Abstract:
A low noise patient cable has a plurality of emitter wires configured to communicate a drive signal between a monitor and at least one emitter. A plurality of detector wires is also configured to communicate a physiological signal between a detector responsive to the emitter and the monitor. A polymer layer is disposed around, and adapted to conduct a triboelectric charge away from, the detector wires.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]    The present application claims priority benefit under 35 U.S.C. §119(e) from U.S. Provisional Application No. 60/346,725, filed Jan. 7, 2002, entitled “Low Noise Patient Cable,” which is incorporated herein by reference. 
     
    
     
       BACKGROUND OF THE INVENTION  
         [0002]    Pulse oximetry is a widely accepted noninvasive procedure for measuring the oxygen saturation level of arterial blood, an indicator of a person&#39;s oxygen supply. Early detection of low blood oxygen level is of crucial importance in the medical field, for example in critical care and surgical applications, because an insufficient supply of oxygen can result in brain damage and death in a matter of minutes. A pulse oximetry system consists of a sensor applied to a patient, a monitor, and a patient cable connecting the sensor and the monitor. The monitor may be a standalone device or may be incorporated as a module or built-in portion of a multiparameter patient monitoring system. A monitor typically provides a numerical readout of the patient&#39;s oxygen saturation, a numerical readout of pulse rate, and an audible indication of each pulse. In addition, the monitor may display the patient&#39;s plethysmograph, which provides a visual display of the patient&#39;s pulse contour and pulse rate.  
         SUMMARY OF THE INVENTION  
         [0003]    One aspect of a low noise patient cable is a plurality of emitter wires configured to communicate a drive signal between a monitor and at least one emitter. A plurality of detector wires is also configured to communicate a physiological signal between a detector, which is responsive to energy received from the at least one emitter, and the monitor. A polymer layer is disposed around, and adapted to conduct a triboelectric charge away from, the detector wires. In one embodiment, the detector wires are configured as a twisted pair and the polymer layer is coextruded with the twisted pair. In a particular embodiment, the polymer layer is a conductive PVC, which may be coextruded to a diameter in the range of about 0.055 to about 0.061 inches and that may also utilize a flexible conductive vinyl.  
           [0004]    Another aspect of a low noise patient cable is a method comprising the steps of twisting a pair of detector wires, coextruding the wires with a conductive polymer to form a polymer layer disposed around the insulator of each of the wires, extending a pair of emitter wires proximate the detector wires and disposing an outer jacket around the detector wires and the emitter wires so as to form a patient cable. In one embodiment, the method further comprises the steps of disposing an inner shield around the polymer layer and disposing an inner jacket around the inner shield, where the inner shield and the inner jacket are configured so as to be between the detector wires and the emitter wires. In a particular embodiment, the method further comprises the step of disposing an outer shield around the inner jacket and the emitter wires, where the outer shield is configured so as to be encased by the jacket.  
           [0005]    Yet another aspect of a low noise patient cable is a detector wire means for conducting a physiological signal between a sensor and a monitor. A polymer means for conducting triboelectric charge is coextruded with the detector wire means. Further an emitter wire means for conducting a drive signal between the monitor and the sensor is jacketed with the detector wire means. In one embodiment, the low noise patient cable further comprises a first conductive means for shielding the detector wire means, which is jacketed with the detector wire means, and a second conductive means for shielding the emitter wire means, which is jacketed with the emitter wire means, the detector wire means and the first conductive means. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0006]    [0006]FIG. 1 is a schematic diagram of a prior art pulse oximetry system;  
         [0007]    FIGS.  2 A-B are a cross-section and cutaway side-view, respectively, of a prior art patient cable; and  
         [0008]    FIGS.  3 A-B are a cross-section and cutaway side-view, respectively, of a low noise patient cable.  
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0009]    [0009]FIG. 1 illustrates the functions of a pulse oximetry system  100 . The sensor  110  has both red and infrared (IR) light-emitting diode (LED) emitters  112  and a photodiode detector  114 . The monitor  160  has LED drivers  162 , a front-end  164  and a signal processor  168 . The monitor  160  determines oxygen saturation by computing the differential absorption by arterial blood of the two wavelengths emitted by the sensor emitters  112 , as is well-known in the art. The LED drivers  162  provide drive current which alternately activates the red and IR LED emitters  112 . The patient cable  200  conducts the LED drive current over drive wires  250  connecting the LED drivers  162  to the LED emitters  112 . The photodiode detector  114  generates a signal corresponding to the red and IR light energy attenuated from transmission through a tissue site. The patient cable  200  conducts the detector signal over detector wires  260  connecting the detector  114  to the front-end  164 . The front-end  164  has input circuitry for amplification, filtering and digitization of the detector signal, which is then input to the signal processor  168 . The signal processor  168  calculates a ratio of detected red and infrared intensities, and an arterial oxygen saturation value is empirically determined based on that ratio. A pulse oximetry sensor is described in U.S. Pat. No. 6,088,607 entitled Low Noise Optical Probe, which is assigned to the assignee of the present invention and incorporated by reference herein. A pulse oximetry signal processor is described in U.S. Pat. No. 6,081,735 entitled Signal Processing Apparatus, which is assigned to the assignee of the present invention and incorporated by reference herein.  
