Abstract:
Apparatus for monitoring oxygen saturation levels in tissue for a miniature wireless disposable optical tissue oximeter to are disclosed. According to one aspect of the present invention, a sensor contains a first light source, a second light source, a photodetector, and a skin contact detector. Once skin contact is detected, the first light source emits light in the near infrared region, and the second light source emits light in the visible red region. The emitted light passes through a transparent layer of an adhesive fixation unit, and enters the underlying tissue, where a portion of the light is absorbed by tissue chromophores, including oxygenated hemoglobin and deoxygenated hemoglobin, and reflected back out of the tissue into the photodetector. The oxygen saturation of the tissue under the sensor is then calculated. The oxygen saturation measurements are wirelessly transmitted to a remote display device, such as a smartphone running a smartphone software application which receives the measurements and displays them in numeric, graphical, and audible form. In addition, the smartphone software application may relay the data to the Internet for remote viewing on a web site or remote transfer to a hospital patient data system.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims the benefit under 35 U.S.C. §119(e) of U.S. Provisional Application No. 61/546,664, filed Oct. 13, 2011, the disclosure of which is hereby incorporated herein by reference in its entirety. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    1. Field of Invention 
         [0003]    The present invention relates generally to optical systems that monitor oxygen levels in tissue. More specifically, the present invention relates to a miniature wireless disposable optical tissue oximeter to monitor oxygen levels in tissue for use in shock trauma and exercise training applications. 
         [0004]    2. Description of the Related Art 
         [0005]    Near-infrared spectroscopy has been used for non-invasive measurement of various physiological properties in animal and human subjects. The basic principle underlying the near-infrared spectroscopy is that physiological tissues include various highly-scattering chromophores to the near-infrared waves with relatively low absorption. Many substances in a medium may interact or interfere with the near-infrared light waves propagating therethrough. Human tissues, for example, include numerous chromophores such as oxygenated hemoglobin, deoxygenated hemoglobin, water, lipid, and cytochrome, where the hemoglobins are the dominant chromophores in the spectrum range of approximately 700 nm to approximately 900 nm. Accordingly, the near-infrared spectroscope has been applied to measure oxygen levels in the physiological medium such as tissue hemoglobin oxygen saturation and total hemoglobin concentrations. 
         [0006]    Various techniques have been developed for the near-infrared spectroscopy, e.g., time-resolved spectroscopy (TRS), phase modulation spectroscopy (PMS), and continuous wave spectroscopy (CWS). In a homogeneous and semi-infinite model, both TRS and PMS have been used to obtain spectra of an absorption coefficient and reduced scattering coefficient of the physiological medium by solving a photon diffusion equation, and to calculate concentrations of oxygenated and deoxygenated hemoglobins as well as tissue oxygen saturation. CWS has generally been designed to solve a modified Beer-Lambert equation and to measure changes in the concentrations of oxygenated and deoxygenated hemoglobins. 
         [0007]    Despite their capability of providing the hemoglobin concentrations as well as the oxygen saturation, one major drawback of TRS and PMS is that the equipment is bulky and expensive. CWS may be manufactured at a lower cost but limited in its utility because it cannot compute the oxygen saturation from the changes in the concentrations of oxygenated and deoxygenated hemoglobins. 
         [0008]    Optical Diffusion Imaging and Spectroscopy (ODIS) allows tissue to be characterized based on measurements of photon scattering and absorption. In tissue such as human tissue, near infrared light is highly scattered and minimally absorbed. Optical diffusion imaging is achieved by sending optical signals into tissue and measuring the corresponding diffuse reflectance or transmittance on the tissue surface. 
         [0009]    Scattering is caused by the heterogeneous structure of a tissue and, therefore, is an indicator of the density of a cell and the nuclear size of the cell. Absorption is caused by interaction with chromophores. ODIS emits light into tissue through a sensor. The position of the light source which emits the light and a detector which detects the light allows a depth of measurement to be determined. A ratio of oxyhemoglobin and deoxyhemoglobin may be used to allow for substantially real-time measurement of oxygen, e.g., oxygen saturation levels. 
