Abstract:
A sensor provides pulse oximetry measurements from the presenting portion of a fetus. In particular, a spiral probe is designed to attach the sensor to the fetal scalp. In one sensor configuration, a light emitting region of the probe embedded in the scalp in conjunction with a light detector located at the scalp surface measures absorption from a larger volume of the scalp tissue than conventional fetal sensors. In another sensor configuration, light emitting and light collecting regions of the probe embedded in the scalp are angled with respect to the scalp surface to measure absorption from a larger volume and deeper layers of the scalp tissue than conventional fetal sensors. These sensors increase the likelihood of measuring blood volume changes occurring in larger arterioles versus smaller arterioles or capillaries, yielding a representative measurement of central arterial oxygen saturation. These sensors also reduce the calibration errors caused by a low blood fraction. Localized arteriolar flow is stimulated with heat or vasodilating substances to reduce the effects of localized oxygen consumption and to increase blood fraction. A three-wavelength sensor is utilized to detect a low blood fraction condition.

Description:
PRIORITY CLAIM 
     This application claims the benefit of U.S. Provisional Application No. 60/092,644, filed Jul. 13, 1998. 
    
    
     BACKGROUND OF THE INVENTION 
     Physicians have long relied on intrapartum fetal surveillance for an early warning of complications arising during labor. The ultimate goal of fetal monitoring is to prevent damage to the most vital and sensitive organs, such as the brain and the heart, by detecting a decreased oxygen supply to these organs before the onset of cell damage. Some causes of fetal hypoxia are umbilical cord compression, placental insufficiency or hypertonia of the uterus. Early examples of fetal monitoring are intermittent auscultation of fetal heartbeat, electronic monitoring of fetal ECG and heart rate, and scalp blood pH. These techniques are based on the assumption that fetal hypoxia, leads to fetal acidemia and also to specific pathologic fetal ECG and heart rate patterns. These indirect techniques, however, are unsatisfactory because it is only after hypoxia has occurred for some time that it is reflected in adverse changes in the heart rate or blood pH. 
     More recently, fetal assessment has evolved to the direct measurement of fetal oxygen status using pulse oximetry. Pulse oximetry instrumentation, which provides a real-time measurement of arterial oxygen saturation, has become the standard of care for patient vital sign monitoring during anesthesia and in neonatal and adult critical care. A pulse oximetry system consists of a sensor attached to a patient, a monitor, and a cable connecting the sensor and monitor. The sensor typically has red and infrared light emitting diodes that illuminate a tissue site and a photodiode detector that measures the intensity of that light after absorption by the pulsatile vascular bed at the tissue site. From these measurements, the oxygen saturation of arterial blood can be calculated. 
     SUMMARY OF THE INVENTION 
     Pulse oximetry as applied to fetal intrapartum monitoring must overcome several significant and interrelated obstacles not faced by pulse oximetry as applied to adults, children, infants and neonates. These obstacles include attaching the sensor to a readily accessible tissue site, obtaining a representative measurement of central arterial oxygen saturation at that site, and calibrating the sensor. Pulse oximetry sensors are conventionally attached, for example, to an adult finger or a neonate foot using a self-adhesive mechanism that wraps around the tissue site. Sensor attachment to a fetus in this manner is impractical if not impossible. The uterine environment is fluid filled and the skin of the fetus is coated with vernix, an oily substance. Further, the presenting portion of the fetus is typically the crown of the head, which yields only the fetal scalp as a readily accessible tissue site. A number of mechanisms have been developed to overcome these impediments to attachment of a pulse oximetry sensor to the fetus. These include suction cups, clamps and vacuum devices for scalp attachment. There are also devices that slide beyond the fetus presenting portion, wedging between the uterine wall and the fetus. 
     FIG. 1 illustrates a scalp attachment mechanism used in conjunction with a fetal ECG sensor but also applicable to fetal pulse oximetry. The sensor assembly  100  consists of a fetal sensor  110 , a drive tube  120 , a guide tube  130 , and interconnecting conductors  140 . The fetal sensor  110  has a spiral probe  112  attached to a sensor base  114 . The probe  112  is utilized to attach the sensor  110  to the fetal scalp and also functions as an ECG electrode. The sensor base  114  is removably connected to the drive tube  120  by a fin  116  that fits within slots  122  of the drive tube  120 . The sensor  110  and connected drive tube  120  are movably contained within the guide tube  130 . The interconnecting wires  140  are attached at one end to the sensor base  114 , and one of the conductors  140  is electrically connected to the probe  112 . The other end of the conductors  140  are threaded through the inside of the drive tube  120  and the guide tube  130 , extending from the end of the drive tube  120  opposite the sensor  110 . 
