Abstract:
Disclosed is methodology and apparatus for producing an asymmetrical filter for use with implantable medical devices, and in other input filtering environments. Differing forward and reverse characteristic responses are provided by inserting a low value resistor in series with heart connecting leads so that EMI input protection may be provided without significantly reducing energy transfer from the protected device. Improved protection against voltage transients is provided with present arrangements of differentiated series impedance. Higher frequency energy is allowed out of a subject device than is allowed into such device, which allows for attenuation of undesired frequency ranges entering the filter while allowing output pulses to exit without distortion.

Description:
PRIORITY CLAIM  
       [0001]     This application claims the benefit of previously filed U.S. Provisional Patent Application entitled “ASYMMETRICAL FILTER” and assigned U.S. Ser. No. 60/728,263, filed Oct. 19, 2005, and which is incorporated herein by reference for all purposes. 
     
    
     FIELD OF THE INVENTION  
       [0002]     The presently disclosed technology relates to the provision of filters and their applications to device input and output filtering. The present technology has particular applicability to implantable medical devices.  
       BACKGROUND OF THE INVENTION  
       [0003]     Heart pacemakers and other implantable medical devices are constructed having an outer housing within which the necessary electronic components are contained. Such outer housing must be formed of a material which is compatible with being placed inside the human body. It is also generally desirable to shield the electronics within such housing from external sources of electromagnetic interference (EMI). Titanium is often utilized to satisfy such dual requirements of biocompatibility and shielding.  
         [0004]     At least one elongate lead (i.e., a wire) will generally extend from the electronics within such an outer housing to a desired location inside a user&#39;s body. While the outer housing may shield the internal electronics from direct EMI radiation, steps may also be taken to inhibit transmission of EMI along such elongated lead itself. For example, selected capacitive and/or inductive components may be mounted on a circuit board along with the other internal electronics in order to provide EMI filtering. Alternatively, a so-called feed-through style filter may be provided at the location where the elongated lead passes through the outer housing. Examples of such feed-through filter subject matter are shown and described in U.S. Pat. Nos. 5,999,398, and 6,459,935 B1, both assigned to the owner of the present subject matter, and the disclosures of which patents are fully incorporated herein by reference, for all purposes.  
         [0005]     U.S. Pat. No. 5,999,398 to Makl et al. entitled “FEED-THROUGH FILTER ASSEMBLY HAVING VARISTOR AND CAPACITOR STRUCTURE” describes a feed-through filter assembly such as may be used in an implantable medical device. The assembly includes a conductive mounting element which may be hermetically sealed to an outer housing of the implantable medical device. In many embodiments, the conductive mounting element will be a conductive canister in which a feed-through filter structure is located. Alternatively, the conductive mounting element may include a suitable subplate structure. Because the filter structure exhibits varistor and capacitive characteristics, effective transient suppression and interference filtering is achieved in a single package. Secondary filtering may be provided downstream of the filter assembly for additional interference filtering at lower frequencies.  
         [0006]     U.S. Pat. No. 6,459,935 B1 to Piersma entitled “INTEGRATED FILTER FEED-THRU” describes a multi-leaded, filter feed-thru assembly for implantable medical devices, such as heart pacemakers, defibrillators, and neurostimulators, which integrates both multi-element semiconductor devices and passive component devices, or multi-element combinations thereof, together with a discoidal capacitive filter device to provide filtration of electromagnetic interference is provided. The assembly additionally provides for the suppression of high voltage transients from defibrillation and electrocautery procedures, as well as providing additional circuit and network functions.  
         [0007]     Although pacemaker signals are relatively low voltage, capacitors utilized in feed-through filtering arrangements must often be constructed to withstand relatively high voltage levels. This is to ensure that the capacitor does not become damaged if subjected to voltage transients, such as those which may be caused by defibrillation pulses. Particular concern is raised if some or all of the same leads used as output from certain implantable medical devices are also used as input leads for associated measurement technology because the energy exiting certain implantable devices may be higher than that entering the devices as input signals.  
