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IMPLANTABLE PRESSURE SENSOR AND
METHOD OF FABRICATION
FIELD OF THE INVENTION
The present invention relates to a body implantable pres- 5 sure sensor attached to an endocardial lead for implantation in a heart chamber or cardiac blood vessel for sensing blood pressure and providing blood pressure signals to an implanted or external hemodynamic monitor and/or therapy delivery device and method of fabrication thereof. 10
DESCRIPTION OF THE BACKGROUND ART
Efforts have been underway for many years to develop implantable pressure transducers and sensors for temporary or chronic use in a body organ or vessel and systems for recording absolute pressure. Many different designs and operating systems have been proposed and placed into temporary or chronic use with patients. Indwelling pressure sensors for temporary use of a few days or weeks are available, and many designs of chronically or permanently 2Q implantable pressure sensors have been placed in clinical use.
U.S. Pat. No. 4,023,562 describes a pressure transducer comprising a piezoresistive bridge of four, orthogonally disposed, semiconductor strain gauges formed interiorly on 25 a single crystal silicon diaphragm area of a silicon base. A protective silicon cover is bonded to the base around the periphery of the diaphragm area to form a sealed, evacuated chamber. Deflection of the diaphragm due to ambient pressure changes is detected by the changes in resistance of the 30 strain gauges. Because the change in resistance is so small, a high current is required to detect the voltage change due to the resistance change. The high current requirements render the piezoresistive bridge unsuitable for long term use with an implanted power source. High gain amplifiers that are 35 subject to drift over time are also required to amplify the resistance-related voltage change.
Other semiconductor pressure transducers employ CMOS IC technology in the fabrication of pressure responsive silicon diaphragm bearing capacitive plates that are spaced 40 from stationary plates. The change in capacitance due to pressure waves acting on the diaphragm is measured, typically through a bridge circuit, as disclosed, for example, in the article "A Design of Capacitive Pressure Transducer" by Ko et al., in IEEE Proc. Symp. Biosensors, 1984, p. 32. 45 Again, fabrication for long term implantation and stability is complicated. In addition, differential capacitive plate, fluid filled pressure transducers employing thin metal or ceramic diaphragms have also been proposed for large scale industrial process control applications as disclosed, for example, 50 in the article, "A ceramic differential-pressure transducer" by Graeger et al., Philips Tech. Rev., 43:4:86-93, February 1987. The large scale of such pressure transducers does not lend itself to miniaturization for chronic implantation.
Piezoelectric crystal or piezoresistive pressure transduc- 55 ers mounted at or near the distal tips of pacing leads, for pacing applications, or catheters for monitoring applications, are described in U.S. Pat. Nos. 4,407,296, 4,432,372, 4,485,813, 4,858,615, 4,967,755, and 5,324,326, and PCT Publication No. WO 94/13200, for example. The so desirable characteristics and applications for patient use of such lead or catheter bearing, indwelling pressure sensors are described in these and other patents and the literature in the field. Generally, the piezoelectric or piezoresistive transducers have to be sealed hermetically from blood. 65
The '296 patent discloses sealing the piezoresistive bridge elements within an oil filled chamber having a deflectable
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diaphragm so that blood pressure fluctuations can deflect the diaphragm and be transmitted by the oil to the pressure transducer. In one embodiment disclosed therein, a sealed inner chamber at a calibrated pressure is provided on one side of the piezoresistive bridge transducer, and the oil bears against the other side of the transducer. The incompressible oil is admitted into the chamber through a fill tube in a vacuum filling process to fill the chamber between the transducer and the diaphragm. The fill tube is then crimped.
The crimping of the end of the fill tube is not necessarily sufficient to prevent leakage of the oil from the chamber. Such pressure sensor leads are intended to be chronically implanted in patients for years or decades. The function of the pressure transducer will be impaired if the oil leaks from the chamber and is displaced by air or body fluids over such a period of time.
Moreover, the configuration of the pressure transducer housing employed in the '296 patent locates the diaphragm perpendicular to the lead body axis and facing distally. This constitutes a bulky transducer module and prevents its use at a location more proximal to the distal end of the lead body.
