IMPLANTABLE MEDICAL DEVICE WITH ENCLOSED PHYSIOLOGICAL PARAMETER SENSORS OR TELEMETRY LINK

Abstract: An implantable medical device (10), such as a cardiac pacer (12), defibrillator or drug delivery system, includes a container housing (14) the required power source (16) and circuitry and a header portion (20) molded or glued to the container housing. Sensors, including physiological parameter sensors (32, 34, 36) as may be necessary to control and implement the operation of the implantable device, or a telemetry link, or both, are disposed and sealed within the header (20). The header may include electromagnetic focusing devices to enhance the performance of the sensors. The sensors may include two pulse oximetry sensors that provide differential measurements to improve detection of arterial blood flow.

Description: IMPLANTABLE MEDICAL DEVICE WITH ENCLOSED PHYSIOLOGICAL PARAMETER SENSORS OR TELEMETRY LINK Field of the Invention The present invention relates generally to implantable medical devices, such as cardiac pacemakers, defibrillators and any of a variety of drug delivery systems. More particularly, the invention relates to such implantable treatment devices in which telemetry transducers or physiological parameter sensors are contained and sealed within the header of the implantable device. Background of the Invention Implantable medical devices having electrical circuit components are well known in medical science. Some of the most common forms of such implantable devices are pacemakers and defibrillators. Additionally, implantable drug delivery systems are available for supplying needed medication for treatment of disease or for responding to the physiological demands of a patient in an emergency situation. A pacemaker (or "pacer" as it is commonly labelled) is an implantable medical device which delivers electrical pulses to an electrode that is implanted adjacent the patient's heart in order to stimulate the heart so that it will beat at a desired rate. A normal human heart contains a natural pacemaker by which rhythmic electrical excitation is developed. If die body's pacemaker performs correctly, blood is oxygenated in the lungs and efficiently pumped by the heart to the body's oxygen-demanding tissues. However, when the body's natural pacemaker malfunctions, due to age or disease, an implantable pacemaker often is required to properly stimulate the heart. An in-depth explanation of certain cardiac physiology and pacemaker theory of operation is provided in U.S. Patent No. 4,830,006. In recent years, "rate-responsive" pacemakers have been developed which, in response to a sensed physiological parameter, will automatically change the rate at which the pacemaker provides stimulating pulses to the heart. The physiological parameter provides some indication of whether the heart rate should increase or decrease, as dictated by the physiological needs of the patient. For example, where the patient is at rest, the rate-responsive pacemaker will maintain a normal or base rate of, for example, 60-70 pulses per minute. However, if the sensed physiological parameter indicates that the patient is exercising, then there is a need for the heart to beat much faster, and the pacemaker will respond by stimulating the heart to beat at a higher rate, for example, 100-110 beats per minute. Similarly, implantable defibrillators sense physiological parameters in order to determine when to supply a defibrillating shock to a patient's heart. Ventricular fibrillation is a condition characterized by rapid, chaotic electrical and mechanical activity of the heart's excitable myocardial tissue, and results in an instantaneous cessation of blood flow from the heart due to the uncoordinated or ineffectual action of the ventricles. Defibrillation is a technique employed to terminate fibrillation by applying one or more high energy electrical pulses to the heart in an effort to overwhelm the chaotic contractions of individual tissue sections and to restore the synchronized contraction of the total mass of tissue. Likewise, implantable drug delivery systems also may rely upon physiological parameter sensors to provide signals that may be processed internally in order to determine when, and in what amount, a stored drug is to be delivered into the patient's body. In the treatment of certain diseases, it is desirable to administer a drug into a particular location within the body where the drug will be roost effective in combating the localized disease. As another example, in treating cardiac arrhythmias, it is sometimes desirable to deliver the drug directly to the heart. In other applications of implantable drug delivery systems, the location at which the drug is introduced into the body is not critical, and the body's circulatory system is relied on to carry the administered drug to all parts of the body. Drugs that may effectively be administered by implantable delivery systems include insulin, glucose, heparin or any of a variety of chemotherapeutic agents. Because the condition requiring the use of an implantable device may drastically impair the patient's quality of life or, in some instances, is a life threatening condition, having reliable indicators of physiological parameters is imperative. Physiological parameter sensors and activity parameter sensors that have been employed in association with implantable devices, or that those in the art have suggested may be employed, include those that sense respiration rate, blood oxygen saturation level, temperature, blood pressure, pH, length of the Q-T interval, the length of the P-R interval, thoracic impedance changes, nerve activity, biochemical concentrations (such as enzymes and glucose) and motion or acceleration. Regardless of the sensed parameter, most prior art implantable devices have relied upon physiological parameter sensors that are positioned remotely from the implantable device. For example, U.S. Patent No. 4,886,064 discloses sensors that are implanted remotely from a pacemaker and that wirelessly transmit to the pacemaker signals that indicate or correlate to a sensed parameter. In many other prior art implantable devices, however, the remote sensors are interconnected with the implantable device by means of electrical leads or conductors that are encased in a catheter that extends between the remote sensor and die implantable device. For example, U.S. Patent No. 4,903,701 discloses an oxygen sensor that is located remotely from the pacemaker and that is mounted on the electrical leads used to transmit the generated pulse from the pacemaker to the stimulating electrode. U.S. Patent No. 4,763,655 discloses a temperature sensor and a blood oxygen sensor that are implanted remotely from the pacemaker housing. The sensors are coupled with the pacemaker circuitry by conductors encased in a catheter. Designs for implantable devices that rely upon remote sensors pose significant problems. First, radiofrequency transmission, as suggested by U.S. Patent No. 4,886, 064, typically requires transmitters and receivers that substantially increase the volume, weight and complexity of the pacemaker and sensor. These characteristics are generally