Patent Publication Number: US-6210346-B1

Title: Method for inserting an intracranial catheter and for monitoring intracranial pressure in a mammal

Description:
This application is a division of application Ser. No. 08/130,634 filed Oct. 1, 1993, now abandoned, which is a continuation of Ser. No. 07/622,250 filed on Dec. 4, 1990 (now abandoned), which was a continuation-in-part of Ser. No. 07/419,938 filed on Oct. 11, 1989 (now abandoned). 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a catheter assembly for monitoring intracranial fluid pressure and removing intracranial fluid, and a method of use. 
     2. Description of the Prior Art 
     The skull is a bony housing of fixed volume containing three types of matter—blood, brain and cerebrospinal fluid—each of which occupies a portion of that volume. If the portion of the volume of one of the three increases without a concomitant fall in the portions of the other two, the intracranial pressure increases. Because the brain has a limited capacity to adapt to increases in intracranial volume, once the limit has been met, small increases in volume cause significant increases in intracranial pressure. 
     Maintaining cerebral blood flow to provide adequate oxygen and glucose to the brain is critical. However, an increase in the volume of the brain or of cerebral spinal fluid as the result of trauma, and the like, correspondingly constricts the flow of blood and can stop it completely if the increase in fluid is large enough to herniate the brain at the base of the skull. In such cases, death results. 
     Because it is not practical to monitor cerebral blood flow at the bedside, an algorithm based upon the relationship between mean arterial pressure, intracranial pressure and the cerebral perfusion pressure is used to calculate blood flow in the brain (CPP=MAP−ICP). Thus, a satisfactory means for monitoring intracranial pressure by placement of a pressure transducer into the intraventricular, subarachnoid or intraparentchymal spaces is of utmost importance to the management of head injury cases and has long been sought. 
     Assemblies for monitoring pressure at various locations within the body are known. The earliest pressure monitoring devices utilize a pressure sensitive diaphragm in contact with a column of sterile fluid contained within a catheter inserted into the blood vessel, the brain, or other area containing a fluid pressure of interest. Pressure exerted by the fluid, for instance the cerebrospinal or cephalorachitic liquid, is transmitted through the fluid column within the catheter to an external pressure transducer that transforms the pressure signal into an analog or digital form suitable for readout on a monitoring device, such as that commonly used to monitor blood pressure. However, pressure monitoring devices that utilize a column of fluid are easily contaminated with bacteria or air bubbles. Air bubbles in the line distort the pressure readings and bacterial contamination of the fluid may inadvertently expose the patient to sepsis. 
     The fluid coupled systems used to monitor intracranial pressure access the compartments of the brain by means of a ventricular catheter or bolts placed through the skull. Since the pressures to be monitored are relatively low (0-50 mm Hg), the hydrostatic effects of the fluid column can compromise the readings. Additionally, the fluid column can affect the frequency response of the system. 
     To eliminate the risks and disadvantages inherent in catheters employing a fluid column for transmitting the pressure reading, improved pressure monitoring devices have been developed that couple the pressure sensitive diaphragm located at the distal end of the catheter with an electrical or optical means for generating a pressure signal and transmitting it to the proximal end of the catheter, and thence to the pressure transducer and monitor. The electrical pressure monitoring diaphragms are typically fitted with a miniaturized Wheatstone bridge strain gauge comprising a series of resistors whose resistance is modified in proportion to the distance from the zero position the diaphragm of the pressure sensor is displaced by the applied pressure. Electrical pressure sensors are commonly employed in hospitals for continuous monitoring of blood pressure and the like. For this reason, hospitals employ monitors adapted to receiving an electrical output from the Wheatstone bridge pressure sensor and transforming it into a pressure reading using well known technology. Thus, an intracranial pressure monitoring catheter employing an electrical pressure sensor could be plugged directly into the pressure, monitor found in most hospital rooms without the need for an expensive intervening transducer to modify the signal into a format compatible with the monitor. 
     However, in designing a pressure monitoring assembly for monitoring intracranial pressure, special considerations are required. Inherent in all electrical pressure monitoring sensors is a risk of electrical shock that may render them unsuitable for insertion into the interior of the brain. Introducing electrical currents into the brain risks permanent damage. 
     Optical pressure monitoring transducers avoid this risk. Optical pressure sensors generally employ a light reflective diaphragm placed at the distal tip of an optical fiber. Displacement of the reflective diaphragm by applied pressure changes the intensity and/or other spectral characteristics of the reflected light signal, depending upon the type of reflective sensor used. For instance, U.S. Pat. No. 5,065,010 to Knute, issued Nov. 12, 1991, discloses an optical pressure catheter having a set of optical fibers for transmitting a light beam to and from a transducer which modulates the intensity of the reflected light in accordance with the sensed pressure. A photosensor comprising a bellows compressible by pressure is located at the distal end of the catheter and a photodetector located at the proximal end of the catheter measures the modulated intensity of the returned beam and produces a corresponding measurement signal. However, optical transducers operating upon the principles of intensity modulation suffer from the drawback that any curvature of the fiber optic extraneously reduces the intensity of the reflected light. 
     To overcome this source of error the Camino catheter preferably also contains a second set of optical fibers for transmitting a reference light beam to and from the location of the sensor. The reference light beam is sent to a second photosensor that measures the intensity of the returned reference light beam and produces a correction signal that compensates for variations in transmittance caused by bending of the catheter. 
     One of the disadvantages of the Camino system for monitoring intracranial pressure is that a dedicated stand alone interface module, such as that manufactured by Camino Laboratories, is required to display the pressure and communicate with various commercial patient monitors. Zeroing is also dependent upon the interface module, which “reads” the characteristics of the individual sensor and provides for zeroing by means of a screw type adjustment. Additionally, to reduce the error caused by bending the optical fibers, intracranial pressure catheters that rely upon modulated intensity of the reflected beam must be very rigid in construction and are therefore inserted into the skull via a bolt. The most reliable intensity modulation catheters, since they require four optical fibers, are larger, more invasive, and therefore inherently more dangerous, than is desirable. 
     Optical pressure transducers that modulate the wavelength of the reflected light in accordance with the variable to be measured are also known. For instance, U.S. Pat. Nos. 4,329,058 and 4,678,904, which are hereby incorporated by reference in their entirety, describe an optical transducer having an optically resonant sensor by which the wavelength of the reflected light is modified if the reflective diaphragm is deflected by applied pressure from its zero position. This kind of pressure transducer incorporates a Fabry-Perot interferometer in the reflective sensor. 
     The Fabry-Perot interferometer operates according to well-known principles whereby the gap between two reflective surfaces causes a plurality of reflections and splittings of a single beam of incident light, such that constructive and destructive interference of the components of the incident light beam may occur numerous times. Inasmuch as an inherent phase reversal occurs when light is reflected from a more dense medium to a less dense medium (for example, when it passes through the diaphragm of the optical sensor to the air or other medium in the Fabry-Perot gap), it is possible for the main reflected light beams to cancel in a gap having a width equal to a multiple of half wavelengths of the incident light. Light beams transmitted through the Fabry-Perot gap and the surface of the sensor (those not having half wavelengths in multiples of the gap width) undergo an even number of reflections, so that, in the event of the phase reversal above described, the even number of phase reversals produces no net phase reversal. These light beams transmitted through the gap are in constructive interference with each other and are therefore transmitted through the gap and returned to the photosensor assembly for processing. Under conditions of high reflectivity, even a small variation in the frequency of light caused by passage through the Fabry-Perot interferometer dramatically reduces transmission of the frequency-altered light beams. 
     Based upon these principles (See Handbook of Physics, 2d, published by McGraw-Hill Section 7, Chapter 5, Part 6 for further information), a Fabry-Perot sensor having a displaceable diaphragm can be used to monitor a physical variable such as applied pressure, temperature, gas density or pH value. In operation, the gap width varies as a function of the physical parameter to be measured. Therefore, the width in the gap corresponding to an applied pressure can be used to measure the pressure. As is described in full detail in U.S. Pat. No. 4,908,474, photodetector circuits can readily compare the incident and reflected light beams to determine the width of the Fabry-Perot gap and compute the magnitude of the sensed variable therefrom. For example, the Model 1400 multisensor system manufactured by Metricor (Woodenville, Wash.) can be used with a Fabry-Perot sensor to detect pressure readings generated by as little as a single Angstrom of change in the gap width. 
