., by a technique of remote transmission of information, without galvanic contact. Until now, telemetry has primarily been carried out by magnetic coupling between coils in the implanted device and the programmer, which is a technique known as “process by induction.” This technique has certain disadvantages, however, because of the low range of an inductive coupling, which necessitates placing a “telemetry head” containing a coil in the vicinity of the implantation site of the active implantable medical device. 
   Implementation of a different nongalvanic coupling technique has been proposed, using the two components of an electromagnetic wave produced by emitting/receiving circuits operating in the field of radio frequencies (RF), typically at frequencies around a few hundred MHz. This technique, known as RF telemetry, makes it possible to program or interrogate implants at distances greater than 3 meters, and thus carry out information exchanges without having to use a telemetry head, and even without intervention of an external operator. U.S. Patent Application Publication Nos. US2003/0114897 and US2003/0149459 describe implants and programmers equipped with such RF telemetry circuits. These RF circuits require, however, a current supply that is greater than what is necessary for the other circuits of the implant (e.g., the stimulation and detection circuits). For example, the current consumption of an RF circuit can exceed 3 mA during emission phases. 
   In the case of defibrillators, taking into account the significant amount of current required by circuits used to apply shock therapy, the batteries used have low internal resistance and can supply without difficulty currents of about a few mA. On the other hand, pacemakers and similar devices, such as multisite or resynchronization devices, are generally supplied by small-size lithium-iodine batteries (or their equivalent), taking into account the low operating current required by the stimulation and detection circuits. These batteries have an internal resistance of about 100 Ω at the beginning of their life, which can increase to 1 kΩ, 2 kΩ, or more as the battery discharges. This internal resistance is not a problem for circuits with low consumption, but can prevent one from being able to provide RF circuits with the required level of current. 
   A first solution is to use a different type of battery, for example, a reduced size lithium-manganese (LiMnO 2 ) weldable button battery with low impedance. There are such batteries whose characteristics are: diameter 12.2 mm, height 1.4 mm, capacity 27 mA/h, nominal voltage 3 V, self-discharge maximum 1% per annum, and which can provide currents of several mA. The current of RF circuits is exclusively provided by the button battery. When the button battery can no longer provide the current, the lithium battery of the pacemaker can provide a low current of 10 μA, which allows the transmitter-receiver to work in pulsated mode. The peak current is provided by a capacitor belonging to a supply circuit controlling the voltage of the battery. 
   OBJECTS AND SUMMARY OF THE INVENTION 
   The present invention provides a novel solution to the above-identified problem, which does not require recourse to an additional battery, due to a circuit making it possible to provide to an RF telemetry circuit incorporated in an implant the high current necessary for operation. 
   For this purpose, the device of the invention, which includes a principal circuit, an auxiliary RF telemetry circuit, and a supply battery for the principal and auxiliary circuits, comprises, between the supply battery and the auxiliary circuit, a regulating circuit including an accumulator of electric power, coupled with the auxiliary circuit to deliver a current ready to feed this auxiliary circuit, and a load circuit coupled with the supply battery to maintain this accumulator with a predetermined level of load. The accumulator can be a rechargeable battery or a condenser. When the voltage corresponding to the predetermined level of load is higher than the voltage delivered by the supply battery, the load circuit includes a voltage multiplying stage. 
   Advantageously, the load circuit is a circuit with intermittent and cyclic operation. The cyclic report/ratio can be a variable report/ratio function of the internal resistance of the supply battery, with the relative duration of the feeding cycles of the regulating circuit decreasing when the aforementioned resistance or level of load increases. The load circuit can stop the load of the accumulator when the terminal voltage level of the accumulator reaches a predetermined upper limit, when the charging current of the accumulator reaches a predetermined lower limit, or after completion of a given maximum duration. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     One now will describe an example of implementation of the device of the present invention, by reference to the annexed drawings, wherein the same numerical references indicate identical elements from one figure to another and: 
       FIG. 1  is a simplified circuit diagram of the various elements constituting the feeding circuit of the invention; 
       FIG. 2  shows details of the voltage multiplier of the circuit of  FIG. 1 ; and 
       FIGS. 3 and 4  show the charge and discharge configurations of the voltage multiplier of  FIG. 2  according to the commutation states of the various switches. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   One now will describe an embodiment of the device of the invention, which can in particular be applied to the active implantable medical devices marketed by ELA Medical, Montrouge, France, such as the Symphony and Rhapsody-branded devices. These are devices with a programmable microprocessor comprising circuits to receive, format, and treat electric signals collected by implanted electrodes, and to deliver stimulation impulses to those electrodes. Adaptation of these devices to the implementation of the functions of the present invention is deemed to be within the ability of persons of ordinary skill in the art, and will not be described in detail (with regard to its software aspects, the invention can be implemented by suitable programming of the operating software of the pacemaker). 
