OF A SENSOR

The invention is generally related to sensors and more particularly, is related to calibrating sensing systems while they are in use.

Monitoring intracranial pressure can be critical to the treatment of a head injury. The resultant swelling of intracranial tissues can cause additional injury and even death to the patient unless the excess pressure is relieved. An intracranial pressure sensor can assist the attending physician in management of the head injury case by providing information concerning the pressure affecting the brain. Through continuous pressure monitoring, the physician is better informed and can take action should the pressure reach an unacceptable level or indicate other conditions of the brain.

In many cases, the pressure sensor must be left in place for an extended period; for example, five days. During this time, the sensor system may drift out of calibration and its readings become less accurate. One solution to correct calibration drift is to remove the pressure sensor and re-calibrate the sensor system while the sensor is removed. However, removing the pressure sensor for re-calibration is not desirable due to the increased possibility of infection of the patient and additional trauma. At the same time though, it is desirable to undertake re-calibration of the pressure sensor during these extended periods of use so that any calibration drift of die sensor is detected and eliminated.

In general, two broad classes of pressure sensing catheters are used for intracranial pressure monitoring. The conventional method employs a hollow tube that is introduced through the cranium into a fluid space in the brain and is in fluid communication with a strain gauge external to the patient. This fluid filled system has the advantage of the possibility of re-calibration when in use because the transducer is located outside die patient and can therefore be isolated from patient pressure; for example, through the use of a valve. However, this system has the disadvantages of increased risk of infection to the patient and sensitivity to patient movement. A more recent approach employs a miniaturized transducer placed at d e distal end of a cadieter. This transducer-equipped distal end of die catheter is inserted

through the cranium and into the brain to monitor pressures. While this approach has the advantages of a lessened risk of patient infection and relative insensitivity to patient movement, me ability to perform in vivo re-calibration has not been available in a manner that at the same time provides cost effectiveness and increased accuracy. A transducer tipped catheter that could be re-calibrated in vivo would provide the physician with the added assurance of correct pressure data over long periods of time. Many pressure sensors employ a diaphragm as a primary transducer. This diaphragm moves in response to changes in patient pressure, referenced to atmospheric. Moving the diaphragm against a physical stop, positioned at die diaphragm zero pressure position, by applying calibration pressure to die side of the diaphragm mat normally is exposed to atmospheric pressure has been proposed as a means of in vivo re-calibration. In this arrangement a halt in the change in output of the secondary transducer that is sensitive to diaphragm displacement indicates that the stop has been reached by die diaphragm, and therefore the differential zero pressure position, has been reached. With the diaphragm in this position die display is tiien re-zeroed as necessary. A technique of this type is disclosed in international PCT patent application WO 91/05575.

Manufacturing a pressure sensor assembly such that a pressure sensor diaphragm is always located against a stop at atmospheric (zero differential) pressure presents some manufacturing challenges related to die positioning of components. If the diaphragm does not rest against die stop at atmospheric pressure, forcing it against the stop during the in vivo re-calibration will result in inaccuracy. The small size required for medical applications (for example 0.050 inches (1.27 mm) diameter), me small diaphragm displacements involved (for example 0.001 inches (0.025 mm) or less for 250 mm Hg differential pressure) and die need to position die diaphragm in a sensitive region of die transfer function of die secondary transducer all contribute to the difficulty in locating a stop at die diaphragm zero position. This difficulty results in increased manufacturing complexity and cost. In the case where such pressure sensors are meant to be disposable, an increased manufacturing expense is undesirable. Hence, those concerned witii the use of sensors have recognized die need for an improved, relatively simple, economical, durable, and reliable re-calibrating system

that permits re-calibration of die sensor system while in vivo. Those concerned have also recognized the desirability of providing such a re-calibrating system mat is relatively inexpensive to manufacture yet provides improved accuracy in pressure sensors. The present invention fulfills these needs and others.