         [0008]     Additional exemplary information regarding filtering technology may be found in other patents, published patent applications, and publications, including Published U.S. Patent Applications 2005/0197677 A1, entitled “APPARATUS AND PROCESS FOR REDUCING THE SUSCEPTABILITY OF ACTIVE IMPLANTABLE MEDICAL DEVICES TO MEDICAL PROCEDURES SUCH AS MAGNETIC RESONANCE IMAGING” by Stevenson, published Sep. 8, 2005; and 2004/0263174 A1, entitled “MAGNETIC RESONANCE IMAGING INTERFERENCE IMMUNE DEVICE” by Gray et al., published Dec. 30, 2004; and U.S. Pat. No. 6,795,730 B2, entitled “MRI-RESISTANT IMPLANTABLE DEVICE” by Connelly et al.; U.S. Pat. No. 6,424,234 B1, entitled “ELECTROMAGNETIC INTERFERENCE (EMI) FILTER AND PROCESS FOR PROVIDING ELECTROMAGNETIC COMPATIBILITY OF AN ELECTRONIC DEVICE WHILE IN THE PRESENCE OF AN ELECTROMAGNETIC EMITTER OPERATING AT THE SAME FREQUENCY” by Stevenson; U.S. Pat. No. 5,896,267, entitled “SUBSTRATE MOUNTED FILTER FOR FEEDTHROUGH DEVICES” by Hittman et al.; and PUBLICATIONS: “Heart Devices May Be Safe for MRI Scan,”  R &amp;  D Digest , October 2004; “New Pacemakers Prove MRI-Proof,”  Spectrum Online , Oct. 21, 2004; and “Advances in MRI-Safe Technology: Interview with Michael L. Weiner, CEO, Biophan Technologies, Inc.,”  EPlab Digest , July 2004.  
         [0009]     While various implementations of feedthrough filter assemblies have been developed for use in association with medical implants and other input and/or output filtering applications, no design has emerged that generally encompasses all of the desired characteristics as hereafter presented in accordance with the subject technology.  
       SUMMARY OF THE INVENTION  
       [0010]     The present subject matter recognizes and addresses several of the foregoing issues, and others concerning certain aspects of feedthrough filtering. Thus, broadly speaking, an object of certain embodiments of the presently disclosed technology is to provide an improved design for certain components associated with the implementation of feedthrough filters and, more particularly, with feedthrough filters employable in association with implantable medical devices and other input and/or output filtering related technology.  
         [0011]     Aspects of certain exemplary embodiments of the present subject matter relate to the provision of an asymmetrical filtering arrangement capable of passing different energy levels in the respective forward and reverse directions (i.e., having different input versus output characteristics).  
         [0012]     Aspects of other exemplary embodiments of the present subject matter provide protective features for a signal input portion of a measuring device while permitting high energy output from the device.  
         [0013]     Still further aspects of yet still other embodiments of the present subject matter provide enhancements to feedthrough filtering configurations allowing for improved operation of associated electronic equipment when exposed to external sources of electromagnetic interference (EMI).  
         [0014]     Yet further aspects of other embodiments of the present subject matter provide enhancements to feedthrough filtering configurations, for improved protection against voltage transients, and in the context of present arrangements of differentiated series impedance, such that higher frequency energy is allowed out of a subject device than is allowed into such device, which allows for attenuation of undesired frequency ranges entering the filter while allowing output pulses to exit without distortion.  
         [0015]     Still further, it is to be understood that the present technology equally applies to the resulting devices and structures disclosed and/or discussed herewith, as well as the corresponding involved methodologies.  
