Patent Application
20110144722
The various embodiments described herein generally relate to implantable leads, and more particularly to MRI-safe implantable leads.
An implantable medical device is implanted in a patient to, among other things, monitor electrical activity of a heart and to deliver appropriate electrical and/or drug therapy, as required. Implantable medical devices (“IMDs”) include for example, pacemakers, cardioverters, defibrillators, implantable cardioverter defibrillators, an appetite or pain suppression device, and the like. The electrical therapy produced by an IMD may include, for example, pacing pulses, cardioverting pulses, and/or defibrillator pulses to reverse arrhythmias (e.g. tachycardias and bradycardias) or to stimulate the contraction of cardiac tissue (e.g. cardiac pacing) to return the heart to its normal sinus rhythm.
A body implantable lead forms an electrical connection between a patient's anatomy and the IMD. The lead includes a lead body comprising a tubular, flexible biocompatible, biostable insulative sheath or housing, such as formed of silicone rubber, polyurethane or other suitable polymer. One example of a lead body is a bipolar lead having a tip electrode and a ring sensing electrode. Generally bipolar leads include two coaxial conductors with insulation therebetween that are carried within the insulative housing. Another example of a lead body is a cardioverter/defibrillator lead that includes a sensing ring, a shocking right ventricle (RV) electrode, a shocking superior vena cava (SVC) electrode and a tip sensing/pacing electrode. The lead includes a multi-lumen body, each lumen of which carries a separate conductor through the lead body to each of the sensing ring, RV electrode, SVC electrode and tip electrode.
Magnetic resonance imaging (MRI) is commonly used as an efficient technique in the diagnosis of many injuries and disorders. MRI scanners provide a non-invasive method for the examination of internal structure and function. During operation, the MRI scanner creates a static magnetic field, a gradient magnetic field and a radio frequency (RF) magnetic field. The static magnetic field may have field strength of between 0.2 and 3.0 Tesla. A nominal value of 1.5 Tesla is approximately equal to 15,000 Gauss. The time varying or gradient magnetic field may have a maximum strength of approximately 40 milli-Tesla/meter. The RF magnetic field may have a frequency between 8 and 215 MHz. For example, up to 20,000 watts may be produced at 64 MHz in a static magnetic field of 1.5 Tesla.
A concern has arisen regarding the potential interaction between the MRI environment and implantable leads. In particular, implantable leads may experience RF-induced current. The RF induced current has been found to raise the temperature in the leads to undesirable levels.
Heretofore, leads have been proposed as MRI-safe. These MRI-safe leads are coupled to, or have housed therein, a discrete resonant tuning module. The resonant tuning module includes a control circuit for determining a resonance frequency of the implantable device and an adjustable impedance circuit to change the combined resonant frequency of the medical device and the lead. The resonant circuit includes an inductor (L) alone or coupled in parallel with a capacitor (C) to form a discrete LC circuit. The inductance and capacitance values of the inductor and capacitor are tuned approximately to the frequency of an expected RF magnetic field in an MRI scanner.
Using self resonant inductors alone in a distal portion of the lead has improved electrical performance. However, the resonant current induced at RF frequencies and the resistance of the conductors and the electrodes in a lead continue to cause self resonant inductors to heat, particularly in leads that utilize PEEK (i.e. Polyetheretherketones) headers.
Existing self resonant inductors use a coil structure that is sufficiently large to afford a large amount of inductance. The large amount of inductance is needed to satisfy desired impedance requirements at the RF frequencies. As the number of turns in the inductor increase, the DC resistance and RF resistance increase which then elevates component heating.
Conventional LC resonant structures couple a capacitor in parallel with the coil electrode wire that extends along the length of the lead. The coil electrode wire functions as an inductor that extends along an entire length of the lead. The amount of inductance and capacitance necessary to tune to a given resonant frequency are generally inversely related. As the inductance is increased, the capacitance can be decreased and vise versa. In the past, it has been difficult to develop an LC architecture that is able to exhibit sufficient inductance and capacitance, still fit within a lead and afford sufficient remaining room in the lead for other lead components.
Thus, it remains challenging to implement discrete LC and L circuits within leads while still meeting performance requirements. For example, circuit size is a challenge as there is a continued desire to provide circuits that are small enough to be packaged inside the distal portion of a lead, without making the LC or L circuits too small whereby they experience very localized heating.