         [0010]    The measurement of oxygen saturation levels in tissue has proven useful in a number of application areas, including the assessment of trauma patients who may experience a loss of circulatory volume due to internal or external bleeding. As blood loss progresses, the body initially compensates by shifting blood out of the limbs, by means of peripheral vasoconstriction, into the central circulation to preserve blood flow to the brain and to the internal organs. Peripheral vasoconstriction causes a drop in the measured peripheral tissue oxygen saturation. Early detection of this drop in tissue oxygen saturation allows for early intervention. As blood loss progresses further, the body additionally compensates by increasing the heart rate in an attempt to maintain normal blood pressure for perfusing the brain and internal organs. When blood loss becomes extreme, these compensatory mechanisms are no longer sufficient and the blood pressure falls, resulting in a shock state. Blood perfusion to organs is then impaired, and can result in stroke and permanent organ injury known as Multiple Organ Dysfunction Syndrome (MODS) leading to complications such as kidney failure, liver failure, and ischemic bowel infarction. The mortality rate for traumatic shock patients who arrive at major urban trauma centers has been reported to be 50%. 
         [0011]    Another application area in which the measurement of oxygen saturation levels in tissue has proven useful is in fitness training. During training, such as for athletes using a bicycle ergometer where a cyclist is presented with increasing levels of work in stages, a point is reached in which the tissue oxygen saturation begins to drop below the established baseline. This is the breakpoint beyond which the muscle becomes increasingly hypoxic and transitions from aerobic metabolism to anaerobic metabolism. This is also the point at which serum lactate begins to rise above its established baseline and is known as the Lactate Breakpoint or Lactate Threshold. Studies show that endurance athletes achieve the highest performance when they do not exceed their Lactate Breakpoint during their weeks of training. Also, as athletes become more physically fit from training, this increase in fitness can be detected by means of an increase in their Lactate Breakpoint. Tissue oxygen saturation measurements can be used to determine the Lactate Breakpoint. 
         [0012]    Existing ODIS systems are bulky an expensive, and therefore limited in utility for mobile applications such as fitness training and traumatic shock monitoring in settings including ambulance, helicopter, trauma center, Emergency Department (ED), or Intensive Care Units (ICU) of hospitals. In these settings, patients may be separated from physicians by considerable distances, and wireless transmission of oxygen saturation data allows early awareness of traumatic shock so that more timely decisions can be made regarding where the patient should be taken, and which medical staff members should be alerted to receive the patient. In both traumatic shock and fitness training applications, due to space limitations in mobile settings, the tissue oximeter must be as small as possible. 
         [0013]    Therefore, what is needed is a miniaturized wireless tissue oximeter that is inexpensive to manufacture, and can be worn in mobile settings to wirelessly transmit immediate and continuous tissue oxygen saturation readings for both local and remote use. 
       SUMMARY OF THE INVENTION 
       [0014]    The present invention relates to a miniature wireless disposable optical tissue oximeter to monitor oxygen levels in tissue for use in shock trauma and exercise training applications. The oximeter measures local tissue oxygen saturation (S t O 2 ) using near-infrared spectroscopy. The measurement is non-invasive, immediate and continuous. 
         [0015]    In one embodiment the wireless disposable optical tissue oximeter consists of a wireless oximeter in a miniature form contained within an adhesive fixation unit and worn on the hand. The entire self-contained oximeter is very small in size, and therefore can be easily worn and used in ambulance, helicopter, trauma center, Emergency Department (ED), or Intensive Care Unit (ICU) of a hospital. The entire oximeter is disposable. The adhesive fixation unit is applied to the thenar eminence at the base of the thumb, and wrapped around to the back of the hand. The portion of the adhesive fixation unit that is located over the thenar eminence contains a sensor which in turn is connected to a programmable system on a chip. The system on a chip is powered by a battery, and communicates with a wireless transceiver and antenna unit. 