     The changes in intensity or in wavelength in reflected light beams can be converted to a signal compatible with conventional monitors for display as a digital readout or printout by a suitable pressure transducer. However, known pressure transducers having this capability, such as those useful for monitoring intracranial pressure are expensive and cumbersome. For instance, U.S. Pat. Nos. 4,611,600, 4,703,174, and 4,705,047 disclose various types of transducer circuits suitable for receiving reflected light beams and processing them to yield signals to the monitor that indicate the value of the parameter of interest. However, there is great need for an inexpensive optical pressure transducer, preferably one employing modern techniques of microcircuitry, that operates off the output voltage of the common hospital pressure monitor and produces a modified electrical signal (such as that supplied by the output from a Wheatstone bridge electrical strain gauge) suitable for input to the same bedside monitor. 
     Intracranial pressure monitoring devices are needed to monitor dangerously high intracranial pressures. One means of reducing elevated intracranial pressure is to drain off cerebrospinal fluid. Thus, a need exists, not only for intracranial pressure monitors, but also for devices for draining fluid from the brain, without inflicting unnecessary damage. 
     The pressure monitoring device must be inserted into the interior of the skull through a drillhole. If a second hole is drilled to insert a drain, the risk of trauma is obviously doubled. Thus, the need exists for a single device capable of simultaneously monitoring intracranial pressure and draining cerebrospinal fluid. 
     Simultaneous optical pressure monitoring and drainage assemblies are known. For instance, the Camino intensity modulation sensor can be used with an auxiliary drain inserted into the skull through a bolt. However, the assembly is subject to breakage, requires a large drillhole to accommodate the bolt, and is assembled out of standard connectors that are subject to leakage and, therefore, provide sources of infection. However, an optical pressure monitoring and drainage assembly is highly desirable because the accuracy of fluid column intracranial sensors is destroyed by simultaneous drainage of cerebrospinal fluids. What is needed is an integral fiber optic pressure transducer and simultaneous drainage system that interfaces with conventional monitors via a low cost processor. 
     All pressure monitoring systems require calibration. In pressure monitoring systems utilizing two fluid reservoirs separated by a diaphragm, such as the fluid column pressure sensor, one reservoir is usually in pressure communication with the local atmosphere, while the other, the applied pressure, is connected to the pressure source to be measured. If the atmospheric pressure is also placed momentarily on the applied pressure input, then the diaphragm moves to the zero point location, and the zero pressure offset error can be measured. However, an implanted catheter, such as an intracranial catheter, cannot be removed for calibration and replaced without introducing the risk of infection. Thus, calibration must be accomplished in situ within the brain. 
     One means of calibrating a pressure sensor is to provide a substitute pressure transducer system for generating a known test pressure that is displayed on the monitoring device as a calibrated output indicating the level of the known pressure. Another method is that employed by the Model CT/6FB catheter tip pressure transducer manufactured by Medical Measurements Incorporated. In this system the sensor, which includes a mechanical stop to indicate the zero pressure location of the transducer, is depressurized in vivo and calibrated using a micromanometer to provide known test pressures. Thus the zero pressure error and calibration error can be determined. For pressure sensors used in monitoring intracranial pressure, it is particularly desirable to calibrate the sensor in situ so that the sensor can be left in place for up to five days, thereby minimizing the risks of infection and the like. Therefore, the need exists for new and improved pressure monitoring systems such as a dual lumen catheter for monitoring fluid pressure via a first lumen while simultaneously withdrawing an amount of the fluid being measured or infusing a second fluid through a second lumen. And for monitoring intracranial pressure, what is particularly needed is an integrated intracranial pressure monitor and drain assembly, preferably one employing a Fabry-Perot sensor and providing an output signal to standard hospital monitors that “looks like” the signal generated by Wheatstone bridge strain gauge sensors. 
     SUMMARY 
     In accordance with one aspect of the present invention, a method of measuring cerebrospinal pressure using a catheter is disclosed. The method includes a technique for inserting and maintaining an intracranial pressure monitoring drainage catheter within a skull of a patient to reduce the potential for infection at the site of a bore hole through the skull. The catheter includes a flexible catheter body having a stylet lumen then extends longitudinally through at least a distal portion thereof and terminates in a close distal end. A stylet insertion aperture is formed in the side wall of the catheter body a spaced distance from the distal end. A pressure sensing component is provided on the distal end of the catheter body, and communicates with a pressure monitoring connector formed on the proximal end. The method includes forming a first scalp incision and a bore hole through the first scalp incision, and forming a second scalp incision at a spaced distance from the first scalp incision. The distal end of catheter is advanced from the second scalp incision through first scalp incision and far enough to expose the stylet insertion aperture. The stylet is then inserted into the stylet lumen through the stylet insertion aperture to rigidify the distal end of the catheter. With the help of the stylet, the distal end of the catheter is passed through the bore hole such that the pressure sensing component is located intracranially. The stylet is then removed and the first scalp incision closed over the catheter body to reduce the potential for infection at that site. The catheter further includes a fluid drainage inlet aperture at the distal end of the catheter, a drainage outlet aperture formed at the proximal end of the catheter body, and a drainage lumen extending between the drainage inlet aperture and the drainage outlet aperture. The method further includes draining cerebrospinal fluid from within the patient&#39;s skull via the drainage inlet aperture, drainage lumen, and drainage outlet aperture. 
     The catheter of this invention is used in a pressure monitoring assembly that comprises, in basic form, a catheter housing having at least one lumen containing a pressure sensor means for generating a pressure signal, which sensor is in communication with a transducer means for converting the output pressure signal from the sensor into electrical signals. Optionally, the sensor has a vent to provide fluid communication with a reference pressure via the first lumen. The catheter usually contains a second lumen that serves as a fluid conduit for infusing or draining fluids through the catheter while monitoring pressure. In addition, the second lumen is adapted to receive a stylet, which is inserted therein as a stiffening agent while the catheter is placed into the brain or other location of operation. 
     In a preferred embodiment of the present invention, the catheter is a multiple lumen catheter having at least three lumens, the pressure sensing lumen, the drainage lumen and a lumen for insertion of a stylet. Preferably, the stylet lumen has a side port located between the distal end and the proximal end of the catheter to receive the stylet. The stylet lumen is closed at the distal end. Thus, a stylet can be inserted into the side port and fed through the lumen toward the distal tip of the catheter and used as a stiffening agent while the catheter is being advanced into the patient&#39;s brain. 
     In another preferred embodiment of the present invention, the catheter has six lumens, a pressure sensing lumen, three drainage lumens, a stylet lumen with the side port for insertion of the stylet and a lumen for a tensile member or stiffener. 
     Preferably, the above-described catheter is an integral intracranial pressure monitoring and drainage catheter used in a pressure monitoring assembly that comprises, in basic form, a light source, one or more light transmission means, one or more spectral modulation sensors having optically resonant structures, a photodetector means for converting the output light from the spectral modulation sensor(s) into electrical signals of the type generated by a Wheatstone bridge piezoresistive strain gauge, and a monitor means for receiving the signals and displaying the pressure measurement(s). The catheter allows for simultaneous intracranial pressure monitoring and drainage of cerebrospinal fluids. 
     It is a feature of the preferred embodiment that the monitor provides the excitation voltage to the photodetector and that the photodetector scales the excitation voltage to create the measurement signal(s), which are returned to the monitor to be displayed as the pressure measurements. 
    
    
     BRIEF DESCRIPTION OF THE FIGURES 
     FIG. 1 is a partial sectional illustration of one preferred embodiment of the catheter and stylet of the present invention. 
     FIG. 2 is a cross-sectional view of another embodiment of the catheter of the present invention. 
     FIG. 3 is a schematic illustration of the catheter assembly employing a spectral modulation sensor. 
     FIG. 4 is an illustration of a spectral modulation sensor. 
     FIG. 5 is a graphic representation of a reflectivity curve for an optically resonant structure. 
     FIG. 6 shows a portion of the curve of FIG. 5 during operation of the spectral modulation sensor. 
     FIG. 7 is a schematic illustration of the catheter assembly employing the spectral modulation sensor with a ratiometric photodetector. 
     FIG. 8 is a schematic illustration of the catheter assembly employing two spectral modulation sensors with a ratiometric photodetector. 