     When using the sensor assembly  100  to attach the sensor  110  to a fetus, a physician first inserts the guide tube  130  into the mother&#39;s birth canal toward the cervix until the guide tube forward end  132  makes contact with the fetal head. Holding the forward end  132  stationary, the physician then inserts the drive tube  120  and attached sensor  110  into the guide tube  130 , pushing the rear end of the drive tube  120  forward until the spiral probe  112  makes contact with the fetal scalp. The physician then rotates the drive tube  120  causing the spiral probe  112  to embed into the fetal epidermis. Thereafter, the physician removes the guide tube  130  and drive tube  120  from the mother, sliding these tubes off the conductors  140 , leaving the sensor attached to the fetus with the wires  140  extending from the mother. The conductors  140  are then attached to a heart rate monitor. 
     Mere attachment of a pulse oximetry sensor to the fetal scalp, however, does not insure that the sensor can measure a representative value of central arterial oxygen saturation from that site. There are many potential tissue sites for scalp attachment of a fetal sensor, but conventional fetal sensors are prone to inconsistent, site-dependent saturation readings. Further, conventional fetal sensors are prone to measurements of oxygen saturation that are dependent on localized oxygen consumption and, therefore, may not be representative of central arterial oxygen supply. These problems are the result of the nonuniform vascularization of the scalp, as explained with respect to FIG. 2A, below. Further, based upon the various presentations of the fetal head, uterine, cervical and vaginal pressures to the head may be unequally applied, resulting in portions of the scalp having compromised perfusion. 
     FIG. 2A depicts the large arteries of the scalp, which are located in the deeper tissue layers. Unlike an adult fingertip or a neonatal foot, the fetal scalp does not provide a specific tissue site with a readily located large artery from which to take pulse oximetry measurements. As shown in FIG. 2A, the scalp contains a web of large arteries separated by significant areas perfused only by branching small arteries, arterioles and capillaries. Because arterial vascularization of the scalp is not uniform, different scalp sites yield measurements taken from various sized arteries and under conditions of differing blood volumes with respect to tissue volume (blood fraction). This, in turn, affects the measured saturation, as described below with respect to FIG.  2 B. 
     FIG. 2B is adapted from  Microvascular and Tissue Oxygen Distribution,  M. Intaglietta, P. Johnson, and R. Winslow, Cardiovascular Research, Elsevier Science 1996, which depicts the distribution of oxygen in the arterioles starting from the larger A1 vessels to the smallest A4 precapillary vessels and capillaries. FIG. 2B is composed of interconnected graphs  210 ,  260 . The graph  210  has an x-axis  212  that corresponds to pO 2  and a y-axis  214  that corresponds blood vessel type. The graph  260  has an x-axis  262  that also corresponds to pO 2  and is aligned with the x-axis  212  of graph  210 . The y-axis  264  of graph  260  corresponds to oxygen saturation. The length of the bars  218  of graph  210  indicate the pO 2  of the blood according to vessel size. The oxygen dissociation curve  268  in the graph  260  illustrates the oxygen binding characteristics of blood hemoglobin. 
     FIG. 2B shows that the oxygen saturation of blood in the microcirculation is dependent on vessel size, indicating the role of the various vessels with respect to tissue oxygenation. In particular, blood flowing through the smaller arterioles and capillaries has been partially desaturated by vessel and localized tissue oxygen consumption. Whereas larger arterioles contain more highly saturated blood reflective of the central oxygen supply. FIGS. 2A and 2B demonstrate that a fetal pulse oximetry sensor that measures a relatively small tissue volume or only superficial tissue layers containing capillaries is less likely to obtain a representative oxygen saturation measurement and more likely to suffer site dependent variations. Further, such a sensor may measure a tissue site with a low blood fraction that renders the pulse oximeter calibration curve invalid. 
     To compute peripheral arterial oxygen saturation, denoted Sp a O 2 , pulse oximetry relies on the differential light absorption of oxygenated hemoglobin, HbO 2 , and deoxygenated hemoglobin, Hb, to compute their respective concentrations in the arterial blood. This differential absorption is measured at the red and infrared wavelengths of the sensor. In addition, pulse oximetry relies on the pulsatile nature of arterial blood to differentiate hemoglobin absorption from absorption of other constituents in the surrounding tissues. Light absorption between systole and diastole varies due to the blood volume change from the inflow and outflow of arterial blood at a peripheral tissue site. This tissue site might further comprise skin, muscle, bone, venous blood, fat, pigment, etc., each of which also absorbs light. It is assumed that the background absorption due to these surrounding tissues is invariant and can be ignored. Accordingly, blood oxygen saturation measurements are based upon a ratio of the time-varying or AC portion of the detected red and infrared signals with respect to the time-invariant or DC portion. This AC/DC ratio normalizes the signals and accounts for variations in light path lengths through the measured tissue. Further, a ratio of the normalized absorption at the red wavelength over the normalized absorption at the infrared wavelength is computed: 
     