undesirable in an implantable device where, for patient comfort, small size and light weight packages are desired goals. Second, remote sensors often need specialized catheters and are susceptible to fixation and migration problems. Another drawback of employing remotely-positioned sensors, at least those that are implanted within or adjacent to the patient's heart, is that should such a sensor fail, delicate surgical intervention may be required to remove and replace the faulty sensor. By contrast, pacemakers and many other implantable devices are typically positioned in an easily accessible location just beneath the patient's skin, and can be accessed and replaced without the risk of life-threatening or extremely costly surgery. Also of significance, because of the required electrical connections between the remote sensors and the internal circuitry within the implantable device, the device is susceptible to infiltration by corrosive body fluids. Any such infiltration will almost instantaneously disable the electrical circuitry in the device. Thus, the locations at which the sensor's leads penetrate the housing of the implantable device must be sealed to prevent infiltration of body fluids. Those involved in the medical arts already have confronted the problem of preventing fluid from infiltrating into an implantable device at the locations where external leads attach to the device housing. As mentioned previously, it is conventional practice to surgically implant a stimulating electrode adjacent the heart and to interconnect the electrode to the pacemaker via conducting leads. This arrangement is shown, for example, in U.S. Patent No. 4,903,701. Additionally, certain implantable medication delivery systems require that electrical conductors interconnect an implantable device containing a power source and control circuitry and a remotely positioned drug dispensing device as, for example, disclosed in U.S. Patent No. 5,041,107. It is, of course, important that all such leads be securely attached to the implantable device to prevent the leads from becoming inadvertently decoupled. At the same time, because pacemakers and drug delivery systems require periodic replacement, and because this replacement procedure ideally is accomplished without disturbing any remotely implanted electrode or other device, the connections between the leads and the implantable device housing must be readily disconnectable. It is critical, of course, that the lead termination and attachment mechanisms prevent infiltration of any fluids into the implantable device. To date, it has been common in the design of implantable devices to provide the device with a header portion which includes one or more terminals for landing and terminating any external leads. The header, which may be made of an epoxy material, supports and insulates the terminals. Internal conductors interconnect the terminal in the header portion with the electrical circuitry contained within the housing of the implantable device. Examples of such headers are shown in U.S. Patent No. 5,282,841 and 4,072,154. Where physiological parameter sensors (rather than electrodes) are implanted remotely from the implantable device, similar such termination means must be provided for landing and terminating the conductors that communicate signals between these remote sensors and die implantable device. In part due to die various problems and disadvantages of positioning sensors remotely from the pacemaker or drug delivery device with which they are associated, it has been suggested that the sensors be housed within the implantable treatment device itself. For example, U.S. Patent No. 5,040,533 proposes an implantable cardiovascular treatment device having self-contained sensors and a window formed in the wall of the container to permit the sensors that are housed within the container to detect or measure a physiological parameter through the window. Although proposed as a solution to some of the aforementioned problems associated with external sensors, the "windowed" pacemaker presents additional and even more pronounced sealing problems. Because the interface between the window and the walls of the container must prevent infiltration by body fluids, a complete and enduring seal must be devised and installed along the entire perimeter of the window. Providing such a seal in the walls of the container presents significant manufacturing difficulties, especially considering d e small size of the container that is typically employed in pacemakers. Further, the additional components and manufacturing time and effort that are needed to provide such a sealed window in the walls of the implantable device would increase substantially the cost of manufacturing such a device. Thus, despite significant advances in the art that have been made over the years, there remains a need for an implantable device capable of housing and protecting any of a variety of types of physiological parameter sensors. The device must permit the sensors to carry out their intended functions and, simultaneously, must seal the devices from exposure to corrosive body fluids. Especially well received would be an implantable device that would be no larger than those presently employed and that would not require significant additional retooling in order to manufacture a new and specialized housing, such as one requiring that a window be formed in what would otherwise be a continuous wall. Ideally, the new device would not create additional sealing problems and could employ known and reliable electrical power supplies and circuitry. Preferably, the implantable device would also house a telemetry link through which data and instructions could be communicated to and from the implantable device. Summary of the Invention In accordance with the present invention, there is provided an implantable medical device having a container for housing electrical circuitry and a header that closes and seals the container. One or more sensors, which may be physiological parameter sensors or telemetry transducers, or both, are retained and sealed within die header. The composition of the header is, in part, dependent upon die particular sensor employed. That is, the material from which the header is formed must allow the sensor to detect or measure through the header die desired physiological parameter. Depending upon the particular sensor, the invention may include a header formed of epoxy, plastic, glass or ceramic. The invention also may include electromagnetic focusing devices that are formed on or attached to the header to improve or enhance sensor performance. Depending upon the desired application for the implantable device, die invention may include any of a variety of sensors that are presently available or that may become available for detecting or measuring relevant physiological parameters. In certain applications, such sensors may include photoemitters or photodetectors. In one particular embodiment of die invention, such photoemitters and detectors are arranged to comprise an oximetry sensor for detecting blood oxygen saturation. A particularly advantageous form of the invention employs two