Abstract:
The disclosure is directed to a system and method for monitoring ICP within a patient on a continuous or periodic basis over an extended period of time. In some situations, a care-giver may want to record ICP measurements over a longer period of time to obtain trend data. A system for monitoring ICP includes a shroud-like, inductive power transmitted element designed to surround at least a substantial portion of a patient&#39;s head and power an implanted ICP monitor. The shroud-like element may be a table-mounted device that arcs over the width of the table or bed, providing room for the patient&#39;s head.

Description:
[0001]     This application claims the benefit of U.S. provisional application No. 60/589,347, filed Jul. 20, 2004, the entire content of which is incorporated herein by reference. 
     
    
     FIELD OF THE INVENTION  
       [0002]     The invention relates to medical devices and, more particularly, devices for draining cerebral spinal fluid.  
       BACKGROUND  
       [0003]     Hydrocephalus is an excess accumulation of cerebrospinal fluid (CSF) in the ventricles of the brain. This fluid, which protects, nourishes and cleanses the brain and spinal cord, is manufactured daily in the ventricles. Buildup of CSF occurs when the fluid cannot flow freely throughout the ventricles and the central nervous system due to various forms of blockage. Except in very rare cases, hydrocephalus is a life-long condition that can only be controlled, not cured, through medical intervention. There are a number of accepted treatments available for hydrocephalus, most of which involve the surgical implantation of a shunt. The shunt diverts CSF from the brain ventricles to another part of the patient&#39;s body.  
         [0004]     Elevated intracranial pressure (ICP) can be a problem for patients suffering from chronic hydrocephalus, as well as patients with brain injuries or other diseases that cause an acute accumulation of CSF. An ICP monitor provides an indication of ICP so that a care-giver can intervene in the event ICP becomes too high. For example, a care-giver may adjust a valve associated with a shunt, administer medication or take other action to relieve elevated ICP levels. An external ICP monitor may be coupled to a catheter that extends into the cranium. Alternatively, the ICP monitor may form part of an implanted ventricular shunt catheter, or be implanted independently of the ventricular shunt catheter.  
         [0005]     Implantable telemetric ICP monitors are equipped to sense ICP and transmit wireless signals representing the sensed ICP level. Typically, an implantable ICP monitor does not include a battery or data storage. Instead, the ICP monitoring device is ordinarily powered inductively by an external device, and provides an instantaneous “snap-shot” of ICP at a particular point in time. In this case, the ICP includes a pressure sensor, monitoring circuitry, a wireless transmitter, and an inductive power interface. The inductive power interface receives inductively coupled energy and generates power for the sensor and transmitter.  
         [0006]     Table 1 below lists documents that disclose implantable telemetric ICP monitors. U.S. Pat. No. 4,519,401 to Ko et al. describes a battery-powered implantable ICP monitor with low power pressure sensing circuitry and wireless telemetry. U.S. Pat. No. 6,113,553 to Chubbuck describes an implantable, inductively powered ICP monitor providing wireless telemetry. U.S. Pat. No. 6,533,733 to Ericson et al. describes an implantable ICP monitor that can be powered by an internal power source or an inductively coupled, external power source. U.S. Pat. No. 6,248,080 to Miesel et al. describes an implantable, battery powered ICP monitor with wireless telemetry.  
                       TABLE 1                       Patent               Number   Inventors   Title                   4,519,401   Ko et al.   Pressure telemetry implant       6,113,553   Chubbuck   Telemetric intracranial pressure monitoring               system       6,533,733   Ericson   Implantable device for in-vivo intracranial and           et al.   cerebrospinal fluid pressure monitoring       6,248,080   Miesel   Intracranial monitoring and therapy delivery           et al.   control device, system and method                  
 
         [0007]     All documents listed in Table 1 above are hereby incorporated by reference herein in their respective entireties. As those of ordinary skill in the art will appreciate readily upon reading the Summary, Detailed Description and claims set forth below, many of the devices and methods disclosed in the patents of Table 1 may be modified advantageously by using the techniques of the present invention.  