       
           R =( Red   AC   /Red   DC )/( IR   AC   /IR   DC )  (1)  
       
     
     where Red AC  and IR AC  are the root-mean-square (RMS) of the corresponding time-varying signals. This “red-over-infrared, ratio-of-ratios” cancels the pulsatile signal. The desired Sp a O 2  measurement is then computed from this ratio. 
     Conventionally, the relationship between the quantity measured by pulse oximeters, R, and the desired oxygen saturation measurement, Sp a O 2 , is determined by statistical regression of experimental measurements obtained from human volunteers using calibrated measurements of oxygen saturation. In a pulse oximeter device, this empirical relationship can be stored as a “calibration curve” in a read-only memory (ROM) look-up table so that Sp a O 2  can be directly derived from R. This calibration curve is qualitatively justified by the Beer-Lambert&#39;s law of absorption, outlined below, which can yield an approximation to the calibration curve. However, the assumptions underlying Beer-Lambert&#39;s law may be invalid for conventional fetal pulse oximetry sensors under certain conditions. 
     According to the Beer-Lambert law of absorption, the intensity of light transmitted through an absorbing medium is given by: 
     
       
           I=I   0  exp(−Σ N   i=1 ε i,λ   c   i   x   i )  (2)  
       
     
     where I 0  is the intensity of the incident light, ε i,λ  is the absorption coefficient of the i th  constituent at a particular wavelength λ, c i  is the concentration coefficient of the i th  constituent and x i  is the optical path length of the i th  constituent. As stated above, assuming the absorption contribution by all constituents but the arterial blood is constant, taking the natural logarithm of both sides of equation (2) and removing time invariant terms yields: 
     
       
         ln( I )=−[ε HbO2,λ   C   HbO2 +ε Hb,λ   C   hb   ]x ( t )  (3)  
       
     
     Measurements taken at both red and infrared wavelengths yield: 
     
       
           RD ( t )=−[ε HbO2,RD   C   HbO2 +ε Hb,RD   C   hb   ]x   RD ( t )  (4)  
       
     
     
       
           IR ( t )=−[ε HbO2,IR   C   HbO2 +ε Hb,IR   C   hb   ]x   IR ( t )  (5)  
       
     
     Taking the ratio R=RD(t)/IR(t) and assuming x RD (t)≈x IR (t) yields: 
     
       
           R =[ε HbO2,RD   C   HbO2 +ε Hb,RD   C   Hb ]/[ε HbO2,IR   C   HbO2 +ε HB,IR   C   Hb ]  (6)  
       
     
     The relationship between arterial oxygen saturation and hemoglobin concentration can be expressed as: 
     
       
           Sp   a O 2   =C   HbO2 /( C   Hb   +C   HbO2 )  (7)  
       
     
     Assuming that: 
     
       
           C   HbO2   +C   Hb =1  (8)  
       
     
     then equation (7) can be solved in terms of R: 
     
       
           Sp   a O 2   =[Rε   Hb,IR −ε Hb,RD ]/[(ε HbO2,RD −ε Hb,RD )+ R (ε Hb,IR −ε HbO2,IR )]  (9)  
       
     
     Thus, Beer-Lambert&#39;s law indicates that there is a fixed relationship between oxygen saturation and the measured value R. It is this relationship that is expressed as the calibration curve stored in the pulse oximeter, as described above. 
     Beer-Lambert&#39;s law is based on an absorption model and does not account for tissue scattering. Blood significantly absorbs the red and infrared wavelengths of interest. Thus, when there is a sufficient blood fraction, the volume of blood as compared with the volume of other tissues, the average photon path length is relatively short, and scattering only has a second-order effect on the intensity of the detected signal. When the blood fraction is small, however, scattering effects cannot be ignored. Using a photon-diffusion model, the relationship between R and SpaO2 can be expressed as: 
     
       
           Sp   a O 2   =[Rε   Hb,IR   −Kε   Hb,RD   ]/[K (ε HbO2,RD −ε Hb,RD )+ R (ε Hb,IR −ε HbO2,IR )]  (10)  
       