oximetry sensors that are positioned within the header so mat their respective photodetectors face in different directions. This arrangement yields separate signals which can be processed in a way to eliminate the deleterious effects that movement may have on the ability of a single oximetry sensor to make the desired measurement of oxygen saturation of the arterial and mixed venous blood flows. In embodiments of ie invention that employ such photodetectors, the invention will include a translucent or transparent header, for example a header made from a material mat is transparent to wavelength in the infra-red and red portions of the optical spectrum. In another embodiment of die invention, the header includes sensors that are capacitively or resistively coupled to the fluid and tissue outside die housing. Such sensors include a conductive plate disposed in the header a predetermined distance from the outer surface of the header. In embodiments of die invention including capacitively coupled sensors, the header may be formed of a material having a high dielectric constant and low volume resistivity, such as, for example, a dielectric constant of 3.8 and a volume resistivity greater than 1 x 10 13 ohm-cm. Where the sensors are resistively coupled, selected portions of the header may have a low volume resistivity of less than 1 x 10° ohm-cm. Without regard to the type of sensor, the invention may be employed in pacemakers, defibrillators or implantable drug delivery systems. In these implantable devices, die invention includes a sensor evaluation circuit to receive signals from the sensor that are indicative of d e measured physiological parameter. The invention further includes within the container a control circuit, such as a microprocessor, and an output circuit. The output from the sensor evaluation circuit is received by die control circuit which, in accordance with a predetermined algorithm, will signal die output circuit, such as a pulse generator, to generate an electrical pulse of a duration and magnitude required for properly stimulating the heart or dispensing an appropriate dose of medication. The present invention having self-contained sensors eliminates die risky or dangerous surgical intervention that may otherwise be required to remove remotely positioned sensors which tail. Further, because the invention takes advantage of a header such as that typically employed in present day pacemakers, the tried-and-true arrangement used in sealing conventional pacemakers may also be used when implementing d e present invention. Likewise, no specialized housing or container for the implantable device circuitry is required when practicing the invention. Instead, conventional components and manufacturing techniques may be employed, and no additional interfaces are created which require complicated or cosdy seal designs. Accordingly, die present invention comprises a combination of features and advantages which enable it to substantially advance die technology associated wid implantable medical treatment devices. The characteristics and advantages of the present invention described above, as well as additional features and benefits, will be readily apparent to those skilled in die art upon reading die following detailed description and referring to the accompanying drawings. Brief Description of the Drawings For a detailed description of die preferred embodiments of the invention, reference will now be made to the accompanying drawings wherein: Figure 1 is an elevational view of a pacemaker made in accordance with the present invention. Figure 2 is a schematic block diagram of the pacemaker shown in Figure 1. Figure 3 is a schematic block diagram of another embodiment of die present invention employing an oximetry sensor embedded within the header of the pacer shown in Figure 1. Figure 4 is a partial elevation view showing another alternative embodiment of die present invention. Figure 5 is a sectional view taken along line 5-5 in Figure 4. Figure 6 is a partial elevation view showing another alternative embodiment of me present invention. Figure 7 is a sectional view taken along line 7-7 in Figure 6. Figure 8 is a partial elevational view of still another alternative embodiment of the present invention having an oximetry sensor embedded within the header of an implantable device. Figure 9 is a top view of the implantable device shown in Figure 8. Figure 10 is a schematic block diagram showing another alternative embodiment of d e present invention having conductive plates embedded widiin the header of an implantable device. Figure 11 is a schematic diagram showing the equivalent circuit corresponding to a portion of the circuit shown in Figure 10. Figure 12 is an elevational view of another alternative embodiment of d e present invention as employed in an implantable iontophoretic drug delivery system. Figure 13 is an elevational view of another alternative embodiment of die present invention employed in another type of implantable drug delivery system. Figure 14 is a top view of the drug delivery system shown in Figure 13. Figure 15 is a partial elevational view of another alternative embodiment of the present invention with certain components shown in a schematic block diagram form. Description of the Preferred Embodiments Presently-preferred embodiments of die invention are shown in the above-identified figures and described in detail below. In describing diese various embodiments, like or identical reference numerals are used to identify common or similar elements. Referring now to Figure 1, there is shown an implantable device 10 which, in die embodiment shown in Figure 1, is adapted for use as a self-contained implantable cardiac pacer 12. Pacer 12 comprises a can or housing 14 which contains an electrical energy source, such as battery 16, and a hybrid circuit 18 which includes electrical circuitry for generating an electrical pulse to stimulate a patient's heart in a predetermined manner. Pacer 12 further includes a header 20 which is molded or glued to housing 14 and serves as a termination point for the electrical conductors that are used to transmit the generated pulse to the patient's heart. Imbedded widiin header 20 is terminal 22 and physiological parameter sensors 32,34,36. Terminal 22 is electrically interconnected widi the hybrid circuitry 18 by conductors 28 which pass through conventional feedthrough 30. Similarly, sensors 32,34,36 are interconnected to the sensor circuitry 62 (Figure 2) contained on die hybrid circuit 18 via conductors 42,44,46. It is to be understood that conductors 42,44 and 46 shown in Figure 1 may each comprise two, three or more discrete conductors depending upon the requirements of the particular sensor employed. Conductors 42,44 and 46 pass from header 20 into housing 14 via feedthroughs 37,38 and 39, respectively. The battery 16 is interconnected widi the hybrid circuitry 18 via conductors 17 in a conventional manner. An electrode 50 used to stimulate the heart is interconnected with pacer 12 via conductors (not shown) that are sheathed in a flexible catheter 52. The proximal end of d e conductors contained widiin catheter 52 terminate