SUMMARY OF THE INVENTION Briefly and in general terms, the present invention in one aspect is directed to a calibration system and method that permit accurate in vivo calibration of a sensor system. In a more detailed aspect of die invention, a calibration system is provided for calibrating a sensor system having a first sensor that moves within a range when sensing a physiological parameter. The sensor system also has a processing system that monitors die position of die first sensor and provides an output signal representative of die physiological parameter based on die monitored position of die first sensor. The calibration system comprises a stop device located so as to restrict die first sensor to a known position outside die sensing movement range when die sensor is engaged witii the stop device. Additionally, a controllable force generator is provided tiiat selectively applies force to the first sensor to move it into contact widi the stop device. A force gauge measures me force applied by die force generator and provides a measured force signal representative of die force applied by die force generator to move the first sensor into contact with the stop device. A memory has stored therein a reference force required to move die first sensor from a selected position, at which the first sensor was subjected to a first force, to the stop device. The processing system controls the force generator to apply force to the first sensor to move it from an unknown position resulting from an unknown force acting upon die first sensor to die stop device, receives the measured force signal from the force gauge, retrieves the reference force, and determines die value of the unknown force based on comparing the reference force to die measured force.

In another aspect, the first sensor movement range comprises a maximum position at which the first sensor would be located should the physiological parameter reach a maximum predetermined level and die stop device is located outside die first sensor movement range and beyond die maximum position. In a further aspect, the

force generator selectively applies a reduced pressure against the first sensor to urge it into contact witii the stop device.

In yet a further aspect, the first sensor is located in an enclosed area tiiat surrounds one end of die first sensor and a vent tube is coupled to d e enclosed area. The reduced pressure is applied to the first sensor through the vent tube. In further aspects in accordance with die inventions, die first sensor is mounted in an outer housing, die outer housing having a first opening for transferring the physiological parameter to the first sensor, the outer housing having a second opening. An inner housing is mounted in the second opening of the first housing at a selected position and provides die stop device and an access port through which the force may be applied by die force generator to die first sensor, wherein die inner housing and associated stop device are mounted opposite the first opening and beyond die maximum position. Additionally, in anotiier aspect, the processing system additionally controls the force generator to apply force to move me first sensing element into contact with the stop device.

In yet a further aspect, die calibration system is further for calibrating a sensor system that additionally comprises a second sensor that determines the position of die first sensor in sensing the physiological parameter, the second sensor being responsive to d e distance of d e first sensor from the second sensor. The calibration system further comprises a first housing within which the first sensor is mounted and a second housing having die stop device. The first housing and second housing are movable in relation to each other along die direction of movement of the first sensor to permit the sensing movement range of die first sensor and to locate the stop surface outside tiiat movement range. The second housing is movable in relation to the second sensor so ' tiiat the first sensor may be located at a desired distance from the second sensor.

In a calibration metiiod in accordance witii an aspect of die invention, a sensor system having a first sensor that moves within a range when sensing a physiological parameter may be calibrated. The sensor system also having a processing system that monitors the position of die first sensor and provides an output signal representative of die physiological parameter based on die monitored position of die first sensor. The method of calibrating in one aspect comprising the steps of locating a stop device to

restrict the first sensor to a known position outside die sensing movement range when die sensor is engaged witii the stop device, selectively applying force to the first sensor to move it into contact with the stop device, measuring the force applied by die force generator and providing a measured force signal representative of die force applied by die force generator to move the first sensor into contact with d e stop device, storing in a memory a reference force used to move die first sensor from a selected position at which the first sensor was subjected to a first force to the stop device, controlling the force generator to apply force to the first sensor to move it from an unknown position resulting from an unknown force acting upon die first sensor to the stop device, receiving the measured force signal from the force gauge, retrieving die reference force from the memory, and deterrnining die value of the unknown force based on comparing the reference force to the measured force.

In a mounting system in accordance witii the invention, tiiere is provided a sensor mounting system for mounting a first sensor that moves within a range when sensing a physiological parameter and a second sensor that determines the position of die first sensor; the response of the sensor system being dependent upon the positioning of die first and second sensors with respect to each other. The mounting system comprises a first housing within which the first sensor is mounted and a second housing within which a stop device is located so as to restrict the first sensor to a known position outside its sensing movement range when the sensor is engaged witii the stop device. Wherein the first housing and second housing are movable in relation to each other along die direction of movement of die first sensor to provide for die sensing movement range of the first sensor and to locate the stop surface outside that movement range so that the first sensor is located at d e particular position and further, wherein the second and first housings are movable in relation to die second sensor so that die first sensor may be located at die particular position in respect to die first sensor.