       SUMMARY OF THE INVENTION  
       [0008]     In general, the invention is directed to a system and method for monitoring ICP within a patient on a continuous or periodic basis over an extended period of time using an inductive power element that extends over a substantial portion of a patient&#39;s head to inductively power an implanted ICP monitor. The inductive power element also may serve as a telemetry antenna to receive wireless telemetry signals transmitted by the ICP monitor. The inductive power element may be shroud-like, and define an opening to receive at least a portion of the patient&#39;s head.  
         [0009]     Various embodiments of the present invention provide solutions to one or more problems existing in the prior art with respect to prior art systems for ICP monitoring. These problems include the inconvenience and discomfort associated with external, catheter-based ICP monitors, and the intermittent nature of measurements obtained by conventional implanted telemetric ICP monitors. Typically, an implantable, telemetric ICP monitor does not include a battery or data storage, and instead must be powered inductively by an external device. Hence, an ICP monitor may provide only an instantaneous “snap-shot” of ICP at a particular point in time at which the IPC monitor is powered. Consequently, it is difficult for a care-giver to obtain continuous ICP measurements over an extended period of time using an implanted, telemetric ICP monitor. As a further problem, a care-giver is unable to detect significantly elevated ICP levels that may occur between intermittent measurements. The inability to detect elevated ICP levels between measurements can expose the patient to health risks.  
         [0010]     Various embodiments of the present invention are capable of solving at least one of the foregoing problems. When embodied in a system or method for monitoring vital signs, the invention includes features that facilitate the continuous or periodic measurement of ICP over an extended period of time without the need for a persistent, catheter-based ICP monitor. In this manner, the invention enables a care-giver to obtain measurements from an implanted ICP monitor on a more continuous basis. The ability to obtain measurements on a more continuous basis permits generation and analysis of a larger body of data that may be useful in diagnosis and care decisions. For example, in some situations, a care-giver may want to record ICP measurements over a longer period of time to obtain trend data. In addition, continuous or periodic measurements permit the detection of elevated levels of ICP, and the delivery of therapy to relieve or otherwise reduce health risks posed by such levels.  
         [0011]     In accordance with the invention, a system for monitoring ICP includes an element designed to extend over at least a substantial portion of a patient&#39;s head. In some embodiments, the element may be a table- or bed-mounted device that arcs over the width of the table or bed, providing room for the patient&#39;s head. In some cases, the patient may sleep with his head within an opening defined by the element.  
         [0012]     The element is electrically conductive and transmits inductive energy to power an ICP monitor implanted in the patient&#39;s head. In addition, the element may serve as an antenna to telemetrically receive information transmitted by the power ICP monitoring device. In this manner, the system can both power the ICP monitor and receive ICP information on a substantially continuous or periodic basis. The element may define an opening sufficiently small to permit reliable inductive power transfer and telemetry with the implantable ICP monitor, yet large enough to comfortably accommodate the patient&#39;s head.  
         [0013]     An external monitor may be provided, e.g., at the patient&#39;s bedside, to receive and process the measurement information received by the element from the implanted ICP monitoring device. The external monitor may be capable of storing received information, and may include ports for download, display or other output of the information. In some embodiments, the external monitor may be a vital signs monitor that accepts inputs from a variety of different vital signs sensors.  
         [0014]     In one embodiment, the invention provides a system for monitoring intracranial pressure (ICP), the system comprising an implantable ICP monitor for implantation in a head of a patient, an inductive power transmitting element sized to extend over at least a substantial portion of the head of a patient and inductively power the ICP monitor, and an external monitor to receive the transmitted ICP signal from the ICP monitor.  