at connector 24 which includes a male connector pin 26. Pin 26 is inserted into terminal 22 within header 20 so as to electrically interconnect electrode 50 wid die pulse generation circuitry contained the hybrid circuit 18. The elements used to interconnect external electrode 50 widi circuitry within pacer 12 are conventional and well understood by diose skilled in this art. Further details concerning such interconnections are shown, for example, in U.S. Patent Nos. 5,076,270 and 4,860,750, the disclosures of which are incorporated herein by mis reference. Connector 24 includes an outer covering 25 of silicon rubber or another material which is both resilient and insulative. Likewise, catheter 52 is covered wid an insulative material throughout its length for insulating die electrical conductors that are contained therein. Pacer housing 14 is made of a biocompatible, corrosion-resistant metal such as stainless steel or titanium. The header 20 preferably mounts on a welded housing 14 after the hybrid circuit 18 and battery 16 are assembled widiin die interior of the housing according to conventional techniques. Header 20 may be formed from any of a large number of biocompatible materials capable of retaining and insulating sensors 32,34 and 36 and terminal 22. Header 20 must also have a transmissivity for electromagnetic energy as required by the particular sensor or communication mechanism employed in die implantable device 10. The header 20 therefore must be constructed of a suitable material that conducts electromagnetic energy without excessive absorption or reflection, thereby allowing the embedded sensor to transmit and receive electromagnetic energy to and from a point external to the header 20. For many applications, header 20 preferably is made of an epoxy resin or similar thermosetting polymer material which is formed in situ over terminal 22 and sensors 32,34 and 36. Header 20 is formed so as to include a bore 23 for receiving connector 24. Accordingly, sensors 32,34,36, terminal 22, as well as portions of conductors 28,42,44 and 46 are supported and imbedded widiin die cured resin, which also serves to insulate die encφsulated components. In addition to epoxy, other material suitable for header 20 include glass, plastics and elastomers, such as Dow Chemical's Pellethane and ceramic materials such as sapphire. Referring now to Figure 2, the various circuits mounted on hybrid circuit 18 and employed in this embodiment of the invention are shown in block diagram form. As shown, included on die hybrid circuit 18 is pulse generation circuitry 60 for generating the electrical pulse which is delivered through pacing electrode 50 for stimulating the patient's heart. Any of a variety of pulse generation circuits can be employed in the present invention including, for example, those disclosed in U.S. Patent Nos. 5,318,596, 5,040,534 and 4,830,006, the disclosures of which are incorporated herein in their entireties by this reference. It should be understood, however, that die present invention is not limited to any particular pulse generation circuit. Hybrid circuit 18 further includes sensor circuitry 62 for controlling, receiving, conditioning and processing the signals sent to and received from sensors 32,34 and 36, which are disposed widiin header 20. The sensor evaluation and control circuitry 62 also includes die components necessary to supply power to the sensors 32,34,36 and, as shown and described widi reference to Figure 3 below, may provide die control and logic necessary to time when various data are gathered or received. A memory circuit 64 also is mounted on die hybrid circuit 18 for storing various data, such as baseline data and algorithms which may be preprogrammed into memory circuit 64 or which may be programmed through an external programmer. A microprocessor 66 also is preferably included for receiving data from sensor evaluation and control circuitry 62 and for controlling the rates at which pulses are generated by pulse generator 60. Timer 69 and rate controller 68 are also included in the hybrid circuit 18 and interconnected between microprocessor 66 and pulse generator 60. Together, microprocessor 66, rate controller 68 and timer 69 cooperate to increase and decrease the rate that pulses are generated by pulse generator 60 in accordance with a predetermined schedule of rate increases or decreases diat are preprogrammed into microprocessor 66. Rather than including discrete circuits for timer 69 and rate controller 68, these circuits preferably may be combined in a single integrated circuit package with microprocessor 66. Sensors 32,34 and 36 are interconnected to sensor evaluation and control circuitry 62 via conductors 42,44,46 (Fig. 1). When eπφloyed in pacer 12, sensors 32,34 and 36 may be any of a variety of sensors capable of sensing various physiological parameters whose value correlate to heart rate. As previously discussed, die physiological parameters that may be sensed by implantable device 10 and evaluated for use in stimulating the heart, providing a defibrillizing pulse or dispensing needed medication include, for example, blood oxygen saturation, respiration rate, pulse rate, temperature, pH value of the blood, the natural atrial rate and the QT interval. While three sensors 32,34 and 36 are shown embedded widiin header 20 in Figure 1, die invention is not limited to a particular number of sensors. For example, in certain applications, only a single sensor may be required or desirable. In other instances, more than one of the same kind of sensor may be employed in the header 20. In such instances, the plurality of sensors may be employed to provide a differential measurement of the sensed physiological parameter. Alternatively, or in addition to such an arrangement, an additional sensor of the same type may be employed as a backup or redundant sensor, so as to delay or eliminate the need for surgical intervention that might otherwise be required to replace die implantable device 10 should critical sensors fail. It should also be understood that the invention may include more man a single type of sensor embedded widiin header 20. For example, referring to Figure 1, sensors 32 and 34 may be used to detect blood oxygen saturation, while sensor 36 may be a temperature sensor or a sensor for measuring the rate of blood flow. The types of sensors that may be embedded widiin header 20 are numerous; the type and number of sensors positioned widiin header 20 are limited only by the size of the implantable device 10 and header 20. There is shown in Figure 3 a more particular embodiment of the invention in which a number of electroptic devices or odier devices for emitting and sensing electromagnetic radiation are embedded widiin a translucent or transparent header of an implantable device 10 for emitting and receiving signals corresponding to sensed physiological parameters. For convenience, this embodiment of d e invention also will be described for use widi a pacer 12, the specifics of which were previously discussed. It is to be understood, of course, that this invention can similarly be used in a drug delivery system or any other implantable