The novel features that are believed to be characteristic of die invention togetiier with further objects and advantages tiiereof will be more readily understood from the following descriptions considered in connection with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS FIGURE 1 is a block diagram of a transducer calibration system in accordance witii the invention;

FIG. 2 is a partially cutaway perspective view of a pressure sensor with a part of a calibration system in accordance with the invention;

FIG. 3 is a perspective view of an instrument that includes various aspects of a transducer calibration system in accordance witii the invention;

FIG. 4 illustrates a cutaway view of the distal end of die catheter and calibration system of FIG. 2 showing the pressure sensing element at a position in the normal range of movement;

HG. 5 illustrates a cutaway view of the distal end of die catiieter and calibration system of FIG. 2 showing the pressure sensing element at the stop surface that comprises the calibration position;

FIG. 6 presents a diagram showing the movement range of the sensor including both normal operation movement and calibration movement;

FIG. 7 presents a graph of the operation of the light transducer showing the light intensity versus distance from die sensor;

FIG. 8 is a flow chart illustrating an in vitro calibration method in accordance with the principles of the invention; and HG. 9 is a flow chart illustrating an in vivo calibration method in accordance with d e principles of the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In the following description, like reference numerals are used to refer to like or corresponding elements in the different figures of the drawings. Referring now to the drawings with more particularity, in FIG. 1 there is shown a block diagram of a transducer calibration system 10 in accordance witii die principles of the present invention. The system 10 is connected to a catiieter 12 having a pressure transducer

14 mounted in its distal tip. In this case, the catiieter 12 is usable for intracranial pressure monitoring apphcations and includes a transducer 14 having a bellows that moves in response to sensed intracranial pressure. Such a transducer configuration is

shown in FIG. 2 where the distal tip opening 16 is oil-filled 17 for transmission of the pressure of intracranial fluids to d e bellows 18. The bellows 18 moves longitudinally in the catheter 12 body in response to intracranial pressure experienced and an optical fiber system 42 monitors the bellows position and provides a position signal as will be described in more detail below.

Referring again to FIG. 1, the catheter 12 provides an output signal representative of the position of die bellows and a preamplifier 20 is placed in die line for amplifying the signal. Such a catiieter with a fiber optic transducer system is available from Camino Laboratories, Inc., San Diego, California and has a model no. of 110-4B. The coupling block 68 at the proximal end of die catiieter may contain light-to-electrical and electrical-to-light transducers and otiier devices, as discussed below.

The output signal from the catheter 12 is provided via die preamplifier 20 to an instrument 21 having an analog-to-digital converter 22 that provides a corresponding digital signal to a central processing unit 24 ("CPU"). The CPU may take many forms; for example, a commercially available integrated circuit microprocessor having the INTEL model designation of 8086. The CPU 24 provides a video signal to a video processor 26 for display on a video monitor 28. Programs and data for d e CPU 24 are stored in a memory unit 30, such as a magnetic or optical storage device, and communications with the CPU 24 may occur with a keypad or keyboard 32 and with a serial port 34. Data may also be output by the CPU 24 to a printer 36.

Referring now to FIG. 2, the catheter 12 includes a vent tube 38 for venting to atmospheric pressure die space 40 between the fiber optic transducer system 42 and die bellows end 44. The bellows end 44 may be formed with a highly reflective surface to reflect light from an emitter optical fiber 46 to a receiver optical fiber 48. Venting the space 40 permits movement of the bellows 18 without opposing forces that may be developed by air pressure if die space 40 were closed.

Returning now to FIG. 1 , the vent tube 38 is connected at die proximal end of die catiieter 12 to a fluid line 50. The fluid line 50 is used to control the pressure in the vent tube 38. A switch 52 is used to switch the fluid line 50 between an atmospheric pressure P _ATM inlet 54 and pressure provided by a controllable pressure

source 56. The controllable pressure source 56 is capable of applying pressures less than atmospheric. The controllable pressure source may take many forms; for example, a Pancake Cylinder made by Fabco-Air Inc. having a model no. of B-121-X. In another embodiment, a syringe may be used. The pressure source driver 58 controls the pressure source 56 to control die pressure applied to die vent tube 38 and may comprise a lead screw type arrangement or other motor driven system such as a Linear Actuator having a model no. of 36443-12 and stepper motor driver from Haydon Switch & Instrument, Inc.

