Abstract:
The disclosure describes a tube pressure sensor to measure penile tumescence which may be used in a therapeutic penile tumescence control system. The system senses penile pressure and sends the information to a stimulator that is capable of stimulation therapy to control an erectile state, thus treating sexual dysfunction or, more specifically, erectile dysfunction. Measuring penile tumescence pressure is accomplished through the use of a tube placed within the urethra of the penis and attached to a module implanted within the bladder. Pressure on the tube generates an electrical signal that is sent wirelessly to an implanted stimulator connected to a lead positioned near pelvic floor nerves that stimulate erections. An external device may be used to wirelessly send information to the implanted stimulator to start or stop stimulation in order for the patient to conduct normal sexual activity. In addition, pressure information and stimulation information may be recorded and reviewed by a physician for continued therapy monitoring.

Description:
TECHNICAL FIELD 
   The invention relates to implantable medical devices and, more particularly, implantable sensors. 
   BACKGROUND 
   Sexual dysfunction of the penis is a common problem afflicting males of all ages, genders, and races. Erectile dysfunction is a serious condition for many men, and it may include a variety of problems. Some of these problems include the inability to create an erection, incomplete erections and brief erectile periods. These conditions may be associated with nervous system disorders and may be caused by aging, injury, or illness. 
   In some cases, erectile dysfunction can be attributed to improper nerve activity that incompletely stimulates the penis. For example, stimulation from the brain during arousal and sexual activity is responsible for activating an erection. With respect to erectile disorders, the problem may be a lack of sufficient stimulation from the brain or a break in communication of the stimulation. Other disorders may involve dysfunctional parasympathetic function that can be attributed to many factors including illness or injury. Clinical evaluation of erectile dysfunction depends on patient description and possible catheter-based pressure measurements in the clinical setting. 
   Some methods for treating erectile dysfunction include pharmaceutical treatment and electrical stimulation. Delivery of electrical stimulation to nerves running through the pelvic floor may provide an effective therapy for many patients. For example, an implantable neurostimulator may be provided to deliver electrical stimulation to the pudendal or cavernous nerve to induce an erection. 
   SUMMARY 
   The invention is directed to a flexible tube sensor that is implantable to sense penile tumescence, as well as a neurostimulation system and method that make use of such a sensor for alleviation of erectile dysfunction. The sensor includes a thin, flexible tube and a sensing element to detect pressure levels within the tube. The flexible tube may be deployed within the bladder neck or urethra to transduce pressure exerted by the swelling of penile tissue on the urethra as a function of the pressure within the flexible tube. Alternatively, the flexible tube may be deployed within or adjacent to the corpus cavernosa of the penis. In either case, the flexible tube is generally thin and flexible, permitting ready deployment within the penis without significant disruption of sexual or urinary function. 
   Inadequate penile tumescence during sexual arousal, i.e., erectile dysfunction, may be a result of faulty nervous system function of the sexual organs. The flexible tube sensor may provide short- or long-term monitoring of penile pressure for storage and offline analysis by a clinician. In addition, a flexible tube sensor may provide feedback in a closed-loop neurostimulation system to control and sustain a state of erection during the course of sexual activity. 
   Neurostimulation therapy is applied to increase blood flow to the penis, thereby promoting tumescence and causing an erection. An implantable neurostimulator may be responsive to penile pressure signals generated by the flexible tube sensor, as described herein, to provide closed-loop neurostimulation therapy to treat erectile dysfunction. In particular, stimulation parameters can be adjusted in response to the penile pressure signals to sustain a state of erection. 
   In one embodiment, the invention provides an implantable electrical stimulation system comprising an implantable pressure sensor including a flexible tube and a sensing element that senses a pressure level within a penis of a patient based on a pressure level within the tube, and an implantable stimulator that delivers electrical stimulation to the patient based on the sensed pressure level within the penis. 
   In another embodiment, the invention provides a method comprising sensing a pressure level within a penis of a patient based on a pressure level within a flexible tube placed within the penis, and delivering electrical stimulation to the patient via an implanted stimulator based on the sensed pressure level. 
   In an additional embodiment, the invention provides an implantable penile tumescence sensor comprising a flexible tube, a sensing element that senses a pressure level within the flexible tube, a fixation mechanism that positions the flexible tube within a penis of a patient, and circuitry that determines a tumescence level within the penis based on the sensed pressure level. 
   In various embodiments, the invention may provide one or more advantages. For example, the use of a thin, flexible tube sensor permits pressure to be sensed within the narrow, constricted passage of the urethra, or within or adjacent to the corpus cavernosa. In this manner, pressure can be sensed without significantly obstructing or altering the physiological function or the bladder, sexual organs, or urethra. 
   The flexible tube sensor may be coupled to a larger sensor housing that resides within the bladder or abdomen and houses sensor electronics for transducing a pressure level of the tube. In some embodiments, the sensor housing may reside within the penis itself. The flexible tube sensor permits pressure information to be obtained on a continuous or periodic basis as the patient goes about a daily routine and, more importantly, during the course of sexual activity. In addition, the flexible nature of the tube permits the sensor to be implanted in a variety of locations, constructed in variety of shapes and sizes, and flex with the changing shape of the penis. 
   The flexible tube sensor may transmit sensed pressure information to an implantable stimulator to permit dynamic control of the therapy delivered by the stimulator on a closed-loop basis. For example, the stimulator may adjust stimulation parameters, such as amplitude, pulse width or pulse rate, in response to the sensed pressure levels. In this manner, the stimulator can respond to changes in sexual activity and maintain penile tumescence at a desired pressure level. Also, with closed-loop stimulation, the stimulator may generate stimulation parameter adjustments that more effectively target erectile function, thereby enhancing stimulation efficacy. 
   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 
       FIG. 1  is a schematic diagram illustrating an implantable stimulation system, incorporating a penile tumescence sensor, for alleviation of sexual dysfunction. 
       FIG. 2  is an enlarged side view of an implantable pressure sensor with a flexible tube extending through the urethra of a patient. 
       FIG. 3  is an enlarged side view of an implantable pressure sensor with a flexible tube residing within the penis of a patient. 
       FIG. 4  is an enlarged, cross-sectional side view of the implantable pressure sensor of  FIGS. 1 and 2 . 