         [0015]     In comparison to known techniques for monitoring ICP, various embodiments of the invention may provide one or more advantages. For example, the invention enables a care-giver to obtain ICP measurement information over an extended period of time, and even when the patient is sleeping. The ability to obtain ICP measurements on a continuous or periodic basis allows a care-giver to obtain a valuable body of information, and permits the care-giver to detect potentially dangerous ICP levels. Consequently, the invention may contribute to improved patient care. At the same time, an element as described herein can be constructed in a manner that provides the patient with comfort and convenience, relative to catheter-based ICP monitors.  
         [0016]     The details of one or more embodiments of the invention are set forth in the accompanying drawings and the description below. Other features, objects, and advantages of the invention will be apparent from the description and drawings, and from the claims.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0017]      FIG. 1  is a side view illustrating an ICP monitoring system having a shroud-like inductive power and telemetry element in conjunction with a patient, in accordance with an embodiment of the invention.  
         [0018]      FIG. 2  is a rear view illustrating the ICP monitoring system of  FIG. 1 .  
         [0019]      FIG. 3  is a perspective diagram of an ICP monitoring system as shown in  FIG. 1  in conjunction with a patient bed.  
         [0020]      FIG. 4  is an enlarged view of a portion of the shroud-like element of  FIGS. 1-3 , in accordance with an embodiment of the invention.  
         [0021]      FIGS. 5 and 6  are rear views of the ICP monitoring system of  FIG. 1  illustrating alternative shroud designs.  
         [0022]      FIG. 7  is schematic diagram of an ICP monitor implanted in the cranium of a patient.  
         [0023]      FIG. 8  is a cross-sectional side view of the implantable ICP monitor of  FIG. 7 .  
         [0024]      FIG. 9  is a block diagram illustrating an ICP monitoring system in accordance with an embodiment of the invention.  
         [0025]      FIG. 10  is a block diagram illustrating an implantable ICP monitor.  
         [0026]      FIG. 11  is a block diagram illustrating an external power/telemetry unit to power and communicate with an implanted ICP monitor.  
         [0027]      FIG. 12  is a flow diagram illustrating an ICP monitoring method in accordance with an embodiment of the invention. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0028]      FIG. 1  is a side view illustrating an ICP monitoring system  10  in conjunction with a patient  14 , in accordance with an embodiment of the invention. As shown in  FIG. 1 , system  10  includes an implantable ICP monitor  12 , which has been implanted in the cranium of patient  14  to obtain ICP measurements. In addition, system  10  includes a shroud-like element  16  that extends over a substantial portion of the patient&#39;s head, including the portion in which ICP monitor  12  is implanted. As will be described, shroud-like element  16  may serve as both an inductive power transmitting element and a telemetry receiver antenna.  
         [0029]     In some embodiments, shroud-like element  16  defines a tube- or arc-like opening to receive the patient&#39;s head  17 . Shroud-like element  16  may be mounted to a bed  18 , and is constructed, at least partially, from an electrically conductive frame  20 A. In other embodiments, shroud-like element  16  may be mounted to a table, chair or other support platform for patient  14 .  
         [0030]     Electrically conductive frame  20 A may include an insulative substrate and an array of wires or conductive traces that form loops of an electromagnetic coil. As an alternative, electrically conductive frame  20 A may carry an array of separated electromagnetic coils coupled in common to a source of alternating current. In this case, the individual coils contribute to an overall inductive field. In addition, electrically conductive frame  20 A may define an array of apertures  22  such as round or elliptical holes or square or rectangular slots, if desired, to promote ventilation. Apertures  22  also may serve to tune the electromagnetic properties of shroud-like element  16 .  
         [0031]     Shroud-like element  16  is coupled to an inductive power generator (not shown in  FIG. 1 ) to receive an alternating current (ac) signal, which is generates an electromagnetic signal that is transmitted by the inductive coil or coils of electrically conductive frame  20 A of shroud-like device  16  to power implantable ICP monitor  12 . Hence, the shroud-like element  16  serves as an inductive power transmitter for the transfer of energy to ICP monitor  12 . In addition, electrically conductive frame  20 A may serve as an antenna to receive wireless signals transmitted by implantable ICP monitor  12 . The wireless signals convey ICP pressure measurements or other operational or status information associated with implantable ICP monitor.  
