Abstract:
A pulse oximetry sensor comprises emitters configured to transmit light having a plurality of wavelengths into a fleshy medium. A detector is responsive to the emitted light after absorption by constituents of pulsatile blood flowing within the medium so as to generate intensity signals. A sensor head has a light absorbing surface adapted to be disposed proximate the medium. The emitters and the detector are disposed proximate the sensor head. A detector window is defined by the sensor head and configured so as to limit the field-of-view of the detector.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    The present application claims priority benefit under 35 U.S.C. §120 to, and is a continuation of U.S. patent application Ser. No. 11/171,632, filed Jun. 30, 2005 entitled “Cyanotic Infant Sensor,” now U.S. Pat. No. 7,937,128, which claims priority benefit under 35 U.S.C. §119(e) from U.S. Provisional Application No. 60/586,821, filed Jul. 9, 2004, entitled “Cyanotic Infant Sensor.” The present application also incorporates the foregoing disclosures herein by reference. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    Cyanosis is a congenital condition in which blood pumped to the body contains less than normal amounts of oxygen, resulting in a blue discoloration of the skin. The most common cyanotic condition is tetralogy of Fallot, which is characterized by an abnormal opening, or ventricular septal defect, that allows blood to pass from the right ventricle to the left ventricle without going through the lungs; a narrowing, or stenosis, proximate the pulmonary valve, which partially blocks the flow of blood from the right side of the heart to the lungs; a right ventricle that is abnormally muscular; and an aorta that lies directly over the ventricular septal defect. Another cyanotic condition is tricuspid atresia, characterized by a lack of a tricuspid valve and resulting in a lack of blood flow from the right atrium to the right ventricle. Yet another cyanotic condition is transposition of the great arteries, i.e. the aorta originates from the right ventricle, and the pulmonary artery originates from the left ventricle. Hence, most of the blood returning to the heart from the body is pumped back out without first going to the lungs, and most of the blood returning from the lungs goes back to the lungs. 
         [0003]    Pulse oximetry is a useful tool for diagnosing and evaluating cyanotic conditions. A pulse oximeter performs a spectral analysis of the pulsatile component of arterial blood so as to measure oxygen saturation, the relative concentration of oxygenated hemoglobin, along with pulse rate.  FIG. 1  illustrates a pulse oximetry system  100  having a sensor  110  and a monitor  140 . The sensor  110  has emitters  120  and a detector  130  and is attached to a patient at a selected fleshy tissue site, such as a thumb or toe. The emitters  120  project light through the blood vessels and capillaries of the tissue site. The detector  130  is positioned so as to detect the emitted light as it emerges from the tissue site. A pulse oximetry sensor is described in U.S. Pat. No. 6,088,607 entitled “Low Noise Optical Probe,” which is assigned to Masimo Corporation, Irvine, Calif. and incorporated by reference herein. 
         [0004]    Also shown in  FIG. 1 , the monitor  140  has drivers  150 , a controller  160 , a front-end  170 , a signal processor  180 , a display  190 . The drivers  150  alternately activate the emitters  120  as determined by the controller  160 . The front-end  170  conditions and digitizes the resulting current generated by the detector  130 , which is proportional to the intensity of the detected light. The signal processor  180  inputs the conditioned detector signal and determines oxygen saturation, as described below, along with pulse rate. The display  190  provides a numerical readout of a patient&#39;s oxygen saturation and pulse rate. A pulse oximetry monitor is described in U.S. Pat. No. 5,482,036 entitled “Signal Processing Apparatus and Method,” which is assigned to Masimo Corporation, Irvine, Calif. and incorporated by reference herein. 
       SUMMARY OF THE INVENTION 
       [0005]    The Beer-Lambert law provides a simple model that describes a tissue site response to pulse oximetry measurements. The Beer-Lambert law states that the concentration c i  of an absorbent in solution can be determined by the intensity of light transmitted through the solution, knowing the mean pathlength, mpl λ , the intensity of the incident light, I 0,λ , and the extinction coefficient, ε i,λ , at a particular wavelength λ. In generalized form, the Beer-Lambert law is expressed as: 
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         [0000]    where μ a,λ  is the bulk absorption coefficient and represents the probability of absorption per unit length. For conventional pulse oximetry, it is assumed that there are only two significant absorbers, oxygenated hemoglobin (HbO 2 ) and reduced hemoglobin (Hb). Thus, two discrete wavelengths are required to solve EQS. 1-2, e.g. red (RD) and infrared (IR). 