       FIG. 5  is functional block diagram illustrating various components of an exemplary implantable pressure sensor. 
       FIG. 6  is a functional block diagram illustrating various components of an implantable stimulator. 
       FIG. 7  is a schematic diagram illustrating cystoscopic deployment of an implantable pressure sensor via the urethra. 
       FIG. 8  is a schematic diagram illustrating retraction of a deployment device upon fixation of a pressure sensor within a patient&#39;s urinary tract. 
       FIG. 9  is a cross-sectional side view of a distal end of a deployment device during deployment and fixation of a pressure sensor. 
       FIG. 10  is a cross-sectional bottom view of the deployment device of  FIG. 10  before attachment of the pressure sensor. 
       FIG. 11  is a flow chart illustrating a technique for delivery of stimulation therapy to alleviate sexual dysfunction based on closed loop feedback from an implantable pressure sensor. 
   

   DETAILED DESCRIPTION 
     FIG. 1  is a schematic diagram illustrating an implantable stimulation system  10  for alleviation of sexual dysfunction. As shown in  FIG. 1 , system  10  may include an implantable pressure sensor  12 , implantable stimulator  14  and external programmer  16  shown in conjunction with a patient  18 . Pressure sensor  12  senses a pressure level of penis  22  on urethra  20  distal to bladder  24 , and transmits pressure information based on the sensed pressure level to at least one of stimulator  14  and programmer  16  by wireless telemetry. 
   The sensed pressure level represents a level of tumescence of penis  22 , i.e., a level of blood flow into the penis and a resulting level of engorgement. In this manner, pressure sensor  12  permits the erectile state of penis  22  to be monitored. Sensor  12 , stimulator  14  or programmer  16  may record the pressure information. Alternatively, or additionally, stimulator  14  or programmer  16  may generate adjustments to electrical stimulation parameters applied by the stimulator in response to the pressure information, permitting closed loop feedback of erectile state information during the course of sexual activity. 
   In some embodiments, stimulator  14  or programmer  16  may generate adjustments to parameters in response to pressure information to support delivery of electrical stimulation to support distinct phases of sexual activity, and transition between such phases. For example, based on the pressure information obtained by sensor  12 , stimulator  14  or programmer  16  may adjust stimulation parameters to maintain a particular phase of sexual activity, transition from one phase to another, and transition from one phase to a cessation of sexual activity. Examples of distinct phases of sexual activity include arousal, e.g., desire, erection or lubrication, and orgasm or ejaculation. To support distinct phases of sexual activity and progression between phases, sensor  12 , stimulator  14 , and programmer  16  may be configured to operate in conjunction with stimulation devices and techniques described in U.S. patent application Ser. No. 10/441,784, to Martin Gerber, filed May 19, 2003, entitled “TREATMENT OF SEXUAL DYSFUNCTION BY NEUROSTIMULATION,” the entire content of which is incorporated herein by reference. 
     FIG. 2  is a side view illustrating implantable pressure sensor  12  implanted within urethra  20  and bladder  24 . As shown in  FIGS. 1 and 2 , pressure sensor  12  includes a sensor housing  26  and a flexible tube  28  that extends from the housing. Flexible tube  28  includes a closed end  32  and an open end (not shown in  FIG. 1 ). Sensor housing  26  contains a sensing element (not shown in  FIG. 1 ) adjacent the open end of flexible tube  28 . Sensor housing  26  further contains electronics to generate pressure information, and telemetry circuitry for transmission of the information. The sensing element senses the pressure level within flexible tube  28 . Flexible tube  28  may contain a fluid, such as a gas or liquid. 
   As further shown in  FIGS. 1 and 2 , sensor housing  26  may reside within bladder  24 . Sensor housing  26  may be temporarily or permanently attached to an inner wall  27  of bladder  24 , such as the mucosal lining, as will be described. Alternatively, housing  26  may be implanted sub-mucosally. Flexible tube  28  extends away from sensor housing  26 , out of bladder  24  and through urethra  20 . In this manner, flexible tube  28  is positioned to directly sense the pressure level exerted within urethra  20  inside of the shaft of the penis  22 . Yet, flexible tube  28  may be sufficiently thin to avoid significant obstruction of urethra  20  or disruption of the function of other urinary or reproductive structures. 
   As a further alternative, housing  26  may reside outside bladder  24 , in which case flexible tube  28  may extend into bladder  24  and through urethra  20  through a hole formed in the bladder. In this case, housing  26  may be surgically or laparoscopically implanted within the abdomen. Tubes  28  may be surgically or laparoscopically guided through a hole in the wall of bladder  24 . A cystoscope may be used to grab tube  28  and pull it downward through urethra  20 . In some embodiments, housing  26  and its contents may be integrated with stimulator  14 , in which case flexible tube  28  extend from the stimulator housing and into bladder  24 , much like leads carrying stimulation or sense electrodes. 
   With further reference to  FIG. 1 , implantable stimulator  14  includes an electrical lead  15  (partially shown in  FIG. 1 ) carrying one or more electrodes that are placed at a nerve site within the pelvic floor. For example, the electrodes may be positioned to stimulate the prostate parasympathetic nerve, the cavernous nerve, the pudendal nerve, the sacral nerves to support and maintain an erection of penis  22 . In particular, electrical stimulation may be applied to increase penile tumescence, i.e., blood flow into the penis  22 , that enables the patient to achieve an erection and participate in normal sexual activity. Further, the level of stimulation may be modified based on closed-loop feedback from sensor  12  to maintain the tumescence of penis  22  at target level. 
   In this manner, implantable stimulator  14  delivers stimulation therapy to the in order to achieve and maintain desired penile tumescence. At predetermined times, or at patient controlled instances, the external programmer  16  may program stimulator  14  to begin stimulation to achieve an erection. Upon the completion of sexual activity or after a predetermined period of time, stimulator  14  may cease stimulation to allow the erection to subside. 
   During the course of stimulation, stimulator  14  may adjust the stimulation delivered to the patient. For example, adjustment of stimulation parameters may be responsive to pressure information transmitted by implantable pressure sensor  12 . External programmer  16  or implantable stimulator  14  may adjust stimulation parameters, such as amplitude, pulse width, and pulse rate, based on pressure information received from implantable sensor  12 . In this manner, implantable stimulator  14  adjusts stimulation to either increase or reduce penile tumescence based on the actual pressure level sensed within urethra  20 . 
