Method for making a body implantable sensor

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FIELD OF THE INVENTION

The present invention relates generally to implantable pacemaker leads, and more particularly, to an implantable pacemaker lead that can sense at least one physiologic parameter of the body such as oxygen saturation of the blood.

BACKGROUND

The evolution of the modern pacemaker lead may be best understood through a review of the development of the pacemaker itself. The earliest pacemaker simply delivered stimulation pulses at a fixed repetition rate. These were known as "asynchronous" or fixed rate pacemakers. "Unipolar stimulation" was achieved by delivering electrical pulses between the tip electrode of the lead and the pacemaker case. Due to their asynchronous nature, the stimulation pulse often competed with natural rhythms. The "demand" pacemaker included sense amplifiers to enable sensing of natural rhythms. In the presence of natural cardiac signals, the demand pacemaker would inhibit a stimulation pulse. In the absence of natural cardiac signals, the demand pacemaker would deliver stimulation pulses. However, sensing between the tip and the case (referred to as "unipolar sensing") sometimes detected myopotentials; that is, the electrical signals generated by the pectoral muscle tissue. The sensing of myopotentials can falsely inhibit the demand pacemaker.

To solve this problem, bipolar leads were developed. A bipolar lead has two electrodes located within the heart: a tip electrode and a ring electrode. The ring electrode is located approximately one-half inch proximally from the tip electrode. This configuration enabled a significant reduction of myopotential sensing, as well as eliminating any pectoral stimulation. However, depending on the orientation of the lead and the direction of the wavefront, bipolar sensing of cardiac signals would sometimes result in signals that are smaller than unipolar signals. The arrival of unipolar/bipolar programmability in demand pacemakers enabled the physician to noninvasively reprogram the pacemaker's polarity to accommodate the patient's changing conditions.

Modern pacemakers can now alter their stimulation rate to accommodate the patient's exercise or stress needs. These rate-responsive pacemakers employ a variety of sensors to determine the physiological condition of the patient. Physiologic sensors may be located on the pacemaker lead or within the pacemaker itself. Physiologic sensors in use today include: minute volume, temperature, oxygen saturation of the blood, respiration, stroke volume, ventricular gradient, activity, and pre-ejection period (PEP), etc.

The ideal physiologic sensor would be one that provides information about the patient's exercise level or workload, and ideally, will operate in a closed loop fashion. In other words, it should operate to minimize the divergence from the ideal operating point. For this reason, the development of a sensor for monitoring blood oxygen saturation for use with an implantable pacemaker is desirable. Oxygen saturation of the blood provides a direct indication of oxygen consumption of the patient during exercise. Furthermore, oxygen saturation has an inverse relationship with pacing rate. That is, as oxygen saturation decreases due to exercise, the pacing rate will increase so that the divergence from the optimum point is minimized.

The development of an oxygen saturation sensor and circuitry for operating such a sensor incorporated into a pacemaker lead is shown in several references. See, for example, U.S. Pat. No. 4,399,820, to Wirtzfeld et al.; U.S. Pat. No. 4,750,495, to Moore et al.; and U.S. Pat. No. 4,815,469, to Cohen et al.

Unfortunately, problems still exist which have heretofore hindered a widespread clinical use of such a pacing system. One of the major difficulties in developing an oxygen sensing system has been to develop a pacemaker lead having a reliable, hermetically enclosed sensor that can be located within the heart. The typical oxygen sensor in combination with a pacemaker lead includes one or more light-emitting diodes (LEDs), phototransistors and resistors. The prior art suffers from complex circuit designs, which designs are difficult to miniaturize and hermetically encapsule. Also, the process of providing a reliable weld to a relatively large area without damaging the sensor electronics is not an easy task.

Another problem is protecting the oxygen sensor circuitry from overvoltages, such as those seen during cardioversion, defibrillation and electrosurgery. In the event of a high voltage cardioversion or defibrillation pulse, the integrated circuits could be destroyed losing all rate-responsive functionality.

Another potential problem occurs when using one or both of the stimulation conductors as the sensor return conductor. Should the sensor fail or interfere with the stimulation electrodes' functionality, pacing of the heart may be jeopardized. For example, bodily fluids may intrude into the sensor circuitry or a lead fracture may occur at the sensor connection (particularly given the periodic forces that are regularly placed on the lead as it moves or flexes with the heart). Under these failure modes the stimulation electrodes could be impaired or even destroyed, thus losing all the functionality of the lead.