         [0016]    In another embodiment, the adhesive fixation unit portion forms a disposable portion, and the remainder of the oximeter system forms a reusable portion for applications such as fitness training. A reusable sensor is removably attached the disposable adhesive fixation system, which has been adhesively applied over the muscle region to be trained, such as the calf or thigh. The reusable portion contains a sensor which in turn is connected by electrical cable to the remainder of the oximeter consisting of a programmable system on a chip which communicates with a wireless transceiver and antenna unit. The programmable system on a chip is powered by a battery. The reusable portion may be worn on the body by attachment means such as a belt, wrist band, leg band, or clothing clip. The programmable system on a chip calculates and transmits an easy to use consumer-friendly exercise index to allow athletes to adjust their exercise intensity level based on the present invention&#39;s non-invasive tissue oxygen saturation measurements rather than invasive blood lactate measurements. The exercise index value can be displayed numerically or in the form of an easy to understand red, yellow, and green light in which green indicates the intensity of exercise is in the aerobic range and exercise may continue, yellow indicates a transition from an aerobic to an anaerobic state and therefore exercise should be slowed, and red indicates the anaerobic range has been reached and that exercise should stop. 
         [0017]    According to an aspect of the present invention, the sensor contains a first light source, a second light source, a photodetector, and a skin contact detector. Once skin contact is detected by means of the skin contact detector, the first light source emits light in the near infrared region, and the second light source emits light in the visible red region. The emitted light passes through a transparent layer of the adhesive fixation unit, and enters the underlying tissue, where a portion of the light is absorbed by tissue chromophores, including oxygenated hemoglobin and deoxygenated hemoglobin, and reflected back out of the tissue into the photodetector. The oxygen saturation of the tissue under the sensor is then calculated as the ratio of the measured concentration of the oxygenated hemoglobin divided by the total hemoglobin concentration, where the total hemoglobin concentration represents the sum of the measured oxygenated hemoglobin concentration and the measured deoxygenated hemoglobin concentration. 
         [0018]    According to another aspect of the invention, the system on a chip monitors the skin contact sensor, and upon detection of skin contact automatically increases or decreases the intensity of the first and second light sources until the detector produces signals that are in the operating range. The first and second light sources are illuminated sequentially so that a corresponding detector measurement at each wavelength can be uniquely obtained. The system on a chip contains internal digital to analog converters that control the intensity of the first and second light sources, and also contains internal amplifiers and an analog to digital converter to obtain measurements from the photodetector. Furthermore the system on a chip contains a processor, read only memory, read-write memory, and a serial interface to communicate with the wireless transceiver. In addition, the system on a chip receives power from a miniature battery, and contains internal power conversion circuitry to provide supply voltages to the wireless transceiver. 
         [0019]    According to yet another aspect of the invention, the oxygen saturation measurements are wirelessly transmitted to a remote display device, such as a smartphone running a smartphone software application which receives the measurements and displays them in numeric, graphical, and audible form. In addition, the smartphone software application may relay the data to the Internet for remote viewing on a web site or remote transfer to a hospital patient data system. 
         [0020]    These and other advantages of the present invention will become apparent upon reading the following detailed descriptions and studying the various figures of the drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0021]    The invention may best be understood by reference to the following description taken in conjunction with the accompanying drawings in which: 
           [0022]      FIG. 1  is a block diagram representation of a wireless disposable shock trauma monitoring device in accordance with an embodiment of the present invention. 
           [0023]      FIG. 2A  is a diagrammatic top view representation of a wireless disposable shock trauma monitoring device in accordance with an embodiment of the present invention. 
           [0024]      FIG. 2B  is a diagrammatic side view representation of a wireless disposable shock trauma monitoring device in accordance with an embodiment of the present invention. 
           [0025]      FIG. 3  is a perspective view of a wireless disposable shock trauma monitoring device placed on a hand. 
           [0026]      FIG. 4  is a perspective view of a wireless disposable shock trauma monitoring device placed on the calf of a leg. 
           [0027]      FIG. 5  is a process flow diagram in accordance with an embodiment of the present invention. 
           [0028]      FIG. 6  is a time-course plot of tissue oxygen saturation in accordance with an embodiment of the present invention. 