   Pressure sensor  12  may transmit pressure information periodically, e.g., every few seconds, during the course of sexual activity. Alternatively, each pressure measurement may be obtained by pressure sensor  12  in response to a request from stimulator  14  or programmer. In either case, stimulator  14  or programmer  16  may activate pressure sensor  12 , e.g., by wireless telemetry, to commence sensing. In some embodiments, pressure sensor  12  may transmit pressure information when there is an abrupt change in sphincter pressure, e.g., a pressure change that exceeds a predetermined rate threshold, which indicates sexual arousal. In this case, pressure sensor  12  may sense pressure levels at relatively long intervals, and then self-activate sensing at shorter intervals upon detection of the onset of sexual activity. 
   External programmer  16  may be a small, battery-powered, portable device that may accompany the patient  18  throughout the day or only during sexual activity. Programmer  16  may have a simple user interface, such as a button or keypad, and a display or lights. Patient  18  may initiate an erection, i.e., a voluntary increase in penile tumescence, via the user interface. In particular, in response to a command from the patient  18 , programmer  16  may activate stimulator  14  to deliver electrical stimulation therapy. In some embodiments, the length of time for an erection event may be determined by pressing a button a first time to initiate stimulation and a second time when the sexual activity is complete, or by a predetermined length of time permitted by programmer  16  or implantable stimulator  14 . In each case, programmer  16  causes implantable stimulator  14  to temporarily stimulate patient  18  to promote penile tumescence. 
   Implantable stimulator  14  may be constructed with a biocompatible housing, such as titanium or stainless steel, and surgically implanted at a site in patient  18  near the pelvis. The implantation site may be a subcutaneous location in the side of the lower abdomen or the side of the lower back. One or more electrical stimulation leads  15  are connected to implantable stimulator  14  and surgically or percutaneously tunneled to place one or more electrodes carried by a distal end of the lead at a desired nerve site, such as a prostate parasympathetic, pudendal, sacral, or cavernous nerve site. 
   In the example of  FIGS. 1 and 2 , sensor housing  26  of implantable pressure sensor  12  is attached to the inner wall  27  of bladder  24 . However, the attachment site for sensor housing  26  could be anywhere with access to urethra  20 . Also, although a single tube  28  is illustrated for purposes of example, pressure sensor  12  may include multiple tubes or multiple sensors. With a relatively long flexible tube  28 , for example, sensor housing  26  could be positioned at a greater distance from the exit of bladder  24 . 
   Also, in some embodiments, sensor housing  26  may be attached within urethra  20 , e.g., closer to the section of urethra  20  within penis  22 , although attachment of the sensor housing within bladder  24  may be desirable to avoid obstruction of the urethra. In other embodiments, sensor housing  26  could be surgically or laparoscopically implanted outside of bladder  24 . In this case, flexible tube  28  may be coupled to the sensor housing  26  and tunneled through a hole in the wall of bladder  24  and into urethra  20 , either by introduction of the tube through the urethra and upward into the bladder, or by introduction of the tube into the bladder and downward into the urethra. 
     FIG. 3  is an enlarged schematic diagram illustrating the side view of an implantable pressure sensor  12  with a flexible tube  28  residing within the penis  22  of a patient  18 . In the example of  FIG. 3 , sensor  12  and flexible tube  28  are surgically implanted within tissue of penis  22 . Some patients may benefit from implantation of sensor  12  and tube  28  within penis  22  when bladder  24  or urethra  20  are not able to carry a device without obstruction or impaired urinary or sexual function. The corpus cavernosa penis  23  and corpus cavernosa urethrae  21  are structures that swell with blood during arousal and erection. Therefore, placement of the sensor within or adjacent to corpus cavernosa penis  23  or corpus cavernosa urethrae  21  may provide accurate sensing of tumescence within penis  22 . 
   As shown in  FIG. 3 , sensor housing  26  and flexible tube  28  are shown surgically implanted into one of the corpus cavernosa penis  23  segments of penis  22 . Sensor housing  26  may simply lie within the tissue or be attached to the outer lining of corpus cavernosa penis  23 . Sensor housing  26  may be attached by simple sutures or by any of a variety of fixation mechanisms, which will be described in greater detail herein in the context of attachment of the sensor housing within bladder  24 . Once implanted, pressure sensor  12  does not readily move within the tissue. The flexible tube  28  may move with the body of the penis  22  as the penis changes position or expands. Flexible tube  28  may vary in length depending on the size of penis  22  or the placement site of sensor housing  26 . 
   In another embodiment, implantable sensor  12  may be surgically implanted into corpus cavernosum urethrae  21 . The corpus cavernosum urethrae  21  of penis  22  surrounds urethra  20  throughout the body of the penis. Placement of the sensor  12  in corpus cavernosum urethrae  21  may enable tumescence sensing while further minimizing the impact of the sensor during sexual activity. In either case, implantation of sensor  12  within the body of penis  22 , rather than within urethra  20 , may present less risk of obstruction of urine flow. 
     FIG. 4  is an enlarged, cross-sectional side view of implantable pressure sensor  12  of  FIGS. 1 and 2 . As shown in  FIG. 4 , sensor housing  26  receives an open end  34  of flexible tube  28 . A sensing element  36  is mounted within sensor housing  26 , at open end  34 , to sense a pressure level within fluid tube  28 . Sensing element  36  may be coupled to a circuit board  38  within sensor housing  26 . Circuit board  38  carries suitable electronics for processing signals generated by sensing element  36 . In particular, circuit board  38  may include circuitry that determines a tumescence level within penis  22  based on the sensed pressure level obtained from sensing element  36 . 
   In the example of  FIG. 4 , flexible tube  28  is filled with a fluid to transduce the pressure on the tube to sensing element  36 . Inward deformation of flexible tube  28  causes an elevation in the internal pressure of the tube. Sensing element  36  senses the elevation in pressure at open end  34  of flexible tube  28 , and generates a pressure signal that represents the pressure level. Although end  34  is referred to as “open,” it is sealed by sensing element  36 . Consequently, deformation of flexible tube  28  causes a change in the tube volume, and hence pressure changes in the fluid  30  within the tube. 