Another disadvantage of oxygen sensor designs that use the same conductors as for stimulating, is that they exhibit rectification of electrosurgery signals. Thus, the current oxygen sensor designs do not meet the proposed international Cenelac standard. Therefore, it is an objective of the present invention to provide a simple hermetic packaging technique for a physiological sensor in a pacemaker lead, particularly an oxygen saturation sensor.

It is an objective of the present invention to provide a packaging technique for a physiological sensor in a pacemaker lead that does not interfere with basic operation of the pacemaker.

It is an objective of the present invention to provide a physiological sensor in a pacemaker lead that is not affected by electrosurgery signals, a cardioversion pulse, or a defibrillation pulse.

It is an objective of the present invention to provide a physiological sensor in a pacemaker lead that permits either unipolar or bipolar stimulation.

It is further an objective of the present invention to provide a reliable sensor circuit with minimum components which will minimize the overall diameter of the lead.

Finally, it is also an objective that all of the aforesaid advantages and objectives be achieved without incurring any substantial relative disadvantage.

SUMMARY OF THE INVENTION

The disadvantages and limitations of the background art discussed above are overcome by the present invention. The present invention includes a body implantable lead having a proximal connector, a lead body having at least one conductor, and at least one stimulating electrode. In addition, the present invention includes two additional conductors, coupled to a hermetically sealed 'sensor, for sensing at least one physiologic parameter of the body. Advantageously, the conductors coupled to the sensor function independently from the stimulation conductors so that interference with basic operation of the pacemaker is prevented. Overvoltage protection circuitry for protecting the sensor circuitry is located within the pacemaker. Thus, the sensor is unaffected by electrosurgery signals, a cardioversion pulse, or a defibrillation pulse.

In the preferred embodiment, the lead body comprises a multilumen bipolar configuration, that is, a silicone rubber or polyurethane tube with at least four lumens, or holes, therein. Each of the four conductors occupies one of the lumens.

In an alternate embodiment, the lead body comprises a "thin bipolar" configuration in which individual conductors are electrically isolated from each other by a thin electrically insulative, polymer coating. The conductors and the sensor assembly are further insulated by a layer of body compatible material.

In the preferred embodiment, the body implantable lead includes an optical sensor for sensing a specified characteristic of body fluid, such as the oxygen content of blood. In this configuration, a light-emitting source is used to transmit light through a transparent tubular housing to the body tissue. Light that is reflected back from the body due to the oxygen level of the blood is received by a light detector also located within the housing. To prevent light from impinging directly from the light-emitting source to the light detector, an insulating light barrier is disposed therebetween.

In the preferred embodiment, the housing is D-shaped and made of an optically clear material, such as glass, glass ceramic, or an optically clear ceramic (such as alumina, sapphire, ruby, quartz or silica ceramics). The sensor components are mounted onto a microelectronic substrate which is advantageously placed on an inner flat portion of the D-shaped housing. End caps are used to seal the ends of the shell. Advantageously, each end cap has either a metal braze or a glass frit sealing ring and a narrow bore for allowing one of the sensor terminals to pass therethrough. A hermetic seal is easily achieved by heating the sealing ring such that the sealing material reflows between the shell and the end caps. Advantageously, the sensor terminals are sized to fit snugly within the narrow bore. The gap between the sensor terminals and the narrow bore is then sealed by localized welding, or otherwise sealing, the sensor terminals to the end cap.

In the preferred embodiment, at least one end cap includes an inner and an outer cap. The inner cap includes the sealing ring and a channel wide enough to slide the substrate therethrough. Advantageously, the inner cap and the opposite end cap may be simultaneously refired, or otherwise heated, in a firing oven to produce a superior hermetic seal. After the end caps are in place, the substrate is slid through the wide channel of the inner cap onto the flat side of the D-shaped shell. The outer cap is sized to fit snugly within the inner cap and includes a narrow bore for allowing one of the sensor terminals to pass therethrough. After the substrate is in place, the outer cap is hermetically welded to the inner cap using localized welding. Thus, the sensor is reliably and hermetically sealed without subjecting the delicate microelectronic circuits to damaging heat conditions.

In one embodiment, the D-shaped sensor assembly is placed on a carrier. The carrier may comprise a portion of a multilumen lead body which has a flat cavity therein for mounting the D-shaped sensor assembly thereon. In the preferred embodiment, the carrier is a separately molded part with lumens molded therein for making appropriate electrical contact between the lead body and the sensor.