     FIG. 9 is a graphic representation of the spectrally modulated output light from a spectral modulation sensor having multiwavelength input light. 
     FIG. 10 is a cross-sectional view of a spectral modulation sensor having a zero stop. 
     FIG. 11 is an illustration of the sensor of FIG. 10 under applied pressure. 
     FIG. 12 is an illustration with partial cut-away view of the catheter assembly employing an intensity modulation sensor. 
     FIG. 13 is an illustration of a second preferred embodiment of the catheter of the present invention having six lumens, including a stylet lumen. 
     FIG. 14 is an illustration of a stylet for use with the catheter shown in FIG.  13 . 
     FIG. 15 is a cross-sectional view of the catheter shown in FIG. 13 taken along line  15 — 15  of FIG.  13 . 
     FIG. 16 is a schematic illustration of the insertion of a stylet into the side port of the stylet lumen of the catheter of the present invention. 
     FIG. 17 is a schematic illustration of the advancement of the catheter of the present invention into the ventricle of the brain using a stylet inserted into the side port of the stylet lumen. 
     FIG. 18 is a schematic illustration of the removal of the stylet from the side port of the stylet lumen of the catheter of the present invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     In the preferred embodiment, the catheter is a dual lumen catheter adapted for insertion into the ventricular and/or parenchymal spaces of the brain of humans and other mammals for measuring intracranial pressure and for simultaneously draining cerebrospinal fluids. As shown in FIG. 1, the catheter comprises catheter housing  54 , which encloses first lumen  70  and optional second lumen  72 . 
     Catheter housing  54  is an elongate flexible tube closed at the distal end, except for sensor aperture  76 , to facilitate insertion of the catheter into the environment whose pressure is to be monitored. Sensor aperture  76  opens into the distal end of first lumen  70  and provides external fluid access to the pressure sensor. 
     Usually catheter housing  54  is extruded from a biocompatible plastic material, preferably silicone rubber. Catheter housing  54  is elongate, flexible and sized to be passed through a borehole in the skull and received within the brain. 
     First lumen  70  extends continuously from the proximal end to sensor aperture  76  at the distal tip of the catheter housing and has a diameter of between about 0.005 and 0.050 inch for containing a pressure sensor and a means for transmitting to the proximal end of catheter housing  54  a signal containing information regarding the pressure measurement sensed by the sensor. 
     The pressure sensor can be any known pressure sensor device suitable for insertion into a catheter. However, catheters are primarily useful for operations that require the least intrusive means of measuring pressure. Therefore, the pressure sensor is usually selected to minimize the diameter of first lumen  70 . 
     Non-limiting examples of the types of pressure sensors that can be housed within the first lumen fall into four types. The first are fluid column sensors wherein the pressure measurement is transmitted from the distal to the proximal end of the catheter housing via a column of fluid. Usually, a transducer located externally to the catheter housing generates an electrical signal from the measurement signal provided by the fluid column. A second type of sensor employs piezo-resistive strain gauge pressure sensors wherein the change in resistance of small resistors located in the sensor upon a pressure sensitive diaphragm at the distal end of the catheter generates the pressure measurement signal. The electrical pressure measurement signal is transmitted via electrical wires located within the first lumen to a transducer located at the proximal end thereof, usually completely externally to the catheter housing. A third type of pressure sensor employs optical means for generating the pressure measurement signal at the distal end of the catheter housing and for transmitting the signal to the proximal end thereof. Optical pressure sensors usually transmit the pressure signal by means of at least one optical fiber located within the first lumen of the catheter housing. 
     There are two known basic types of optical pressure transducers, those that generate a pressure signal by detecting the degree to which the intensity of a source light beam is modulated when it is reflected from a movable pressure sensitive diaphragm, and those that generate a pressure signal by detecting the constructive and destructive interference in a source light beam when it is reflected from a movable pressure sensitive diaphragm. The latter type of sensor, which employs an optically resonant structure, is preferred herein because such sensors can be made to yield a pressure signal that is linear with pressure over the desired pressure range and because such sensors require a single optical fiber, yet yield measurements of high accuracy. 
     When an optical pressure sensor is housed within the first lumen, as shown in FIG. 1, the sensor transmits the optical measurement signal via an optical fiber means, such as fiber optic cable  98  extending from the first lumen and having located thereon a test port  97  and an optical fiber connector  96  for providing optical communication to a photodetector means (not shown) with which it is in optical communication. 
     As shown in FIG. 1, catheter housing  54  optionally contains a second lumen  72  for transporting fluids between the distal and proximal ends of catheter housing  54 . Second lumen  72  can have any cross-sectional area appropriate to the application for which it is intended. When used as a drain for draining cerebrospinal fluid, for example, lumen  72  has a diameter of between about 0.025 and 0.100 inch and extends continuously from the proximal to the distal end of the catheter housing, opening at the distal end exterior to catheter housing  54  through aperture  68 . At the proximal end optional lumen  72  is fluid tightly attached to drain conduit  66  by any suitable means of attachment  78 , preferably one containing a drainage port such as a luer lock fitting. Drain conduit  66  can be provided by any flexible tubing material, such as common hospital polyvinlychroride tubing. 
     Preferably, catheter housing  54  is about six inches in length, and a plurality of apertures  68  opening into second lumen  72  are spaced along the distal end, preferably along about the extreme half inch of the catheter housing. Most preferably, apertures  68  are bore holes having a diameter of from about 0.005 to 0.050 inch, and the bore holes are arranged in three rows along the length of the catheter housing  54 , most preferably at 0.100 inch intervals for allowing an unobstructed flow of cerebrospinal or other fluid therethrough. 
     Optionally, first lumen  70  is in fluid communication with the sensor contained in first lumen  70  via vent  62  (shown in FIGS. 10 and 11) and is used to vent the sensor to the atmosphere or to apply a reference pressure, such as atmospheric pressure, to the sensor for in situ calibration, as is explained in full hereinafter. 
     In addition, second lumen  72  is optionally adapted to receive therein stylet  74 , which is a slim, stiff rod made of any non-brittle material, such as plastic or metal, and has a handle means, such as a loop  99 , at one end for inserting and removing the stylet. When inserted into second lumen  72 , stylet  74  provides stiffness to the catheter housing while it is being inserted into the brain and can be removed thereafter so that second lumen  72  can be used as a fluid conduit. 
     In one embodiment shown in FIG. 2 in cross section, first lumen  70  and second lumen  72  have the general arrangement of a tube within a tube. The first lumen is adapted for housing one or more fiber optics and is comprised of a tube  75  having a relatively smaller diameter, for example 0.016 inch, than the diameter of the housing  54 , which is, for example 0.170 inch. As is also shown in FIG. 2, first lumen  70  is fixedly attached along one interior side of the larger second lumen  72  such that the remaining open space within the housing forms a second irregularly shaped conduit  66 , the exterior of the first lumen having a single cross-sectional point of tangency with the interior circumference of second lumen  72 . The catheter housing containing the first and second lumens can optionally be coextruded from silicone rubber. In this embodiment of the invention, apertures  68  are preferably arranged in rows, preferably three rows along the length of second lumen  72 , which itself forms the catheter housing, and are preferably located at angles of 90, 180 and 360 degrees around the cross-sectional circumference of second lumen  72  as measured from said point of tangency. When conduit  66  is used for draining or infusing fluids, it is in fluid communication with the exterior of the catheter housing via at least one of apertures  68 . 
     In another embodiment of the catheter of the present invention, at least three lumens are provided, a first lumen for pressure sensing, a second lumen for transporting or draining fluids, and a third lumen for insertion of a stylet used to stiffen the catheter during placement of the catheter into the patient&#39;s brain. 
     In this embodiment, the third lumen extends continuously from the proximal end of the catheter to the distal end. The third lumen is closed at the distal tip by a plug or other suitable means and a side port is provided in the outer peripheral wall of the catheter body in communication with the stylet lumen. The side port is provided at a location proximal to the distal end of the catheter so that when the catheter is inserted into the patient&#39;s body, the side port is located outside the patient&#39;s body just proximal to the insertion site. In other words, the side port should be provided along the catheter body at a point just proximal to the desired insertion depth of the catheter. In the preferred embodiment of the catheter shown in FIG. 13, the side port is located at approximately 4 inches or 15 cm from the distal tip. A stylet can then be inserted into the side port and advanced distally in the lumen until it contacts the closed end of the lumen at the distal tip. 
