Abstract:
A medical monitoring apparatus designed to be implanted in the vascular system is capable of sensing and transmitting via a telemetry link to an external monitor both pressure and temperature information. An internally or externally powered microcircuit component is supported on a stent-like structure and adapted to be placed in the vascular system. Placement in the ventricular septum permits measurement of pressure and temperature in the left ventricle without introducing thrombus generating materials in the left ventricle.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This is a continuation-in-part of application Ser. No. 09/303,634, filed May 3, 1999. 
    
    
     BACKGROUND OF THE INVENTION 
     I. Field of the Invention 
     This invention relates generally to medical apparatus for monitoring physiologic parameters within the body of a human or other animal, and more particularly to an implantable device for chronic monitoring of pressure, flow and temperature within living humans or animals. 
     II. Discussion of the Prior Art 
     In the diagnosis and treatment of various maladies, a variety of devices have been developed which can be implanted within the body and used to monitor various physiologic parameters. With the advent of microminiature circuitry, it has become practical to implant a variety of sensors responsive to various physiologic changes, along with circuitry for the transcutaneous transmission of information from the implanted unit, via a telemetry link to an external recording/display device. For example, in the field of implantable cardiac pacemakers and defibrillators, sensing circuitry is incorporated therein for monitoring a number of physiologic parameters, such as respiratory rate, tidal volume, heart rate, blood temperature, movement, etc. Pacemaker leads have been developed that incorporate pressure transducers and temperature sensors such that the pacing rate of the implanted device can be made to vary in relation to detected changes in blood temperature and blood pressure. 
     In implementing such devices, the electronic circuitry is housed in a body compatible, fluid impervious housing along with a suitable power supply or AC to DC converter and electrical leads are then routed from the implant site and through the vascular system to a location on or in the heart. Because of concern that the presence of a lead in the left ventricular chamber may result in the formation of a thrombus that could break loose and reach the brain and cause stroke or embolize to another peripheral vessel, pacing leads or other devices are seldom inserted into the left ventricle, especially for chronic monitoring or therapy delivery. 
     The ability to measure left ventricular pressure or its surrogate in the ambulatory patient, non-invasively, has great potential in determining the status of heart failure patients, providing an opportunity to modify medical management of ventricular dysfunction very precisely as compared to current clinical practice. Moreover, ambulatory hypertensive patients can be managed more closely when peak systolic and diastolic pressure can be chronically monitored. 
     The ability to measure myocardial temperature with an implanted device and to thereafter telemeter the temperature information to an external monitor will permit cardiac transplant patients to be closely managed. It is believed that rejection in organ transplant patients manifest early as a small tissue temperature elevation due to the inflammatory reaction of rejection. The only presently available method to determine transplant status is to perform a biopsy, an invasive procedure that is sometimes done weekly or more often, and is done in such a patient hundreds of times during that patient&#39;s life. A device for measuring tissue temperature and telemetering the information to an external monitor would limit the number of times such biopsy is required-a significant clinical advance. 
     Myocardial temperature sensing is beneficial in the management of heart failure. Ventriculo-vascular coupling and impedance mismatches manifest themselves as excess heating of the ventricle. By having temperature monitoring available, accurate titration of preload and afterload reducing medication could be achieved to limit myocardial energy output and thereby the heart will perform more efficiently. Therefore, a need exists for a system for chronically monitoring temperature and pressure within the left ventricular and/or atrial chambers of the heart or myocardial tissue. 
     It has also determined that a temperature sensor located in the pulmonary artery branches for sensing lung tissue temperature can provide meaningful information following heart/lung transplant surgery in that an elevated blood or lung tissue temperature in the pulmonary artery or branches may be indicative of the onset of rejection, allowing interventional adjustment in the amount of anti-rejection drug being administered to the patient. We are presently unaware of any temperature sensor that can be chronically implanted to measure temperature changes in blood traversing the pulmonary artery. 
     By locating the monitor implant at other locations within the body, renal, hepatic or pancreas transplant status can be assessed. Locating the device in the peripheral blood vessels can allow assessment of exercise capacity. The monitor may also be used to calculate blood flow using thermodilution techniques. 
