Intracranial monitoring and therapy delivery control device,...

Details Intracranial monitoring and therapy delivery control device,... Intracranial monitoring and therapy delivery control device,...

1999-04-26

2001-06-19

Layno, Carl H. (Department: 3762)

Surgery

Diagnostic testing

Measuring fluid pressure in body

C600S311000

Reexamination Certificate

active

06248080

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates generally to implantable physiologic sensors, and more particularly to intracranial sensors, systems and methods.
BACKGROUND OF THE INVENTION
Implantable medical devices (IMDs) for cardiac monitoring or for delivering therapy typically include one or more sensors positioned in a patient's blood vessel, heart chamber or other portion of the body. Examples of IMDs include heart monitors, therapy delivery devices, pacemakers, implantable pulse generators (IPGs), pacer-cardio-defibrillators (PCDs), implantable cardio-defibrillators (ICDs), cardiomyo-stimulators, nerve stimulators, gastric stimulators, brain stimulators and drug delivery devices. In a cardiac therapy or monitoring context, such IMDs generally include electrodes for sensing cardiac events of interest and sense amplifiers for recording or filtering sensed events. In many currently available IMDs, sensed events such as P-waves and R-waves are employed to control the delivery of therapy in accordance with an operating algorithm. Selected electrogram (EGM) signal segments and sense event histogram data and the like are typically stored in IMD RAM for transfer to an external programmer by telemetric means at a later time.
Efforts have also been made to develop implantable physiologic signal transducers and sensors for monitoring a physiologic condition other than, or in addition to, an EGM, to thereby control delivery of a therapy, or to filter or store data.
In respect of cardiac monitoring, sensing and recording such additional physiologic signals as blood pressure, blood temperature, pH, blood gas type and blood gas concentration signals has been proposed.
One type of ideal physiologic sensor provides information concerning a patient's exercise level or workload and operates in closed loop fashion. In other words, such an ideal physiologic sensor operates to minimize divergence from an ideal operating point or set of points. Blood oxygen saturation provides a direct indication of the amount oxygen consumed by a patient when exercising. In a rate responsive pacing context, oxygen saturation is generally inversely related to pacing rate. That is, as oxygen saturation decreases due to exercise, pacing rates are correspondingly increased so that divergence from the optimum operating point is minimized. In such a fashion a closed loop system capable of monitoring a physiologic parameter and delivering an appropriate therapy is implemented.
Piezoresistive pressure transducers mounted at or near the distal tips of catheters have been employed in such pressure monitoring applications. U.S. Pat. No. 4,023,562 describes a piezoresistive bridge of four, orthogonally disposed, semiconductor strain gauges formed interiorly on 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 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 subject to drift over time are also required to amplify the resistance-related voltage change.
Other semiconductor sensors employ CMOS IC technology in the fabrication of pressure responsive silicon diaphragm bearing capacitive plates that are spaced 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. 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, 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.
Efforts have been underway for years to develop pressure transducers and sensors for temporary or chronic use in a body organ or vessel, including those relating to the measurement or monitoring of intracranial fluid pressure. Many different designs and operating systems have been proposed and placed into temporary or chronic use with patients.
Patients suffering from head trauma, adult head trauma and infantile hydrocephalus and attendant increased intracranial fluid pressure are often difficult to treat successfully. Among other things, this is because the sensors generally employed to sense intracranial pressure often provide a direct path for infectious agents to enter the brain (leading to dangerous intracranial infections), the actual source or cause of the increased intracranial pressure is poorly understood or not understood at all, or the devices and methods employed to sense intracranial pressure are limited in their capabilities, the locations where they may be positioned, or the durations of time over which they may be used.
Various implementations of systems for sensing physiologic parameters are known in the art. Some examples of such sensors and associated methods of sensing may be found in at least some of the patents, patent applications or publications listed in Table 1 below.
TABLE 1
U.S. Pat. No.,
U.S. patent
Issue/
application Ser. No.
Publication/
or Document No.
Inventor(s)
Filing Date
WO 80/01620
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Mortazavi
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Mick
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Millar
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Obel et al.
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5,267,564
Barcel et al.
December 7, 1

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