         [0006]      FIG. 2  shows a graph  200  depicting the relationship between RD/IR  202  and oxygen saturation (SpO 2 )  201 , where RD/IR denotes the ratio of the DC normalized, AC detector responses to red and infrared wavelengths, as is well-known in the art and sometimes referred to as the “ratio-of-ratios.” This relationship can be approximated from Beer-Lambert&#39;s Law, described above. However, it is most accurately determined by statistical regression of experimental measurements obtained from human volunteers and calibrated measurements of oxygen saturation. The result can be depicted as a curve  210 , with measured values of RD/IR shown on an x-axis  202  and corresponding saturation values shown on a y-axis  201 . In a pulse oximeter device, this empirical relationship can be stored in a read-only memory (ROM) for use as a look-up table so that SpO 2  can be directly read-out from an input RD/IR measurement. For example, an RD/IR value of 1.0 corresponding to a point  212  on the calibration curve  210  indicates a resulting SpO 2  value of approximately 85%. 
         [0007]    Accurate and consistent pulse oximetry measurements on cyanotic infants have been difficult to obtain. An assumption inherent in the calibration curve  210  ( FIG. 2 ) is that the mean pathlength ratio for RD and IR is constant across the patient population. That is: 
         [0000]        mpl   RD   /mpl   IR   =C   (3)
 
         [0000]    However, EQ. 3 may not be valid when cyanotic infants are included in that population. The reason may lie in what has been observed as abnormal tissue tone or lack of firmness associated with cyanotic defects, perhaps due to reduced tissue fiber. Such differences in tissue structure may alter the mean pathlength ratio as compared with normal infants. A cyanotic infant sensor addresses these problems by limiting variations in the RD over IR mean pathlength ratio and/or by providing a mean pathlength ratio measure so as to compensate for such variations. Alone or combined, these sensor apparatus and algorithms increase the accuracy and consistency of pulse oximetry measurements for cyanotic infants. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0008]      FIG. 1  is a block diagram of a prior art pulse oximetry system; 
           [0009]      FIG. 2  is an exemplar graph of a conventional calibration curve; 
           [0010]      FIGS. 3A-B  are a perspective and an exploded perspective views, respectively, of a cyanotic infant sensor embodiment; 
           [0011]      FIGS. 4-5  depict cross-sectional views of a tissue site and an attached pulse oximeter sensor, respectively; 
           [0012]      FIG. 6  depicts a cross-sectional view of a tissue site and an attached cyanotic infant sensor; 
           [0013]      FIGS. 7A-B  are plan and cross-sectional sensor head views of a conventional pulse oximeter sensor; 
           [0014]      FIGS. 8-9  are plan and cross-sectional sensor head views of cyanotic infant sensor embodiments; and 
           [0015]      FIG. 10  is an exemplar graph of a calibration surface incorporating a mean pathlength ratio measure. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0016]      FIGS. 3A-B  illustrate one embodiment of a cyanotic infant sensor. The sensor has a light absorbing surface, as described with respect to  FIGS. 4-6 , below. The sensor also has a detector window configured to limit the detector field-of-view (FOV), as described with respect to  FIGS. 7-9 , below. Advantageously, these features limit mean pathlength ratio variations that are particularly manifest in cyanotic patients. 
         [0017]    The sensor emitters and detector are also matched so as to limit variations in the detector red over IR DC response, i.e. RD DC /IR DC , that are not attributed to variations in the mean pathlength ratio (EQ. 3). Such matching advantageously allows for measurement and calibration of the mean pathlength ratio, as described with respect to  FIG. 10 , below. In one embodiment, cyanotic infant sensors  300  are constructed so that: 
         [0000]      λ RD ≈c 1 ; λ IR ≈c 2   (4)
 
         [0000]      I 0,RD /I 0,IR ≈c 3 ; for i DC (RD), i DC (IR)  (5)
 
         [0000]      RD DC /IR DC ≈c 4   (6)
 
         [0000]    That is, sensors  300  are constructed from red LEDs and IR LEDs that are each matched as to wavelength (EQ. 4). The LEDs are further matched as to red over IR intensity for given DC drive currents (EQ. 5). In addition, the sensors  300  are constructed from detectors that are matched as to red over IR DC response (EQ. 6). 
         [0018]    As shown in  FIG. 3A , the sensor  300  has a body  310  physically connecting and providing electrical communication between a sensor head  320  and a connector  330 . The sensor head  320  houses the emitters and detector and attaches to a patient tissue site. The connector mates with a patient cable so as to electrically communicate with a monitor. In one embodiment, a sensor head surface  324  is constructed of light absorbing material. 