Despite the considerable effort that has been expended in designing such pressure sensors, a need exists for a body implantable, durable, long-lived and low power consuming pressure sensor for accurately sensing absolute pressure waves in the body over many years and for deriving body temperature signals in a system for demodulating and storing the signals.
SUMMARY OF THE INVENTION
It is therefore an object of the present invention to provide an improved method for sealing oil filled pressure transducer modules for a chronically implantable pressure sensor lead.
The present invention involves a body implantable pressure sensor attached to an endocardial lead for implantation in a heart chamber or cardiac blood vessel for sensing blood pressure and providing blood pressure signals to an implanted or external hemodynamic monitor and/or therapy delivery device and method of fabrication thereof. A pressure sensor module is formed of an elongated receptacle having an elongated receptacle cavity for receiving a calibrated, micro-machined pressure transducer having a pressure responsive element. The receptacle cavity is covered by a diaphragm disposed alongside the lead body and in parallel with the lead axis. The receptacle cavity is filled with a incompressible oil for transferring pressure forces that are applied to the diaphragm to the pressure transducer. The oil is introduced through a fill port, and the fill port is sealed after the oil is introduced to prevent leakage of the oil from the receptacle cavity and to complete the hermetic sealing of the receptacle cavity. In one embodiment, the fill port comprises a fill tube having a fill tube lumen extending outward of an end wall of the receptacle cavity to a fill tube end, and said sealing step further comprises the steps of crimping the fill tube end to close the fill tube lumen, fitting a fill port cover having an abutting edge over the crimped fill tube end and against the end wall of the receptacle to enclose the crimped fill tube end within a fill port cover cavity, and sealing the abutting edge against the receptacle end wall to hermetically enclose the sealed fill tube end within the fill port cover cavity.
In another embodiment, the fill port is formed with a tapered fill port lumen and is closed by a stopper pressed into it to close or block the fill port lumen and inhibit leakage of oil therethrough. The closed fill port is then enclosed within the fill port cover in the same manner.
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Moreover, the fill tube lumen can be tapered and receive a stopper and these closure techniques can be combined.
The crimping or blocking of the fill tube does not overpressurize the oil within the receptacle cavity. In addition, the sealing of the abutting edge against the receptacle 5 obviates the need for welding of the crimped or blocked fill tube end. The welding energy applied to the fill tube end can overheat the oil within the receptacle cavity that can result in expansion of the oil and damage to the pressure transducer and/or to the receptacle and diaphragm or explosion of the 1° oil from the receptacle cavity in extreme cases.
BRIEF DESCRIPTION OF THE DRAWINGS
These and other objects, advantages and features of the present invention will be appreciated as the same becomes 15 better understood by reference to the following detailed description of the preferred embodiment of the invention when considered in connection with the accompanying drawings, in which like numbered reference numbers designate like parts throughout the figures thereof, and wherein: 20
FIG. 1 shows a plan view of a pressure sensing lead according to the present invention;
FIG. 2 is a side, cross-section view of the pressure sensor module and electrical connections therewith of FIG. 1; 2J
FIG. 3 is a top view of a completed pressure sensor module employed in the pressure sensing lead of FIG. 1;
FIG. 4 is a side view of a completed pressure sensor module employed in the pressure sensing lead of FIG. 1;
FIG. 5 is a bottom view of a completed pressure sensor 30 module employed in the pressure sensing lead of FIG. 1;
FIGS. 6 and 7 are end views of a completed pressure sensor module employed in the pressure sensing lead of FIG. 1;
FIG. 8 is an exploded perspective view of the steps of 35 locating the pressure transducer and associated circuit in the receptacle cavity of a receptacle and the attachment of a cover and diaphragm to the frame surrounding the opening into the receptacle cavity;
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FIG. 9 is a schematic illustration of the filling of the receptacle cavity with incompressible oil;
FIG. 10 is an exploded perspective view of the crimped end of the filler tube and a cover that is fitted over it and sealed to the end wall of the receptacle as shown in FIGS. 45 3-6; and