device. Referring now to Figure 3, pacer 12 includes an oximetry sensor 70 for detecting the level of oxygen saturation in the blood. More specifically, oximetry relates to the portion of circulating hemoglobin that has been saturated widi oxygen. The extent of oxygen saturation is dependent upon the patient's level of activity, and the exchange process between carbon dioxide and oxygen within the blood. The relationship between oxygen saturation of die blood and die pulse rate of a patient having a healthy heart is well understood. Due to this relationship, the present invention may be employed to regulate the pacing rate in response to the sensed blood oxygen level in order to pace the heart at a rate appropriate to the physiological demands of die patient's body. Oximetry sensor 70 is encapsulated widiin header 20 and includes photoemitters 72,74 and photodetector 76. In a preferred embodiment, the photoemitters 72,74 comprise light emitting diodes (LED's). More particularly, LED 72 is chosen to emit light in the red wavelength, for example within the range of 640-660 nm, while photoemitter 74 emits light in the infrared wavelength, such as a wavelength at die isobestic point (approximately 805 nm) or farther into the infrared spectrum, such as within the range of 880-940 nm. LED's 72,74 will be sequentially turned on and off so as to emit light pulses 73,75 in an alternating fashion. Photodetector 76, which may be a conventional phototransistor or photodiode, will receive the optical signals 77 that are reflected from the patient's blood. Thus, the output of photodetector 76 will represent a received signal first from the firing of the red LED 72 and subsequently from the IR LED 74. In the embodiment of die invention shown in Figure 3, sensor evaluation and control circuitry 62 includes circuitry for firing the LED's 72,74 in the proper sequence, and for separately receiving and evaluating the reflected signals. More specifically, sensor circuitry 62 includes an LED driving circuit 92, control logic 90, amplification circuit 78, switch 80 and sample-and-hold circuits 82,84. According to a predetermined sequence, the control logic circuit 90 will signal LED driving circuit 92 to alternately fire LED's 72,74, circuits 90 and 92 being powered by battery 16. The light pulses 73,75 alternately emitted by LED's 72,74, respectively, will be reflected back and received by photodetector 76 as reflected signal 77. The signal 77 received by photodetector 76 is amplified by a conventional amplification circuit 78. A switch 80 is controlled by control logic 90 so as to supply the amplified signal received from photodetector 76 alternately to the appropriate sample-and-hold circuits 82,84, circuit 82 holding die sensed value as detected by die reflection of red light emitted by LED 72, and circuit 84 holding the sensed value as detected by die reflection of IR wavelengths as emitted by LED 74. The outputs from sample-and-hold circuits 82,84 are provided to a multiplexer 86 which multiplexes d e amplified output signal and supplies die signal to an analog-to-digital converter 88. The converted digital signal then is supplied to microprocessor 66 which, in accordance widi a preprogrammed algorithm, causes pulse generator 60 to produce die appropriately timed stimulation to the patient's heart. Other circuitry for receiving and processing signals received by photodetectors, and for thereafter generating appropriate pacing signals in a predetermined manner are known in the art of pulse oximetry. For example, U.S. Patent No. 4,903,701, incorporated herein in its entirely, discloses circuitry associated with oxygen sensor control and pacemaker timing, the powering of the oxygen sensors, telemetry of the sensed oxygen saturation values, timing of sensor operation and die decoding of die signals generated by the oxygen sensor. Thus, the present invention is not limited to the exemplary circuitry described widi reference to Figure 3. To enable the pulse oximetry sensor 70 thus described to operate effectively, it is important that the header 20 permit die desired wavelengths of light to pass through die header widi an optical transmission effective rate of 70%. Accordingly, it is preferred that the material from which header 20 is formed have the following characteristics: high dielectric constant and dielectric strength, USP Class VI biocompatibility, flatness of its optical transmission curve encompassing the desired wavelength, and low changes in optical properties d roughout implant life. Presendy, the most preferred materials for header 20 are Emerson & Cuming Stycase* 1267 or 1269 transparent, high-impact casting resins or Epoxy Technology, Inc. Epo-tek* 301 spectrally transparent epoxy which have an appropriate transmission between 900 nm and 350 nm. Thus, die oximetry sensor 70 and cardiac pacer 12 shown in Figure 3 provides d e advantages offered by pulse oximetry technology but, in contrast to die prior art which typically required specialized leads for housing the LED's and photoreceptors, instead provides a self- contained integrated package that is not susceptible to the mechanical stresses imparted to conventional pacing leads. The oximetry sensor 70 and pacer 12 include windowless enclosures and thus are less likely to experience seal failures or to be infiltrated by corrosive and destructive body fluids. Another alternative embodiment of ie present invention that is useful when employing photoemitters and photodetectors as sensors is shown in Figures 4-7 in which one or more electromagnetic focusing devices 94 are embedded in or formed in or on header 20. Referring first to Figures 4 and 5, focusing device 94 may comprise a preformed lens 96 of glass or other composition. Lens 96 is positioned adjacent the outer surface 95 of header 20 when header 20 is formed such that reflected optical signals 77 are better focused on photodetector 76. More particularly, lens % is selected and positioned widiin header 20 such uiat its focal point is positioned on die light sensing surface 71 of photodetector 76. Similarly, lenses 97 are retained within header 20 adjacent outer surface 95 so as to more precisely focus the light emitted by LED's 72,74 at a focal point a predetermined distance from header surface 95. Referring now to Figures 6 and 7, focusing device 94 may similarly comprise a Fresnel lens 99 that is created by forming a series of concentric grooves 98 on surface 95 of header 20. Grooves 98 may be laser etched or otherwise engraved on die header surface 95 and, like lens 96 shown in Figures 4 and 5, will serve to focus the reflected light signal 77 on the light sensing surface 71 of photodetector 76. Although not shown, a similar lens 99 is preferably formed in header 20 adjacent to LED 72,74 to more precisely focus the light pulses emitted from LED's 72,74. While two distinct electromagnetic focusing devices 94 have been shown in Figures 4-7, it should be understood uiat die present invention is not limited to die use of die lenses 96 or 97. Instead, any of a variety of other means to focus electromagnetic energy may similarly be employed. For example, a disk of transparent or