           [0029]      FIG. 7  is a plot of blood lactate as a function of running speed before and after physical training in accordance with an embodiment of the present invention. 
           [0030]      FIG. 8  is a plot of breakpoint workload derived from tissue oxygen saturation versus breakpoint workload derived from lactate in accordance with an embodiment of the present invention. 
           [0031]      FIG. 9A  is a plot of a first exercise index versus exercise stage in accordance with an embodiment of the present invention. 
           [0032]      FIG. 9B  is a plot of a second exercise index versus exercise stage in accordance with an embodiment of the present invention. 
           [0033]      FIG. 9C  is a plot of a third exercise index versus exercise stage in accordance with an embodiment of the present invention. 
           [0034]      FIG. 9D  is a plot of a fourth exercise index versus exercise stage in accordance with an embodiment of the present invention. 
           [0035]      FIG. 9E  is a plot of a fifth exercise index versus exercise stage in accordance with an embodiment of the present invention. 
           [0036]      FIG. 10A  is a plot of a first exercise index versus lactate in accordance with an embodiment of the present invention. 
           [0037]      FIG. 10B  is a plot of a second exercise index versus lactate in accordance with an embodiment of the present invention. 
           [0038]      FIG. 10C  is a plot of a third exercise index versus lactate in accordance with an embodiment of the present invention. 
           [0039]      FIG. 10D  is a plot of a fourth exercise index versus lactate in accordance with an embodiment of the present invention. 
           [0040]      FIG. 10E  is a plot of a fifth exercise index versus lactate in accordance with an embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
       [0041]    The present invention relates to a miniature wireless disposable optical tissue oximeter to monitor oxygen levels in tissue for use in shock trauma and exercise training applications. The oximeter measures local tissue oxygen saturation (S t O 2 ) using near-infrared spectroscopy. The measurement is non-invasive, immediate and continuous. 
         [0042]      FIG. 1  is a block diagram representation of a wireless disposable shock trauma monitoring system  100  in accordance with an embodiment of the present invention consisting of sensor  101  which contains a first light source  102 , a second light source  103 , and photodetector  104 . The first light source  102  emits a first beam of light in the near infrared region into the tissue, and the second light source  103  emits a second beam of light in the visible red region into the tissue. By way of example, the first light source may emit at a wavelength of 905 nm and the second light source may emit at a wavelength of 660 nm. It should be appreciated, however, that the wavelengths of light produced by light emitting diodes associated with first light source  102  and second light source  103  may vary widely. The first beam of light and the second beam of light enter the tissue, and a portion of each beam is reflected by the tissue and received by photodetector  104 . In addition, sensor  101  contains skin contact detector  105 . By way of example, skin contact detector  105  may consist of a planar conductive element forming a first plate of a capacitor, adjacent to one or more conductive elements forming a second plate of a capacitor. Skin contact detector  105  is electrically insulated from the skin by means of adhesive fixation unit  160 . The total capacitance value between the first plate and the second plate is increased by contact with human skin which serves as an electrical dielectric, and therefore measurement of the capacitive value allows for the detection of skin contact. The skin contact detector is located near first light source  102 , second light source  103 , and photodetector  104  to detect contact with the skin. 
         [0043]    Sensor  101  interconnects with a programmable system on a chip (PSOC)  120 . These connections consist of connection  112  joining the first light source  102  to PSOC  120  through which the PSOC can control the intensity of the first light source, connection  113  joining the second light source  103  to PSOC  120  through which the PSOC can control the intensity of the second light source, connection  114  joining the photodetector  104  to PSOC  120  through which the PSOC can measure the electrical signal from photodetector  104 , and connection  118  joining skin contact detector  105  to PSOC  120  through which the PSOC can detect whether or not the sensor  101  is in contact with skin. 