In another embodiment, the D-shaped sensor assembly is placed onto a bipolar inner lead body. Additional individually insulated conductors for the sensor terminals are then coaxially wound around the inner lead body with an insulating sheath placed thereover.

Advantageously, the D-shaped housing reduces the area that needs to be hermetically sealed by more than half, and thus reduces the overall diameter of the lead. It is well known that small diameter leads are more easily introduced into the vein and easier and more flexible to position in the heart. Therefore, the overall diameter is a critical parameter in developing any new pacemaker lead.

It may therefore be seen that the present invention teaches a simple hermetic packaging technique for a physiologic sensor in a pacemaker lead, thus particularly enabling the production of an oxygen saturation sensor. In addition, the present invention provides a packaging technique for a physiologic sensor in a pacemaker lead that does not interfere with the basic operation of a pacemaker.

The present invention also provides a physiologic sensor in a pacemaker lead that is not affected by electrosurgery signals, a cardioversion pulse, or a defibrillation pulse. The pacemaker lead including the physiologic sensor of the present invention can be configured in either a unipolar or bipolar lead. In addition, the present invention also provides a reliable sensor circuit having the smallest number of components possible, which will minimize the overall diameter of the lead.

Finally, all of the aforesaid advantages and objectives are achieved without incurring any substantial relative disadvantage. It will therefore be perceived that the advantages of the present invention result in an implantable stimulation lead having a reliable hermetically sealed sensor that enables the use of a sophisticated, closed-loop, rate-responsive pacemaker. The present invention thereby enables a higher quality of life for the patient, making the present invention a highly desirable enhancement to implantable cardiac pacemaker therapy.

BRIEF DESCRIPTION OF THE DRAWINGS

Further features and advantages of the present invention may be more readily understood by reference to the following description taken in conjunction with the accompanying drawings, in which:

FIG. 1 is a diagrammatic illustration of the installation of the system of the present invention in the upper chest region of a human being;

FIG. 2 is a schematic block diagram of a pacemaker system incorporating the oxygen sensor of the present invention;

FIG. 3 shows a side view of the body implantable lead assembly of the present invention with the sensor mounted therein;

FIG. 4 shows an isometric view of the preferred housing of the present invention;

FIG. 5 shows a plan view of the housing shown in FIG. 4;

FIG. 6 shows an end view of the housing shown in FIG. 4;

FIG. 7 shows an profile view of the preferred end cap for the housing shown in FIG. 4;

FIG. 8 shows a cross-sectional view of the preferred end cap shown in FIG. 7;

FIG. 9 shows can isometric view of an alternate embodiment end cap which may be used instead of the end cap shown in FIG. 7;

FIG. 10 shows a cross-sectional view of the end cap shown in FIG. 9;

FIG. 11 shows an isometric view of an alternate embodiment end cap which may be used instead of the end cap shown in FIG. 7;

FIG. 12 shows a cross-sectional view of the end cap shown in FIG. 11;

FIG. 13 is a schematic block diagram showing one possible electrical design for the oxygen sensor of the present invention in conjunction with the portion of the pacemaker system operating the oxygen sensor;

FIG. 14 shows a side view of the substrate and some of the electrical components shown in FIG. 13;

FIG. 15 shows a composite top view of the substrate shown in FIG. 14, including the integrated circuit components shown in FIG. 13;

FIG. 16 shows a composite bottom view of the substrate shown in FIG. 14, including the screen printed resistors shown in FIG. 13;

FIG. 17 is an isometric view of the L-shaped barrier;

FIG. 18 is an isometric view of the chair-shaped barrier;

FIG. 19 shows a cross-sectional view of the D-shaped sensor assembly of one embodiment, including the substrate placed within the housing with the preferred end cap of FIG. 7 attached thereto;

FIG. 20 shows an isometric view of an alternate embodiment of the D-shaped sensor assembly, including the substrate placed within the housing with the end caps of the alternate embodiment shown in FIG. 9 attached thereto;

FIG. 21 shows a cross-sectional view of the assembly shown in FIG. 20;

FIG. 22 shows an isometric view of another alternate embodiment of the D-shaped sensor assembly, including the substrate placed within the housing with the end caps of the alternate embodiment shown in FIG. 11 attached thereto;

FIG. 23 shows a cross-sectional view of the assembly shown in FIG. 22;

FIG. 24 shows a cross-sectional view of the D-shaped sensor assembly of the preferred embodiment, including the substrate placed within the housing with at least one of the preferred end caps of FIG. 7 attached thereto;

