Optical catheter calibrating assembly
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to a calibration device and calibrating system for optical catheters used in a catheter oximetry system, and more particularly, it relates to a calibrating device which may remain with the sealed and sterilized distal end of the catheter within a package while the proximal end of the catheter is plugged into a computer or processor in order to perform the calibrating operation. 
     2. Description of the Prior Art 
     A catheter oximetry system provides accurate, continuous, real-time measurement of mixed venous oxygen saturation using multiple wavelength reflection spectrophotometry. The color of red blood cells progressively changes from scarlet to purple as the amount of oxygen the red blood cells are carrying decreases. When light of different selected wavelengths illuminates the blood, the amount of light backscattered, or reflected, at each wavelength depends upon the color, and therefore, oxygen level of the blood. Careful choice of wavelengths in the transmittal light allows accurate measurement of oxygenated hemoglobin with minimal interference by other blood characteristics such as temperature, pH, and hematocrit. 
     Approximately 98% of the oxygen in the blood is chemically combined with hemoglobin in red blood cells. The absorption of red and infrared light substantially differs for oxygenated and deoxygenated hemoglobin, and it varies for different wavelengths of light within this red/infrared spectrum. Therefore, the relative amounts of oxygenated hemoglobin and deoxygenated hemoglobin in the blood can be determined by measuring the relative absorption of light at different selected wavelengths. The percentage of hemoglobin which is in the oxygenated form is defined as the oxygen saturation of the blood in the equation: ##EQU1## where HbO 2  is the oxygenated hemoglobin concentration and Hb is the deoxygenated hemoglobin concentration. 
     A widely used catheter oximetry system consists of three basic components: (1) a disposable fiberoptic pulmonary artery catheter that has a distal end adapted to be inserted into a vein of a patient and that interfaces at its other end with (2) an optical module containing light emitting diodes, a photodetector and associated electronics, which in turn, interfaces with the electrical leads of (3) a computer-based instrument that performs all of the data processing and control functions with displays, alarms and associated read-out devices. The instrument and optical module may be reused many times with different patients, but the catheter is used only with a single patient during a single operation or monitoring process. Thus, the catheters are disposable and are arranged to be separately packaged in sealed aseptic packages each with a specially designed optical connector plug adapted to be plugged into the optical module when the catheter is ready for use. 
     Since the total amount of light reflected back from the blood under test during the catheter oximetry measurements is relatively low, and since variations in the manufacturing of the optical components (particularly the fiberoptics) create differences in transmission which affect the output readings, it is important that each catheter be separately calibrated immediately before it is used so as to relate the actual light intensities received from the sample under test to the unknown concentrations of the substances being quantified in the sample under test. This may be accomplished by initially measuring a given sample of blood with the catheter and then wholly independently measuring the same blood in the laboratory by a different technique in order to match the laboratory calculated actual oxygen saturation content with the instrument calculated content and adjusting the latter accordingly. Such a technique has the obvious disadvantage, however, that the time required for making the laboratory tests causes an undesirable delay between the time of catheter placement and the time at which the oxygen saturation measurements can be utilized with assurance of their correctness. In order to overcome this obvious disadvantage, various techniques have been proposed whereby the catheter is initially calibrated using a reference material such as suspensions of milk of magnesia combined with dyes or filters of various light reflective targets which the distal end of the catheter can be initially directed to and which have known reflectivity characteristics. 
     One method of initial catheter calibration which has found wide acceptance in the field is disclosed in U.S. Pat. No. 4,322,164 to Robert F. Shaw et al. Briefly, this method involves a reference block formed as a solid compliant mass having a plurality of light reflective particles embedded therein. This reference block is received within an enclosed tube, and, in the initial packaging of the catheter, the distal end thereof is inserted into the tube adjacent to but spaced from the reference block and gripped to restrain further movement. The reference block is then spring loaded but restrained by a releasable catch so that it can be released into resilient engagement with the end of the catheter at the time that the calibration measurements are made. Once the calibration readings have been obtained, the catheter can be pulled loose from the tube and reference block and placed in a patient for obtaining blood oxygen saturation measurements in the manner intended. The initial calibration readings are obtained with the reference block and catheter remaining in the package in a sealed and sterilized condition while the connector plug end of the catheter is connected to the optical module and oximetry processor. 