translucent material having a different index of refraction than die remainder of header 20 may likewise be used as a focusing device 94 and embedded widiin or ouierwise retained on header 20. When employing pulse oximetry technology, it is the ratio between oxygen saturation of uie arterial blood flow, and that of die venous return flow, that is important in determining whether die patient's heart is functioning properly. See e.g. Inbar, et al. "Development of a Closed-Loop Pacemaker Controller Regulating Mixed Venous Oxygen Saturation Level", IEEE Trans. BME 35(9) (1988), at 679-690. The oximetry sensor distinguishes die arterial blood flow by sensing die pressure pulses created in die arterial blood flow by the periodic contraction of ie heart. Conventional pulse oximetry sensors are motion intolerant in that movement of die oximetry sensor, as may, for example, be caused by a patient jogging or otherwise exercising, may cause the sensor to provide a signal similar to that caused by die arterial pressure pulses. This is because such exertion may cause relative movement between die sensor and die surrounding tissue. Such motion affects the thickness of tissue between die light source and photodetector, dierefore altering die absorption of light as it travels along die light path. Repetitive motion may mimic arterial pulsation such uiat die oximetry sensor is unable to discern d e difference between pulses resulting from motion and diose resulting from the arterial activity. However, by positioning photoemitters 72,74 and photodetectors 76 in the header 20 of implantable device 10 as described here below, a differential measurement may be taken which can eliminate the problems that sensor motion may cause in conventional oximetry sensors. More specifically, and referring to Figures 8 and 9, two pairs of LED's 72,74 and two pairs of photodetectors 76 are positioned such uiat LED's 72a,74a and photodetector 76a face in a first direction relative to a plane 100 which bisects the pacer 12, while LED's 72b,74b and photodetector 76b face in die opposite direction. In this manner, optical signal 77a detected by photodetector 76a may be compared by microprocessor 66 (Figure 3) to optical signal 77b detected by photodetector 76b for each. Through this comparison, the effect that movement of die pacer 12 would otherwise have on the sensed signals can be eliminated. A simple algorithm used to process these signals in order to eliminate die effects of movement would add die digitized signals detected by photodetectors 76a and 76b while LED's 72a and 72b are on. The same would be done when LED's 74a and 74b are on, providing die two signals necessary for calculating oxygen saturation throughout die blood flow cycle. Anodier embodiment of ie present invention is shown in Figure 10. As previously discussed, die invention is not limited to any particular type of sensor, but instead may be employed widi any device capable of detecting or measuring a physiological parameter useful in properly stimulating ie heart or dispensing needed medication or providing other desirable treatment. In certain instances, it may be advantageous to detect die electrocardiogram of a patient or otherwise make use of a body's tissues and fluids as a bioelectrode in order to measure a physiological parameter. In die embodiment shown in Figure 10, sensors 32,34 comprise electrodes formed by conductive plates 102 that are embedded widiin header 120 of pacer 12. Header 120 is formed of an electrically insulative material having a relatively high dielectric constant. Plates 102 form capacitively-coupled bioelectrodes to sense electrical signals that are related to a desired physiological parameter. As shown schematically in Figure 10, plates 102 are disposed at a predetermined distance from outer surface 124 of header 120 and are connected to high input impedance buffers 104 which, for example, may be J-FET unity-gain buffers or single FET transistors. Because of their small size, buffers 104 may be mounted widiin header 120 immediately adjacent to places 102. Alternatively, buffers 104 may be included as part of the circuitry housed widiin housing 114. The outputs of buffers 104 are connected to an amplification circuit 106 which provides a signal to sensor evaluation circuit 108. The output from evaluation circuit 108 is communicated to a microprocessor or control logic 66. Depending upon predetermined instructions programmed into die control logic 66, die appropriate stimulating pulse will then be generated by pulse generator 60. Figure 11 schematically depicts die equivalent circuit for the sensors 32,34 shown in Figure 10. Bioelectric source 112 may, for example, represent ie electrocardiogram which is capacitively coupled to plates 102 by a network 116 which represents the impedances 118 formed by the patient's blood and various tissues and d e dielectric material from which the header 120 is formed. To provide for die appropriate capacitive coupling, it is important uiat d e volume resistivity of the header material be higher than 1 x 10 12 ohm-cm. When used in a pacer 12 as shown in Figures 10 and 11 and used to sense die electrocardiogram, d e header material preferably will have a dielectric constant above 3 (such as, for example 3.8), a volume resistivity above 1 x 10 12 ohm-cm and die electrodes 102 will be disposed approximately 0.2 mm from the outer surface 124 of header 120. An epoxy that is particularly suited for use in forming header 120 is Emerson & Cuming Stycast* 1267. Alternatively, where a particular physiological parameter may be better sensed by means of a resistive coupling, rather than through die capacitive coupling just described, die material of header 120 may be selectively modified to have a lower dielectric constant. For example, electrically conductive particles such as colloidal platinum may be added to an epoxy header material before it sets, such that the hardened material will have a resistivity of approximately 1 megaohm-cm. As stated above, the present invention may also be employed in implantable, closed-loop drug delivery systems. Conventional such systems are typically one of two types. A first such system is one having a sensor that is implanted remotely from the implantable drug delivery device. An example of such apparatus is disclosed in U.S. Patent No. 5,041,107, the disclosure of which is incorporated herein by this reference. A second type of implantable system is structured such uiat the sensor is housed along widi the drug delivery apparatus. An example of this second type of drug delivery system is shown in U.S. Patent No. 4,373,527, the disclosure of which is also incorporated herein by reference. Referring now to Figure 12, there is shown implantable drug delivery system 130 which generally includes sensor body 132, electrically operable drug delivery mechanism 134 and catheter 136 disposed therebetween. Sensor body 132 generally includes a container 14 which houses battery 16 and hybrid circuit 18 all as previously described widi reference to Figure 3. Sensor body 132 further includes a header 20, also previously described widi reference to Figure 3. Header 20 seals