     Referring to FIG. 13, there is shown a preferred embodiment of the catheter of the present invention having six lumens. The catheter housing  250  is an elongate flexible tube extruded from a biocompatible plastic material, preferably silicone rubber. Referring to FIG. 15, the catheter housing has a first lumen  254  for sensing pressure, second, third, and fourth lumens  252  for draining or transporting fluids between the distal and proximal ends of the catheter, a fifth lumen  256  for receiving stylet  276  used to place the distal end of the catheter in the patient&#39;s brain, and a sixth lumen  258  containing a tensile member or stiffener. Each lumen extends continuously from the proximal end to the distal end of the catheter housing  250 . 
     All of the lumens within the catheter housing  250  are closed at the distal end except for the pressure sensing lumen  254  which has an opening or sensor aperture  260  at the distal tip of the catheter housing. Each drainage lumen has a plurality of apertures  262  opening into the lumen and spaced along the distal end of the catheter housing, preferably along the extreme half inch of the catheter housing, as described above in connection with FIG.  1 . Accordingly, the catheter housing has three rows of apertures  262  along the length of the catheter, each row opening into a separate drainage lumen  252 . 
     In the preferred embodiment, an optical pressure sensor is housed within the pressure sensing aperture and transmits the optical measurement signal via an optical fiber extending from the sensor through the pressure sensing lumen to the proximal end of the catheter. A connector  264  joins the pressure sensing lumen at the proximal end of the catheter body to optical cable  267 . The optical fiber thus extends from the proximal end of the catheter through the optical cable  267  to optical fiber connector  268 . Optical fiber connector  268  can then be connected to an instrument containing a photodetector means and means for processing the optical signal to display a pressure reading. A calibration port  270  is provided in cable  267  and communicates with the pressure sensing lumen to provide a reference pressure during pressure monitoring and also serves to apply a calibration pressure for in situ calibration of the pressure sensor. 
     Drainage conduit  266  is fluid tightly connected to drainage lumen  262  by connector  264  and a drainage port  272  is provided at the proximal end of conduit  266  for attachment to drainage tubing or a fluid reservoir for receiving the fluids. 
     The stylet lumen or guide lumen  256  is closed at the distal tip by suitable means, such as a silicone plug or the like. A side port or aperture  274  is provided in the catheter housing  254  in communication with the stylet lumen at approximately 4 inches from the distal tip. Thus, stylet  276  shown in FIG. 14 can be inserted into the stylet lumen through side port  274  by holding the handle  278  and advancing the distal end of the stylet  280  toward the distal tip of the catheter housing until the end of the stylet contacts the closed end of the stylet lumen. Stylet  276  is identical to stylet  74  except that it is shorter in length since it is adapted to be used in only the distal portion of the stylet lumen from the side port  274  to the distal tip of the catheter. Whereas stylet  74  is adapted to be inserted through the drainage port  78  shown in FIG.  1  and extends from the proximal end of the drainage lumen to the distal end. 
     Stylet  276  and  74  both serve to stiffen the catheter during insertion of the catheter into the patient&#39;s brain. Referring to FIG. 16, a schematic drawing illustrates the insertion of the stylet  276  into the side port  274  of the stylet lumen  256  in preparation for advancing the catheter through a bore hole in the patient&#39;s skull and through the brain into the ventricle or other area in which the intracranial pressure will be monitored. 
     To insert the catheter, the neurosurgeon may use the following tunnelling technique. The neurosurgeon first makes two incisions in the scalp, a first incision  275  at the desired location for the bore hole and a second incision  277  several inches away from the bore hole location. Bore hole  279  is then drilled into the patient&#39;s skull at the location of the first incision  275 . The flap of skin between the two incisions serves as a tunnel between the scalp and the skull through which the distal end of the catheter is fed before advancing it into the bore hole. The tunnel is designed to minimize or prevent bacterial infection at the sterile site of the bore hole. The contaminants and bacteria that may contact the proximal end of the catheter near the drainage connector or optical connector could reach the bore hole by migrating along the catheter body. The tunnel provides a conduit where the body&#39;s immune system can attack any bacteria or other contaminants that may migrate along the catheter body and, therefore, prevent them from entering the bore hole and infecting the brain. 
     After the distal end of the catheter is fed through the tunnel, the physician pulls it up and away from bore hole  279  as shown in FIG.  16  and then inserts stylet  276  into side port  274 . The physician then inserts the distal end of the catheter into bore hole  279  and advances the catheter into the patient&#39;s brain with the aid of the stylet as shown in FIG.  17 . 
     After the catheter has been advanced to the desired location, stylet  276  is removed as shown in FIG.  18 . The excess length of the catheter can then be pulled back through the tunnel. 
     A suture loop  282  is attached to the catheter  250  near the distal end and can be used to suture the catheter in place once the catheter has been advanced into the patient to the desired location. 
     The preferred pressure sensor housed within first lumen  70  of the catheter shown in FIG. 2 or in lumen  254  of the catheter shown in FIG. 13 is a spectral modulation sensor located at the distal end thereof. The light transmission means comprises an input optical fiber, an optical beam splitter, an optical fiber connector, a sensor optical fiber, and an output optical fiber. The optical beam splitter and optical fiber connector optically connect the sensor optical fiber with the input and output optical fibers. 
     A single optical fiber namely the sensor optical fiber, acts to both convey input light to the spectral modulation sensor and to convey output light reflected therefrom. This simplifies the optical measuring device, reduces its size and cost, and increases its reliability as compared to a system employing two or more optical fibers for conveying light to and from the sensor. Moreover, the use of a single optical fiber reduces the external diameter of the catheter to a minimum and leaves open space surrounding the optical fiber within the first lumen proximal to the sensor, which open space can be used to vent the sensor to a reference pressure via a vent in the sensor substrate, the first lumen and the vent serving as a conduit to the cavity between the reflective surfaces of the sensor. 
     The spectral modulation sensor has as its active element an optically resonant structure, comprising a pair of separated reflective surfaces, with the reflectivity and transmission of the resonant structure being a function of the distance between its reflective surfaces. In a spectral modulation sensor, the optical characteristics of its reflecting surfaces, and the index of refraction of the medium between its reflective surfaces will also affect the reflectivity and transmission of the sensor; therefore, in this invention the said medium is held constant so as to have no effect upon the measurement signal. 
     Thus, if the distance between the reflective surfaces of the spectral modulation sensor&#39;s optically resonant structure is altered by the pressure being measured, then the light reflected and/or transmitted by the spectral modulation sensor will change as a function of the measured pressure. Accordingly, the output light from the spectral modulation sensor will be spectrally modulated by its optically resonant structure as a function of the pressure measured and carries information regarding the physical parameter being measured. The spectrally modulated output light from the spectral modulation sensor is converted into an output electrical signal by the photodetector means which comprises photodetector and amplifier means. The output electrical signal provides an accurate determination of the physical parameter being measured, within a certain range of values for the pressure being measured once the optical measuring device has been calibrated. 
     However, the basic form of the invention may be susceptible to measurement inaccuracies due to two causes. The first cause is changes in the light source intensity or in light transmission intensity due to bending of the optical fibers or due to optical connector light loss. These inaccuracies can be eliminated if the light source is selected to emit light over at least two wavelengths such as is provided by a light emitting diode (LED), for example. A second cause of inaccuracy is spectral modulations in light from light emitting diodes caused by a change in temperature of the diode. 
     The inaccuracies caused by transmission loss and variations in the light source can be corrected automatically by the detection means if the spectrally modulated output light wavelength(s) from the spectral modulation sensor are separated into two spectral components, each of which is separately converted into an electrical signal by photodetector means and then amplified. The two amplified signals are sent to a divider circuit that takes the ratio of the two electrical signals to provide an output signal that is accurate within a certain range of values for the physical parameter being measured, once the optical measuring device has been calibrated. 
     Because changes in the light source intensity or in light transmission intensity due to bending of optical fibers normally effect the two spectral components of the spectral modulation sensor equally, when the two electrical signals corresponding to the two spectral components are divided in the divider circuit, such changes cancel each other out and have no effect on the output signal from the divider circuit. 