         [0032]      FIG. 2  is a rear view illustrating ICP monitoring system  10  of  FIG. 1 .  FIG. 3  is a perspective diagram of an ICP monitoring system  10  in conjunction with patient bed  18 . As shown in  FIGS. 2 and 3 , electrically conductive frame  20 A of shroud-like element  16  may define a tube- or arc-like opening  23  to receive the head  17  of patient  14 . Opening  23  of shroud-like device  16  is sized sufficiently small to permit reliable inductive transfer of power from electrically conductive frame  20 A to implantable ICP monitor  12 , as well as reliable wireless telemetry between the implantable ICP monitor and the electrically conductive frame.  
         [0033]     For example, opening  23  may be sized to provide a distance of approximately 1 cm to 10 cm between implantable ICP monitor  12  in head  17  of patient  14  and an interior surface of element  16 . In this manner, opening  23  can be sized to balance patient comfort with reliable power transfer and wireless telemetry. In some embodiments, electrically conductive frame  20 A may have an adjustable size or height in order to accommodate patients of different sizes.  
         [0034]     Electrically conductive frame  20 A may be constructed as a continuous sheet of conductive material, or include apertures  22 , as described above. As further alternatives, electrically conductive frame  20 A may be constructed as a mesh or cage-like assembly, or carry an array of inductive coils. As shown in  FIGS. 2 and 3 , electrically conductive frame  20 A may be substantially hemispherical in shape, but may be subject to other shapes or sizes. A hemispherical, arc-like shape may be advantageous in terms of minimizing the average distance between electrically conductive frame  20 A and ICP monitor  12 . Also, an arc-like shape may provide more effective coverage when the patient  14  has more than one implanted ICP monitor. However, other shapes for opening  23  may be used, such as rectangular, square, or triangular shapes. As a further alternative, electrically conductive frame  20 A may be constructed to extend only partially over the head  17  of patient  14 .  
         [0035]     In operation, shroud-like element  16  continuously or periodically powers implanted ICP monitor  12 , in which case the ICP monitor continuously or periodically transmits signals representative of ICP levels. Shroud-like element  16  receives the signals and couples them to an external monitor (not shown in  FIGS. 1-3 ) for processing, analysis and presentation. The external monitor may provide a continuous or periodic indication of ICP, and may invoke advisory levels at which an ICP measurement may trigger an alarm or other indicator for the attention of a care-giver.  
         [0036]      FIG. 4  is an enlarged view of a portion of shroud-like element  16  of  FIGS. 1-3 , in accordance with an embodiment of the invention. Electrically conductive frame  20 A of shroud-like element  16  may be constructed in a variety of ways. In the example of  FIG. 4 , however, electrically conductive frame  20 A is constructed to having an insulative substrate  25  with an inductive coil  27  having a plurality of turns  29  and terminals  31 ,  33 . Turns  29  may be embedded wires or conductive traces, and may be formed from a variety of conductive materials, such as copper, silver or platinum. Insulative substrate  25  may be formed from any of a variety of polymeric, dielectric materials, and may be selected to have desired dielectric properties in some embodiments.  
         [0037]     An inductive power generator drives terminals  31 ,  33  to cause turns  29  of inductive coil  27  to generate electromagnetic energy for transfer to implantable ICP monitor  12 . In addition, turns  29  serve to receive telemetry signals from implantable ICP monitor  12 . A telemetry circuit is coupled to terminals  31 ,  33  to process signals received by inductive coil  27  of electrically conductive frame  20 A. As an alternative to the single inductive coil  27  of  FIG. 4 , electrically conductive frame  20 A may include an array of individual coils. In either case, shroud-like element  16  emits electromagnetic energy for inductive transfer to implantable ICP monitor  12  and receives telemetry signals from the ICP monitor.  