Included in the fluid line 55 between the switch 54 and die pressure source 56 is a fitting 60 that permits a pressure gauge 62 to monitor the pressure in the fluid line 55. When the switch 52 is controlled to apply pressure from the controllable pressure source 56 directly to the fluid line 50, the gauge 62 then also measures the pressure in the fluid line 50. The sensed pressure from the pressure gauge 62 is provided to the A-to-D converter 22 to be converted to a digital signal and provided to die CPU 24. The CPU 24 also controls the switch 52 in the fluid line to switch between the vent 54 and d e controllable pressure source 56 in accordance with an embodiment of the metiiod of the invention, as is described in further detail below. A digital control signal from the CPU 24 is applied to a digital-to-analog ("D-to-A") converter 64 for conversion into an analog signal for control of the switch 52. In both the cases of the switch 52 and die gauge 62, A-to-D and D-to-A converters are shown separately. However, other devices may be used in which the converters are incorporated into the device itself. For example, a pressure gauge may be used that outputs a digital signal rather than an analog signal.

Thus, as shown in FIG. 1 , the CPU 24 monitors the pressure in die fluid line 50 connected to the vent tube 38 of the catheter 12, controls the fluid line switch 52 to apply either atmospheric pressure P _ATM or the pressure generated by the controllable pressure source 56 to the fluid line 50, controls the pressure applied to die fluid line 50 by the pressure source 56 through the driver 58, and monitors die position of die bellows 18 of die catiieter 12 dirough the output electrical line 66 from the proximal end of the catheter that contains the pressure sense data from the optical fiber transducer system 42.

As discussed briefly above, die controllable pressure source 56 may be a syringe or other variable volume reservoir driven by a stepper motor as the driver unit 58. The reservoir 56 contains a fluid, such as air, and movement of die syringe plunger in the syringe barrel will cause movement of that fluid dirough the fluid line 55 connected to die nozzle of the syringe. A commercially available stepper motor and associated controller may be used provided tiiat tiiey are satisfactorily responsive to control signals to cause discrete changes in the pressure applied by die syringe at the resolution desired.

The fitting 60 may be a common "T" fitting and die pressure gauge 62 may be model 24PC-FA1D made by Honeywell MicroSwitch having an address of Freeport, IL. The switch 52 may be a solenoid valve such as die one having a model no. of 401- 37 and available from Humphrey Products Co. having an address of Kalamazoo, MI. The vent device 54 may include a filter to prevent foreign particles from entering the fluid line 50 and vent tube 38 of the catheter 12. The A-to-D and D-to-A converters, CPU, keypad, gauge, switch, fitting, pressure source and driver, vent, memory, serial port, video card and video display may all be located in a single instrument 21 such as that shown in FIG. 1. The instrument 21 includes a housing 82 having a catheter port 84 for coupling to a catheter 12 through die preamplifier 20. The serial port is not shown in the figure and is located at die back, as is die printer port. In anotiier embodiment, the housing 82 may also contain a reduced-size printer, the printed output of which may be provided to die operator through a slot 86 on the front panel 88.

Referring again to FIG. 2, a partially cut-away perspective view of die distal end of die pressure monitoring catheter 12 is shown. In the distal end is mounted a pressure sensor 90 tiiat, in this case, comprises a movable sensing element that is shown as the bellows 18 mounted in an outer housing 92. The pressure of the intracranial or other fluid to be sensed is transmitted to die bellows 18 through the oil 17 located in the inlet port 16. This pressure will cause the bellows 18 to move in accordance witii that pressure. The bellows 18 includes a reflective surface 44 that is efficient in reflecting incident light. Facing the bellows 18 is the main shaft 94 of the catheter 12 that includes die two optical fibers 46 and 48 and d e vent tube 38. Also

included in the pressure sensor 90 is an inner housing 96 having a stop surface 98. The inner housing 96 is attached to die outer housing 92 at the proximal end 100 of the outer housing in one embodiment by epoxy or welding. As is described in more detail below, both die inner 96 and outer 92 housings can be independentiy moved in relation to the main catheter body 94. After they have been located as desired, the outer housing 92 can be welded to die inner housing 96 and die inner housing 96 can be welded to die main catheter body. Thus, two controls over the locations of parts of the transducer systems are provided. As will be described in more detail below, this arrangement greatly facilitates manufacturing the transducer systems and die transducer calibration system and enables increased accuracy in operation of die catheter and the transducer calibration system.

Referring now to FIG. 6, a cross-sectional side view of die catiieter 12 is shown in which die sensing element or bellows 18 is responsive to internal pressure P _int apphed through the vent tube 38 and to external pressure P _ext , such as intracranial pressure, applied dirough the oil 17 disposed in die bellows opening 16. The bellows is mounted such tiiat its sensing range does not include contact with die stop surface. In the embodiment shown, die stop surface is proximal to the bellows.