   Flexible tube  28  may be formed from a variety of flexible materials, including polyurethane or silicone. The flexibility of tube  28  permits the tube to conform to contours within urethra  20 , or penis  22 , and deform in response to changes in penis  22  and pressure exerted on urethra  20 . In particular, a rise in penile tumescence results in exertion of pressure inward against the outer wall of urethra  20 . In turn, the inner wall of urethra  20  exerts pressure inward against the outer wall of flexible tube  28 , causing the wall of the tube to deform and compress inward, providing an indication of penile tumescence. 
   Sensing element  36  may include a strain gauge sensor, e.g., formed by thin film deposition on a flexible membrane. Circuit board  38  may include processing electronics to process signals generated by sensing element  36 , and generate pressure information based on the signals monitoring the pressure level of each tube. In addition, circuit board  38  may include telemetry circuitry for wireless telemetry with stimulator  14 , external programmer  16 , or both. 
   Sensing elements  36 , in some embodiments, may be constructed as a membrane that carries a resistive strain gauge or piezoelectric element selected to be effective as a pressure transducer. Upon deformation of the membrane, in response to pressure levels within their respective tubes, sensing element  36  produces an electrical signal. When penile pressure increases, the flexible tube  28  deforms and the pressure inside the tube increases. The higher pressure forces the membrane within sensing element  36  to deform, thus producing an electrical signal change and enabling implanted pressure sensor  12  to measure pressure and, indirectly, penile tumescence. 
   Fluid  30  contained within the tube may be a liquid or gas, or a combination of liquid and gas. For example, flexible tube  28  could be filled with saline, distilled water, oxygen, air or any other biocompatible fluid. Preferably, the fluid  30  within the tubes is generally non-compressible. Fluid  30  tends to exhibit an elevation in pressure as the walls of tube  28  are deformed during engorgement of penis  22 . Conversely, fluid  30  exhibits a reduction in pressure as penis  22  relaxes. In each case, the pressure level is transduced by sensing element  36 , and can be communicated to stimulator  14 , programmer  16 , or both for analysis or closed loop control of stimulation parameters 
   Flexible tube  28  may be provided with different dimensions selected for patients having different anatomical dimensions. In particular, implantable pressure sensor  12  may be constructed with a flexible tube  28  having different lengths or diameters. Different tube lengths maybe necessary given the distance between the attachment site of sensor housing  26  and urethra within penis  22 , either to ensure that flexible tube  28  reaches the distal urethra or does not extend too far down urethra  20 . It may also be important for tube  28  to remain within urethra  20  while the penis is both flaccid and erect. Multiple diameters may also be necessary to allow tube  28  to be placed into both a large or narrow urethra  20 . The dimensions may be fixed for a given pressure sensor  12 , as a complete assembly. Alternatively, tubes of different sizes may be attached to a pressure sensor housing  26  by a physician prior to implantation. 
   In general, flexible tube  28  may have a length of less than approximately 9 cm and more preferably less than approximately 7 cm. In some embodiments, flexible tube  28  may have a length of approximately 0.5 cm to 3 cm. The lengths of tube  28  may vary according to the anatomy of the patient. In addition, tube  28  may have an outer diameter in a range of approximately 1 to 3 mm. The wall of tube  28  may be relatively thin to ensure sufficient deformation and conformability, yet thick enough to ensure structural integrity. As an example, the thickness of the wall of tube  28  may be in a range of approximately 0.1 mm to 0.3 mm. 
   Sensor housing  26  may be made from a biocompatible material such as titanium, stainless steel, or nitinol, or polymeric materials such as silicone or polyurethane. In general, sensor housing  26  contains no external openings, with the exception of the opening to receive flexible tube  28 , thereby protecting sensing element  26  and circuit board  38  from the environment within bladder  24 . The proximal, open end  34  of flexible tube  28  resides within sensor housing  26  while the distal, closed end  32  resides outside of the sensor housing. The opening in sensor housing  26  that receives open end  34  of flexible tube  28  may be sealed to prevent exposure of interior components. 
   Attaching implantable pressure sensor  12  to the mucosal lining of bladder  24  may be accomplished in a variety of ways, but preferably is completed in a manner that will not excessively injure bladder  24 . Preferably, attachment should cause limited inflammation no adverse physiological modification, such as tissue infection or a loss in structural integrity of bladder  24 . However, it is desirable that implantable pressure sensor  12  also be attached securely to the attachment site in order to provide an extended period of measurement without prematurely loosening or detaching from the intended location. 
   As an example, sensor housing  26  may contain a vacuum cavity  39  that permits a vacuum to be drawn by a vacuum channel  40 . The vacuum is created by a deployment device having a vacuum line in communication with vacuum channel  40 . The vacuum draws a portion  42  of the mucosal lining  44  of bladder  24  into vacuum cavity  39 . Once the portion  42  of mucosal lining  44  is captured within vacuum cavity  39 , a fastening pin  46  is driven into the captured tissue to attach sensor housing  26  within bladder  24 . Fastening pin  46  may be made from, for example, stainless steel, titanium, nitinol, or a high density polymer. The shaft of pin  46  may be smooth or rough, and the tip may be a sharp point to allow for easy penetration into tissue. Fastening pin  46  may be driven into housing  26  and the portion  42  of mucosal lining  44  under pressure, or upon actuation by a push rod, administered by a deployment device. 
   In some embodiments, fastening pin  46  may be manufactured from a degradable material that the breaks down over time, e.g. in the presence of urine, to release implantable pressure sensor  12  within a desired time period after attachment. In still another embodiment, implantable pressure sensor  12  may be attached without the use of a penetrating rod but with a spring-loaded clip to pinch trapped mucosal lining  44  within cavity  39 . A variety of other attachment mechanisms, such as pins, clips, barbs, sutures, helical screws, surgical adhesives, and the like may be used to attach sensor housing  26  to mucosal lining  44  of bladder  24 . Similar attachment mechanisms may be used when implanting sensor  12  within the body of penis  22 , e.g., within or adjacent to corpus cavernosa penis  23  and corpus cavernosa urethrae  21 . 