         [0038]      FIGS. 5 and 6  are rear views of the ICP monitoring system of  FIG. 1  illustrating alternative designs for shroud-like element  16 . As shown in  FIG. 5 , shroud-like element  16  may have an electrically conductive frame  20 B with a quarter-spherical arc that extends only partially over head  17  of patient  14 . In the example of  FIG. 5 , it is desirable to place head  17  of patient  14  in a position at which distance between implanted ICP monitor  12  and electrically conductive frame  20 B. As shown in  FIG. 6 , shroud-like element  16  may have an electrically conductive frame  20 C with a right-angled configuration, including a vertical member  21  and a horizontal member  23 . Horizontal member  23  extends over head  17  of patient  14 . In some embodiments, vertical member  21  may be provided strictly for support of horizontal member  23 . In that case, horizontal member  23  serves as an inductive power transmitter and a telemetry antenna.  
         [0039]      FIG. 7  is schematic diagram of an ICP monitor  12  implanted in the cranium of a patient. ICP monitor  12  may be constructed as a conventional implantable ICP monitor. In some embodiments, ICP monitor  12  may generally conform to a monitor as described in U.S. Pat. No. 6,248,080 to Miesel et al., the entire content of which is incorporated herein by reference. As shown in  FIG. 7 , ICP monitor  12  is implanted beneath scalp  24 , and includes a portion that extends through skull  26  and into brain  28 . For example, ICP monitor  12  may include a pressure sensor probe  30 , a silicone plug  32  and a cap member  34 . Silicone plug  32  fills and substantially seals a burr hole within skull  26 . Probe  30  extends inward from silicone plug  32  and penetrates brain  28 . Cap  34  rests under scalp  24  and over skull  26 .  
         [0040]      FIG. 8  is a cross-sectional side view of the implantable ICP monitor  12  of  FIG. 7 . As shown in  FIG. 8 , probe  30  may include a pressure sensor  38  carried on a circuit board  40 . Pressure sensor  38  may take the form of any of a variety diaphragm sensors, strain gauge sensors, capacitive sensors, piezoelectric sensors, or other sensors used in conventional ICP pressure measurement. Probe  30  may define a hole  42  for fluid communication with the environment with the cranium. Additional circuitry  44  may be provided on circuit board  40  to amplify, filter and process the pressure signal output by pressure sensor  38 . In addition, circuitry  44  may be electrically coupled, via conductors  46 , to an inductive coil  50  within cap  34 . Accordingly, circuitry  44  also may include power generation circuit to convert current induced in coil  50  into operating power, and telemetry circuitry to drive the coil for transmission of signals carrying ICP measurements from pressure sensor  38 .  
         [0041]      FIG. 9  is a block diagram illustrating an ICP monitoring system  12  in accordance with an embodiment of the invention. As shown in  FIG. 9 , an external power/telemetry unit  52  generates power to drive shroud-like element  16 . Implantable ICP monitor  12  receives power by inductive transfer from shroud-like element  16 . In addition, ICP monitor  12  then transmits ICP measurement signals, which are received by shroud-like element  16  as an antenna. External power/telemetry unit  52  receives the ICP measurement signals from shroud-like element  16  for further processing, analysis and presentation to care-givers. In this manner, shroud-like element  16  provides a persistent link between implantable ICP monitor  12  and external power/telemetry unit  52  for continuous or periodic ICP monitoring.  
         [0042]      FIG. 10  is a block diagram illustrating an implantable ICP monitor  12 . As shown in  FIG. 10 , implantable ICP monitor  12  includes pressure sensor  38 , inductive coil  50 , monitor circuitry  54 , telemetry circuitry  55  and power conversion circuitry  56 . Pressure sensor  38  senses intracranial pressure and generates an ICP measurement signal. The ICP measurement signal may be transmitted substantially in real-time, or buffered within ICP monitor  12  for a period of time. In some embodiments, ICP monitor  12  may support bi-directional communication and may be configured to transmit pressure measurement signals in response to an interrogation request transmitted by external power/telemetry unit  52 .  