     
     where K=K(Σ IR , Σ R , α IR , α R , d) 
     adapted from  Simple Photon Diffusion Analysis of the Effects of Multiple Scattering on Pulse Oximetry,  Joseph M. Schmitt, IEEE Transactions on Biomedical Engineering, December 1991. Equations (9) and (10) differ by the term K appearing in the numerator and denominator of equation (10). K is a function of tissue thickness, d, and the optical properties of the tissue medium, including the wavelength-dependent scattering coefficients, Σ, and attenuation coefficients, α, of the tissue. The photon diffusion model accounts for the distances traversed by source photons before they are captured by the detector, as determined by both absorption and scattering mechanisms in the tissue. By contrast, in the Beer-Lambert model, the optical path length at the red and infrared wavelengths is assumed to be a constant independent of the tissue optical properties. The photon-diffusion model predicts a blood-fraction dependent calibration curve. 
     FIG. 2C illustrates a graph  280 , also adapted from the Schmitt reference cited above, which shows the effect of a change in blood fraction on the pulse oximeter calibration curve. The graph  280  has an x-axis  282  corresponding to oxygen saturation and a y-axis  284  corresponding to the measured ratio, R. A first curve  292  represents the calibration curve for a low blood volume (1%) and a second curve  294  represents the calibration curve for a high blood volume (5%). A fetal sensor that measures a small tissue volume or only the surface tissue layers could be measuring a relatively low blood fraction or a relatively large blood fraction depending on sensor placement. FIG. 2C illustrates that such a sensor may provide measurements that do not correspond to the calibration curve of the connected pulse oximeter. Correspondingly, such a sensor configuration would obtain saturation readings offset from the actual saturation. Further, these saturations could be time varying, as tourniquet-like pressures on the head during labor alter the flow of blood to the tissue site. 
     FIGS. 3,  4 A and  4 B illustrate several fetal pulse oximetry sensor configurations that are inherently susceptible to the problems described above. These sensors can be classified as either reflectance mode or transmission mode sensors. Reflectance mode sensors have the emitters and detector placed next to each other on the tissue site. Transmission mode sensors have the emitters and detector on opposite sides of a tissue site. Both reflectance and transmission mode sensors transmit light into a pulsatile vascular bed, where it is absorbed, reflected, and scattered by tissue and blood. With reflectance mode sensors, only that fraction of emitted light that is reflected back to the detector is relevant. With transmission mode sensors, that fraction of emitted light that is not absorbed or scattered away from the detector is relevant. 
     FIG. 3 depicts the configuration of a reflectance mode pulse oximetry sensor attached to a fetal scalp  10 . Emitters  360  and a detector  370  are co-located against the scalp surface  12 . Light transmitted from the emitters  360  follows the paths  380  in reaching the detector  370 . As a result of dependence on scattering, the volume of measured fetal tissue is relatively small and limited to the surface layers of the scalp. Further, reflectance mode sensors have several drawbacks. A gap between sensor and skin may result in interference from ambient light. In the birth canal, however, there is an absence of ambient light interference. Nevertheless, a portion of light from the emitters may be reflected straight back from the skin surface, resulting in oxygen saturation readings that are falsely low. Also, reflectance mode sensors inherently have a weaker detected signal and a correspondingly lower signal-to-noise ratio resulting in less accuracy than transmission mode sensors. 
     FIG. 4A illustrates the configuration of a transmission-mode pulse oximetry sensor attached to a fetal scalp. Emitters  360  and a detector  370  are longitudinally embedded within the fetal scalp  10 , that is, in a plane parallel to the scalp surface  12 . As with the reflectance mode sensor configuration described above, this sensor configuration also measures a relatively small tissue volume. Emitted light follows the paths  380  from the emitters  360  to the detector  370 , illuminating only the tissue layers proximate the plane of the emitters  360  and the detector  370 . Thus, deeper scalp layers cannot be measured without deeper insertion of the spiral probe and the accompanying risk of injury. Also, interfering light following a back-scattered path  382  from the scalp surface may be detected. 
     FIG. 4B illustrates another transmission mode sensor configuration. Emitters  360  are positioned against the scalp surface  12  and a detector  370  is embedded in the fetal scalp  10 . In contrast to the sensor configurations of FIGS. 3 and 4A, the emitters  360  and detector  370  are transverse to the scalp, that is, in a plane perpendicular to the scalp surface  12 . Thus, emitted light follows the paths  380  along tissue layers extending from the scalp surface  12  to the furthest extent of the detector  370 . The cross-sectional area of the detector  370 , however, is inherently limited to avoid excessive trauma to the scalp. As a result, the measured tissue volume, which is a function of the detector active area, is also limited. 
     The fetal pulse oximetry sensor according to the present invention addresses the above-stated problems inherent in conventional fetal oximetry sensors. Sensor configurations are described that measure a larger tissue volume or measure a deeper tissue layer in order to incorporate the larger arterioles and a larger blood fraction in the derivation of oxygen saturation. Also, methods are described for stimulating localized arterial flow to increase blood fraction and overcome the effects of localized oxygen consumption. Further, a method using an additional wavelength is described for detecting inadequate blood fraction. 
     One aspect of the invention is a sensor comprising a base and a probe having a first portion proximate the base and a second portion distal the base. The probe second portion has a light emitting region, and there is a light collecting region distal the light emitting region. The probe second portion is embeddable within a tissue site so that light transmitted from the emitting region is received at the light collecting region after passing through the tissue site. 
     In one embodiment, the light collecting region comprises a detector located proximate the base. Encapsulated within the base is a generally planar substrate having a first side proximate the probe and a second side distal the probe. The detector is mounted to the first side of the substrate and an emitter is mounted on the substrate proximate the probe first portion so that light is transmitted from the emitter and reflected within the probe to the light emitting region. In particular, the emitter is mounted on the substrate second side, and the probe first portion extends through the substrate from the first side to the emitter on the second side. The emitter may be flush mounted or end-mounted to the substrate second side. In this embodiment, the substrate also functions as a light shield between the emitter and the detector. Alternatively, the emitter is mounted on the substrate first side, and a light shield separates the emitter and the detector. 