     FIG. 5  is functional block diagram illustrating various components of an exemplary implantable pressure sensor  12 . In the example of  FIG. 5 , implantable pressure sensor  12  includes a sensing element  36 , processor  48 , memory  50 , telemetry interface  52 , and power source  54 . Sensor  36  transforms pressure levels produced by mechanical deformation from tube  28  into electrical signals representative of penile tumescence. The electrical signals may be amplified, filtered, and otherwise processed as appropriate by electronics within sensor  12 . In some embodiments, the signals may be converted to digital values and processed by processor  48  before being saved to memory  50  or sent to implantable stimulator  14  as pressure information via telemetry interface  52 . 
   Memory  50  stores instructions for execution by processor  48  and pressure information generated by sensing element  36 . Pressure information may then be sent to implantable stimulator  14  or external programmer  16  for long-term storage and retrieval by a user. Memory  50  may include separate memories for storing instructions and pressure information. In addition, processor  48  and memory  50  may implement loop recorder functionality in which processor  48  overwrites the oldest contents within the memory with new data as storage limits are met. 
   In some embodiments, sensor  26  may be deployed purely as a diagnostic device to obtain and store penile tumescence measurements over a period of time. In particular, sensor  26  may be used to diagnose a patient&#39;s condition in order to determine whether the patient suffers from erectile dysfunction, the degree the dysfunction, and whether electrical stimulation therapy may be desirable. In each case, sensor  26  is entirely ambulatory and requires little or no setup by the patient  18 . Instead, sensor  26  simply accompanies patient  18  throughout his daily routine. Loop recorder functionality may be especially desirable for monitoring of penile tumescence over an extended period of time. Following implantation of stimulator  14 , sensor  26  may function as both a diagnostic device and a closed loop feedback device for the stimulator. 
   Processor  48  controls telemetry interface  52  to send pressure information to implantable stimulator  14  or programmer  16  on a continuous basis, at periodic intervals, or upon request from the implantable stimulator or programmer. Wireless telemetry may be accomplished by radio frequency (RF) communication or proximal inductive interaction of pressure sensor  12  with stimulator  14  or programmer  16 . 
   Power source  54  delivers operating power to the components of implantable pressure sensor  12 . Power source  54  may include a battery and a power generation circuit to produce the operating power. In some embodiments, the battery may be rechargeable to allow extended operation Recharging may be accomplished through proximal inductive interaction between an external charger and an inductive charging coil within sensor  12 . In some embodiments, power requirements may be small enough to allow sensor  12  to utilize patient motion and implement a kinetic energy-scavenging device to trickle charge a rechargeable battery. In other embodiments, traditional batteries may be used for a limited period of time. As a further alternative, an external inductive power supply could transcutaneously power sensor  12  whenever pressure measurements are needed or desired. 
     FIG. 6  is a functional block diagram illustrating various components of an implantable stimulator  14 . In the example of  FIG. 6 , stimulator  14  includes a processor  56 , memory  58 , stimulation pulse generator  60 , telemetry interface  62 , and power source  64 . Memory  58  stores instructions for execution by processor  56 , stimulation therapy data, and pressure information received from pressure sensor  12  via telemetry interface. Pressure information is received from pressure sensor  12  and may be recorded for long-term storage and retrieval by a user, or adjustment of stimulation parameters, such as amplitude, pulse width or pulse rate. Memory  58  may include a single memory, or separate memories for storing instructions, stimulation parameter sets, and pressure information. 
   Processor  56  controls stimulation pulse generator  60  to deliver electrical stimulation therapy via one or more leads  15 . Processor  56  also controls telemetry interface  62  to send information to stimulator  14 , programmer  16 , or both, and optionally receive information. Based on pressure information received from sensor  12 , processor  56  interprets the information and determines whether any therapy parameter adjustments should be made. For example, processor  56  may compare the pressure level to one or more thresholds, and then take action to adjust stimulation parameters based on the pressure level. Information may be received from sensor  12  on a continuous basis, at periodic intervals, or upon request from stimulator  14  or external programmer  16 . Alternatively, or additionally, pressure sensor  12  may transmit pressure information when there is an abrupt change in the pressure level, e.g., at the onset of sexual arousal. 
   Processor  56  modifies parameter values stored in memory  58  in response to pressure information from sensor  12 , either independently or in response to programming changes from external programmer  16 . In other words, stimulator  14  may directly control its own parameters in response to information obtained from sensor  12 . Alternatively, programmer  16  may direct the parameter adjustments. Stimulation pulse generator  60  provides electrical stimulation according to the stored parameter values via a lead  15  implanted proximate to a nerve, such as a prostate parasympathetic nerve. Processor  56  determines any parameter adjustments based on the pressure information obtained form sensor  12 , and loads the adjustments into memory  58  for use in delivery of stimulation. 
   As an example, if the pressure information indicates an inadequate tumescence pressure during a desired erectile event, processor  56  may increase the amplitude, pulse width or pulse rate of the electrical stimulation applied by stimulation pulse generator  60  to increase stimulation intensity, and thereby increase penile tumescence. If tumescence pressure is adequate, processor  56  may implement a cycle of downward adjustments in stimulation intensity until tumescence pressure becomes inadequate, and then incrementally increase the stimulation upward until tumescence pressure is again adequate. In this way, processor  56  converges toward an optimum level of stimulation. Although processor  56  is generally described in this example as adjusting stimulation parameters, it is noted that the adjustments may be generated by external programmer  16 , as mentioned above. Stimulator  14  may deliver stimulation pulses with different parameters for different phases of sexual activity, such as arousal and ejaculation. For a first phase of arousal, stimulator  14  may deliver neurostimulation pulses at a frequency in the range of approximately 50 to 150 Hz, and more preferably approximately 70 to 100 Hz. Each pulse for the first phase may have an amplitude in the range of approximately 1 to 10 volts, and more preferably approximately 2 to 5 volts, and a pulse width in the range of approximately 100 to 400 microseconds, and more preferably approximately 200 to 300 microseconds. The duration of the first phase of neurostimulation may depend on a detected transition to the second phase, which may be indicated by sensed tumescence. 