The reflective surface 44 faces the two optical fibers 46 and 48 to reflect tight provided by d e emitter fiber 46 back to the receiver fiber 48 in a manner well known to those skilled in the art. By processing the reflected light, the position of die reflective surface 44 can be determined and tints, the pressure sensed by d e transducer 90. The vent tube or lumen 38 couples the controlled pressure fluid from me controllable pressure source 56 or air at atmospheric pressure to the inside part of the housing 92 as selected by the switch 52 shown in FIG. 1. Referring now to FIG. 5, the bellows 18 has been moved to a second position which in this case, is the calibration position against the stop surface 98 provided by die inner housing 96. This stop surface 98 is a physical stop that provides a known and repeatable position for the reflective surface 44. This stop surface 98 is also located outside the sensing range of movement of the bellows 18 under normal operational pressures. Therefore, additional force is required to move the bellows 18 to the calibration position where the reflective surface 44 abuts the stop surface 98.

Force in the form of a pressure o-_ αe proximal side P _int of the bellows lower than the pressure on die distal side P _e)rt must be apphed. To return the bellows to a point within its normal range of movement, the pressure P _int must be relieved. Thus, by controlling die pressure apphed to die vent lumen 38, die reflective surface 44 of the bellows 18 can be moved to a known position; i.e., the stop surface 98, for use in conducting a calibration operation.

The use of die stop surface 98 at a known point outside die normal range of movement of die bellows 18 results in a highly accurate point of reference. By previously measuring the amount of pressure reduction required to move me bellows 18 from its position at atmospheric pressure to the calibration position and storing that amount of pressure reduction in a memory, it may be referred to while the sensor is in vivo for use in a calibration operation. Then when in vivo calibration is desired, die amount of pressure reduction required to move the bellows from its present unknown position to the calibration position is determined and this pressure is compared to the stored pressure as follows:

where:

P _M = the change in pressure necessary to move the bellows from the unknown position to the stop position (measured pressure) Pu = the pressure acting on the bellows (patient pressure) tiiat moves it from the equilibrium position to the unknown position (unknown pressure) P _R = the change in pressure necessary to move the bellows from me equilibrium position to the stop position By rearranging equation 1, the calculation needed to derive die unknown pressure is more apparent:

Pu = ^P R ^~ *u < ^• <»

Thus by storing the reference pressure P _R determined in vitro, and tiien measuring the

pressure P _M needed to move the sensor to die stop position, the unknown pressure P _υ can be determined. In die embodiment shown in FIG. 1, the reference pressure P _R would be stored in die memory 30 by the processor 24 and retrieved when performing a calibration procedure. It may also be stored in a memory device, such as an EEPROM, accompanying the catheter so that the catheter can be connected to another instrument while in use. Such a memory device may be located in the termination block 68 at the proximal end of die catheter (FIG. 1). The P _R may be determined just before use of die sensor on a patient. The processor would tiien use it and die measured pressure P _M to calculate the unknown pressure Pu as shown in equation (2). The calculated unknown pressure P _y would d en be used to correct any zero shift that resulted in error in the displayed pressure.

From a review of FIGS. 4 and 5, it will be noted tiiat two sensors are involved. The first sensor is the bellows or pressure sensing element. The second sensor is d e light system established by the two optical fibers 46 and 48. As described above, die pressure sensing element must be positioned properly so tiiat tiiere exists a sensing movement range and a stop or reference position beyond die sensing range. In this case, the reference position is at a point beyond the position the sensor would be in if it experienced the maximum expected sensing pressure. However, die hght system also has inherent response characteristics that must be taken into account when manufacturing the system shown in these figures.

Because one sensor, whether it be die pressure sensing device or the hght sensing system, rarely has the same response characteristics as another sensor, each sensor needs to be calibrated during manufacture. The arrangement of inner and outer housings in accordance with the invention satisfies a long standing need to do so efficiently, accurately, and at decreased expense.