     In another embodiment, a light collecting region is disposed in the second probe portion so that the light emitting region and the light collecting region are in a plane substantially parallel to the tissue site surface. The light collecting region and the light emitting region are angled relative to the plane of the light emitting and light collecting regions. In this manner, the sensor advantageously measures deeper tissue layers. A generally planar substrate is encapsulated within the base with a first side proximate the probe and a second side distal the probe. The detector and the emitter are mounted to the substrate proximate the probe first portion. In particular, the emitter and the detector are mounted on the same side of the substrate with a light shield separating the emitter and the detector. The detector and the emitter may be mounted to the substrate second side, with the probe first portion extending through the substrate from the first side to the second side so that light transmitted from the emitter is reflected within the probe to the light emitting region and light received at the light collecting region is reflected within the probe to the detector. The emitter and the detector may be generally flush-mounted to the substrate or end-mounted. The base further comprises a light absorbing material proximate the base surface that contacts the tissue site. 
     Another aspect of the present invention is a pulse oximetry sensor method comprising the steps of embedding an emitting region within a tissue site so that light from the emitting region illuminates the tissue site and positioning a detector proximate to the tissue site so as to receive light passing through the tissue site from the emitting region. A light collecting region of the detector is of substantially greater area than the emitting region. A larger collecting region outside the tissue site advantageously provides for the measurement of a larger tissue volume without increasing the area of the emitting region embedded within the tissue site, which would result in greater tissue damage when the sensor probe is inserted into the tissue site. The sensor method may also comprise transmitting light from an emitter located proximate the tissue site to the emitting region. The sensor method may further comprise shielding the detector from the emitter so that the detector substantially receives light only after passing through the tissue site. In addition, the sensor method may comprise heating the tissue site to stimulate the flow of arterial blood to the tissue site. Alternatively, arterial flow may be stimulated by applying a vasodilating substance to the tissue site. 
     Yet another aspect of the present invention is a pulse oximetry sensor method comprising the steps of embedding an emitting region within a tissue site so that light from the emitting region illuminates the site and also embedding a collecting region within the tissue site distal the emitting region so as to receive light passing through the tissue site from the emitting region. The emitting region and the collecting region are angled away from a surface of the tissue site. In this manner, measurements are obtained from the deeper layers of the tissue site. The sensor method may also comprise transmitting light from an emitter located proximate the tissue site to the emitting region and transmitting light to a detector located proximate the tissue site from the collecting region. The sensor method may further comprise absorbing light from the emitting region that is reflected from the surface of the tissue site so that substantially no reflected light is received at the collecting region. Also, the sensor method may comprise the additional step of shielding the detector from the emitter so that the detector substantially receives light only after passing through the tissue site. 
     A further aspect of the current invention is a pulse oximetry sensor method comprising measuring a first intensity ratio from a first pair of wavelengths illuminating a tissue site, and measuring a second intensity ratio from a second pair of wavelengths illuminating the tissue site. Applying a first calibration curve to the first intensity ratio yields a first saturation value. Similarly, applying a second calibration curve to the second intensity ratio yields a second saturation value. Detecting a low blood fraction condition at the tissue site is accomplished from an examination of the difference between the first saturation value and the second saturation value. 
     An additional aspect of the pulse oximetry sensor according to the present invention is an emitting means for illuminating a tissue site and a collecting means for receiving light from the emitting means after passing through the tissue site so as to measure characteristics of the tissue site. Further, there is a probe means for embedding at least a portion of the emitting means within the tissue site and for attaching the collecting means distal the emitting means. In one embodiment, the collecting means may comprise a detecting means attachable proximate the tissue site. Also the emitting means may comprise a light generating means attachable proximate the tissue site and a transmitting means for conveying light from the generating means to a light emitting region of the probe means. The sensor may also have a shielding means for blocking direct light between the light generating means and the detecting means. In another embodiment, the probe means may also comprise a means for embedding at least a portion of the collecting means within the tissue site. In this embodiment, there may be an angling means for measuring tissue layers distal said probe means. The sensor may also have a transmitting means for conveying light from a generating means to a light emitting region of the probe means and a receiving means for conveying light from a light collecting region of the probe means to a light detecting means attachable proximate the tissue site. There may also be an absorbing means for preventing light from reaching the light collecting region after reflection from the surface of the tissue site. Also included may be a shielding means for blocking direct light between the light generating means and the light detecting means. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention will be described in detail below in connection with the following drawing figures in which: 
     FIG. 1 is an illustration of a prior art fetal ECG sensor having a spiral probe for attachment to the fetal scalp; 
     FIG. 2A is an illustration depicting the location of the larger arteries of the scalp; 
     FIG. 2B is a graph of oxygen saturation as a function of arteriolar size and type; 
     FIG. 2C is a graph of pulse oximetry calibration curves as a function of blood fraction; 