   For a second phase of ejaculation, stimulator  14  may deliver neurostimulation pulses at a frequency in the range of approximately 1 to 5 Hz, or in the range of approximately 25 to 35 Hz. Each pulse for the second phase may have an amplitude in the range of approximately 1 to 10 volts, and more preferably approximately 2 to 5 volts, and a pulse width in the range of approximately 200 to 700 microseconds, and more preferably approximately 400 to 500 microseconds. 
   The adequacy of tumescence pressure is determined by reference to the pressure information obtained from sensor  12 . Penile pressure may change due to a variety of factors, such as normal nervous activity or arousal. Hence, for a given set of stimulation parameters, the efficacy of stimulation may vary in terms of tumescence pressure, due to changes in the physiological condition of the patient. For this reason, the continuous or periodic availability of pressure information from implantable sensor  12  is highly desirable in order to maintain an optimal level of stimulation in support of sexual activity. 
   With the pressure information provided by sensor  12 , stimulator  14  is able to respond to changes in penile tumescence with dynamic adjustments in the stimulation parameters delivered to the patient  18 . In particular, processor  56  is able to adjust parameters in order to maintain erection of penis  22  and thereby avoid prematurely ceasing sexual activity. In some cases, the adjustment may be nearly instantaneous. If pressure sensor  12  indicates an abrupt change in tumescence pressure, stimulator  14  can quickly respond by more vigorously stimulating one or more selected nerve sites to increase penile tumescence. 
   In general, if the tumescence of penis  22  is not reaching the target pressure, processor  56  may dynamically increase the level of therapy to be delivered. Conversely, if the tumescence of penis  22  is consistently achieving target pressure, processor  56  may incrementally reduce stimulation, e.g., to conserve power resources. 
   As in the case of sensor  12 , wireless telemetry in stimulator  14  may be accomplished by radio frequency (RF) communication or proximal inductive interaction of pressure stimulator  14  with implantable pressure sensor  12  or external programmer  16 . Accordingly, telemetry interface  62  may be similar to telemetry interface  52 . Also, power source  64  of stimulator  14  may be constructed somewhat similarly to power source  54 . For example, power source  64  may be a rechargeable or non-rechargeable battery, or alternatively take the form of a transcutaneous inductive power interface. 
     FIG. 7  is a schematic diagram illustrating cystoscopic deployment of an implantable pressure sensor  12  via the urethra  20  using a deployment device  66 . Pressure sensor  12  may be surgically implanted. However, cystoscopic implantation via urethra is generally more desirable in terms of patient trauma, recovery time, and infection risk. In the example of  FIG. 7 , deployment device  66  includes a distal head  68 , a delivery sheath  69  and a control handle  70 . Deployment device  66  may be manufactured from disposable materials for single use applications or more durable materials for multiple applications capable of withstanding sterilization between patients. 
   As shown in  FIG. 7 , distal head  68  includes a cavity  72  that retains sensor housing  26  of implantable pressure sensor  12  for delivery to a desired attachment site within bladder  24 . Sensor housing  26  may be held within cavity  72  by a friction fit, vacuum pressure, or a mechanical attachment. In each case, once distal head  68  reaches the attachment site, sensor housing  26  may be detached. Sheath  69  is attached to distal head  68  and is steerable to navigate urethra  20  and guide the distal head into position. In some embodiments, sheath  69  and distal head  68  may include cystoscopic viewing components to permit visualization of the attachment site. In other cases, external visualization techniques such as ultrasound may be used. Sheath  68  may include one or more steering mechanisms, such as wires, shape memory components, or the like, to permit the distal region adjacent distal head  68  to turn abruptly for access to the mucosal lining of bladder  24 . 
   A control handle  70  is attached to sheath  69  to aid the physician in manually maneuvering deployment device  66  throughout urethra  20  and bladder  24 . Control handle  70  may have a one or more controls that enable the physician to contort sheath  69  and allow for deployment device  66  to attach pressure sensor housing  26  to the mucosal lining of bladder  24  and then release the sensor housing to complete implantation. A vacuum source  74  supplies negative pressure to a vacuum line within sheath  69  to draw tissue into the vacuum cavity defined by sensor housing  66 . A positive pressure source  76  supplies positive pressure to a drive a fastening pin into the tissue captured in the vacuum cavity. 
   Deployment device  66  enters patient urethra  20  to deliver pressure sensor  12  and implant it within bladder  24 . First, the physician must guide distal head  68  through the opening of urethra  20  in patient  18 . Second, distal head  68  continues to glide up urethra  20  and into bladder  24 , for access to an appropriate site to attach pressure sensor  12 . Using actuators built into control handle  70 , sheath  69  is bent to angle distal head  68  into position. Again, sheath  69  may be steered using control wires, shape memory alloys or the like. As pressure sensor  12  is guided into place against the mucosal wall  44  of bladder  24 , a physician actuates control handle  70  to attach sensor  12  to mucosal wall  44  and then release the attached sensor. Upon attachment, pressure sensor  12  is implanted within bladder  24  of patient  18  and deployment device  66  is free to exit the bladder. Exemplary methods for attachment and release of sensor  12 , including the use of both vacuum pressure and positive pressure, will be described in greater detail below. Although  FIG. 7  depicts cystoscopic deployment of pressure sensor  12 , surgical or laparoscopic implantation techniques alternatively may be used. 
     FIG. 8  is a schematic diagram illustrating retraction of deployment device  66  upon fixation of pressure sensor  12  within the urinary tract of patient  18 . Once the sensor  12  is released, flexible tube  28  remains attached to sensor housing  26 . During removal of deployment device  66 , tube  28  maintains its position through the neck of bladder  24 . As deployment device  66  is removed, tube  28  passes through a guide channel formed in the deployment device. The guide channel ensures that flexible tube  28  remains pinned between distal head  68  and the wall of bladder  24 . As distal head  68  slides through urethra  20 , however, flexible tube  28  releases from deployment device  66  and is left in place within the urethra in the region of penis  22 . Deployment device  66  may then be completely withdrawn past the remainder of urethra  20 . In the example of  FIG. 8 , flexible tube  28  is suspended by device housing  26 , which is attached to mucosal wall  44 , and is held in place by pressure exerted against the urethral wall by urinary sphincter  22 . In other embodiments, tube  28  may be kept in place using other techniques such as actively fixing tube  28  to the side of urethra  20 , e.g., with sutures or other anchor mechanisms. 