FIG. 25 shows an end view of the preferred carrier used for mounting the D-shaped sensor;

FIG. 26 shows a plan view of the carrier shown in FIG. 25;

FIG. 27 shows a side view of the housing mounted onto the carrier of FIG. 25;

FIG. 28 shows an end view of the housing mounted onto the carrier of FIG. 25;

FIG. 29 shows an end view of the multilumen lead body;

FIG. 30 shows a partial cross-sectional view of the multilumen lead body in the area of the sensor;

FIG. 31 shows an isometric view of an alternate carrier;

FIG. 32 shows a cross-sectional side view of the housing mounted onto the alternate carrier of FIG. 31;

FIG. 33 shows a cross-sectional side view of the housing mounted on the "thin bipolar" lead body;

FIG. 34 shows an end view of the housing mounted onto the alternate carrier of FIG. 33;

FIG. 35 shows a first alternate embodiment for attaching the sensor housing onto the "thin bipolar" lead body;

FIG. 36 shows a second alternate embodiment for attaching the sensor housing onto the "thin bipolar" lead body;

FIG. 37 shows a third alternate embodiment for attaching the sensor housing onto the "thin bipolar" lead body;

FIG. 38 shows a profile view of a multipolar connector subassembly for the lead;

FIG. 39 shows an end view of the multipolar connector subassembly shown in FIG. 38;

FIG. 40 shows a cross-sectional profile view of a multipolar connector subassembly shown in FIG. 38;

FIG. 41 shows a profile view of the multipolar connector subassembly, including the sealing rings; and

FIG. 42 shows a profile view of the multipolar connector assembly, including the sealing rings and the protective sleeve.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The following description is of the best mode presently contemplated for practicing the invention. This description is not to be taken in a limiting sense but is made merely for the purpose of describing the general principles of the invention. The scope of the invention should be ascertained with reference to the issued claims. In the description of the invention that follows, like numerals or reference designators will be used to refer to like parts or elements throughout.

Although the preferred embodiment of the present invention is directed towards the construction and hermetic sealing of an oxygen saturation sensor, the present invention is not limited to an oxygen saturation sensor. Any physiologic sensor that would be desirable to locate on a lead could be mounted by utilizing this method.

Before describing the present invention in detail, it will be helpful to have a basic understanding of a rate-responsive pacemaker. In a typical application, a pacemaker lead 10 is connected to a rate-responsive pacemaker 12, as illustrated in FIG. 1. The rate-responsive pacemaker 12 is shown implanted in the right upper chest cavity. The pacemakers lead 10 is electrically and mechanically connected to the pacemaker 12. The pacemaker lead 10 is introduced into the heart 18 through a vein, with a distal tip electrode 14 of the pacemaker lead 10 being located in the right ventricle 16 of the heart 18.

The pacemaker lead 10 illustrated in FIG. 1 is shown connected to bipolar lead. Bipolar stimulation is achieved between the tip electrode 14 and a ring electrode 20 approximately one-half inch from the tip electrode 14. Although a bipolar lead is shown in the preferred embodiment, it is evident to one skilled in the art that a unipolar lead could also be used, if desired. In addition, the pacemaker 12 illustrated is a single-chamber pacemaker, although the principles of the present invention are equally applicable to both single- and dual-chamber pacemakers.

An oxygen sensor 22 is positioned within an area of a living body where blood is able to come in contact with the light energy emitted by the oxygen sensor 22. The oxygen sensor 22 may be placed either within a vein that is carrying blood back to the heart 18, within the right atrium 24, or within the right ventricle 16 itself. In the preferred embodiment, the oxygen sensor 22 is positioned on the pacemaker lead 10 proximal to the ring electrode 20 so as to place the oxygen sensor 22 within the right atrium 24 of the heart 18. It is believed that sensing oxygen saturation of the blood within the right atrium is a more sensitive indicator of exercise. Further, when positioned properly within the heart 18, the pacemaker lead 10 is curved in a manner that causes the oxygen sensor 22 to face blood just prior to the blood's passage through the tricuspid valve 46 of the heart 18. For a complete discussion of the use of an oxygen sensor placed in the right atrium as the control mechanism for a rate-responsive pacemaker, see U.S. Pat. No. 5,076,271, issued 12/31/91, which is assigned to the assignee of the present invention, and is hereby incorporated herein by reference.