     FIG. 3 is a cross-sectional depiction of a reflectance mode, pulse oximetry sensor attached to the surface of a fetal scalp; 
     FIG. 4A is a cross-sectional depiction of a transmission mode, pulse oximetry sensor located within the tissue of a fetal scalp and longitudinally-oriented with respect to the scalp surface; 
     FIG. 4B is a cross-sectional depiction of a transmission mode pulse oximetry sensor, transversely-oriented with respect to the scalp surface, where the detector is located within the tissue of a fetal scalp and the emitters are attached to the surface of the scalp; 
     FIG. 5 is an illustration of a fetal pulse oximetry system, depicting the fetal sensor, patient cable and monitor; 
     FIG. 6 is a cross-sectional view depicting one embodiment of a fetal pulse oximetry sensor according to the present invention, illustrating an emitter flush-mounted to the backside of a base substrate; 
     FIG. 7 is a cross-sectional depiction of the fetal pulse oximetry sensor of FIG. 6, illustrating a transverse orientation to the fetal scalp, an emitting region located within the tissue of a fetal scalp, and a detector attached to the surface of the scalp; 
     FIG. 8 is a cross-sectional view of another embodiment of a fetal pulse oximetry sensor, illustrating an emitter edge-mounted to a base substrate; 
     FIG. 9 is a cross-sectional view of yet another embodiment of a fetal pulse oximetry sensor, illustrating a detector and emitters co-located on the probe side of a base substrate; 
     FIG. 10A is a cross-sectional view of another embodiment of a fetal pulse oximetry sensor, illustrating a probe having twin spiral needles with angled light emitting and detecting openings at the probe tip; 
     FIG. 10B is a cross-sectional view of a further embodiment of a fetal pulse oximetry sensor, illustrating a probe containing emitters and a detector mounted in the probe adjacent angled slots located along the probe; 
     FIG. 11 is a cross-sectional depiction of the pulse oximetry sensor of FIGS. 10A and 10B, illustrating emitting and detecting regions located within the tissue of the fetal scalp and obliquely-oriented to the scalp surface; and 
     FIG. 12 is a graph depicting calibration curves for two pairs of wavelengths. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 5 illustrates a fetal pulse oximetry system  500 . The system  500  includes a fetal sensor  540 , a patient cable  560  and a pulse oximetry monitor  580 . The fetal sensor  540  includes a sensor base  510 , a spiral probe  520 , and a pigtail  530 . The spiral probe  520  is attached to a front end  512  of the sensor base  510  and extends away from the base  510 . The pigtail  530  is connected to a backend  514  of the sensor base  510  at one end and extends from the sensor base  510  to a position external to the mother, terminating in a patient cable connector  532 . The probe  520  attaches to the fetal scalp as described above with respect to FIG.  1 . That is, the probe  520  is screwed into the presenting portion of fetus, specifically the fetal scalp  10 . The patient cable  560  connects to the pigtail  530  at one end and to the monitor  580  at the other end and transmits signals between the monitor  580  and the sensor  540 . The monitor  580  has a connector  582  for receiving one end of the patient cable  560 . The monitor  580  controls the sensor  540  and processes intensity signals from the sensor  540 , providing a display  584  of the resulting oxygen saturation, pulse rate and plethysmograph. 
     FIG. 6 illustrates one embodiment of the fetal pulse oximetry sensor  540 . The sensor  540  has a sensor base  510 , probe  520  and pigtail  530  as described above. The sensor base  510  is constructed of a substrate  610  encased in an encapsulant  640 . The substrate  610  has a first side  612  facing the probe  520  and a second side  614  facing the pigtail  530 . One end of the pigtail  530  is attached to the second side  614  of the substrate. Individual conductors  632  of the pigtail  530  are electrically connected to the substrate, providing electrical communication between these conductors and the components on the first side  612 , through “vias,” and the second side  614  of the substrate. The pigtail  530  is partially encapsulated, further securing it to the substrate  610  and sensor base  510 . 
     A detector package  650  is mounted on the first side  612  of the substrate  610  and an emitter package  660  is mounted on the second side  614  of the substrate  610 . The detector package  650  contains a photodiode detector chip  652  mounted to leads  654  and enclosed in an encapsulant  658 . The detector package is mounted so that the active, light collecting region of the photodiode  652  faces the probe  520  and detects light from the direction shown by the arrows. The detector package leads  654  are electrically connected to the substrate  610 . The emitter package  660  contains a pair of light emitting diodes (LEDs)  662  encased in an encapsulant  668 , one of which emits a narrow band of red wavelength light and the other of which emits a narrow band of infrared wavelength light. The LEDs  662  are connected back-to-back and in parallel with the emitter package leads  664 . The emitter package  660  is mounted so that the active regions of the LEDs  662  face into the substrate  610 , generating light into the probe  520  in the direction shown by the arrow. The emitter package leads  664  are electrically connected to the substrate  610 . The detector package  650  and emitter package  660  are advantageously mounted on opposite sides  612 ,  614  of the substrate  610  so that the substrate  610  also functions as a light shield. This prevents light leaking from the LEDs  662  from directly reaching the photodiode  652  without first passing through perfused tissue. 
     The probe  520  is hollow and constructed of a highly reflective material, such as stainless steel. One probe end  622  is mounted through the substrate  610  so that the inner diameter  626  of the probe  520  encompasses both of the LEDs  662 . The probe  520  is partially encapsulated, further securing it to the substrate  610  and the sensor base  510 . So constructed, the LEDs  662  can transmit light into the hollow portion of the probe  520 . This light is then transmitted through the substrate  610  and reflected around the probe spirals and out the other probe end  624 . This probe end  624  is cut at an oblique angle, forming a sharp tip  628 , which can easily penetrate fetal scalp tissue. The cut is also made to form an opening  629  facing generally downward and toward the center portion of the substrate  610  where the detector package  650  is located. The probe opening  629  is sealed with a material, such as an epoxy, that is transparent to the red and infrared LED wavelengths. In this manner, light from the LEDs  662  is transmitted through the opening  629  and yet tissue is prevented from accumulating within the hollow probe portion proximate the opening  629 . Thus, the opening  629  is a light emitting region of the probe  520 . 