and insulates physiological parameter sensors 138J 39,140 as well as terminal 22. The hybrid circuit 18 includes sensor evaluation circuit 141, memory component 142, microprocessor 144 and a pulse generator 146. The configuration of sensor evaluation circuit 141 is dependent upon the type of physiological parameter sensors 138-140 that are employed. In turn, die choice of physiological parameter sensors 138-140 will, in part, be dependent upon die particular drug diat is to be administered via implantable drug delivery system 130. Although die present invention is not limited to any particular implantable drug delivery system, it may be used advantageously in an implantable insulin delivery system. The determining factor in whether to administer insulin is die measure of glucose circulating in die patient's bloodstream. As widi oxygen saturation, measurements of circulating glucose may be made by photoelectric means. For example, in the implantable system 130 shown in Figure 12, devices 138 and 139 may be LED's 148,150 chosen to emit light in the wavelengths of 9.68 micrometers and 3.42 micrometers, respectively. Sensor 140 may comprise phototransistor which will receive reflected light from that emitted by LED's 148,150. The signals from phototransistor 139 may be sampled by sensor evaluation circuit 141 which, in uiis embodiment, would be substantially identical to die sensor evaluation circuit 62 described widi reference to Figure 3. Based on die values sensed and evaluated, microprocessor 144 will cause pulse generator 146 to generate an electrical pulse diat will be transmitted through the conductors contained in catheter 136 to drug delivery mechanism 134 which will dispense die appropriate amount of insulin response thereto. A drug delivery system of die type having die sensor housed widi die drug delivery apparatus and which employs the present invention is shown in Figures 13 and 14. As shown, implantable drug delivery system 160 generally includes container 162 and header portion 164. Container 162 is made of a biocompatible material such as stainless steel or titanium and is generally divided internally into four compartments or chambers: an inlet chamber 165; a reservoir chamber 166; a pump chamber 167; and an electronics chamber 168. Inlet chamber 165 contains a supply port 170 which includes a self-sealing membrane 171. To supply insulin or other appropriate drug to drug delivery system 160, the drug is injected by means of a hypodermic needle diat is inserted through membrane 171. Inlet chamber 165 is in fluid communication widi reservoir 166 and, when required, die drug will be drawn from inlet chamber into reservoir 166 dirough a fluid duct (not shown). Pump chamber 167 includes a pump 169 which, when receiving the appropriate signal from the circuitry contained widiin electronics chamber 168, will pump the appropriate amount of medication from reservoir chamber 166 into the patient's body via port 174. Housing 162 includes an aperture 176 (Figure 14) into electronics chamber 168. Header 164 closes aperture 176 and seals die physiological parameter sensors 176,177, 178 therein. Electronics chamber 168 houses batteries 16 and hybrid circuit 18. The hybrid circuit 18 contains the sensor evaluation circuitry, memory, microprocessor and pulse generator as previously described widi reference to Figure 12. As described previously, die invention may be employed widi any of a variety of physiological parameter sensors. Where such sensors employ LED's and phototransistors or other photoelectric means, die material used in forming header 164 will be selected to as to have the optical properties necessary to achieve the desired light transmission. For example, an epoxy having die physical characteristics described previously with reference to header 20 in Figure 3 is well suited for use in die implantable drug delivery system shown in Figures 13 and 14. The principles of die present invention can also be employed as a telemetry link used to transmit data or instructions to and from the implantable device. Referring to Figure 15, implantable device 180 is shown having a container 14 and header 20. Container 14 houses a battery (not shown) and a hybrid circuit 18 which contains the electronics required both to fulfill die primary function of ie implantable device, for example, cardiac pacing or drug delivery, as well as the components required to communicate between die implantable device and data transmitters and receivers diat may be located both outside die body or implanted remotely from die implantable device 180. The telemetry link 182 is contained widiin header 20 and can comprise any of a number of types of transmitters and receivers, the details of which will depend on the type of communications desired. Particularly suited for use in die present invention is communication link 182 which employs a photoemitter such as LED 184 and a photodetector such as phototransistor 186. LED 184 and phototransistor 186 permit an optical link between implantable device 180 and external probe 190 diat, when placed against the patient's skin 192 at a location adjacent to header 20 can transmit instructions via optical signal 194. Those instructions are received at photodetector 186 and transmitted to signal conditioning and LED driver circuitry 196, the output of which may be used to reprogram microprocessor 198 or be stored in memory package 199. Similarly, data gathered by implantable device 180 may be transmitted to probe 190 by means of LED 184 which, in response to the appropriate signal from microprocessor 198 and signal conditioning and LED driver circuit 196, will transmit optical signal 193 to probe 190. As will be understood by those skilled in die art, where implantable device 180 includes a telemetry link 182 as thus described, header 20 should have die optical qualities and properties such as those previously described for header 20 shown in Figure 3. It should also be understood diat die telemetry link 182 is not limited to a photoelectric means, but instead may send and receive telemetry by other means, for example, by transmission of radiofrequency signals, in which event, antennas, rather than photoemitters and photodetectors, would be employed in header 20. Similarly, the invention is not limited to a means for communicating widi an external probe 190, but instead may be configured to communicate wi i odier remotely positioned implantable devices diat may, for example, include physiological parameter sensors such as contemplated in U.S. Patent No. 4,886,064. While preferred embodiments of uiis invention have been shown and described, modifications thereof can be made by one skilled in die art without departing from the spirit or teaching of uiis invention. The embodiments described herein are exemplary only and are not limiting. Many variations and modifications of the system and apparatus are possible and are widiin die scope of the invention. Accordingly, die scope of protection is not limited to die embodiments described herein, but is only limited by die claims which follow, the scope of which shall include all equivalents of die subject matter of the claims.