     To self-correct for temperature caused inaccuracies in the apparatus, the optical fiber beam splitter contains a filter that separates the spectrally modulated output light wavelengths from the spectral modulation sensor into two spectral components. This filter is selected to incorporate a temperature effect upon the spectrum of light emitted from it that matches or “tracks” the spectral temperature effect of the light emitting diode with which it is in optical communication. By this matching of the spectral shift in the LED with that in the filter, the detection means provides a self-correcting device for removing any spectral modulation in the output light caused by a change in temperature of the diode during operation of the apparatus. 
     Thus, the invention features a self-correcting detection means requiring no adjustment by the operator during operation. 
     Turning now to FIG. 3, the first embodiment of the integral pressure sensor and fluid transport catheter assembly is schematically illustrated. Light source  10  is for providing suitable measuring input light, which may be monochromatic or which may be dispersed over two or more adjacent or nonadjacent wavelengths bands. Light source  10  can comprise a single source of monochromatic light, such as a laser or a lasing diode, a source of two or more wavelengths such as a light emitting diode, and/or suitable optical filters to provide the desired input light. Power for light source  10  is provided by any suitable source of electrical power through conventional electrical connection means  11 . 
     Input light from light source  10  is coupled into sensor optical fiber  14  via input optical fiber  12 , optical beam splitter  18  and optical fiber connector  20 . 
     Optically connected to the end of optical sensor fiber  14 , and receiving input light therefrom, is a spectral modulation sensor  22  having an optional substrate  9  supporting an optically resonant structure  21 . If used, substrate  9  preferably has a thickness about equal to the diameter of sensor optical fiber  14 . Improved sensitivity of optically resonant structure  21  results if substrate  9  is used because light entering optically resonant structure  21  from sensor optical fiber  14  is relatively more collimated than if substrate  9  were omitted and sensor  22  were secured directly to the end of sensor optical fiber  14 . If substrate  9  is omitted, optically resonant structure  21  is secured directly to the end of sensor optical fiber  14 . Substrate  9  serves as an aid in the manufacture of sensor  22  and in the assembly of optically resonant structure  21  to the end of sensor optical fiber  14 , since as well known, optically resonant structure  21  can be less than 1 micron thick. Preferably, substrate  9  does not play a part in spectrally modulating the measuring input light to optically resonant structure  21 , but it could do so. 
     Optically resonant structure  21  comprises, in general, a pair of separated reflective surfaces  13  and  15  with the reflectivity and transmission of the optically resonant structure being a function of its optically physical sensitive characteristics, such as the distance between reflective surfaces  13  and  15 , the optical characteristics of reflective surfaces  13  and  15 , and the index of refraction of any medium located within cavity  17  between reflective surfaces  13  and  15 . In this invention, the output light from sensor  22  is spectrally modulated by optically resonant structure  21  as a function of the distance between reflective surfaces  13  and  15 . For measuring fluid pressure, it is further preferred that the medium in cavity  17  be either a vacuum or air, both of which have an index of refraction of one and therefore have no effect upon the reflectivity and transmission of the optically resonant structure. 
     Spectrally modulated output light from sensor  22  travels sequentially through sensor optical fiber  14 , optical fiber connector  20 , optical beam splitter  18  and output optical fiber  16  where it is optically coupled to photodetector  30  in photodetector assembly  50 . Photodetector assembly  50  comprises photodetector  30 , and amplifier  34 , which receives an output signal from photodetector  30  via electrical connection means  32 . The amplified output signal from amplifier  34  is delivered to output terminal  36 , and provides a measurement of the sensed pressure. 
     Power is provided from any suitable source to photodetector  30  and amplifier  34  through electrical connection means  31  and  33 , respectively. 
     Spectral modulation sensor  22  is shown in FIG. 4 in greater detail. Sensor  22  preferably comprises a cylindrical substrate  24  defining a cylindrical cavity  26  and has a cover  28  which covers cavity  26 . Cavity  26  has a reflective bottom surface  27  while cover  28  has a reflective inner surface  29  and an outer surface  25 , surfaces  27  and  29  being parallel. It is preferred, but not required, that the thickness of substrate  24  between bottom  27  and the end of sensor optical fiber  14  be as was discussed regarding substrate  9  of sensor  22  with regard to FIG.  3 . 
     If an absolute pressure sensor is desired, then cavity  26  is preferably evacuated, cover  28  providing it with a fluidtight seal. Alternatively, if a differential pressure sensor  22  is desired, cavity  26  is in fluid communication via optional conduit  41  and the first lumen (not shown) with a source of pressure, such as atmospheric pressure, while a source of applied pressure bears against outside surface  25  of cover  28 . Naturally, optional conduit  41  is omitted if sensor  22  is to be an absolute pressure sensor. 
     Reflective surfaces  27  and  29  are preferably coated with about 100 to 200 Angstroms of high refractive index transparent media, for reasons explained hereinafter. 
     Cavity  26 , its reflective bottom  27 , and reflective innersurface  29  of cover  28  form optically resonant structure  21 . As alternative constructions for sensor  22 , it is of course possible to form cavity  26  in cover  28  rather than in substrate  24 ; or even to form cavity  26  partially in substrate  24  and partially in cover  28 , without departing from the scope of the invention. 
     The method of making sensor  22  and its optically resonant structure is described in detail in U.S. Pat. No. 4,678,904, which is incorporated herein by reference in its entirety. It should be noted here, however, that when cavity  26  is evacuated, a getter structure (not shown) in the form of a ring of deposited layers of chrome and iron is optionally deposited in the bottom of each cavity  26  to absorb residual gases or subsequent outgassing in cavity  26  after the initial evacuation thereof is performed. The optional getter structure comprises a donut shaped ring of the deposited chrome and iron layers with an inner diameter of about 140 microns and an outer diameter of about 190 microns centered in bottom  27  of cavity  26 . Thus, getter rings leave a clear central portion of about 140 microns in diameter in the bottom of each cavity  26  through which light can pass. After cover  28  is bonded to cavity  26 , the getter ring removes gases from cavity  26 . 
     When in operation as an absolute pressure sensor, sensor  22  is subjected to an external pressure such that cover  28  over evacuated cavity  26  is bowed inwardly toward the reflective bottom  27  of cavity  26  to a greater or lesser degree depending upon the amount of external pressure. As the external pressure on cover  28  increases, such bowing increases; and as the external pressure decreases, the bowing decreases and becomes zero when the external pressure is zero. 
     Thus, as the external pressure on cover  28  increases and decreases the distance between reflective surfaces  27  and  29  of optically resonant structure  21  alters. Changes in the said distance of as little as one Angstrom in length can be detected by the catheter assembly of this invention. 
     When sensor  22  is operated as a differential pressure sensor, one source of pressure is fluidly communicated to cavity  26 , such as through conduit  41  and the first lumen, while surface  25  of cover  28  is exposed to a second source of pressure. Cover  28  will bow inwardly towards reflective bottom  27  of cavity  26  when pressure on surface  25  of cover  28  exceeds that within cavity  26 , the amount of bowing depending upon the pressure differential, and will not bow at all when the pressure differential is zero. However, when the pressure within cavity  26  is greater than that on surface  25  of cover  26 , cover  28  will bow outwardly, the amount of bowing again depending on the pressure differential. 
     Whether sensor  22 , is used as an absolute or differential pressure sensor, bowing of cover  28  in response to applied pressure alters the distance between reflective surfaces  27  and  29 , which in turn produces corresponding microshifts in the reflectivity curve and operating segment(s) of optically resonant structure  21  as a function of the pressure to which sensor  22  is subjected, as will be described in greater detail hereinafter. As a result, output light from sensor  22  is spectrally modulated as a function of the pressure to which sensor  22  is subjected and carries accurate information regarding such pressure. 