         [0016]     In one present exemplary embodiment, a filtered feedthrough capacitor arrangement is provided for use in a patient-implantable medical environment, and may comprise a feedthrough capacitor, a feedthrough filter, and an asymmetrical filter. In such exemplary arrangement, such exemplary feedthrough capacitor may have a main body and at least a first lead emerging from one side of such main body and a second lead emerging from another side of such main body. Such exemplary feedthrough filter may be associated with at least one of said feedthrough capacitor leads; and such exemplary asymmetrical filter may be associated with such at least one of the feedthrough capacitor leads, and electrically situated between such feedthough capacitor and such feedthrough filter. Such asymmetrical filter in such exemplary arrangement may preferably further include a relatively low value resistance for providing differing forward and reverse characteristic responses of the filtered feedthrough capacitor arrangement.  
         [0017]     Still further, in various specific arrangements of such exemplary present embodiment, such feedthrough capacitor first and second leads may comprise respective sets of pluralities of leads, while such exemplary asymmetrical filter may comprise a resistor mounting substrate, with a corresponding plurality of relatively low value resistors supported thereon, and associated with one of the respective sets of plural leads of said feedthrough capacitor, each of said resistors having a respective value generally no greater than a predetermined value. In certain of such arrangements, such resistors may each comprise a thick film resistor supported on such asymmetrical filter substrate, and respectively interconnecting with one of such feedthrough capacitor leads and a lead associated with such feedthrough filter. In other, alternative arrangements, such resistors may each comprise one of a plurality of relatively low value, wire wound resistors supported on such substrate.  
         [0018]     In yet another present arrangement of a present embodiment, a filter circuit may be provided for use electrically interposed between a feedthrough capacitor and a feedthrough filter of the type having plural respective leads, and provided for use in association with implantable medical devices and other input and/or output filtering related environments. Such exemplary filter circuit may preferably comprise a resistor mounting substrate; and a plurality of predetermined relatively small value resistors supported on such substrate and arranged for connection in series with input or output leads of a further device, collectively providing with an associated feedthrough capacitor and feedthrough filter an asymmetrical filtering arrangement capable of passing different energy levels in respective forward and reverse directions.  
         [0019]     Still further present embodiments relate to corresponding methodology, for example such as an exemplary method for improving EMI and voltage transient protection for implantable medical devices, including providing differing forward and reverse characteristic responses thereof by inserting a predetermined relatively low value resistance in series with biocompatible connecting leads of the implantable device, so that EMI input protection may be provided thereto without significantly reducing energy transfer from the protected implantable device.  
         [0020]     Additional objects and advantages of the present subject matter are set forth in, or will be apparent to those of ordinary skill in the art from, the detailed description herein. Also, it should be further appreciated by those of ordinary skill in the art that modifications and variations to the specifically illustrated, referenced, and discussed features and/or steps hereof may be practiced in various embodiments and uses of the disclosed technology without departing from the spirit and scope thereof, by virtue of present reference thereto. Such variations may include, but are not limited to, substitution of equivalent means, steps, features, or materials for those shown, referenced, or discussed, and the functional, operational, or positional reversal of various parts, features, steps, or the like.  
         [0021]     Still further, it is to be understood that different embodiments, as well as different presently preferred embodiments, of this technology may include various combinations or configurations of presently disclosed steps, features or elements, or their equivalents (including combinations of features or configurations thereof not expressly shown in the figures or stated in the detailed description). 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0022]     A full and enabling description of the present subject matter, including the best mode thereof, directed to one of ordinary skill in the art, is set forth in the specification, which makes reference to the appended figures, in which:  
         [0023]      FIG. 1  illustrates a partial schematic diagram and equivalent circuit of an asymmetrical filter associated with an exemplary feedthrough filter in accordance with the present technology;  
         [0024]      FIG. 2  is a graphical representation of the input and output response of an exemplary asymmetrical filter, such as that of present  FIG. 1 , constructed in accordance with the present technology;  
         [0025]      FIG. 3  is a graphical representation of the forward and reverse insertion voltage loss of an exemplary asymmetrical filter, such as that of present  FIG. 1 , constructed in accordance with the present technology;  
         [0026]     FIGS.  4 ( a ) and  4 ( b ) are respectively side elevation and top plan views of a first embodiment of an exemplary present resistor configuration for use with an exemplary asymmetrical filter constructed in accordance with the present technology;  
         [0027]      FIG. 5  is a partially exploded view of a second embodiment of an exemplary present resistor configuration for use with an exemplary asymmetrical filter constructed in accordance with the present technology;  
         [0028]     FIGS.  6 ( a ) and  6 ( b ), respectively, are top plan and side cross section views of a second exemplary embodiment of a present resistor configuration for use with an exemplary asymmetrical filter constructed in accordance with the present technology; and  
         [0029]      FIG. 7  is a side elevation view (with partial cutaway illustrations) of an exemplary present resistor configuration combined with an exemplary feedthrough capacitor in accordance with the present technology. 