A need remains for a self resonant inductor solution that avoids undue heating at the header assembly or along the lead. It would be further desirable to provide an improved implantable medical lead that may be operated in an MRI environment without the generation of significant heat in the lead. Furthermore, other desirable features and characteristics will become apparent from the subsequent detailed description of the invention and the appended claims, taken in conjunction with the accompanying drawings and this background of the invention.
In accordance with an embodiment, an implantable lead is provided that comprises a lead body extending along a longitudinal axis and that includes a distal end, a proximal end and a lumen within the lead body. The lead includes a header assembly provided at the distal end of the lead body. The header assembly includes a tissue engaging end. The lead also includes an electrode provided on the header assembly. The electrode is configured to deliver stimulating pulses. The lead also includes an electrode conductor provided within the lumen of the lead body and extending from the electrode to the proximal end of the lead body. An LC resonant component is provided in at least one of the lead body and the header assembly. The LC resonant component comprises a capacitor having an elongated shape that extends along the longitudinal axis of the lead body. The capacitor has a core that is located about the longitudinal axis of the lead body. The LC resonant component further comprises an inductor wire wound in multiple turns about an exterior surface of the capacitor to form an inductor.
Optionally, the LC resonant component may be located within the header assembly or at an intermediate location along a length of the lead body. The inductor wire extends concentrically about the capacitor and includes at least one insulated filar. The inductor and capacitor are connected in parallel with one another and tuned to a resonant frequency of an MR scanner.
In accordance with an embodiment, the capacitor has first and second sets of conductive plates that are arranged along the longitudinal axis of the lead body. The first and second sets of conductive plates are interleaved with one another. The conductive plates may be oriented orthogonal to the longitudinal axis of the lead body and wrapped about the longitudinal axis. Optionally, the LC resonant component may include an insulated elongated core located about the longitudinal axis of the lead body, where the conductive plates circumferentially wrap about the elongated core. The capacitor may have a tubular shape that is centered along the longitudinal axis of the lead body. The core of the capacitor may be centered along the longitudinal axis of the lead body.
Optionally, the electrode conductor may be shaped as a coiled conductor. The LC resonant component may have a lumen therethrough with the coiled conductor extending through the lumen in the LC resonant component. The parallel combination of the inductor wire and capacitor are joined in series with the coiled conductor. The inductor wire is physically separate and distinct from the electrode conductor, with the inductor wire being joined at a connecting node to the electrode conductor. The inductor wire and electrode conductor are wound in separate coil shapes having different corresponding inner diameters, turn densities along the longitudinal axis, and turn pitches oriented with respect to the longitudinal axis. At least one of the inner diameter, turn density and turn pitch of the inductor wire of the inductor differs from the inner diameter, turn density and turn pitch of the electrode conductor.
In accordance with an alternative embodiment, an implantable medical device is provided that comprises a processor, a pulse generator for generating stimulating pulses and an implantable lead. The lead comprises a lead body extending along a longitudinal axis and that includes a header assembly provided at the distal end of the lead body. The header assembly includes a tissue engaging end. The lead also includes an electrode provided on the header assembly. The electrode is configured to deliver stimulating pulses. The lead also includes an electrode conductor provided within the lumen of the lead body and extending from the electrode to the proximal end of the lead body. An LC resonant component is provided in at least one of the lead body and the header assembly. The LC resonant component comprises a capacitor having an elongated shape that extends along the longitudinal axis of the lead body. The capacitor has a core that is located about the longitudinal axis of the lead body. The LC resonant component further comprises an inductor wire wound in multiple turns about an exterior surface of the capacitor to form an inductor.
Optionally, the medical device 18 may be implanted elsewhere, such as in the patient's abdomen, neck, pelvis regions, etc. In the illustrated embodiment, the lead 12 is a pacing and sensing lead. However, other types of leads may be used in alternative embodiments, such as neuromodulation leads, defibrillation leads, patient monitoring leads and the like. Although the following embodiments are described principally in the context of a pacemaker/defibrillator unit capable of sensing and/or pacing pulse delivery, the medical system 10 may be applied to other IMD structures. As further examples, embodiments may be implemented in leads for devices that suppress an individual's appetite, stimulate the patients nervous or muscular systems, stimulate the patient's brain functions, reduce or offset pain associated with chronic conditions and control motor skills for handicap individuals, and the like.