     To make and use sensor  22 , a brief summary of its theory of reflectivity is required. The reflectivity R of parallel plane optically resonant structure  21  is well known to be a periodic function of the wavelength of its input light—a common characteristic of optically resonant structures. FIG. 5 shows a typical reflectivity curve  40  of parallel plane optically resonant structure  21  having a given set of physical characteristics. It should be particularly noted that reflectivity curve  40  also represents the reflectivity curve of optically resonant structure  21  when the reflective surfaces are not parallel (i.e., when the moveable diaphragm is not in the zero position). Reflectivity R of parallel plane optically resonant structure  21  is known to be given by the equation: 
     
       
           R =1 −s   2 /((1− r ) 2 +4 r   2  sin 2  (theta))  
       
     
     Where s=(s 1 ,s 2 ) 0.5  and r=(r 1 ,r 2 ) 0.5    
     The quantities s 1 ,s 2  are, respectively, the transmittances of reflective surfaces  13  and  15  while r 1 ,r 2  are, respectively, the reflectances of reflected surfaces  13  and  15  (or  27  and  29 ) as seen from within parallel plane optically resonant structure  21 . The angle theta in the sine term of the above equation is known to be given by: 
     theta=2(pi)nt cos (phi)/lambda+e 
     Where: 
     n=the refractive index of medium  17  between reflective surfaces  13  and  15 ; 
     t=the distance between reflective surfaces  13  and  15 ; 
     phi=the angle of light reflection between reflective surfaces  13  and  15 ; 
     lambda=the wavelength of input light impinging on optically resonant structure  21 ; 
     e=any phase shift caused by reflection from either reflective surface  13  or  15 . 
     Parallel plane optically resonant structure  21  is designed so that parameter group nt cos (phi)/lambda changes in response to applied pressure as distance t changes. Thus, for any selected wavelength of input light, parallel plane optically resonant structure  21  will exhibit varying reflectivity as a function of pressure. 
     FIG. 6, which shows an enlarged portion of reflectivity curve  40 , illustrates the effect on reflectivity curve  40  when distance t between the reflective surfaces of its particular corresponding optically resonant structure  21  is altered. 
     As is also seen in FIG. 6, reflectivity curve  40  of optically resonant structure  21  is shifted to the left, with respect to any particular selected wavelength L 1  of input light from light source  10 , to become reflectivity curve  40 ′ when the distance between its reflective surfaces  13  and  15  is reduced; and it is shifted to the right with respect to L 1  to become reflectivity curve  40 ″ when the distance between its reflective surfaces  13  and  15  is increased. 
     By way of nonlimiting example, a resonance cycle on reflectivity curve  40  of any optically resonant structure  21  is AB, as seen in FIG.  6 . 
     In general the reflectivity curve of any optically resonant structure  21  shifts left and/or right with respect to wavelength L 1  as a function of any change in the distance between its reflective surfaces, and hence as a function of any change in the measured pressure, such shifts being termed herein microshifts. The resonant cycle of the reflectivity curve of any optically resonant structure  21  is defined as one complete cycle on its reflectivity curve. There are, of course, a plurality of such resonance cycles on its reflectivity curve since a resonance cycle may start anywhere on its reflectivity curve. 
     It is known that accurate measurements of pressure can be made using operating segments of the reflectivity curve of the particular optical resonant structure  21  that are less than about one resonant cycle in length and by using operating segment microshifts which are also less than about one resonance cycle in length at the measuring input light wavelength(s). As the reflectivity curve is cyclic, there are a plurality of such operating segments on it. 
     By way of nonlimiting example, if the operating segment of reflectivity curve  40  of FIG. 6 were A″B″ it is of course, one resonance cycle in length. Further, if in response to sensed pressure, operating segment A″B″ were microshifted to the left until point B″ intersected wavelength L 3 , then the microshift of operating segment A″B″ would be one resonance cycle in length. 
     With regard to the embodiment of the invention wherein light source  10  delivers monochromatic input light of a wavelength, such as L 1 , wavelength L 1  does not change in response to sensed pressure. However, from an inspection of FIG. 6 it is seen that for any given intensity of any input light of wavelength L 1  to sensor  22 , the output intensity at wavelength L 1  will be different when distance t is altered in response to sensed pressure, as compared to distance t when sensed pressure is zero. Thus, operating segment AC of optically resonant structure  21  undergoes a microshift to the left and/or right of operating segment AC (to become A′C′ and A″C″) in response to the pressure being measured. 
     Thus, sensor  22  is a spectral modulation sensor whose optically resonant structure  21  modulates input light of wavelength L 1  as a function of the pressure being measured and produces modulated output light of light wavelength L 1  that carries information regarding the pressure being measured. This information is converted by photodetector  30  and amplifier  34 , as has been described, into an electrical output measuring signal carrying the same information. 
     Naturally, the wavelength(s) of input light from light source  10  and the distance between reflective surfaces  13  and  15  of optically resonant structure  21  are selected such that the wavelength(s) of input measuring light fall at least substantially within the desired operating segment of its reflectivity curve, such as AC for example, over the desired operating range of values for pressure being measured. Thus, as seen in FIG. 6, wavelength L 1  remained within operating segment AC, despite operating segment AC being microshifted by optically resonant structure  21  in response to sensed pressure to become A′C′ and/or A″C″. 
     Preferably, the operating segment, the wavelength(s) and/or amplitude(s) of the input measuring light are selected to obtain an operating segment of greatest length which will yield unambiguous spectrally modulated output light over the range of values of interest for the pressure being measured. 
     For example, for a monochromatic input light of wavelength L 1 , changes in the output light of optically resonant structure  21  are maximized when operating segment AC extends, as illustrated in FIG. 6, between any maxima and adjacent minima (or vice versa) on its reflectivity curve  40 , and when the pressure being measured alters optically resonant structure  21  such that its operating segment AC intersects the input measuring light wavelength L 1 , i.e., is microshifted a full half resonance cycle. Most preferably, only the linear portion of the operating segment is used. So that the spectrally modulated output light bears a unique one to one relationship to the pressure being measured over the range of values of interest. 
     To maximize sensitivity, it is desirable to increase the maximum to minimum reflectivity difference of optically resonant structure  21 , such as between AC in FIG.  6 . This is done by coating at least one of reflectivity surfaces  13  and  15  with an appropriate thickness, say 100 to 200 Angstroms, for example, of a high refractive index transparent media that increases surface reflectivity, such as rutile, titanium dioxide, cubic zirconia or silicon. 
     An alternative and preferred embodiment of the invention is illustrated in FIG.  7 . In this embodiment, light source  48  is chosen to produce at least two light wavelengths and may be, by way of example, a light emitting diode (LED) which emits a band of light wavelength(s) centered at approximately 810 nm, such as part #MFOE1202, made by the Motorola Company. Power for light source  48  is provided by any suitable source of electrical power through electrical connection means  49 . 
     Spectrally modulated output light from output optical fiber  16  is directed onto a beam splitter  51  which divides it into two output light beams. The first output light beam from beam splitter  51  passes through short pass optical filter  52 , which transmit to an appreciable degree only light having a wavelength shorter than a given, preselected wavelength. The short wavelength output light band from filter  52  is converted to a first electrical signal by photodetector  154  and then conveyed to amplifier  156  by electrical connection means  158  to be amplified by amplifier  156 . Said preselected wavelengths may be, by way of example, the wavelength of light from light source  48  of highest intensity, such as the wavelength of 810 nm, for the particular LED mentioned above. 
     The second output light beam from beam splitter  51  passes through long pass optical filter  160 , which transmits to an appreciable degree only light having a wavelength longer than said given, preselected wavelength. The long wavelength output light band from filter  160  is converted to a second electrical signal by photodetector  162  and then conveyed by electrical connection means  164  to amplifier  166  to be amplified by amplifier  166 . Amplifiers  156 , 166  preferably amplify their respective signals to the same degree. 
     The amplified first and second electrical signals from amplifiers  156 , 166  are conveyed to a divider circuit  168  by electrical connection means  170  and  172 . Divider circuit  168  takes a ratio thereof and provides an output measuring signal to electrical connection means  174 . The output measuring signal carries information regarding the physical parameter being measured. 
     Power for photodetectors  154 , 162 , amplifiers  156 , 166  and divider circuit  168  are provided by any suitable source of electrical power through electrical connections means  176 , 178 , 180 , 182 , 184 , respectively. Preferably, however, power to the photodetector assembly is provided by a hospital bedside monitor such that the excitation voltage to the photodetector assembly is scaled by amplifiers  156 , 166  and divider circuit  168  to provide an output signal to the monitor. 
     In the most preferred embodiment of the invention, the pressure monitoring apparatus employs a multiwavelength light source providing at least two wavelengths of light and a second, reference sensor having a second photodetector for producing a reference signal by operations that repeat the operations described above with reference to the photodetector means depicted in FIG.  7 . The photodetector assembly compares the reference signal with the measurement signal to produce an absolute measurement signal that is free from transmission and temperature errors. 