     
    
       [0030]     Repeat use of reference characters throughout the present specification and appended drawings is intended to represent same or analogous features, elements, or steps of the present subject matter.  
       DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0031]     As discussed in the Summary of the Invention section, the present subject matter is particularly concerned with certain aspects of feedthrough filtering employable in association with implantable medical devices and related technology and methodology. More particularly, the present subject matter is concerned with an improved asymmetrical filter designed to provide differing forward and reverse energy flow characteristics, and is concerned with corresponding methodologies.  
         [0032]     Selected combinations of aspects of the disclosed technology correspond to a plurality of different embodiments of the present subject matter. It should be noted that each of the exemplary embodiments presented and discussed herein should not insinuate limitations of the present subject matter. Features or steps illustrated or described as part of one embodiment may be used in combination with aspects of another embodiment to yield yet further embodiments. In additional, certain features may be interchanged with similar devices or features not expressly mentioned which perform the same or similar functions.  
         [0033]     Reference will now be made in detail to exemplary presently preferred embodiments involving the subject asymmetrical filter. Referring now to the drawings,  FIG. 1  schematically illustrates a partial schematic diagram and equivalent circuit diagram of an exemplary asymmetrical filter generally  100  associated with a feedthrough filter  110  in accordance with the present technology.  
         [0034]     Asymmetrical filter  100  may be formed, for example, by inserting a small value resistor  130  in series between heart lead  140  and a filter circuit  110 . Filter circuit  110  is coupled between small value resistor  130  and an input terminal of a device schematically represented by equivalent circuit  150 . Heart lead  140  may correspond to, or represent, one or more electrical leads coupled directly to a patient&#39;s heart for application of, for example, a pacing signal from a pacemaker, as well as for detection of naturally occurring heart related electrical signals.  
         [0035]     In the present discussion, reference is made to the use of the asymmetrical filter in accordance with the present subject matter in association with a pacemaker. It should be well understood, however, by those of ordinary skill in the art, that the present subject matter is not so limited, as the disclosed subject matter may be applied in other environments as well. For example, asymmetrical filtering may provide certain advantages when used in association with other medical devices including, for example, heart monitors, defibrillators, and neurostimulators. Asymmetrical filtering as disclosed herein may also be applied in other environments where both high and low level signals may be applied to a common signal transmission medium. Non-exhaustive examples of such include data line transceivers and radio frequency (RF) transceivers.  
         [0036]     As illustrated in  FIG. 2 , it has been found that insertion of a small valued resistor  130  in series between heart lead  140  and filter circuit  110  provides differing filter responses with respect to whether a signal is being applied to the asymmetrical filter and passed to, for example, measuring circuitry or being sent out through the asymmetrical filter and applied to, for example, heart lead  140 .  