         [0043]     Coil  50  receives electromagnetic energy from shroud-like element  16 , which induces current in the coil. Hence, ICP monitor  12  is powered by inductive telemetric transmission of energy. Power conversion circuitry  56  converts current induced in inductive coil  50  into operating power for pressure sensor  38 , monitor circuitry  54  and telemetry circuitry  55 . For example, power circuit  56  may include an ac/dc conversion circuit, such as a rectifier, that converts the ac current induced in coil  50  into dc operating power. The electromagnetic energy transmitted by shroud-like element  16 , and hence the ac current induced in coil  50 , may reside within any frequency range suitable for effective inductive transfer of energy, as is known in the art. For example, transmission frequencies of approximately 100 kHz to several MHz may be suitable for inductive telemetric energy transfer, although other frequencies may be used. Wireless signals generated by ICP monitor  12  may reside within the telemetric power frequency range, or any other frequency ranges suitable for reliable communication. In some embodiments, shroud-like element  16  serves as both an integrated power source and signal receiver for ICP monitor  12 .  
         [0044]     Power conversion circuitry  56  also may include a capacitor or other storage device to store a dc potential as a source of operating power. The capacitor may store energy temporarily to power ICP monitor  12 , e.g., only during the time that coil  50  receives energy from shroud-like element  16 . Alternatively, a battery may be provide to power ICP monitor  12  over an extended period of time. In some embodiments, power conversion circuitry  56  may generally correspond to similar circuitry described in U.S. Pat. No. 6,731,976 to Penn et al., the entire content of which is incorporated herein by reference.  
         [0045]     Monitor circuitry  54  filters, amplifies, and processes the ICP measurement signal, as necessary. Telemetry circuitry  55  then generates telemetry signals for wireless transmission to external power/telemetry unit  52 , using inductive coil  50  and shroud-like element  16  as antennas. Hence, inductive coil  50  and shroud-like element  16  serve as inductive transfer elements for purposes of both power transfer and telemetry. Telemetry circuitry  55  includes appropriate amplifier, filtering and modulation circuitry to convert the ICP measurement signal into a telemetry signal.  
         [0046]      FIG. 11  is a block diagram illustrating an external power/telemetry unit  52  to power and communicate with an implanted ICP monitor  12 . As shown in  FIG. 11 , external power/telemetry unit  52  may include a processor  58 , a user input device  60 , display  62 , memory  64 , inductive power generator  66  and telemetry interface  68 .  
         [0047]     Processor  58  controls the operation of the various components of external power/telemetry unit  52 . For example, processor  58  controls inductive power generator  66  and telemetry interface  68 , and handles processing and storage of information obtained from implantable ICP monitor  12 . Processor  58  may include one or more microprocessors, digital signal processors (DSPs), application-specific integrated circuits (ASICs), field-programmable gate arrays (FPGAs), or other equivalent logic circuitry.  
         [0048]     Processor  58  also may accept input from user input device  60 , e.g., to select different formats, or time or amplitude scales, for presentation of ICP information on display  62 . Display  62  may include any of a variety of different displays, such as a liquid crystal display (LCD), plasma display, or cathode ray tube (CRT) display. In addition, processor  58  may archive ICP information within memory  64  for retrieval or transmission to other devices, such as remote monitors distributed within a network.  
         [0049]     Memory  64  may include any magnetic, electronic, or optical media, such as random access memory (RAM), read-only memory (ROM), electronically-erasable programmable ROM (EEPROM), flash memory, or the like, or a combination thereof. Memory  64  may store program instructions that, when executed by processor  58 , cause the processor to perform the functions ascribed to it herein. For example, memory  64  may store instructions for processor  58  to execute in support of control of wireless telemetry interface  68  and control of, and processing of information obtained from implantable ICP monitor  12 . Memory  64  may include separate memories for storage of instructions and archived ICP information.  