     FIG. 7 depicts a sensor configuration  700  corresponding to the sensor  540  (FIG. 6) described above. Specifically, with the sensor  540  (FIG. 6) attached to a fetal scalp, the detector  650  (FIG. 6) is positioned such that a light detecting region  720  is located at the scalp surface  12 . Also, the probe opening  629  (FIG. 6) is positioned such that a light emitting region  710  is located within the scalp  10 . Light transmitted from the emitting region  710  follows the paths  730  to the detecting region, measuring a relatively large tissue volume compared to the sensor configurations depicted in FIGS. 3,  4 A and  4 B. In particular, the sensor  700  is a transmission-mode configuration not unlike the adult fingertip sensors and in stark contrast to the reflectance-mode configuration depicted in FIG.  3 . Also, the detecting region  720  is located outside the scalp  10  in contrast to the longitudinal configuration of FIG.  4 A and the transverse configuration of FIG. 4B each having an embedded detector. Thus, the cross-section area of the light detecting region  720  is unconstrained by considerations of tissue trama, advantageously allowing the detecting region  720  to collect light transmitted through a relatively large tissue volume. 
     FIG. 8 depicts an alternative embodiment to the sensor depicted in FIG.  6 . By comparison, this sensor  540  has a sensor base  510 , probe  520  and pigtail  530 . The pigtail  530  is as described above. The sensor base  510  is constructed of a substrate  610  encased in an encapsulant  640 , also as previously described. A detector package  650  is mounted on the first side  612  of the substrate  610  and an emitter package  660  is mounted on the second side  614  of the substrate  610 , as previously described. By contrast, however, the emitter package  660  is end-mounted to the second side  614  of the substrate  610 . Further, one end portion  822  of the probe  520  spirals through the substrate  610  so that the inner diameter  626  of the probe  520  encompasses both of the LEDs  662  (FIG.  6 ). The remainder of the probe  510  is as described above with respect to FIG.  6 . 
     FIG. 9 depicts another alternative embodiment to the sensor depicted in FIG.  6 . Again, this sensor  540  has a sensor base  510 , probe  520  and pigtail  530 . The pigtail  530  is as described above. The sensor base  510  is constructed of a substrate  610  encased in an encapsulant  640 , also previously described. By contrast with the embodiments described above, the detector package  650  and the emitter package  660  are mounted on the first side  612  of the substrate  610 . Because the substrate  610  does not separate the LEDs from the photodiode, a light barrier  910  is installed between the emitter package  660  and detector package  650 . The emitter package  660  is mounted so that the active regions of the LEDs  662  face away from the substrate  610 . One probe end  922  is mounted adjacent the emitter package  660  so that the inner diameter  626  of the probe  520  encompasses both of the LEDs  662  (FIG.  6 ). The remainder of the probe  510  is as described above with respect to FIG.  6 . 
     FIG. 10A illustrates another embodiment of the fetal pulse oximetry sensor  540 . The sensor  540  has a sensor base  510 , probe  520  and pigtail  530 . The pigtail  530  is as described above. The sensor base  510  is constructed of a substrate  610  having a first side  612  and a second side  614 , also as described above. A detector package  650  and an emitter package  660 , described above, are mounted on the second side  614  of the substrate. The probe  520 , however, is distinct from the embodiments described above. 
     The probe  520  is constructed of two hollow spiral needles  1022 ,  1024  of highly reflective material. At one end of the probe  510 , each needle  1022 ,  1024  is mounted through the substrate  610 . A first needle  1022  is terminated at the detector package  650 . A second needle  1024  is terminated at the emitter package  660  so that its inner diameter  1026  encompasses both of the LEDs  662  (FIG.  6 ). At the other end of the probe  510 , each needle  1022 ,  1024  is cut at an oblique angle, forming sharp tips  1028 , which can easily penetrate fetal scalp tissue. The needles are also cut to form openings  1029  facing generally inward and upward at an angle to the sensor base  510 . The opening  1029  at the end of the first needle  1022  creates a light-detecting region. The opening  1029  at the end of the second needle  1024  creates a light-emitting region. The probe openings  1029  are sealed as described above. A light absorbing material  1060  covers the face of the sensor base  510  proximate the probe  520  to prevent photons emitted at one opening  1029  from being reflected off the base  510  and detected at the other opening  1029 . 
     FIG. 10B illustrates yet another embodiment of the fetal pulse oximetry sensor  540 . The sensor  540  has a sensor base  510 , probe  520  and pigtail  530  as described above with respect to FIG.  5 . The probe  510  comprises a single, hollow spiral needle. The probe end  1042  is solid and cut at an oblique angle, forming a sharp solid tip, which can easily penetrate fetal scalp tissue. A pair of slots  1044  form openings along the probe. The slots  1044  are located proximate the probe end  1042  and on opposite portions of one loop of the probe spiral. The slots  1044  are oriented to face generally inward and upward at an angle to the sensor base  510 . Mounted inside the probe  510  proximate the slots  1044  are LED chips and a photodiode chip (not shown). The LEDs are mounted so as to transmit light through one of the slots  1044 , creating a light-emitting region at that slot  1044 . The photodiode is mounted so as to collect light through the other one of the slots  1044 , creating a light-detecting region at that slot  1044 . A light absorbing material  1060  covers the face of the sensor base  510  proximate the probe  520  to prevent photons emitted at one slot  1044  from being reflected off the base  510  and detected at the other slot  1044 . The LEDs and photodiode are connected to the substrate  610  via conductors  1012  threaded through the hollow portion of the probe  520  allowing drive current from the pulse oximetry monitor  580  (FIG. 5) to activate the LED chips via the pigtail  530 . Similarly, an intensity signal detected by the photodiode chip is received by the pulse oximetry monitor  580  (FIG. 5) via the pigtail  530 . 
     FIG. 11 depicts a sensor configuration  1100  corresponding to the sensor  540  described above in FIGS. 10A and 10B. Specifically, with the sensor  540  (FIGS. 10A-B) attached to a fetal scalp, the probe openings  629  (FIG. 10A) or probe slots  1044  (FIG. 10B) are positioned such that a light emitting region  1110  and light collecting region  1120  are located within the scalp  10 . Light transmitted from the emitting region  1110  follows the paths  1130  to the detecting region  1120 , advantageously measuring a relatively large tissue volume and deeper tissue layers compared to the sensor configurations depicted in FIGS. 3,  4 A,  4 B. In particular, the sensor  1100  is a transmission-mode configuration not unlike the adult fingertip sensors and in stark contrast to the reflectance-mode configuration depicted in FIG.  3 . Also, the angled emitting region  1110  and detecting region  1120  advantageously allow the detecting region  720  to collect light transmitted through a relatively larger and deeper tissue volume than the strictly longitudinal or transverse configuration of FIGS. 4A and 4B respectively. Further, the absorbing layer  1140  avoids the backscattering interference depicted in FIG.  4 A. 