         [0037]     As illustrated in  FIG. 2 , a pair of response curves  210 ,  220  illustrate, respectively, the overall input and output responses from an exemplary asymmetrical filter constructed in accordance with the present subject matter. From a review of such response curves  210 ,  220 , it will be appreciated that the input response curve  210  experiences significantly higher insertion loss than the output response curve  220 , particularly at frequencies above about 2 MHz. Such asymmetric response provides improved protection from EMI applied to device  150  while at the same time limiting impact on any output signal from device  150 . In this exemplary embodiment of the present subject matter, the small value resistor  130  had a value of 15 ohms.  
         [0038]     Also, in such exemplary embodiment, the resistance, capacitance, and inductance values within representative filter circuit  110  may be practiced as follows. Variations may also be practiced, for specific embodiments in accordance with the present subject matter. The represented initial inductance value may be 0.1 nH, while the inductance represented on either side of node  112  may each be 0.083 nH, while the inductance going to ground (beneath node  112 ) may be 0.028 nH. The exemplary resistance in such ground leg may be about 0.229 ohms, while the resistance on either side of node  112  may be 0.2 ohms. The exemplary capacitance in the ground leg below node  112  may be 3553.407 pF.  
         [0039]     With reference now to  FIG. 3 , representative response curves are illustrated for a further exemplary embodiment of an asymmetrical filter constructed in accordance with the present subject matter. In the embodiment represented by  FIG. 3 , the small value resistor  130  ( FIG. 1 ) had a value of 20 ohms. In this exemplary embodiment, the forward measurement (representative response curve  310 ) exhibits a significantly higher insertion loss than the reverse measurement (representative response curve  320 ), particularly at frequencies above about 2 MHz, in a manner similar to that illustrated in  FIG. 2 . Response curves such as shown in  FIG. 3  in conjunction with a small value resistor  130  value of 20 ohms are the results of measurements made in a gain phase mode, with a 1 M ohm input impedance, as will be well understood by those of ordinary skill in the art.  
         [0040]     With reference now to FIGS.  4 ( a ) and  4 ( b ), there are illustrated side elevation and top plan views of an exemplary embodiment of a present resistor mounting substrate generally  400  as may be used to support a plurality of small value resistors  440 ,  442 ,  444  for connection in series with input/output leads of an implantable medical device, for example, a pacemaker.  
         [0041]     In the illustrated exemplary embodiment, resistor mounting substrate  400  may correspond to a ceramic substrate  430  on which are mounted a number of thick film resistors  440 ,  442 ,  444  such that the resistors are coupled to respective connection pin pairs  410 / 420 ;  412 / 422 ; and  414 / 424 . Such present exemplary resistor mounting substrate  400  may be coupled, in some instances, to known feedthrough capacitor structures, as will be more fully illustrated and discussed with reference to  FIG. 7 .  
         [0042]     With reference now to FIGS.  5 ,  6 ( a ) and  6 ( b ), a further exemplary embodiment of a present resistor mounting substrate generally  600  in accordance with the present subject matter. First with reference to  FIG. 5 , there is illustrated a second exemplary embodiment of a resistor usable as the small value resistor  130  ( FIG. 1 ) to produce an asymmetrical filtering response. In this exemplary embodiment, resistor  530  corresponds to a wound wire resistor and is configured such that respective end portions generally  532  and  534  of the wound wire are inserted into respective conductive termination tubes  542  and  544 . In an exemplary configuration, wire wound resistor  530  may correspond to about 42 turns of nichrome wire (3 mil coated resistor wire, non-magnetic) wound around a 15 mil mandrel to produce a resistance value of about 15 ohms. With such exemplary values, the resulting coil would be expected to be about 120 mils long. Conductive termination tubes  542  and  544  may correspond to Platinum/Rhodium (Pt/Rh) tubes, for example, with about 12 mil outside diameters and 4 mil inside diameters, while the end portion  534  of the wound wire may be gas tungsten arc (TIG) welded onto end  546  so as to form a hermetic and positive joint.  