         [0010]    In a pulse oximetry system, the detector typically generates a low-level signal that is susceptible to corruption from various noise sources, such as electromagnetic interference (EMI) and internal noise sources that originate in the sensor, the patient cable and the monitor. One internal noise source is due to the triboelectric effect, which is the static charge generated when two materials are rubbed together. Triboelectric noise is induced in the detector signal when, for example, the detector wires of the patient cable rub together, such as when the patient cable is flexed or is impacted. Triboelectric noise spikes can be orders of magnitude larger than the detector signal.  
         [0011]    FIGS.  2 A-B illustrate a patient cable  200  designed for a pulse oximetry system  100  (FIG. 1). The patient cable  200  has an outer jacket  210 , an outer shield  220 , an inner jacket  230 , a graphite coating  240 , detector wires  250  configured as a twisted pair, emitter wires  260  and textile fillers  270 . The twisted pair  250  has detector conductors  252  and associated insulation  254 . The emitter wires  260  have emitter conductors  262  and associated insulation  264 . The shield  220  and the twisted pair configuration of the detector wires  250  reduce noise due to EMI and crosstalk. Because of the proximity of the twisted pair insulation  254 , however, the detector wires  250  are prone to rubbing and, hence, triboelectric noise. The graphite coating  240  provides a conductive layer along the outside of the detector wires  240 , reducing triboelectric noise by draining the triboelectric induced charge away from the detector wire insulation  254 .  
         [0012]    The coating  240  is formed by drawing the twisted pair  250  through a solvent bath containing graphite. The solvent is allowed to evaporate, depositing the conductive graphite coating  240  on the twisted pair  250 . A deposited graphite coating  240 , however, has several drawbacks. The coating  240  is difficult to precisely manufacture because the deposition process is difficult to control. As a result, the cable  200  itself is relatively expensive to manufacture. Also, preparation of the cable  200  for connector attachment involves cutting and stripping the cable layers to expose the conductors  252 ,  262 , which are difficult procedures to perform. In particular, the deposited coating  240  has to be selectively cleaned-off with a solvent and mechanical abrasion to expose the conductor ends  252 , which is time consuming and which may subject the cable  200  to damage.  
         [0013]    FIGS.  3 A-B illustrates a low noise patient cable  300 , which has an outer jacket  310 , an outer shield  320 , an inner jacket  330 , an inner shield  340 , a polymer layer  350 , detector wires  360  configured as a twisted pair, emitter wires  370  and textile fillers  380 . The twisted pair  360  has detector conductors  362  and associated insulation  364 . The emitter wires  370  have emitter conductors  372  and associated insulation  374 . The low noise patient cable  300  functions in a pulse oximetry system  100  (FIG. 1) in a manner similar to that of the patient cable  200  (FIG. 2) described above. In particular, the emitter wires  370  electrically connect the LED drivers  162  (FIG. 1) to the LEDs  112  (FIG. 1), and the twisted pair  360  electrically connects the detector  114  (FIG. 1) to the monitor front-end  164  (FIG. 1). Further, the shields  320 ,  340  and twisted pair  360  reduce EMI and crosstalk. The polymer layer  350 , however, is advantageously disposed around the detector wires  360  instead of a graphite coating as described with respect to FIG. 2, above. The polymer layer  350  is formed by coextruding the twisted pair  360  with a conductive polymer. In one embodiment, the polymer layer  350  is a conductive PVC. In a particular embodiment, the conductive PVC utilizes a flexible conductive vinyl compound, such as Abbey #100-1 available from Abbey Plastic Corporation and is coextruded to a diameter in the range of about 0.058±0.003 inches.  
         [0014]    A coextruded conductive polymer has several advantages over a deposited graphite coating for reducing triboelectric noise. As with the graphite coating, the polymer layer  350  drains the triboelectric induced charge away from the detector wire insulation  364 . The coextrusion process, however, is easier to control and less expensive accordingly. Further, during cable preparation for connector attachment the polymer layer  350  can be easily cut from the twisted pair  360 . In addition, better triboelectric noise reduction can be achieved with the polymer layer  350  than with a graphite coating.  
         [0015]    In addition to the foregoing, disposing the polymer layer  350  around the twisted pair of detector wires  360  has several advantages over disposing the polymer layer  350  around individual wires. For example, disposal around the twisted pair can be less expensive than disposal around individual wires and can produce an end product cable having a smaller diameter. Moreover, disposal around the twisted pair in the embodiment of the low noise patient cable  300  being used for at least pulse oximetry, can increase the eventual signal quality output from signal processing circuitry, such as, for example, a differential amplifier. For example, formation of the polymer layer  350  in a manner that maintains the close physical proximity of the twisted pair tends to ensure external noise applied to the patient cable  300  is applied substantially equally (or common) to each conductor of the twisted pair. Thus, the differential amplifier (not shown) of the monitor  160  can effectively filter the applied external noise through, for example, the amplifier&#39;s common mode rejection.  
         [0016]    The low noise patient cable has been disclosed in detail in connection with various embodiments. These embodiments are disclosed by way of examples only and are not intended to limit the scope of the claims that follow. One of ordinary skill in the art will appreciate many variations and modifications.