     Most preferably, as shown schematically in FIG. 8, the photodetector assembly means  100  splits light received from a single multiwavelength source  48  by means of beamsplitter  102  and steers split beams  114  and  116 , each containing at least two light wavelengths, to spectral modulation sensors  22 A and  22 B by means of optical fibers  16 A and  16 B (not shown), respectively. 
     Sensor  22 A is located at the distal tip of the catheter assembly (not shown) and sensor  22 B is a reference transducer located elsewhere. Each of the sensors contains an optically resonant structure  21 A and  21 B consisting of two parallel reflecting surfaces  27 A,  29 A and  27 B,  29 B separated by a hollow cavity  26 A and  26 B to form a Fabry-Perot gap. The cavities of sensors  22 A and  22 B are maintained at a vacuum, while a pressure to be measured is applied to the pressure sensitive surface  25 A of sensor  22 A. 
     The reflected light beams from sensors  22 A and  22 B are transmitted via fiber optics  16 A and  16 B and beamsplitters  104 ,  106 ,  108 ,  110  and  112  to separate photodetectors  30 A and  30 B as described above. Each photodetector comprises means  52 A and  52 B and  60 A and  60 B for integrating the intensity of the reflected light beam directed thereto over a first and a second different wavelength bands. Preferably, means  52 A and  60 A are a first pair of short pass and long pass optical filters centered on a preselected wavelength from the first wavelength band of beam  114  from light source  48 , and means  52 B and  60 B are a second pair of optical filters centered on a different preselected wavelength selected from the second wavelength band of beam  116 . 
     The integrated signals are sent via connector means  94 A, 96 A, 94 B, 96 B to means  90 A and  90 B respectively for taking the ratio of the two integrated signals to produce from each reflected light beam a normalized signal output at connectors  92 A and  92 B. The normalized signals are independent of the absolute intensity of the reflected light beam. (For example photodetector  30 A takes the ratio of the integrated signals from short pass and long pass filters  52 A and  60 A.) The resulting signal from each photodetector represents a measure only of the distance between the reflecting surfaces of the sensor from which it emanates. The normalized signals output at terminals  92 A and  92 B are then transmitted via connectors  93 A and  93 B to means  116  for subtracting the signal transmitted via connector  93 A from the signal transmitted via connector  93 B to yield a signal representing the difference between the applied pressure and the reference pressure output at terminal  118 . 
     It is especially preferred that photodetectors  52 A  60 A, 52 B, 60 B, divider means  90 A, 90 B and means  117  for subtracting the signals output from the divider means be such that when the input voltage received at input terminal  120  is the same as the output voltage provided by any of the common types of hospital monitor devices, which can be either direct, alternating, or pulsed, the said photodetector assembly “scales” the input voltage by the above described operations thereon to generate an electrical signal at output terminal  118  that can be returned to the monitor for display as a pressure measurement reading. In other words, the preferred photodetector assembly produces an output signal that “looks” like one produced by a Wheatstone bridge piezoresistive electrical pressure sensor. 
     When the pressure applied to reference sensor  22 B is atmospheric and sensor  22 A is vented to atmospheric pressure via conduit  41  and first lumen  70 , the resultant measurement signal represents the gauge pressure at surface  25 A of sensor  22 A. 
     Any commercial photodetector device capable of performing the above operations can be used as photodetectors  30 A and  30 B, for example, the Model 1400 Multisensor System manufactured by Metricor Corporation (Woodenville, Wash.). 
     This technique of spectroscopic normalization is sensitive only to variations in the source light beam, with spectral microshifts appearing as pressure changes for an individual sensor. By using the same light source for both sensors and computing the difference as described above to find the pressure, any microshifts in the spectrum that might occur in the source beam during the course of the monitoring period will affect both reflected beams equally and thus cancel at the ratio taking step. 
     As has been mentioned, the catheter assembly employing a multiwavelength light source  48  is self-correcting for changes in the intensity of light source  48  and changes in light transmission intensity due to bending of optical fibers and due to light loss in optical connectors. Both short and long wavelength output light band from optical filters  52  and  160  (FIG. 7) and  52 A,  52 B,  60 A, and  60 B (FIG. 8) are effected equally by intensity changes. Accordingly, when their respective amplified first and second electrical signals are divided in divider circuit  168  (FIG.  7 ). or divider means  90 A and  90 B (FIG.  8 ), such changes cancel each other out and have no effect on the output measuring light signal therefrom. Such dividing of two signals is known as ratiometric signal processing. 
     Moreover, to self correct for signal errors resulting from temperature effects upon the LED&#39;s, optical filters  52 ,  160 ,  52 A,  52 B,  60 A and  60 B are selected such that temperature changes produce a microshift in their respective output light band equal to and in the same direction as is produced by temperature in the LED. Similarly, beamsplitters  108  and  112  optionally can contain filters selected to accomplish the same goal with regard to source  48 , when it is an LED. Therefore, the microshift in the operating signal produced by temperature effect in the LED is cancelled by the corresponding microshift produced in the operating segment by selection of the filter. Thus, photodetector assembly means  50  and  100  are self-correcting for any microshift in output signal caused by effects other than those caused by changes in the measured pressure. 
     It is within the scope of the present invention to eliminate the inaccuracies in the output measuring signal discussed above by having photodetector assembly means  100  take the ratio of the electrical signals corresponding to any two different portions of the output light from the sensors, even wherein one portion may be the entire output light from the sensor. 
     Light source  48  can comprise at least two sources of monochromatic light such as lasers or laser diodes, if a minimum of two wavelengths are required. Alternatively, it could comprise one or more sources of a plurality of wavelengths, such as LED&#39;s or white light sources, along with suitable optical filters, as needed, to provide at least two input measuring light wavelengths and/or wavelength bands. 
     However, it is preferred for simplicity that light source  48  be a single LED whose band width is substantially less than the resonant cycle length of the reflectivity curve of optically resonant structure  21 . 
     The theoretical constraints on operation of the ratiometric preferred embodiments of the spectral modulation sensor require different operational parameters to avoid ambiguity in the output signal than are required with monochromatic light. The ratiometric embodiments, it will be remembered, require at least two different input measuring light wavelengths to the photodetector assembly. 
     It is known that when multiwavelength source light is used, the output light signal from spectral modulation sensor  22  contains a minimum of ambiguity when the maximum length and the maximum microshifting of the operating segment on its reflectivity curve both approach but do not exceed about one full resonance cycle. This criteria is different than that for the embodiment of the invention employing monochromatic input measuring light. In that case the maximum length and maximum microshifting of the operating segment of the reflectivity curve should not exceed one-half of a resonance cycle. Of course, the length and microshifting of the operating segment may be selected to be considerably less than the maximum allowable so as to improve linearity in the output light signal of optical resonance structure  21 . 
     The intensity curve of the output light of multiwavelength light source  48  is shown in FIG. 9 by curve  86 , wherein light source  48  is a single LED. Wavelength L 2  is, by way of example, selected to be at or near the most intense output wavelength of light from said light source. Also schematically shown in FIG. 9 is an enlarged portion of reflectivity curve  40  taken from FIG. 5, which illustrates by way of example, operating segment DE on reflectivity curve  40  on optically resonant structure  21  and corresponding reflectivity curve  40 ′ and operating segments D′E′ which result from a shift of reflectivity curve  40  to the left due to the influence of sensed pressure on the distance between the reflective surfaces of optically resonant structure  21 . Reflectivity curve  40  and its operating segment DE could also be shifted to the right in the same manner, but this is not illustrated in FIG. 9 for clarity. 
     The optically resonant structures used in this preferred embodiment of the invention and described in U.S. Pat. No. 4,678,904 are fabricated from a silicon wafer using photolithography and micromachining processes. These sensors are remarkably free from the phenomenon commonly known as “drift” wherein the “zero” reading of the transducer varies over time. When the spectral modulation sensors are employed in an intracranial pressure monitoring assembly the reliability and constancy of the pressure reading is of utmost importance. Dangerously elevated pressures in the brain are usually on the order of only about 50 mm Hg so that small inaccuracies in pressure readings may have dangerous consequences. 
     However, all pressure sensors are subject in some degree to inaccuracies wherein the monitor reads some pressure other than zero in the absence of any applied pressure and to calibration error, wherein the transducer reading is larger or smaller over the range of measured pressure than it should be. But an intracranial catheter cannot be removed to check the accuracy of the reading without subjecting the patient to increased risks of trauma and infection. 
