Surgery – Diagnostic testing – Cardiovascular
Reexamination Certificate
1999-04-29
2001-01-09
O'Connor, Cary (Department: 3736)
Surgery
Diagnostic testing
Cardiovascular
C600S488000, C324S678000, C702S052000, C073S724000
Reexamination Certificate
active
06171252
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates generally to pressure sensors adapted for use with an implantable medical device. More particularly, the present invention pertains to a pressure sensor with increased sensitivity that responds to conditions of blood pressure and provides pressure related signals to an implanted hemodynamic monitor and/or cardiac pacemaker or pacemaker/cardioverter/defibrillator. It also can be employed in any implantable medical device which uses a capacitive pickoff sensor.
BACKGROUND OF THE INVENTION
Various implantable medical devices have been developed that receive information from one or more physiologic sensors or transducers. One such physiologic transducer is a pressure sensor that transduces blood pressure into corresponding electrical signals. The electrical signals produced by the pressure sensor are acquired by an implantable medical device via a lead coupled therebetween.
The sensitivity of a body implantable pressure sensor is an important design characteristic. The sensitivity characteristics of an implantable pressure sensor have a direct impact on the signal-to-noise ratio of the pressure sensor signal transmitted to the implantable medical device via the lead coupled therebetween. Increasing the strength of the signal produced by the implantable pressure sensor would therefore appear desirable, as such an increase would result in improved signal-to-noise characteristics and increased integrity of pressure data transmission.
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. 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 implantable pressure sensors have been placed in clinical use.
Many indwelling pressure sensors employ a piezoelectric element or a piezoresistive element as a pressure transducer. Piezoelectric crystals or piezoresistive pressure transducers 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. 4,023,562; 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 desirable characteristics and applications for patient use of such leads or catheter bearing, indwelling pressure sensors are described in these and other patents and the literature in the field.
Other semiconductor sensors employ complimentary metal oxide semiconductor (CMOS) integrated circuit (IC) technology in the fabrication of pressure responsive silicon diaphragm bearing capacitive plates that are spaced from stationary plates. A change in capacitance due to pressure waves acting upon the diaphragm is measured, typically through use of 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. Fabrication for long term implantation and stability is, however, 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, in the article “A ceramic differential-pressure transducer” by Graeger et al., Philips Tech. Rev., 43:4:8693, Feb. 1987. The large scale of such pressure transducers does not lend itself to miniaturization for chronic implantation.
Improved capacitive pressure sensor implementations are disclosed in U.S. Pat. Nos. 5,564,434 and 5,535,752 to Halperin, both of which are incorporated herein by reference in their respective entireties. The '434 and '752 patents disclose a capacitive pressure and temperature sensing system for providing signals representative of absolute pressure and temperature of a body fluid. A sensor module includes a pickoff capacitor and a reference capacitor. The pickoff capacitor changes in capacitance in response to fluid pressure changes, while the reference capacitor is relatively insensitive to fluid pressure changes.
A constant current source provides for charging and discharging of the pickoff and reference capacitors. A pulse generator generates pickoff and reference timing pulses separating pressure related and temperature related charge time intervals of the pickoff and reference capacitors which vary as a function of the charge current and capacitance changes of the pickoff capacitor. Although the capacitive pressure sensing systems disclosed in the '434 and '752 patents provide for accurate sensing of body fluid pressure, a pressure sensing approach having increased sensitivity may be desirable in certain applications.
Despite the considerable effort that has been expended in designing such pressure sensors, a need continues to exist for a body implantable pressure sensor for accurately sensing the pressure of a body fluid which is not subject to mechanical limitations of conventional capacitive transducers. There continues to exist a need for a pressure sensing system that provides for improved sensitivity beyond that provided by a conventional mechanical pressure-to-capacitance transducer. The present invention fulfills these and other needs.
SUMMARY OF THE INVENTION
The present invention is directed to an apparatus and method for obtaining pressure data using a body implantable pressure sensor. The pressure sensor may be provided in a lead for use with a body implantable medical device. Alternatively, the pressure sensor may be incorporated into an implantable device locatable anywhere within the body, such as the heart, brain or other organ or vessel, which transduces pressure of a body fluid to corresponding electrical signals. The pressure sensor may communicate pressure data to a receiving device provided internal to, or externally of, the body, such as an implantable pace generator or an external programmer system, respectively.
The pressure sensor includes a measurement capacitor which is responsive to pressure changes of a body fluid. A technique employing non-constant, multiple current source charging of the measurement capacitor provides for a significant increase in sensitivity to changes in body fluid pressure. Obtaining pressure data using a pressure sensor in accordance with the principles of the present invention involves charging a measurement capacitor at a first charge rate during a first charge period of an integration cycle. During a second charge period of the integration cycle, the measurement capacitor is charged at a second charge rate. The first charge rate is preferably greater than the second charge rate. The duration of the first charge period or of the second charge period may be varied.
Using the measurement capacitor, a signal indicative of a pressure change of a body fluid imparted to the measurement capacitor is produced. The signal indicative of a body fluid pressure change may be a signal indicative of a change in a time of integration. The signal, indicative of a time of integration, has a sensitivity to pressure changes which is greater than the mechanical capacitive sensitivity of the measurement capacitor to such pressure changes.
More particularly, a relative change in a time of integration, which may be characterized by &Dgr;t/T, in response to a body fluid pressure change is greater than a relative change in the capacitance of the measurement capacitor, which may be characterized by &Dgr;C/C, in response to the pressure change, where &Dgr;t represents the change in integration time during the integration cycle of duration T, and &Dgr;C represents the change in capacitance, C, of the measurement capacitor. The signal indicative of a body fluid pressure change may alternatively be a signal indicative of a change in capacitance of the measurement capacitor, such as a voltage signal or a signal h
Atlass Michael B.
Medtronic Inc.
Natnithithadha Navin
O'Connor Cary
Patton Harold R.
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