A connector assembly 28 is provided at the proximal end portion 24 of the lead 12. The connector assembly 28 is configured to be inserted into a receiving orifice in the IMD 18. The connector assembly 28 includes first and second electrical terminals 30, 32 each being connected to respective electrical conductors, such as pacing and sensing electrical conductors, within the lead 12.
A header assembly 40 is provided at the distal end portion 22 of the lead 12. The header assembly 40 includes a tip electrode 42 at the distal end portion 22 and a ring electrode 44 proximate to the distal end portion 22. The tip electrode 42 is electrically connected to the first electrical terminal 30. The ring electrode 44 is connected to the second electrical terminal 32. In an alternative embodiment, the header assembly 40 may include only the tip electrode 42 without a corresponding ring electrode. Optionally, the header assembly 40 may include a heat spreader 38 thereabout to convey thermal energy away from the header assembly 40.
The header assembly 40 includes a fixation mechanism 46 that functions to interlock the lead 12 within the cardiac tissue at the implantation site and thereby prevent inadvertent displacement of the distal end portion 22 once the lead 12 is implanted. In the illustrated embodiment, the fixation mechanism 46 is represented by a screw-in helix that penetrates the cardiac tissue to anchor the lead 12 thereto.
The housing 50 is formed of an insulator and is electrically inactive such that the housing 50 does not interact electrically with the cardiac tissue of the patient. Optionally, the housing 50 may be fabricated from a suitable insulative, biocompatible, biostable material. Alternatively, the housing 50 may be fabricated from a biocompatible, biostable metal or metal alloy having an insulative coating surrounding all portions of the housing 50 that may engage the cardiac tissue of the patient. Optionally, the housing 50 may include at least one fluoro-marker (not shown), or other suitable means, for identifying a position of the distal end portion 22 during and/or after implantation within the patient.
The housing 50 includes a rear section 47 and a main body 51 formed integral with one another along the axis 56. The rear section 47 includes an internal lumen 48 that is open at the lead mating end 52. The main body 51 includes a chamber 49 that is joined at one end to the internal lumen 48 and is open at the tissue engaging end 54. The tip electrode 42 is secured on the main body 51 of the housing 50 at the tissue engaging end 52. The tip electrode 42 has an opening 53 through which the fixation mechanism 46 moves. The fixation mechanism 46 of the header assembly 40 is advanced in the direction of arrow A to an extended position to penetrate, and become fixed to, the heart 16 upon implantation. The fixation mechanism 46 is retracted in the direction of arrow B until enclosed in the header assembly 40 to facilitate implantation to a desired location.
The header assembly 40 may retain various electrodes and sensors used by the implanted medical system 10 (shown in
The rear section 47 of the housing 50 receives the inner conductor 34 within the inner lumen 48. A guide member 60 is provided within the chamber 49 of the main body 51. The guide member 60 moves in the directions of arrows A and B within the chamber 49 with the fixation mechanism 46. The guide member 60 includes a rearward extension 62, a central body 63 and a forward extension 64 arranged along the longitudinal axis 56. The central body 63 holds an LC resonant component 120. The LC resonant component 120 includes a capacitor 122 centered along the longitudinal axis 56 and an inductor wound concentrically about the capacitor 122. The rearward extension 62 holds a transition pin 58. The inner conductor 34 terminates on the transition pin 58. The transition pin 58 is connected to a segment 35 of the LC resonant component 120 that extends within the rearward extension 62. The fixation mechanism 46 is secured to and held on the forward extension 64.
The capacitor 122 and the inductor 150 are electrically connected in parallel with one another to form an LC resonant circuit. The LC resonant circuit is connected in series at one end with the inner conductor 34 and at the other end with the tip electrode 42 through the segments 35 and 37, respectively. The LC resonant circuit may be tuned by setting the capacitance and inductance to desired levels. The LC resonant circuit may be tuned to a resonance frequency of 64 MHz, 128 MHz and the like, based on the MRI scanner(s) contemplated for use therewith.