     While the aforedescribed catheter calibration scheme has met with considerable success, there have been some problems from time to time. Thus, it may be inconvenient for the doctor or nurse to perform the separate operation of releasing the reference block into engagement with the catheter tip, or, such operation may fail or expose the catheter to possible contamination prior to its actual time of use. 
     SUMMARY OF THE INVENTION 
     With the optical catheter assembly package of the present invention, a catheter having transmitting and receiving light guides therein is arranged to be packaged in a tray with the distal end of the catheter being received in a calibrating device. The calibrating device includes a tubular enclosure within which a reference element is urged into compliant engagement with the distal end of the catheter and with means being provided for tightly gripping and holding the catheter to the enclosure so that the catheter and calibrating device are ready for an immediate calibration operation in the package without any additional movement of either catheter or reference block being required. 
     The package is sealed with a cover material that encloses the catheter and calibrating device in the tray in a sealed and sterile condition. When the catheter is ready for use, a portion of the sealing material can be removed while the remainder is left in its sealed and sterile condition so as to only expose the optical connector at the proximal end of the catheter permitting it to be connected to an oximetry system to provide a calibration reading for the catheter--all while the distal end and insertable major length of the catheter remains in a sealed, sterile condition. Upon the conclusion of the calibration process and the recording of the results within the oximetry system, the remainder of the sealing material can be removed, the calibrating device readily removed from the distal end of the catheter, and the catheter placed directly in the patient for continuous blood oxygen saturation readings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is an isometric view of the calibrating device of the present invention with the distal end of an oximetry catheter being shown inserted and clamped therein. 
     FIG. 2 is an enlarged section taken along line 2--2 of FIG. 1. 
     FIG. 3 is an enlarged longitudinal section through the calibrating device and catheter of FIG. 1. 
     FIGS. 4-6 are schematic views illustrating the packaging of the catheter and calibrating device of the present invention and particularly showing the manner in which the calibrating operation is carried out. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The calibrating device 10 of the present invention is shown in FIGS. 1-3 wherein it will be seen to be comprised of a cylinder 12 which is closed at one end 13 and open at the other end thereof (FIG. 3). Received within the open end of the cylinder 12 is a plug 15 which is provided near the center thereof with a circumferencially extending rib 17 adapted to be received within a detent 18 just within the open end of the cylinder 12. The plug is inserted in the cylinder during the assembly of the calibrating device, and, as indicated in FIG. 3, it is snapped into snug engagement therewith. 
     The distal end of an optical catheter 20 is adapted to be inserted through the plug 15 within an axial passage 22 extending therethrough. As seen in FIG. 3, the axial passage 22 is just slightly larger than the outer diameter of the catheter and its balloon 21 so as to snugly confine the catheter therewithin. Positioned within the inner end of the tubular member 12 is a reference block 24 and a coil spring 25 with the spring urging the reference block into firm engagement with the flat distal end 20a of the catheter 20 at a position spaced inwardly from the end of the plug 15. 
     The reference block 24 is the same as that shown and described in the aforementioned prior U.S. Pat. No. 4,322,164 to Robert F. Shaw et al. Briefly, the reference block comprises a solid cylindrical element formed of a siicone resin and having a plurality of tiny particles scattered throughout its mass to provide scattering and reflecting surfaces for the light beams transmitted by the catheter 20. The particles will typically have dimensions within the range of from about 0.02 to about 20 microns and should be uniformly dispersed within the solid mass of the reference block 24. The mass is translucent in nature and has compliant characteristics at the surface thereof so that it will yield when pressed against the rigid surface 20a of the catheter thereby insuring a snug fit which will not become easily dislodged during handling of the catheter and attached calibrating device. 
     As shown in FIGS. 1-3 the catheter 20 comprises a conventional optical catheter useful in oximetry measurements having a pair of separated lumens with a transmitting light guide 28 formed of a single fiber and a receiving light guide 29 likewise formed of a single fiber extending side-by-side along the length of the catheter to an exposed position at the flattened surface 20a at the very end of the catheter. Light carried along the transmitting fiber 28 is directed into the reference block 24 where it is backscattered and a portion thereof is reflected back into the receiving fiber 29 for transmission back to the oximetry processing apparatus to provide readings useful for calibrating the catheter and associated optical components. 