FIG. 11 is an exploded perspective view of a ball fitted into the fill tube lumen to seal the fill tube and a cover that is fitted over it and sealed to the end wall of the receptacle as shown in FIGS. 3-6. 50
DETAILED DESCRIPTION OF THE
PREFERRED EMBODIMENTS OF THE
INVENTION
A preferred embodiment of the present invention is 55 described herein that includes a pressure sensor module that is integrally formed into a pressure sensing lead adapted to be used for sensing blood pressure in a heart chamber and also preferably includes a pace/sense electrode and electrical lead connections of a pacing lead. However, it will be 60 understood that the pressure sensing lead may be constructed as described herein without the pace/sense electrode or that it can be constructed with a cardioversion/ defibrillation electrode. In addition, it may be included in a lead bearing another physiological sensor. 65
FIG. 1 illustrates an exemplary pressure sensing lead in accordance with the present invention. The pressure sensing
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lead is formed of a proximal connector assembly 20 extending between an elongated proximal lead body section 18 to a pressure sensor module 16 and a distal lead body section 14 extending between the pressure sensor module 16 and the distal end of the lead. The lead has a lead body axis, and a distal location for attachment with a pressure sensor module 16 is defined. A pace/sense electrode 10 is located at the distal end of the pressure sensing lead and is coupled by one lead conductor extending the full length of the proximal and distal lead body sections 18 and 14, respectively to a connector pin element 22. Distal electrode 10 is used in conjunction with a further indifferent electrode to apply pacing pulses to the heart tissue or to sense cardiac electrical activity. Electrode 10 is held in engagement with heart tissue by means of passive fixation, flexible tines 12, which engage with the trabeculae in the apex of the ventricle of the heart or in the atrial appendage. A flexible insulating sheath 14 extends from electrode 10 to the distal location where pressure sensor module 16 is located. In a bipolar pace/sense lead configuration, the exterior surface of pressure sensor module 16 can be coupled with an indifferent, ring shaped, pace/sense electrode.
The proximal lead body section 18 that extends proximally from pressure sensor module 16 to the connector assembly 20 located at the proximal end of the lead surrounds three conductors and a stylet lumen. The three conductors are preferably each separately insulated and are wound in a common diameter coil and in an interleaved manner as shown in commonly assigned U.S. Pat. Nos. 5,007,435 and 5,040,544, both incorporated by reference. The three conductors surround a stylet lumen which extends from a lumen of the connector pin element 22 through the proximal lead body section 18, alongside the pressure sensor module 16, and through the distal lead body section 14 to the lead distal end. A stylet 30 having a proximal end knob 32 is shown inserted into the stylet lumen and extending through the length of the lead body to stiffen the lead body and aid in introduction of the distal tip electrode 10 to a site in a heart chamber in a manner well known in the art.
The construction of the proximal lead body section 18 is depicted in FIG. 2. The three conductors are depicted as an outer, multi-polar wire coil 52 and an inner, single pole coiled wire conductor 56 that surrounds the stylet lumen. The outer, multi-polar wire coil 52 is separated from the inner, single pole wire coil 56 by an insulating sheath 58. An outer insulating sheath surrounds the outer, multi-polar wire coil 52. The outer, multi-polar wire coil 52 comprises a pair of coiled wire conductors 48 and 50 that are separately insulated and wound in an interleaved manner.
Connector assembly 20, described in more detail in the above-incorporated '435 patent, is formed with the connector pin element 22 and with two connector ring elements 24 and 26. Connector ring elements 26 and 24 are coupled with coiled wire conductors that extend through the proximal lead body section to and are electrically connected with the pressure sensor transducer IC within pressure sensor module 16. An anchoring sleeve 28 that is used to secure the lead at its point of venous insertion site surrounds the proximal lead body section 18.
The pressure sensor module 16 includes a pressure transducer and electronic IC located within an oil filled receptacle cavity of a receptacle that is covered by a diaphragm as described below. The pressure sensor module can be used for various applications including simply recording and monitoring pressure data wherever it is located, or for sensing blood pressure within the heart in particular. Such measurements can be used to modulate the pacing rate of a
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