Claims: Claims What is claimed is: 1. An implantable medical device for implantation into a living body, said device comprising: a container having a chamber housing electrical components therein; a header attached to said chamber; at least one sensor sealed widiin said header; and leads electrically interconnecting said sensor and said electrical components. 2. The iπφlantable device of claim 1 wherein said sensor comprises a physiological parameter sensor. 3. The implantable device of claim 1 wherein said sensor comprises a telemetry transducer for communicating with a device remote from said implantable device. 4. The implantable device of claim 1 wherein said header is made of epoxy. 5. The implantable device of claim 1 wherein said header is made of a ceramic material. 6. The implantable device of claim 1 wherein said header is made of plastic. 7. The implantable device of claim 1 wherein said header is made of glass. 8. The implantable device of claim 1 wherein said header is translucent. 9. The implantable device of claim 1 wherein said header is transparent. 10. The implantable device of claim 1 wherein selected portions of said header are made of a material having a volume resistivity less than 10 megaohm cm. 11. The implantable device of claim 1 wherein said header is made of a material having a volume resistivity higher than 1 x 10 12 ohm-cm. 12. The implantable device of claim 1 wherein said header has a dielectric constant greater than 3. 13. The implantable device of claim 1 wherein said header has high optical transmissivity to wavelengths between 350 nanometers and 900 nanometers. 14. The iπφlantable device of claim 1 wherein said header has high optical transmissivity to wavelengths between 3 micrometers and 10 micrometers. 15. The iπφlantable device of claim 2 wherein said sensor comprises a photodetector capable of receiving dirough said header an optical signal indicative of d e measure of a physiological parameter. 16. The implantable device of claim 15 wherein said sensor further comprises a photoemitter capable of emitting through said header an optical signal to be received as reflected light by said photodetector. 17. The implantable device of claim 2 wherein said sensor comprises an oximetry sensor. 18. The implantable device of claim 1 further comprising an electromagnetic focusing device on said header. 19. The implantable device of claim 18 wherein said electromagnetic focusing device comprises a body of material having an index of refraction that is different from the index of refraction of said header. 20. The implantable device of claim 18 wherein said header includes an outer surface, and wherein said electromagnetic focusing device comprises a plurality of grooves formed in said outer surface of said header. 21. The implantable device of claim 18 wherein said sensor includes a sensing surface, and wherein said electromagnetic focusing device comprises a lens attached to said header, said lens having a focal point adjacent to said sensing surface of said sensor. 22. The implantable device of claim 1 wherein said header includes an outer surface, and wherein said sensor comprises a conductive plate disposed in said header a predetermined distance from said outer surface. 23. The implantable device of claim 22 wherein said predetermined distance is widiin die range of approximately 0.1 mm to 2.0 mm. 24. The implantable device of claim 1 wherein said header includes a plurality of sensors. 25. The iπφlantable device of claim 24 wherein said plurality of sensors include a first physiological parameter sensor and a second physiological parameter sensor, and wherein said first and second sensors measure different physiological parameters. 26. The iπφlantable device of claim 25 wherein said plurality of sensors include a physiological parameter sensor and a telemetry link. 27. The implantable device of claim 24 wherein said plurality of sensors include a first oximetry sensor having a first photodetector widi a first light-sensing surface and a second oximetry sensor having a second photodetector widi a second light-sensing surface, said first and second photodetectors disposed in said header so that said first and second light-sensing surfaces face in different directions. 28. The implantable device of claim 27 wherein said light-sensing surface of said first photodetector faces in a direction substantially 180 degrees apart from the direction faced by said light-sensing surface of said second photodetector. 29. An implantable medical device for implantation in a body, said device comprising: a housing for containing electrical components; an encapsulated module attached to said housing; a sensor disposed in said encapsulated module and capable of sensing a predetermined physiological parameter. 30. An implantable medical device as in claim 29, wherein said encapsulated module is constructed of a material with a volume resistivity higher than 1 x 10 12 ohm-cm. 31. An implantable medical as in claim 29, wherein said encapsulated module is constructed of a material with a high optical transmissivity to wavelengths between 350 nanometers and 900 nanometers. 32. An implantable medical as in claim 29, wherein said encapsulated module is constructed of a material widi a high optical transmissivity to wavelengths between 3 micrometers and 10 micrometers. 33. An implantable medical device as in claim 29, wherein said header is constructed of a material with a dielectric constant greater than 3.0. 34. An iπφlantable device as in claim 29, wherein selected portions of said header are constructed of a material with a volume resistivity less than 10 megaohm-cm. 35. An implantable medical device comprising: a sensor responsive to a physiological parameter and capable of producing a first signal indicative of die measure of the sensed physiological parameter; a control circuit responsive to said first signal and capable of generating a control signal according to a predetermined algorithm relating said control signal to the measure of said sensed physiological parameter; an output circuit responsive to said control signal and capable of producing an electrical pulse of a predetermined duration and magnitude in response to a control signal; a container housing said control circuit and said output circuit; a header closing and sealing said container, wherein said sensor is disposed widiin said header; and means for coupling said sensor in said header to said control circuit in said container. 36. An implantable medical device comprising: a container having a body portion housing a pulse generation circuit, a sensor evaluation circuit and a microprocessor for receiving signals from said sensor evaluation circuit and controlling said pulse generation circuit; said container further including an opening in said body portion housing and a header portion positioned adjacent said body portion housing, in die region of said opening; a sensor disposed in said header portion and in electrical communication with said sensor evaluation circuit in said body portion; and a terminal disposed in said header portion and electrically insulated from said sensor, said terminal being electrically interconnected widi said pulse generation circuit and being adapted for landing and terminating an electrical lead. 37. A pulse oximetry sensor comprising: a first pulse oximeter including a first photodetector having a light-sensing surface; a second pulse oximeter including a second photodetector having a light-sensing surface; and a body encapsulating and sealing said first and said second pulse oximeters in a predetermined orientation such that said light-sensing surfaces of said first and second photodetector face in different directions.

Inventors: PATRICK PAUL J, PRUTCHI DAVID, PATRICK, PAUL, J., PRUTCHI, DAVID
Applicants: INTERMEDICS INC, INTERMEDICS, INC.
Application year: 1996
Main classification: A61N1
IPC codes: A61B5, A61M37, A61N1