In die system shown, one optical fiber 46 emits Hght to be reflected by the reflective surface 44 of the bellows. The other optical fiber 48 receives that reflected light. Characteristics of the received hght may be used to determine die position of the pressure sensor 18 and the pressure sensed. Referring now to FIG. 7, a sample response graph 106 of a hght sensing system is shown. To more clearly explain the points to be made by die graph, a drawing of the distal ends 104 of die optical fibers

has been positioned at die zero distance point on d e horizontal axis. A bellows 18 has been drawn below tiiat same axis and its range of movement indicated by lines intersecting the plotted intensity 106 graph line. The lines labeled P _N indicate die normal or sensing range of movement of the bellows. That is, the range of movement in response to the specified range of pressures that may be sensed with tiiat sensor. In the case shown, the bellows 18 has been positioned so tiiat the normal range of movement P _N occurs in a high slope portion of die response curve 106 of the hght sensor 42. The total range of movement of die bellows including its position at the stop surface is indicated by P _τ . As is also apparent, the position 108 at die stop surface also falls in a high slope portion of die graph 106. Therefore, the bellows 18 must be located away from the optical fiber ends 104 by a certain distance to be in the high slope range of operation of the hght sensor 42. This distance varies from hght sensor to hght sensor.

Thus, two sensors having independent response characteristics need to be calibrated during manufacture. It is desirable to provide a system for cahbrating each sensor independently so tiiat interaction between the two during calibration is minimized. That is, it is desirable that die calibration of one eitiier not at all or only minimally depend on the calibration of the other.

In die case of the first sensor, or bellows in this embodiment, d e bellows must have a full range of movement when subjected to die specified sensing pressures in the apphcations to which it will be apphed. Additionally, it must have a stop or reference position beyond tiiat specified sensing pressure range. The stop surface must be located at a particular position to accomplish the latter goal. However, that entire range of movement of the bellows, including die stop surface position, must fall, in this embodiment, witiiin a linear response range for tiiis particular optical system 42. Achieving these two calibration objectives is accomphshed in the arrangement in accordance witii die invention by moving the inner housing into the outer housing until the inner housing contacts the bellows. The bellows is then subjected to a pressure greater than the highest expected pressure but witiiin die range of pressure that can be generated by the controllable pressure source. Because die inner housing started out in contact with die bellows before the pressure was apphed, d e bellows has moved d e

inner housing in relation to the outer housing during application of the pressure and die bellows is still in contact with die stop surface 98. The housings can now be bonded together by an adhesive such as epoxy or by welding or otiier means.

Then die second sensor; i.e., the hght transducer, may be calibrated during manufacture by moving die bonded outer/inner housing combination along the shaft 94 of the catiieter until the optimum optical response is realized when the bellows is moved diroughout its total range of movement. The inner housing 96 can then be bonded to die catheter shaft 94 by use of an adhesive or by welding. As can be appreciated, die arrangement provided by die invention results in an ability to quickly and accurately cahbrate the transducer during manufacture regardless of die performance differences between individual pressure sensors and optical sensors. Where in most cases the small size of the components would cause severe difficulties during die manufacturing process, the arrangement of the system in accordance with die invention results in relative ease and allows for increased accuracy during manufacture.

Referring now to both FIGS. 8 and 1, an embodiment of an aspect of die method of die invention is presented in the form of a flow chart. In the procedure shown, die transducer 90 is initially calibrated, such as during manufacture or immediately before use on a patient. The switch 52 is opened 110 to vent the vent tube 38 to atmospheric pressure P _ATM . A known pressure such as atmospheric pressure P _ATM is also apphed to the bellows as an external pressure P _ext 112. The pressure source 56 is then reset 114, such as by moving it to its minimum stroke position and die switch is closed so tiiat only pressure from the pressure source 20 is coupled to die proximal side of the bellows 18 as an internal pressure P^. At the same time, the data value SI in the CPU 24 is set to zero. Next die pressure source 56 decreases 116 the pressure in the vent tube 38 by an incremental amount while the P^ is measured 118 by the gauge 62. The catheter output is sampled 120 and is low pass filtered 122. The maximum pressure indicated by die catheter sample is obtained 124 from this incremental pressure change and is compared 126 to die last pressure value SI. If the difference D exceeds 128 a predetermined threshold value, such as 0.5 mV, the data value SI is reset 130 to that maximum pressure measured and the process of

incrementaUy varying the pressure is repeated.

In the case where the maximum pressure value D 128 does not change by the predetermined amount, die processor concludes that the reflective surface 44 is against the stop surface 98 and reads 132 d e pressure gauge 62 to determine die reference pressure P _R . This measured P _R is stored 134 in a memory 30 for later reference. The catheter may now be used.