   In a preferred embodiment, sheath  69  and distal head  68  may be disposable. Disposable devices that come into contact with patient  18  tissues and fluids greatly decrease the possibility of infection in implantable devices. Control handle  70  does not come into contact with body fluids of patient  18  and may be used for multiple patients. In another embodiment, the entire deployment device  66  may be manufactured out of robust materials intended for multiple uses. The device would then need to be sterilizable between uses. In still a further embodiment, the features of distal head  68  may be incorporated into pressure sensor  12 . In this configuration, pressure sensor  12  may be larger in size but would include the necessary elements for attachment within the device. After attachment, the entire sensor would detach from sheath  69 , making removal of deployment device  66  easier on patient  18 . 
   After the useful life of implantable pressure sensor  12  is complete or it is no longer needed within patient  18 , it can be removed from patient  18  in some manner. As an example, deployment device  66  may be reinserted into patient  18 , navigated into bladder  24 , and reattached to pressure sensor  12 . Deployment device  66  may then be withdrawn from the bladder  24  and urethra  20 , explanting sensor  12 , including housing  26  and flexible tube  28 , from patient  18 . In another embodiment, as mentioned with respect to  FIG. 3 , the attachment method of pressure sensor  12  to bladder  24  may involve degradable materials, such as a biodegradable fixation pin. After a certain period of time exposed to urine in the bladder  24 , the fixation material may structurally degrade and allow pressure sensor  12  to be released from the mucosal wall  44  of bladder  24 . In some embodiments, sensor  12  may be sized sufficiently small to follow urine out of the bladder, urethra, and body during an urination event. In other embodiments, sensor housing  26  or tube  28  may carry a suture-like loop that can be hooked by a catheter with a hooking element to withdraw the entire assembly from patient  18  via urethra  20 . In still further embodiments, such a loop may be long enough to extend out of the urethra, so that the loop can be grabbed with an external device or the human hand to pull the sensor  12  out of the patient. 
     FIG. 9  is a cross-sectional side view of distal head  68  of deployment device  66  during deployment and fixation of pressure sensor  12 . In the example of  FIG. 9 , distal head  68  includes a vacuum line  78  and a positive pressure line  80 . Vacuum line  78  is coupled to vacuum source  74  via a tube or lumen extending along the length of sheath  69 . Similarly, positive pressure line  80  is coupled to positive pressure source  76  via a tube or lumen extending along the length of sheath  69 . Vacuum line  78  is in fluid communication with vacuum cavity  39 , and permits the physician to draw a vacuum and thereby capture a portion  42  of mucosal lining  44  within the vacuum cavity. Although vacuum line  78  is shown as being coupled laterally to vacuum cavity  39 , the vacuum line could access the vacuum cavity from another direction, such as the top of the vacuum cavity. Positive pressure line  80  permits the physician to apply a pulse of high pressure fluid, such as a liquid or a gas, to drive fixation pin  46  into sensor housing  26  and through the portion  42  of mucosal lining  44 . Pin  46  thereby fixes sensor housing  26  to mucosal lining  44 . In some embodiments, a membrane mounted over an opening of positive pressure line  80  may be punctured by pin  46 . 
   Flexible tube  28  resides within a channel of sheath  69  prior to detachment or sensor  12  from distal head  68 . Once fixation pin  46  attaches sensor  12  to bladder  24 , vacuum line  78  is no longer needed. However, in some embodiments, vacuum line  78  may be used to detach pressure sensor  12  from distal head  68  of deployment device  66 . By terminating vacuum pressure, or briefly applying positive pressure through vacuum line  78 , for example, head  68  may separate from sensor  12  due to the force of the air pressure. In this manner, vacuum line  78  may aid in detachment of sensor  12  prior to withdrawal of deployment device  66 . 
   As described previously in  FIG. 4 , fixation pin  46  punctures mucosal lining  44  for fixation of sensor  12 . While the force of this fixation may vary with patient  18 , deployment device  66  provides adequate force for delivery of pin  46 . In an exemplary embodiment, positive pressure line  80  is completely sealed and filled with a biocompatible fluid, such as water, saline solution or air. Sealing the end of positive pressure line  80  is a head  82  on fixation pin  46 . Head  82  is generally able to move within positive pressure line  80  much like a piston. Force to push fixation pin  46  through the portion  42  of mucosal lining  44  captured in vacuum cavity  39  is created by application of a pulse of increased fluid pressure within positive pressure line  80 . For example, the physician may control positive pressure source  76  via control handle  70 . This simple delivery method may provide high levels of force, allow multiple curves and bends in sheath  69 , and enable a positive pressure line  80  of many shapes and sizes. In some embodiments, a membrane sealing line  80  may be punctured by pin  46 . 
   In an alternative embodiment, a flexible, but generally incompressible, wire may placed within positive pressure line  80  and used to force fixation pin  46  through the captured portion  42  of mucosal lining  44 . This wire presents compressive force from control handle  70  directly to the head  82  of fixation nail  46 . This method may eliminate any safety risk of pressurized fluids entering patient  18  or, in some embodiments, permit retraction of pin  46  after an unsuccessful fixation attempt. The flexible wire may be attached to pin  46  and pulled back to remove the pin from capture mucosal tissue  42 . The flexible wire may be sheared from fixation nail  46  for detachment purposes as distal head  68  releases sensor  12 . This detachment may be facilitated by a shearing element or low shear stress of the wire. 
   In  FIG. 9 , deployment device  66  illustrates flexible tube  28  on the same end of housing  26  as sheath  69 , while the fixation structures are located in the opposite, or distal end of distal head  68 . In some embodiments, it may be necessary for pressure sensor  12  to be deployed with tube  28  located at the distal end of head  68  and the fixation structures located near sheath  69 . In still other embodiments, the fixation structures and tube  28  may be located on the same end of pressure sensor  12 . 
   In some embodiments, deployment device  66  may include a small endoscopic camera in the distal head  68 . The camera may enable the physician to better guide deployment device  66  through urethra  20  and to a desired attachment location of bladder  24  in less time with more accuracy. Images may be displayed using video fed to a display monitor. 