     One of ordinary skill will appreciate that there are many variations in the sensors of FIGS. 6,  8 ,  9 ,  10 A and  10 B within the scope of this invention. The light emitting region  629  of FIGS. 6,  8  and  9  can be at a slot along a hollow probe that transmits light reflected inside the probe from emitters located external to the probe. As an alternative, the light emitting region  629  may be the end of a fiber optic located at a probe opening at the probe tip or a slot along the probe, where the fiber optic is mounted inside the probe and transmits light from emitters located external to the probe and coupled to the fiber optic. As another alternative, the light emitting region  629  may be emitters mounted inside a probe, with the surface of the emitters located at a probe opening at the probe tip or a slot along the probe. Conductors located inside the probe electrically connect the emitters to emitter drivers, which are located external to the probe. 
     The light emitting and detecting regions  1029  of FIG. 10A may be fiber optics having ends located at a probe opening at the tip or at slots along the probe, which transmit light between fiber optic ends and a detector or emitters located external to the probe. Further, the probe  520  of FIG. 10A may comprise a single needle  1022  having multiple openings located at or near the needle tip  1028  or elsewhere along the length of the needle  1022 , with the detecting region and emitting region located at these openings. As described above, the detecting and emitting regions of such a single needle may be openings that transmit or receive light reflected inside a hollow needle, the ends of fiber optics that transmit or receive light, or the light sensitive surface of a needle-mounted detector and the light transmitting surfaces of needle-mounted emitters. 
     One of ordinary skill will also appreciate that the substrate  610  and pigtail  530  of FIGS. 6,  8 ,  9 ,  10 A, and  10 B may also be constructed in a variety of ways. The substrate  610  may be made of any number of materials suitable for mounting conductive traces and electronic components, such as standard circuit board material or ceramics with individually mounted components. Alternatively, the substrate may be an integrated circuit or a hybrid circuit. The pigtail  530  may be, for example, a cable of individual conductors or a flex circuit. 
     FIG. 12 shows a graph  1200  that illustrates detection of inadequate blood fraction using an additional wavelength. The graph  1200  has an x-axis  1210  corresponding to the measured ratio, R, and a y-axis  1220  corresponding to oxygen saturation. A first calibration curve  1230  corresponds to the measured signals at a first red wavelength, λ1, and an infrared wavelength, λ3. A second calibration curve  1240  corresponds to the measured signals at a second red wavelength, λ2, and the infrared wavelength, λ3. A first actual curve  1250  corresponds to a shift in the first calibration curve due to reduced blood fraction, as depicted in FIG. 2C. A second actual curve  1260  corresponds to a shift in the second calibration curve, also due to reduced blood fraction. 
     As shown FIG. 12, if the tissue site has a blood fraction corresponding to the “average” physiological conditions for which the first calibration curve and the second calibration curve were derived, then measurements made at wavelengths λ1 and λ3 should match measurements made at wavelengths λ2 and λ3. For example, if the first and second calibration curves  1230 ,  1240  are valid, a saturation reading  1222  of 80% would be indicated by a measured ratio  1232  at λ1 and λ3 of about 1.35 and a measured ratio  1242  at λ2 and λ3 of about 0.65. That is, measurements taken at either set of wavelengths would yield the same oxygen saturation reading. 
     By contrast, a low blood fraction condition would result in a shift in the actual relationships between Sp a O 2  and R from the calibration curves  1230 ,  1240  to the actual curves  1250 ,  1260 , as indicated by FIG. 2C above. Thus, a saturation value  1222  of 80% would result in a measured ratio  1252  of about 1.75 at λ1 and λ3 and a measured ratio  1262  of about 0.8 at λ2 and λ3. However, the calibration curves  1230 ,  1240  would translate these ratio measurements into a saturation reading  1224  of 73% at λ1 and λ3 and a saturation reading  1226  of 67% at λ2 and λ3. Because these saturation readings  1224 ,  1226  must be approximately the same for either set of wavelengths, the pulse oximeter would interpret this discrepancy δ (shaded) as an indication that blood fraction conditions are such that the stored calibration curves are invalid, resulting in erroneously low saturation readings. Therefore, the pulse oximeter would effectively detect a low blood fraction condition. 
     Localized arteriolar flow can also be stimulated to avoid a localized measurement of oxygen saturation and to increase the measured blood fraction. Hyperemia, or the increased flow of arterial blood to the capillaries, is effected by causing the opening of precapillary sphincters localized to the tissue proximate the fetal sensor. In one embodiment, the sensor body  510  (FIG. 5) and probe  520  (FIG. 5) are heated to a range between 40° C. and 43° C. The heating is accomplished with a thermistor mounted to the substrate  610  (FIG.  6 ). The thermistor current is supplied from the monitor  580  (FIG. 5) via the pigtail  530  (FIG. 5) and patient cable  560  (FIG.  5 ). The thermistor voltage is monitored by the monitor, also via the pigtail and patient cable. The monitor adjusts the heat generated by the thermistor by regulating the thermistor supply current. The sensor heat is measured by the monitor from the thermistor resistance, which is simply related to the supplied current and the measured voltage by Ohm&#39;s law. The characteristics of this thermistor feedback control loop, such as stability and response time, are determined by the control processor within the monitor. 
     In another embodiment, localized precapillary sphincters are opened by the topical application of vasodilating substances, such as thurfyl nicotinate or histamine iontophoresis. For example, just prior to the insertion of the sensor  540  (FIG. 5) in the birth canal for scalp attachment, the probe  520  (FIG. 5) is dipped in a solution of nicotinic acid. 
     The fetal pulse oximetry sensor has been disclosed in detail in connection with various embodiments of the present invention. These embodiments are disclosed by way of examples only and are not to limit the scope of the present invention, which is defined by the claims that follow. One of ordinary skill in the art will appreciate many variations and modifications within the scope of this invention.