   In  FIG. 1 , reference  10  indicates generally the RF telemetry circuits, which require a relatively high supply current (several mA), in particular during emission phases of the modulated signal. To deliver such a supply current, the invention proposes supplying these RF circuits starting from an accumulator  12 , itself charged by the supply battery  14  of the implanted device by means of a regulating circuit  16 . The supply battery  14  also supplies other circuits of the device (e.g., the detection and stimulation circuits). 
   Accumulator  12  can be an accumulator of the lithium-ion type, of which there are models of reduced size having characteristics compatible with the supply requirements for RF circuits in implanted devices, typically: capacity 10 mA/h, internal resistance 25 Ω uninterrupted and 8 Ω into alternate, self-discharge maximum of 15% per annum, and rechargeable 250 times with a maximum loss of capacity of 14%. Such accumulators are in particular manufactured by the company Quallion LLC, Sylmar, Calif., USA. Alternatively, the lithium-ion accumulator can be replaced by a condenser of very strong rated capacity, typically about 1 Farad. 
   The lithium-ion accumulators present a nominal voltage of 4 V at full load, which can then decrease to a value of about 3 V. Because the lithium-iodine batteries used in cardiac pacemakers have a nominal voltage of about 2.8 V, this voltage is insufficient to charge the accumulator  12  and it is therefore necessary to use an intermediate stage voltage multiplier  18 , making it possible to deliver to the accumulator a charging voltage of 2.8V×1.5=4.2 V. This voltage multiplier  18  is connected to the supply battery  14  by a switch  20  and to the accumulator  12  by a switch  22 . Its operation, and thus the load of the accumulator  12 , is controlled by a control circuit  24 , which includes a load checking circuit  26  whose entry is connected to a reference voltage standard V ref  and to the point between voltage divider resistors  28 ,  30 , which gives an indication of the terminal voltage of accumulator  12  and is brought into service by closing switch  32 . 
   The internal structure of the voltage multiplier  18  is illustrated in  FIG. 2 . It includes an entry  34  connected via switch  20  to the supply battery  14 , making it possible to charge a first condenser  36  by closing a switch  38 . This same entry also makes it possible to charge two condensers  40 ,  42  assembled in series, by closing a switch  44 . In addition, a switch  46  makes it possible to connect the point between condensers  40 ,  42  to the point between condenser  36  and switch  38 . Lastly, a switch  48  makes it possible to short-circuit the circuit formed by condensers  40  and  42 . 
   In the initial phase, corresponding to the configuration of  FIG. 3 , switches  20 ,  38 , and  44  are closed, while switches  22 ,  46 , and  48  are open. Condenser  36  is thus charged with the voltage of the battery (2.8 V) and condensers  40  and  42  are each charged with half of this voltage (1.4 V). In the subsequent phase, switches  20 ,  38 , and  44  are opened, while switches  22 ,  46 , and  48  are closed. Condensers  40  and  42  are then in parallel, and the voltage on their terminals (1.4 V) is added to the boundaries of condenser  36  (2.8 V), thus giving an exit voltage of 2.8+1.4=4.2 V. This voltage of 4.2 V produced by the voltage multiplier  18  is used to charge accumulator  12 , with a charging current which can vary from 2 to 0.1 mA, for example, according to changes in the internal resistance of the battery  14 . 
   Advantageously, this load of the accumulator  12  is operated in an intermittent and cyclic way, for example, with a 25% load during a cycle of 1 second, the remaining 75% being devoted to the supply of the other circuits (e.g. the detection and stimulation circuits) of the device. 
   Advantageously, the cyclic report/ratio (25% in the example above) is a variable report/ratio, a function of the internal resistance of the supply battery  14  (the duration of the phases of load becoming shorter when internal resistance increases) and/or of the load level of accumulator  12  (the duration of the cycles of load decreasing as the accumulator  12  approaches its level of maximum loading). 
   The load of the accumulator  12  continues thus until reaching a predetermined level, for example, when the load checking circuit  26  detects that the terminal voltage of the accumulator has reached 4 V. The load checking circuit  26  then operates to suspend the load until the terminal voltage of the accumulator  12  has fallen below a given threshold due to energy consumption by the RF circuits. 
   The load also can be stopped according to other criteria, for example, when the charging current reaches a low limit because of accumulator  12 , or at the end of a given maximum duration, for example, at the end of 100 hours for 10 mA of charging current. 
   RF circuits  10 , supplied with the energy stored in accumulator  12 , could be fed satisfactorily with a relatively significant output current, for example from 3 to 20 mA. 
   To take into account the difference between the terminal voltage of the accumulator (about 3 to 4 V according to the level of load) and the level of nominal voltage required for the supply of RF components (typically between 1.8 and 3 V), use of an adapted regulator is envisaged, for example, a linear or self-inductive regulator, to generate the supply voltage wanted with a suitable capacity while running. 
   One skilled in the art will appreciate that the present invention can be practiced by other than the described embodiments which are presented for purposes of illustration and not of limitation.