     FIG. 10  is a cross-sectional bottom view of the deployment device  66  of  FIG. 9  before attachment of pressure sensor  12 . As shown in  FIG. 10 , distal head  68  includes proximal tube channel  84  to accommodate flexible tube  28  during placement of sensor  12  and distal tube channel  86  to accommodate the flexible tube during retraction of deployment device  66 . In addition, sheath  69  includes a sheath channel  88  to accommodate flexible tube  28 . Channels  84 ,  86 ,  88  serve to retain tube  28  during delivery of sensor  12  to an attachment site. 
   Distal head  68  is rounded on both sides at the distal end to permit easier entry of deployment device into areas of patient  18 . Head  68  may also be lubricated before delivery to facilitate ease of navigation. On the proximal end of head  68 , proximal tube channel  84  runs through the head for unimpeded removal of tube  28  during detachment of pressure sensor  12 . This channel may be U-shaped, e.g. closed on 3 sides. In some embodiments, proximal tube channel  84  may be an enclosed hole in which tube  28  resides and glides through upon deployment device  30  removal. 
   Sheath channel  88  is formed within sheath  69  to allow tube  28  to stay in place during delivery of pressure sensor  12 . In this embodiment, tube  28  is only partially retained within channel  88 . In some embodiments, sheath channel  88  may be deeper to allow tube  28  to lie completely within sheath  69 , whereas others may include a completely enclosed channel that tube  28  must glide out of after attachment. 
   Distal channel  86  in distal end of head housing  68  is not used by tube  28  before attachment. The purpose of this open channel is to allow tube  28  to glide through it while head  68  is removed from bladder  24 . As head  68  slides back past pressure sensor  12 , tube  28  will slide through channel  86  and head housing  68  will keep tube  28  between the wall of bladder  24  and head  68  until head  68  has been removed beyond sphincter  22 . Tube  28  may then be ensured correct placing through sphincter  22 . 
   Some embodiments of tube  28  include multiple length and diameter combinations which would lead to modifications in channels  84 ,  86  and  88 . These channels herein may be of different diameters or lengths to properly house tube  28 . One embodiment may include flexible housing channels to accommodate a wide variety of tube  28  dimensions. Further embodiments of deployment device  30  may contain modified channel locations in head housing  68 . These locations may be needed to place tube  28  from different locations, particularly if fixing implantable sensor  12  at different sites within bladder  24  or urethra  20 . 
     FIG. 11  is a flow chart illustrating a technique for delivery of stimulation therapy based on closed loop feedback from an implantable pressure sensor. In the example of  FIG. 11 , implantable stimulator  14  makes use of information obtained from implantable pressure sensor  12  and external programmer  16 . A patient  18  activates stimulator ( 90 ) by entering a command via a user interface associated with external programmer  16 . The command indicates that the patient would like to commence sexual activity. In response to the command, programmer  16  activates stimulator  14  ( 90 ) to deliver stimulation therapy. 
   During the course of stimulation therapy, sensor  12  senses the tumescence level of penis  22  ( 92 ), and transmits information indicative of the tumescence level to stimulator  14 , programmer  16  or both. The tumescence level correlates with a pressure level sensed by sensor  12 , either within urethra  20  or within the body of penis  22 . If stimulator  14  or programmer  16  determines that the tumescence level is below an applicable threshold ( 94 ), indicating an inadequate erectile state, one or more stimulation parameters are adjusted ( 96 ) to provide more vigorous stimulation. The adjustment may be made directly by stimulator  14  or in response to an adjustment command or reprogramming by programmer  16 . 
   Upon delivery of the adjusted stimulation ( 98 ), stimulator  14  or programmer  16  determines whether the patient  18  wants to sustain the erection ( 100 ), or whether sexual activity has terminated. Patient  18  may terminate sexual activity by entry of a command via a user interface associated with programmer  16 . If sustained erection is desired, the process continues with tumescence sensing ( 92 ), threshold comparison ( 94 ), adjustment of stimulation parameters ( 96 ) and delivery of adjusted stimulation ( 98 ). 
   In some embodiments, as mentioned previously, pressure sensor  12  may be used exclusively for monitoring pressure without providing feedback for stimulation therapy. In this case, pressure sensor  12  simply collects data and either stores it locally, or sends it to an external programmer. Pressure may be measured continuously, intermittently or at the request of external programmer  16 . These embodiments may be used for disease diagnosis or condition monitoring and may allow a patient to avoid frequent clinic visits and uncomfortable procedures while acquiring more extensive and more accurate pressure data during sexual activity. 
   Although the invention has been generally described in conjunction with implantable neurostimulation devices, a tube-based tumescence sensor  12  may also be used with other implantable medical devices, implantable drug delivery devices, which may be configured to treat sexual dysfunction. In particular, tumescence levels sensed by a pressure sensor  12  may be used to trigger and control delivery of any of a variety of drugs capable of achieving arousal in a male or female patient. Prostaglandin, Alprostdil, Tadalafil, Sildenafil, Vardenfil are examples of drugs that could be infused, e.g., by intracavernous injection, to elicit an erection in a male patient. Approximate dosages for some of the above drugs are: Alprostdil—10 to 250 micrograms, Sildenafil—10 to 250 micrograms, and Apormorphine—10 to 250 micrograms. The tumescence levels obtained by sensor  12  may be used to trigger drug delivery, control the rate of delivery of the drug, or control the overall amount of drug delivered to the patient, e.g., to achieve and maintain an erection during a first phase of sexual activity. A suitable drug delivery system is described in the aforementioned pending application to Gerber. 
   Various embodiments of the described invention may include processors that are realized by microprocessors, Application-Specific Integrated Circuits (ASIC), Field-Programmable Gate Array (FPGA), or other equivalent integrated or discrete logic circuitry. The processor may also utilize several different types of storage methods to hold computer-readable instructions for the device operation and data storage. These memory or storage media may include a type of hard disk, random access memory (RAM), or flash memory, e.g. Compact Flash or Smart Media. Each storage option may be chosen depending on the embodiment of the invention. While the implantable stimulator and implantable pressure sensor may contain permanent memory, the patient or clinician programmer may contain a more portable removable memory type to enable easy data transfer for offline data analysis. 
   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.