         [0050]     Telemetry interface  68  may include a wireless radio frequency (RF) receiver to permit reception of information transmitted by implanted ICP monitor  12 . In some embodiments, ICP monitor  12  may be equipped for bi-directional communication, and may be responsive to commands transmitted via telemetry interface  68 . In each case, telemetry interface  68  includes an antenna, in the form of shroud-like element  16 , which is located proximate to a patient&#39;s head to ensure reliable telemetry.  
         [0051]     Inductive power generator  66  applies current to shroud-like element  16  to support inductive power transfer to implanted ICP monitor  12 . Although energy transfer between shroud-like element  16  and ICP monitor  12  may be relatively inefficient, external power/telemetry unit  52  preferably is coupled to a line power supply. As an example, inductive power generator  66  may drive shroud-like element  16  with a high frequency, ac signal having an amplitude sufficient for reliable telemetric energy transfer. In response to the ac signal, shroud-like element  16  transmits inductive energy to power ICP monitor  12 . Telemetric energy transfer for implantable monitors is well known in the art.  
         [0052]     Hence, external power/telemetry unit  52  enables ICP monitor  12  to be operated passively. In other words, all of the power for operation of ICP monitor  12  is provided by external power/telemetry unit  52 . Yet, in accordance with the invention, shroud-like element  16  permits the power from external power/telemetry unit  52  to be coupled to ICP monitor on a continuous basis. In this manner, ICP measurements can be obtained on a substantially continuous or periodic basis, as desired.  
         [0053]      FIG. 12  is a flow diagram illustrating an ICP monitoring method in accordance with an embodiment of the invention. As shown in  FIG. 12 , external power/telemetry unit  52  transmits power to shroud-like element  16  to generate an electromagnetic field for transfer of energy from the shroud-like element to implantable ICP monitor  12  ( 70 ). External power/telemetry unit  52  then monitors telemetry output from shroud-like element  16  ( 72 ), which serves as a telemetry antenna for signals transmitted by ICP monitor  12 . External power/telemetry unit  52  records a continuous record of ICP measurements based on the telemetry output of shroud-like element  16  ( 74 ).  
         [0054]     In some embodiments, external power/telemetry unit  52  may invoke advisory levels to provide a care-giver with an indication when levels indicated by the measurement signals exceed a threshold level or deviate from a particular range. For example, external power/telemetry unit  52  may compare the ICP measurement to a threshold and, if the ICP measurement exceeds the threshold ( 76 ), generate an advisory ( 78 ), which may be in the form of a visual or audible alarm, alert or other conspicuous message. For example, the threshold level may be selected to alert a care-giver to the presence of ICP levels that could endanger a patient&#39;s health.  
         [0055]     Accordingly, the ability to obtain ICP measurements on a continuous or periodic basis allows a care-giver to obtain a valuable body of information, and permits the care-giver to detect potentially dangerous ICP levels. Consequently, an ICP monitoring system  10  as described herein may contribute to improved patient care. At the same time, a shroud-like element  16  can be constructed in a manner that provides the patient with comfort and convenience.  
         [0056]     The preceding specific embodiments are illustrative of the practice of the invention. It is to be understood, therefore, that other expedients known to those skilled in the art or disclosed herein may be employed without departing from the invention or the scope of the claims.  
         [0057]     In the claims, means-plus-function clauses are intended to cover the structures described herein as performing the recited function and not only structural equivalents but also equivalent structures. Thus, although a nail and a screw may not be structural equivalents in that a nail employs a cylindrical surface to secure wooden parts together, whereas a screw employs a helical surface, in the environment of fastening wooden parts a nail and a screw are equivalent structures.  
         [0058]     Many embodiments of the invention have been described. Various modifications may be made without departing from the scope of the claims. These and other embodiments are within the scope of the following claims.