     From the foregoing, it can be seen a need exists for an implantable sensor especially designed for placement in a selected portion of a patient&#39;s vascular system and which can be used to chronically transmit pressure and/or temperature data to an external monitor/display unit so that a medical professional can more readily diagnosis and treat various medical conditions. It is principal object of the present invention to fulfill this need. 
     SUMMARY OF THE INVENTION 
     In accordance with the present invention there is provided a medical monitoring apparatus that comprises a support member that is adapted for chronic implantation at a predetermined location within the vascular system of a living human or other animal. One or more sensor devices are affixed to the support means for sensing at least one measurable physiologic parameter. The apparatus further includes a means for telemetrically transmitting signals representative of the sensed parameter percutaneously to an external signal receiver. In accordance with one embodiment of the invention, the support means may comprise a self-expanding or balloon expandable tubular stent that is adapted for chronic implantation at a predetermined location in the vascular system and affixed to the tubular stent is an electronic circuit for measuring a physiologic parameter. The electronic circuit means also includes a means for telemetrically transmitting signals representative of the sensed parameter percutaneously to a signal receiver external to the body of the living animal. 
     To measure left ventricular pressure/temperature, the apparatus of the present invention may be placed in an puncture made through the ventricular septum with the stent being anchored in this opening, such that the pressure/temperature sensor is exposed to blood or tissue in the left ventricle. An anchoring arrangement is provided on the stent to prevent the normal pumping action of the heart from displacing the implanted stent. To prevent blood flow through the tubular stent, the lumen thereof may be packed with a fibrous material for occluding the opening. The electronics module may also be located in the lumen if occlusion is desired. 
     If the stent device is to be placed in the pulmonary or some other artery of a patient, the anchoring means may comprise a series of hooks that become engaged with the inner wall of the artery when the stent is allowed to or made to expand radially during its implantation. 
    
    
     DESCRIPTION OF THE DRAWINGS 
     The foregoing features, objects and advantages of the invention will become apparent to those skilled in the art from the following detailed description of a preferred embodiment, especially when considered in conjunction with the accompanying drawings in which like numerals in the several views refer to corresponding parts. 
     FIG. 1 is a perspective view showing a tubular stent as a support member for an electronic circuit package for sensing and telemetrically transmitting sensed pressure and temperature data and powered by an implantable power pack; 
     FIG. 2 is an end view of the device of FIG. 1; 
     FIG. 3 is a sectioned view through the heart showing the monitor device of the present invention located in the ventricular and atrial septum; 
     FIG. 4 is a schematic diagram illustrating apparatus for applying power to an implanted unit percutaneously; and 
     FIG. 5 is a block diagram of the integrated circuit chip forming a part of the implantable monitor apparatus. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring first to FIG. 1, there is illustrated a first embodiment of a temperature/pressure monitoring device adapted for placement at a desired location within the vascular system of a living animal. It is seen to comprise a support member  10 , here shown as a self expandable or balloon expandable stent, to which is attached an electronics module  12  that is adapted to be powered by an implantable power source  14  connected to it by means of conductors  16 . The power source  14  is preferably a lithium-iodide battery contained within a body fluid impervious housing  18 . The electronic circuitry comprising the module  12  is also contained within a body fluid impervious housing  20  having sealed electrical feed-throughs  22  to which the conductors  16  are attached for bringing DC power into the module. 
     As will be further explained, associated with the electronics module  20  are one or more sensors for detecting changes in a physiologic parameter such as blood temperature, blood pressure or flow. The sensor may comprise a pressure sensor of the type described in the Brockway et al. U.S. Pat. No. 4,846,191, either alone or in combination with a thermistor temperature transducer and a Doppler flow sensor. 
     Formed on opposed ends of the stent  10  are retention elements, shown in FIG. 2 as hooks  26  which are adapted to engage tissue to prevent migration of the device from its desired implant site. The need for retention elements is, of course, somewhat dependent on the location selected for the implant. 