         [0043]     Referring now to FIGS.  6 ( a ) and  6 ( b ), there are illustrated top plan and side cross section views of a second embodiment of a resistor mounting substrate generally  600  in accordance with the present subject matter. As may more readily be seen in  FIG. 6 ( a ), a plurality of wire wound resistors  610 ,  612 ,  614 , and  616  are mounted on support substrate  630  and coupled by way of lands  640 ,  642 , away from fillets  650 ,  652 . Solderable wires  660 ,  662  (shown in  FIG. 6 ( b )) may be soldered to selected fillets as at solder locations  664 ,  666  to provide connection to a feedthrough capacitor structure, such as will be described with reference to  FIG. 7 . Support substrate  630  may correspond to a ceramic substrate, although it is to be strictly understood that any other suitable support structure material may be employed.  
         [0044]     With reference now to  FIG. 7 , an exemplary assembly generally  700  of an exemplary asymmetrical filter in accordance with the present subject matter will be described. As illustrated per this embodiment, a known feedthrough capacitor structure generally  710  is provided with leads  712 ,  714 ,  716 , and  718  that may, in fact, correspond in number to more or less than the number of leads presently illustrated. The specific structure of the representative feedthrough capacitor is not a limitation of the present subject matter, but as an example only, such structure may generally correspond to that illustrated in U.S. Pat. Nos. 5,999,398 and 6,459,935 B1, referenced above.  
         [0045]     An exemplary asymmetry circuit board  720  constructed in accordance with the previously described exemplary configurations may be mounted to feedthrough capacitor  710  leads  716 ,  718  such that the asymmetry circuit board  720 , by way of associated resistors (like resistor arrangements or equivalent thereto per other present figures but not separately shown in  FIG. 7 ), may provide a mechanism for inserting a small value resistor in series with the feedthrough capacitor leads. Low thermal conductivity leads  722 ,  724  soldered to fillets on asymmetry circuit board  720  provide a connection pathway to a standard filter assembly  730 , for example, such as filter  110  schematically illustrated in  FIG. 1 .  
         [0046]     In the instance of the example of present  FIG. 7 , low thermal conductivity leads  722  and  724  may have a thermal conductivity rating of less than 8 British thermal unit IT  per hour foot degree Fahrenheit (i.e., BTU/hrftF). Such leads  722  and  724  may comprise, for example, Inconel, Titanium or Zirconium alloys, to allow laser welding or other forms of heat treatment for welding without causing significant heat transfer to any circuitry, such as on exemplary circuit board  720 . By contrast, in such exemplary embodiment, the thermal conductivity of the leads  712  and  714  may be above 42 BTU/hrftF, and such leads may comprise Platinum or an equivalent material.  
         [0047]     Further in conjunction with the present exemplary configurations of FIGS.  6 ( a ),  6 ( b ) and  7 , present exemplary circuit board  720  may have an outside diameter of about 130 mils while leads  722  and  724  are about 95 mils apart and leads  712  and  714  are about 66 leads apart, in the illustrated exemplary embodiments. Also, it will be understood by those of ordinary skill in the art that the side elevation view of  FIG. 7  in fact only illustrates half of the leads that would be utilized in an actual embodiment.  
         [0048]     Various dimensions, materials, and characteristics may be practiced in the foregoing exemplary embodiments, as understood by those of ordinary skill in the art, for use in particular embodiments, without departing from the spirit or scope of the present subject matter. In addition, all presently referenced dimensions, materials and characteristics are intended as exemplary values, within the broader aspects of the present subject matter, and not intended as limitations thereto.  
         [0049]     While the present subject matter has been described in detail with respect to specific embodiments thereof, it will be appreciated that those skilled in the art, upon attaining an understanding of the foregoing may readily adapt the present technology for alterations or additions to, variations of, and/or equivalents to such embodiments. Accordingly, the scope of the present disclosure is by way of example rather than by way of limitation, and the subject disclosure does not preclude inclusion of such modifications, variations, and/or additions to the present subject matter as would be readily apparent to one of ordinary skill in the art.