         [0019]    As shown in  FIG. 3B , the sensor  300  has a face tape  330 , a flex circuit  340  and a base tape  360 , with the flex circuit  340  disposed between the face tape  330  and the base tape  360 . The flex circuit  340  has a detector  342 , an emitter  344  with at least two light emitting diodes (LEDs), an information element  346 , and contacts  348  disposed on a connector tab  349 . Neonatal sensors having a detector, LEDs, an information element, contacts and connector tab are described in U.S. Pat. No. 6,256,523 entitled “Low-Noise Optical Probes,” which is assigned to Masimo Corporation, Irvine, Calif. and incorporated by reference herein. In one embodiment, the face tape  350  and base tape  360  are constructed of Betham tape having attached polyethylene head tapes  351 ,  361 . In a particular embodiment, the base head tape  361  is made of black polyethylene, and the face head tape  351  is made of white polyethylene. In one embodiment, a clear tape layer is disposed on the base head tape  361  tissue side over the detector window  362 . The base head tape  361  has a detector window  362  and an emitter window  364  each allowing light to pass through the base head tape  361 . In one embodiment, the base head tape  361  has a 4 mil thickness and the flex circuit has a 10 mil thickness. The combined 14 mil material thickness functions to limit the detector FOV, as described with respect to  FIGS. 6 and 8 , below. 
         [0020]      FIGS. 4-6  illustrate some of the pathlength control aspects of a cyanotic infant sensor  300 .  FIG. 4  depicts a fleshy tissue site  10  for sensor attachment, such as a finger or thumb  400 . The tissue  10  has an epidermis  12 , a dermis  14 , subcutaneous and other soft tissue  16  and bone  18 . 
         [0021]      FIG. 5  depicts a conventional pulse oximetry sensor  20  having a detector  22 , an emitter  24  and a tape  26  attached to the fleshy tissue  10 . Transmitted light  30  propagating from the emitter  24  to the detector  22  that results in a significant contribution to pulse oximetry measurements passes through and is absorbed by the pulsatile blood in the dermis  14 . A portion of the transmitted light  30  is scattered out of the epidermis  12  and reflected by the tape  26  back into the fleshy tissue  10 . The detector field-of-view (FOV)  40  is relatively wide and, as a result, the detector responds to transmitted light  30  that has propagated, at least in part, outside of the fleshy tissue  10 . 
         [0022]      FIG. 6  depicts a cyanotic infant sensor  300  that is configured to limit variations in the mean pathlength ratio. In particular, the sensor  300  has a light absorbing tape inner surface  324  that reduces transmitted light reflection back into the tissue site  10 , as described with respect to  FIGS. 3A-B , above. Further, the detector  342  has a limited FOV  50  so as to reduce the detection of transmitted light that has propagated outside of the tissue site  10 , as described in detail with respect to  FIGS. 7-9 , below. 
         [0023]      FIGS. 8-9  illustrate cyanotic infant sensor embodiments having a limited detector field-of-view (FOV).  FIGS. 7A-B  illustrate a conventional sensor  700  having a tape portion  760 , a detector window  762  and a detector  742  having a relatively wide FOV  701 . In particular, the window thickness does little to restrict the FOV.  FIGS. 8A-B  illustrate one embodiment of a cyanotic infant sensor  300  having a material portion  360 , a detector window  362  and a detector  342  having a restricted FOV  801 . In particular, the material thickness  360  functions to define the FOV  801 . In one embodiment, the material thickness  360  comprises a flex circuit thickness and a base head tape thickness, as described with respect to  FIG. 3B , above.  FIGS. 9A-B  illustrate another embodiment of a cyanotic infant sensor  900  having a material portion  960 , a detector window  962  and a detector  942  having a restricted FOV  901 . In particular, an O-ring  980  deposed around the window  962  defines the FOV  901 . 
         [0024]      FIG. 10  depicts an exemplar calibration surface  1000  for a cyanotic infant sensor  300  ( FIGS. 3A-B ) calculated along a DC response ratio axis  1001 , a ratio-of-ratios axis  1003  and a resulting oxygen saturation axis  1005 . Matching the emitters and detectors, as described with respect to  FIG. 3A , above, allows for pathlength calibration. In particular, variations in the detector DC response ratio (RD dc /IR dc ) are attributed to variations in the mean pathlength ratio (EQ. 3). As such, a calibration surface is determined by statistical regression of experimental measurements obtained from human volunteers and calibrated measurements of oxygen saturation, as is done for a conventional calibration curve ( FIG. 2 ). A calculated DC response ratio  1001  in combination with a conventionally calculated ratio-of-ratios  1003  is then used to derive an oxygen saturation  1005  for the calibration surface  1000 . 
         [0025]    A cyanotic infant sensor has been disclosed in detail in connection with various embodiments. These embodiments are disclosed by way of examples only and are not to limit the scope of the claims that follow. One of ordinary skill in art will appreciate many variations and modifications.