During each discrete pressure variance cycle described above, the CPU 24 signals the pressure source 56 to cause incremental decreases in die pressure on die bellows 18. This change in pressure causes the bellows 18 to move in a direction towards die stop surface 98. When the pressure indicated by die optical fiber system ceases to change, the processor then stops this in vitro calibration and stores die amount of pressure P _R required to move die bellows from the position caused by die known external pressure P _ATM to the reference position at the stop surface. In the preferred embodiment, the increments in pressure are approximately 2 mm Hg. In the preferred embodiment, approximately one thousand samples are obtained during a three second period to obtain the maximum transducer value. A digital low pass filter function is provided by the CPU 24 that is used to eliminate noise in the sample data. Digital filtering is well known to those skilled in die art and no further details are given here. Referring now to FIGS. 9 and 1, an embodiment of a process of in vivo ^' calibration of die transducer in accordance witii an aspect of the invention is presented. When recalibration of the system is desired witiiout removal of the catheter from the patient, the CPU 24 receives a signal from a user to perform an in vivo calibration routine, such as by the user pressing a button on the front panel 88 of die instrument 21, such as die "PROGRAM" button 136 (FIG. 5). The CPU 24 tiien resets 140 the controllable pressure source 56 and closes 142 die switch 52 so that only pressure supplied by the pressure source 56 provides P _int (the switch 52 was open to atmospheric pressure P _ATM for operation). The system is equilibrated for some selected time period such as one second 144 to allow d e vent tube 38 pressure to equahze. The CPU 24 samples signals from the catheter 146 for a time necessary to receive a maximum transducer value. In the preferred embodiment, one thousand samples are obtained

during a three second period to obtain the maximum transducer value. The CPU 24 performs a digital low pass filtering function 148 tiiat is used to eliminate noise in the sample data. The maximum transducer monitor value is obtained 150 from the sample data and is assigned die data value SI 152. Next, die CPU 24 controls a series of discrete pressure variance cycles to determine the exact amount of pressure P _int necessary to cause the bellows 18 to contact the stop surface 98. During each discrete pressure variance cycle, the CPU 24 first signals the pressure source 56 to cause incremental decreases 154 in pressure on the proximal side of die bellows. This change in pressure causes die bellows to move in a proximal direction towards d e stop surface 98. During these incremental adjustments in the pressure, signals from the catheter are sampled 156 and digitally low-pass filtered 158 to eliminate noise in the sample data, and die maximum pressure value is obtained from each sample 160. In die preferred embodiment, one thousand data samples are obtained during a three second period to obtain die maximum transducer value.

During in vivo calibration, the heart beat causes continuous, pulsate changes in the sensed pressure. As die systolic (maximum) component is highest in magnitude, it will be affected first by the stop. The maximum transducer monitor value must be measured at a time when the detector is at die maximum systolic pressure position, odierwise die bellows 18 may be positioned too close to die stop surface 98 and clipping of the systohc maximum pressure waveform will occur. Chpping will cause the bellows to move randomly tiiereby making sampling of a maximum transducer monitor value difficult. The CPU 24 receives die pressure waveform from the catheter, tracks the systohc pressure and determines die peak 160. The maximum value obtained 160 is tiien compared to the SI value 162 and if die difference D exceeds a predetermined value 164, such as 0.5 mV, the SI data value is changed to die MAX value 166. The process of incrementally moving the pressure source 56 is repeated.

The above-described discrete pressure variance cycle is repeated until die transducer differential D is less tiian or equal to die predetermined value (0.5 mV). Upon this occurrence, the bellows 18 is considered to have made contact with the stop

surface 98. The CPU 24 then measures 168 the pressure P _M through the gauge 62 required to move die bellows to the stop position from the unknown position. The processor retrieves the reference pressure P _R and calculates the unknown pressure P _y based on d e measured pressure and reference pressure in accordance witii eq. (2) above. The calculated pressure is then displayed and die system is now recahbrated. The switch 52 is then opened for further operation by the catheter.

The entire in vivo calibration process described above is automated and controlled by d e CPU 24 after request by the operator. The process permits rapid recalibration of the sensing system without the need to remove the sensing system from the patient. Because of the unique construction and method of die calibration system in accordance witii the principles of the invention, accuracy and speed in recalibrating result.

While particular forms of the invention have been illustrated and described, it will be apparent that various modifications can be made witiiout departing from the spirit and scope of the invention.