Returning to
The LC resonant component 220 includes a cylindrical capacitor 222 having an elongated shape that extends along the longitudinal axis 218 of the lead body 212. The capacitor 222 has an inner core 226 and an outer layer 228 that are separated by a gap 227 that holds capacitor plates. An inductor 250 is arranged concentrically about outer layer 228 of the capacitor 220. The capacitor 220 and inductor 250 are held within a LC component housing that comprises end caps 260 and 262 and an outer shell 264. The end caps 260 and 262 are positioned at opposite ends of the LC resonant component 220. The outer shell 264 extends in a direction parallel to the longitudinal axis 218 between the end caps 260 and 262. The outer shell 264 is located over an outer surface of the inductor 250. The outer shell 264 circumferentially encloses and surrounds the inductor 250 and capacitor 220.
The inductor 250 is formed from an inductor wire 252 that is wound in multiple turns 256 about an exterior surface 224 of the capacitor 220. The electrode conductor 208 constitutes a coiled conductor that extends along the lead body 212. The inductor 250 and capacitor 220 are joined in parallel with one another and are joined in series with the electrode conductor 208. The inductor wire 252 is physically separate and distinct from the electrode conductor 208. The inductor wire 252 is joined at a connecting node to the electrode conductor 208.
The capacitor 222 has an overall elongated tubular shape in the direction of arrow 246 and a circular cross-section in the direction of arrow 244. By interleaving the capacitor plates 232 and 236, and filling the gap 227 there between with dielectric material 242, the capacitor 222 is able to provide a large capacitance within a small radial form factor (in the direction of arrow 44).
The electrode conductor 208 has an inner diameter 280 and an outer diameter 282. The outer diameter 282 is smaller than the inner diameter of the lumen 225 through the LC resonant component 220. The inner and outer diameters 280 and 282 of the electrode conductor 208 are less than the inner diameter of 270 of the inductor 250. The electrode conductor 208 has a turn pitch 284 which corresponds to the angular orientation 286 of the turns 288 with respect to the longitudinal axis 218. The electrode conductor 208 has a turn density 290 which represents the number of turns 288 per unit of length along the lead body. In the example of
By way of example only, in one embodiment, the capacitor dimensions may be 100 mils in length and 30 mils in diameter. Optionally, the capacitor plates may not be inter-leaved with one another, such as when less capacitance is desired. Optionally, the inductor may represent a coil or spiral inductor located on a tubular shaped printed substrate. Optionally, multiple LC resonant components may be located along the length of the lead. For example, separate LC resonant components may be provided at each electrode.
By way of example, in one embodiment, the dielectric material in the capacitor may be selected to have a high dielectric constant (e.g. 20). When all or a portion of the capacitor is formed from non-bio-compatible material, a hermetic seal may be created about the capacitor, such as from a bio-compatible, non-metal moisture resistant material at the end caps and/or outer shell of the capacitor.
It is to be understood that the above description is intended to be illustrative, and not restrictive. For example, the above-described embodiments (and/or aspects thereof) may be used in combination with each other. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from its scope. Dimensions, types of materials, orientations of the various components, and the number and positions of the various components described herein are intended to define parameters of certain embodiments, and are by no means limiting and are merely exemplary embodiments. Many other embodiments and modifications within the spirit and scope of the claims will be apparent to those of skill in the art upon reviewing the above description. The scope of the invention should, therefore, be determined with reference to the appended claims, along with the full scope of equivalents to which such claims are entitled. In the appended claims, the terms “including” and “in which” are used as the plain-English equivalents of the respective terms “comprising” and “wherein.” Moreover, in the following claims, the terms “first,” “second,” and “third,” etc. are used merely as labels, and are not intended to impose numerical requirements on their objects. Further, the limitations of the following claims are not written in means—plus-function format and are not intended to be interpreted based on 35 U.S.C. §112, sixth paragraph, unless and until such claim limitations expressly use the phrase “means for” followed by a statement of function void of further structure.
This application is related to U.S. patent application Ser. No. 12/613,435, filed Nov. 5, 2009, titled “MRI-COMPATIBLE IMPLANTABLE LEAD HAVING A HEAT SPREADER AND METHOD OF USING SAME” (Attorney Docket A09P1057).