     Therefore, in one embodiment of the invention, the pressure monitoring assembly is designed to allow correction of the offset error and any calibration error without removing the applied pressure and venting the transducer to atmospheric pressure (ie. without removing the catheter from the patient&#39;s brain). The problem is eliminated by providing a means for positioning the pressure sensitive diaphragm of the sensor in the same position as it would occupy under the condition of equal pressures so as to allow for in situ calibration testing. 
     As is shown in FIG. 10, to provide for in situ calibration of the sensor, during fabrication of circular pressure sensitive diaphragm  28 , a ground and polished glass wafer  56 , between about 0.005 and 0.025 inch thick, preferably about 0.011 inch thick, is bonded concentrically thereto. Glass wafer  56  is chemically etched using known methods to form a circular channel  58  concentric to the center point of the wafer, the width of the channel usually being between about 0.002 and 0.010 inch. Etching of the channel leaves an outer concentric raised ring and an inner raised concentrically circular plateau, or “stop”  56 A on the surface of the glass wafer. Glass wafer  56  and substrate  24  are chemically bonded concentrically to surface  25  of circular diaphragm  28  such that the centers of the circular substrate, diaphragm, and glass wafer are coaxially aligned. When so aligned, the surface of “stop”  56 A formed by etching the channel into the glass wafer rests against surface  25  of diaphragm  28 , but is not bonded thereto. The outer radius of channel  58  has a diameter equal to or greater than that of circular cavity  26  etched into substrate  24 . 
     Thus, as is shown in FIG. 10, under zero pressure surface  25  of diaphragm  28  is flush against the surface of the stop  56 A. But, as is shown in FIG. 11, under applied pressure surface  25  of diaphragm  28  is deflected from the stop a distance proportional to or at least functionally related to the amount of applied pressure. 
     Wafer  56  contains aperture  60  therethrough opening into channel  58 , through which aperture an applied pressure impinges upon and deflects diaphragm  28 . Substrate  24  contains aperture  62 , in fluid communication with the first lumen, which houses the sensor, aperture  62  preferably having a cross-sectional area equal to that of aperture  60 , whereby a selected reference pressure, such as atmospheric pressure, impinges upon surface  29  of diaphragm  28  so as to exert a force opposite to that of the applied pressure. When the reference pressure exerted against surface  29  is equal to or greater than that of the applied pressure, as shown in FIG. 10, surface  25  of diaphragm  28  is restored to zero pressure position and rests flush against stop  56 A. 
     An in vivo calibration reading indicating the offset error contained in the reading on the monitor can be obtained by slowly increasing the amount of back pressure applied to surface  29  of diaphragm  28  via vent  62  and the first lumen until the reading on the monitor ceases to fall and attains a constant low value. This constant low value represents the zero error of the system and should be subtracted from all pressure readings in obtaining a true pressure reading. In addition, the calibration accuracy of the transducer system can be obtained without the need to remove it from its location of operation if a manometer or other means of applying a known back pressure is used to measure the amount of pressure applied through aperture  62  against surface  29 . The difference between the known back pressure and the constant pressure reading on the monitor generated when the diaphragm is restored to its zero position can be used to obtain the calibration error of the system. 
     Alternatively, the calibration error can be found by applying a known vacuum to surface  29  of diaphragm  25 , which technique is equivalent to applying a positive pressure to the opposite side of the diaphragm. The displacement caused by the vacuum will result in a pressure being displayed on the monitor that can be evaluated using known methods against the known negative pressure to determine the calibration error. Once known, the operator can use the zero error and the calibration error to obtain the true pressure reading at any time without withdrawing the catheter from its location of operation. 
     The ability to calibrate and zero the instrument in situ offers a great advantage for any application requiring uninterrupted monitoring of the variable of interest, such as the pressure of a fluid stream in a chemical plant, and the like. This ability also has important clinical significance for monitoring intracranial pressure because the intracranial pressure catheter can be left in place for up to about five days according to current clinical practice while periodic checks are made to determine the integrity and accuracy of the system. 
     As shown in FIGS. 10 and 11, aperture  62  and the first lumen can also be used to vent the system to atmospheric pressure to obtain a gauge pressure measurement without contaminating the fluid system because diaphragm  28  serves as a protective barrier between the atmosphere and the fluid system under observation. Atmospheric gauge pressure readings are commonly preferred for monitoring pressure of bodily fluids, such as blood pressure, since the body is at atmospheric pressure. When the fluid system is the bloodstream or the intracranial cavity, and the like, this safety feature is highly desirable. It is also of great importance when monitoring any fluid stream that might be compromised by contact with the constituents of air. 
     A less preferred type of fiber optic pressure sensor adapted for use in the present invention is described in U.S. Pat. No. 5,065,010 and is schematically illustrated in FIG.  12 . The apparatus includes catheter housing  111  comprised of a first lumen  116  having a first set of optical fibers and a second fluid transport lumen  129 . The first set of fibers includes emitter fiber  115  and return fiber  117  for transmitting a first light beam to and from the remote end of the catheter. In this type of pressure transducer, the removable reflective surface is mounted on bellows  119  located at the distal end of the catheter. Reflective surface  121  moves in accordance with pressure and thereby modulates the intensity of the first light beam, not by microshifts in the operating segment of the reflectivity curve of an optically resonant structure, but by scattering of the source light beam caused by movement of the reflective bellows, as is described in detail in European patent application 0127476. This type of optical sensor can be used with monochromatic light or with the multiwavelength light produced by LED  123  and transmitted by the first set of optical fibers to a first photodetector means  125 , for detecting the modulated intensity of the reflected beam caused by movement of bellows  119 . 
     Since bending of the catheter  111  effects the light transmittance of both the emitter fiber  115  and return fiber  117 , the pressure monitoring apparatus optionally includes a reference channel comprised of a second set of optical fibers located within first lumen  116  and extending substantially along its entire length. This reference channel is used to determine the effects of bending on transmittance and to determine the effects of temperature and aging on the efficiency of LED  123  and photodetector  125 . Since the first and second set of optical fibers are substantially coextensive and experience substantially the same degree of bending within catheter  111 , the effect of that bending on their respective transmittances is presumed to be substantially equivalent. 
     A second light beam produced by LED  123  is transmitted by the second set of optical fibers so that variation in transmittance of the fibers and variations in light from the LED can be determined and a corresponding correction made to the intensity signal. The second set of optical fibers includes an emitter fiber  137  for transmitting the second light beam from LED  123  to a location near the distal end of catheter  111  and a return fiber  139  for transmitting the light beam back from that location. A second photodetector means  141  detects the intensity of this reflected light beam, and produces a correction signal on line  143  having an electrical current whose magnitude is proportional to intensity. Preferably, the correction signal is sent to control means  145  for generating a feedback control signal to be sent to light source  123  via power box  145  to compensate for any variation in light intensity caused by transmission losses such as those resulting from bending of the optical fibers and thereby to correct the measurement signal issuing from photodetector  125  at output terminal  146 . 
     To standardize the intensity of the reflectivity of the reference sensor, a translucent droplet  145  of an epoxy containing a white pigment bonds together the remote ends of the second set of fibers and thereby reflects a predetermined fixed proportion of a light beam from the emitter fiber to the return fiber. The epoxy droplet is preferably coated with an opaque silver paint, the opacity preventing movement of adjacent reflective bellows  119  from effecting the reflected beam and the silver color maximizing the intensity of the reflected beam. 
     The above described photodetector assembly can be fashioned by one skilled in the art from commercially available elements. A type of photodetector apparatus suitable for use in this invention is Model OLM manufactured by Camino Laboratories, which interfaces with standard patient monitors. 
     This type of intensity modulation sensor requires that the first lumen be large enough to enclose at least two optical fibers, and four optical fibers are required if the reference channel is used. Therefore, the catheter housing is larger than that required for the spectral modulation sensor, which preferably uses a single optical fiber. For this reason, spectral modulation sensors are favored for use in intracranial pressure monitoring catheters. 
     From the foregoing various further applications, modifications, and adaptations of the invention within the scope of the claims appended hereto will be apparent to those skilled in the art to which it pertains. All embodiments, examples and alternatives, and the like, set forth herein are strictly intended as non-limiting examples.