In FIG. 2, a block diagram is shown illustrating the manner in which the oxygen sensor 22 is connected to control circuitry within the pacemaker 12. Within the pacemaker 12, a sensor drive circuit 30 provides the current pulse used to drive the oxygen sensor 22. Similarly, a sensor process circuit 32 monitors the voltage developed across the sensor terminals 34, 36. Appropriate timing signals 37 are shared between the sensor drive circuit 30 and the sensor process circuit 32. Further, in order to synchronize the sensing function of the oxygen sensor 22 with other events, the sensor drive circuit 30 and the sensor process circuit 32 typically receive timing signals from the pacemaker circuits 42. Timing signals include at least a clock signal 38 and a timing reference (V/R) signal 40 (signifying either that V-stimulation pulse or an R-wave has occurred).

The sensor process circuit 32 shown in FIG. 2 develops a control signal 56 that is representative of the reflectance properties of the blood (and hence relatable to the amount of oxygen that has been sensed within the blood). The control signal 56 is used to control the rate at which the pacemaker 12 delivers a stimulation pulse to the heart 18. Thus, the system shown in FIG. 2 is representative of a rate-responsive pacemaker 12 wherein the pacemaker rate varies as a function of the sensed oxygen content of the blood.

In the preferred embodiment shown in FIG. 3, the pacemaker lead 10 is a bipolar lead. The pacemaker lead 10 includes a lead body 57 having four conductors 48, 50, 52, 54 (FIG. 2) therein. The pacemaker lead 10 further includes a multipolar connector assembly 58 which is designed to mate with the pacemaker 12 by way of a multipolar pacemaker electrode connector 44 (FIG. 2). Thus, the multipolar connector assembly 58 includes four electrical contacts 60, 62, 64 and 66. The electrical contact 60 is connected to the tip electrode 14. The electrical contact 62 is connected to the ring electrode 20. The electrical contacts 64, 66 are connected to a first and second sensor terminal 34, 36, respectively. In a unipolar lead body configuration, a tripolar electrode connector would be employed, thus eliminating the need for contact 62 for the ring electrode 20. In the preferred embodiment, the sensor 22 is combined with the bipolar lead in the area of 59. As is known in the art, sensing cardiac events occurs using the same electrodes as for stimulation. Advantageously, both terminals 34, 36 of the oxygen sensor 22 are connected to separate conductors 48, 50, respectively, of the pacemaker lead 10, which are electrically independent of the conductors 52, 54 which are used for stimulation.

It is believed that the best way of describing the present invention is to describe the apparatus at the lowest level of assembly and then to describe the construction of the body implantable lead.

FIG. 4 shows a tubular "D-shaped" shell 68 which is used for housing the sensor electronics. A plan view and a cross-sectional view of the shell 68 may be seen in FIGS. 5 and 6, respectively. The shell 68 may be made of any hermetic material, such as stainless steel, ceramic, glass, etc. For an oxygen saturation sensor, the shell 68 should be a transparent material, such as glass, a glass ceramic, or an optically clear ceramic. Examples of optically clear ceramics include: alumina, sapphire, ruby, quartz or silica ceramics.

In the preferred embodiment, the thickness of the shell 68 is approximately 0.010 inch, the inner radius is approximately 0.035 inch, with the outer radius therefore being approximately 0.045 inch. This leaves a flat surface 69 on the inside of the shell 68 onto which may be located a microelectronic substrate (not shown). This configuration is ideal since it minimizes the overall diameter of the sensor. The length of the shell 68 is dictated by the size of the microelectronic substrate, which in turn is dictated by the number of components.

In FIGS. 7 and 8 is the preferred embodiment of an end cap 70 that may be used to seal the shell 68. The end cap 70 has an inner cap 72 and an outer cap 82. As seen in FIG. 8, the inner cap 72 comprises a tubular section of metal, preferably 90 percent Platinum and 10 percent Iridium, having a channel 76 therethrough. At one end, the inner cap 72 has a preformed sealing ring 78, either glass frit or a metal, such as gold, sealing ring. At the other end, the inner cap 72 has a protruding lip 80.

The outer cap 82 also comprises a tubular section of metal, preferably made of a material which is 90 percent Platinum and 10 percent Iridium, having a channel 86 therethrough. At one end, the outer cap 82 has a protruding lip 90. The outer diameter of the protruding lip 90 is dimensioned so as to have a snug fit within the protruding lip 80 of the inner cap 72. Adjacent to the protruding lip 90 is a protruding shoulder 92. When mated with the inner cap 72, the protrudi