     The sectional view taken through a heart illustrated in FIG. 3 shows the way in which the present invention can be used to monitor either left ventricular pressure or left atrial pressure on a chronic basis. Here, an incision is made through the ventricular septum  28  or the atrial septum  30  with a device like that shown in FIG. 1 percutaneously implanted via an artery or vein and inserted into the surgically created opening. The support device  10 , itself, may comprise a septal defect occluder fashioned after that described in the Kotula et al. U.S. Pat. No. 5,725,552 but with an electronics module  12  mounted thereon. The sensor element is exposed to the blood in the left ventricle and/or the left atrial chamber depending on the placement of the device. The support device  10  may be delivered by way of a catheter routed through the vascular system into the right ventricle and thence through the surgically created septal opening. When the device  10  is released from the confines of the catheter, it self-expands to a predetermined dumbbell configuration, as illustrated, to maintain it in position in the septal wall. Alternatively, in an open heart surgery, the device of FIG. 1 can be inserted through the myocardium of the left ventricle or left atrium. 
     When disposed in the lumen of a blood vessel, the support device  10  is tubular as shown in FIG. 1, permitting blood flow therethrough. The hooks  26  on opposed ends thereof serve to anchor the device in place in the selected blood vessel. Placement of the stent with its temperature/pressure/flow measuring circuitry in the pulmonary artery or a branch thereof can be used to obtain a good estimation of left ventricular end diastolic pressure which is meaningful in the treatment of CHF and hypertension. It is calibrated by direct comparison with left ventricular pressure measured with an acutely placed pressure sensing catheter. Periodic recalibration can be accomplished via software. 
     FIG. 4 illustrates an alternative embodiment of the invention wherein the implant device may receive its operating power transcutaneously from a programmer transducing head  32  supported on a shoulder strap  34  which keeps the transducing head is oriented in alignment with the implanted device. The transducing head  32  may be of the type used in the telemetry link of a programmable implantable pacemaker allowing the patient to be ambulatory. The transmitting and receiving electronics and the battery power supply therefore may be contained in a case  36  worn on a belt surrounding the patient&#39;s abdomen. Information developed by the sensor  24  of the implant device  10  is telemetered to the external transducer  32  via RF transmission and is fed to the electronic module  36  for signal processing, storage and later analysis. 
     FIG. 5 is a block diagram illustrating the circuitry contained within the housing  20  of the implant device. The output signals from the aforementioned pressure/ temperature/flow transducers can readily be separated into two channels, one for carrying the pressure information and the other for carrying temperature information by appropriate filtering techniques, it being recognized that the output signal from the pressure sensor will be of a significantly greater frequency than that from the temperature sensor. Hence, in FIG. 5, both a pressure sensor  50  and a temperature sensor  52  are illustrated to indicate the dual channel nature, even though a single transducer device may be utilized. The analog output signal from both the pressure sensor  50  and the temperature sensor  52  are applied to an analog-to-digital converter forming a part of the on-board microprocessor  54 . The microprocessor  54  includes an address bus  56 , a data bus  58  and a control bus  60  to which are connected a ROM memory  62 , a RAM memory  64  and an input/output interface  66 . ROM  62  conventionally stores a program executable by the microprocessor  54  while RAM  64  may store programmable constants and intermediate data developed during the execution of the program. The I/O interface is attached to a telemetry circuit  68 , allowing data carried on the data line  58  from the microprocessor and/or the RAM to be transmitted transcutaneously from the patient&#39;s body, represented by dashed-line  70  to an external monitor  72 . The monitor  72  may be conveniently be a lap-top PC having the ability to receive and process the telemetry data from the implant and to deliver programming data to the implant device, via the telemetry link. 
     The temperature transducers illustrated in FIGS. 1 and 2 may comprise a thermistor, or thermocouple or an infrared sensor. A separate piezoelectric device can be utilized as a pressure sensor in a fashion indicated in the Brockway U.S. Pat. No. 4,846,191. It is also contemplated that a separate flow sensor may be made a component of the implantable monitor device or, alternatively, the temperature sensor may be used to assess flow using known thermodilution techniques. 
     This invention has been described herein in considerable detail in order to comply with the patent statutes and to provide those skilled in the art with the information needed to apply the novel principles and to construct and use such specialized components as are required. However, it is to be understood that the invention can be carried out by specifically different equipment and devices, and that various modifications, both as to the equipment and operating procedures, can be accomplished without departing from the scope of the invention itself.