     Since it is critical that the catheter remain in snug engagement with the compliant reference block from the time that it is initially packaged up until and through the time when the calibration readings are obtained, means are provided for insuring that this condition will be maintained. Thus, it will be seen that the outer end of the plug 15 is removed so as to provide a short axially extending section 32 which exposes the longitudinally extending passage 22 through the plug. A pair of prongs 34 are provided at opposed sides of the section 32 of the plug and extend outwardly therefrom. A strap 36 formed of a highly resilient and elastomeric material is stretched between the prongs 34 so as to tightly engage one side of the catheter 20 and force it into tight engagement with the longitudinally exposed section of the passage 22 wherein it will remain until the strap is removed. This is accomplished by providing a pair of apertures 38 (one only being shown in FIG. 1) at one end of the strap which apertures are spaced apart by a distance less than the distance between the prongs 34. One aperture is then forced over one of the prongs 34 and the strap is stretched until the other aperture can be received upon the opposed prong 34. Also, as shown, the strap includes an enlarged tab 40 at the outwardly projecting end thereof which tab is of a size whereby it can be readily gripped between the fibers in order to pull the strap loose from the prongs at the conclusion of the calibration operation in order to release the catheter from the calibrating device. 
     The use of the calibrating device 10 of the present invention in a catheter oximetry system is shown sequentially in FIGS. 4, 5 and 6. With reference to FIG. 4, it will be seen that the catheter 20 is arranged to be packaged within conforming recesses set in a rectangularly shaped plastic tray 42. A piece of plastic sealing material 44 is laid atop the tray and sealed thereto, and the tray and enclosed catheter are then sterilized using conventional sterilization techniques. The distal end of the catheter is connected directly to the calibrating device 10 in the aforedescribed manner and clamped thereto by the strap 36 with the tab 40 of the strap extending to the side in a position adapting it to ready removability. The proximal end of the catheter includes the optical connector plug 46 and a plurality of other conventional output connections including lumen connections for pressure readings, samplings, or infusion, a thermistor connection for cardiac outputs and a mechanism connected to pressurize the balloon 21 at the tip of the catheter--all of such elements being conventional with the details thereof having no relevance with respect to the present invention. 
     As shown in FIG. 5, the first step in the calibration operation is to remove the plastic sealing material 44 from atop the tray to allow the fiberoptic connector plug 46 to be removed and coupled to the computer or processor 48. As can be seen, however, the sealing material 44 is provided with two sections separated by a seam or scoreline 43 whereby only one portion thereof is removed during the initial peeling of the material, as shown in FIG. 5, exposing only the proximal end of the catheter and the connections thereto (including the connector plug 46) but leaving the main body of the catheter, which will later be placed in the patient, within the package in its original sealed and sterilized condition. The connector plug can then be placed in a receptacle in an optical module 50 which provides the electro-optical coupling between the connector plug 44 and the processing circuitry of the computer 48. When this is accomplished, the computer is turned on to provide signals to the optical module 50 creating the light sources which are directed via the coupling 46 down the length of the catheter to the reference block 24 wherein the light is backscattered and reflected back to the optical module. The module then converts these light signals into electrical signals for processing by the computer. In this way the appropriate calibration readings are obtained and stored in the computer. 
     Once the relevant calibration readings have been obtained the catheter is calibrated and immediately ready for use in monitoring the blood oxygenation of a patient. As shown in FIG. 6, the remainder of the sealing material 44 is then removed, and a simple pulling away of the strap 36 from its secured position on the calibrating device 10 leaves the catheter 20 free from its locked engagement therewith. The nurse or doctor can then directly take the catheter and place it in the patient. 
     It will be seen that the calibrating device of the present invention permits the catheter to be directly locked to a calibrating device and packaged in such manner so that no additional steps are required other than to connect the proximal end of the catheter to suitable processing circuitry in order to obtain appropriate calibration readings. Once the readings have been obtained, the catheter is ready for immediate use, and the protective and sealing material can be removed to permit the catheter to be immediately used. It has been found that the packaging method as aforedescribed will stand up under repeated jostling or dropping without dislodging the reference block from the catheter. 
     Although the best mode contemplated for carrying out the present invention has been herein shown and described, it will be apparent that modification and variation can be made without departing from what is regarded to be the subject matter of the invention.