         [0044]    The programmable system on a chip  120  receives power from battery  150  by means of connection  117 . The programmable system on a chip  120  transmits data including tissue oxygen saturation measurements to wireless transceiver  130 , which in turn transmits and receives information from antenna  140  by means of connection  116 . The system on a chip  120  contains internal digital to analog converters that control the intensity of the first and second light sources, and also contains internal amplifiers and an analog to digital converter to obtain measurements from the photodetector. Furthermore the system on a chip  120  contains a processor, read only memory, read-write memory, and a serial interface to communicate with the wireless transceiver. In addition, the system on a chip  120  receives power from a miniature battery, and contains internal power conversion circuitry to provide supply voltages to the wireless transceiver  130 . An example of such a programmable system on a chip is the Cypress Semiconductor PSoC® 5 CY8C55. An example of such a wireless transceiver with connected antenna is the Roving Networks RN42 Bluetooth Transceiver module. 
         [0045]    Adhesive fixation unit  160  contains all of the system components including sensor  101 , programmable system on a chip  120 , battery  150 , wireless transceiver  130 , antenna  140  and their associated interconnections, thereby forming a fully self-contained disposable miniature oximeter system. 
         [0046]      FIG. 2A  is a diagrammatic top view representation of a wireless disposable shock trauma monitoring device  200  in accordance with an embodiment of the present invention. Opaque compartment  201  houses sensor  101  and secures it to biocompatible transparent pressure sensitive adhesive film  210 . Compartment  203  houses the programmable system on a chip  120  and battery  150  and secures it to the transparent pressure sensitive adhesive film  210 . Compartment  205  houses the wireless transceiver  130  and antenna  140  and secures it to the transparent pressure sensitive adhesive film  210 . Compartment  201  is electrically connected to compartment  203  by means of flexible connection  202 . Compartment  203  is electrically connected to compartment  205  by means of flexible connection  204 . An example of such transparent pressure sensitive adhesive film is Scapa RX1402P single coated pressure sensitive adhesive biocompatible 0.003 inch thick polyethylene film. An example of material for compartment  201 , compartment  203 , and compartment  205  is Scapa 0399003 single coated pressure sensitive adhesive biocompatible ⅛ inch thick polyethylene foam, covered by an outer opaque layer of Scapa RX848P biocompatible metallized polypropylene film. 
         [0047]      FIG. 2B  is a diagrammatic side view representation of a wireless disposable shock trauma monitoring device  200  in accordance with an embodiment of the present invention. The emitted light from first light source  102  and second light source  103  within sensor compartment  201  passes through a transparent layer  210  of the adhesive fixation unit, and enters the tissue upon which adhesive fixation layer  210  has been applied, where a portion of the light is absorbed by tissue chromophores, including oxygenated hemoglobin and deoxygenated hemoglobin, and reflected back out of the tissue into photodetector  104  contained within compartment  201 . The oxygen saturation of the tissue under the sensor is then calculated as the ratio of the measured concentration of the oxygenated hemoglobin divided by the total hemoglobin concentration, where the total hemoglobin concentration represents the sum of the measured oxygenated hemoglobin concentration and the measured deoxygenated hemoglobin concentration. 
         [0048]      FIG. 3  is a perspective view of a wireless disposable shock trauma monitoring device placed on and secured to a hand. Transparent pressure sensitive adhesive film  210  is positioned and adhesively secured to the hand such that the long axis of sensor compartment  201  is aligned with the long axis of the thenar eminence  310  of the hand and compartment  203  containing the programmable system on a chip and battery is positioned over the dorsal aspect of the back of the hand. Compartment  201  is electrically connected to compartment  203  by means of flexible connection  202 . 
         [0049]      FIG. 4  is a perspective view of another embodiment of the present invention, in which reusable sensor element  401  containing sensor  101  is removably affixed to compartment  403 . Compartment  403  is permanently affixed to transparent pressure sensitive adhesive film  404  which is adhesively applied the calf of a leg. Reusable sensor element  401  is electrically connected to the programmable system on a chip  120  by means of electrical cable  402 . Compartment  403  and transparent pressure sensitive adhesive film  404  form a disposable adhesive fixation unit. 
         [0050]      FIG. 5  is a process flow diagram in accordance with an embodiment of the present invention, illustrating one method by which the programmable system on a chip  120  can obtain and wirelessly transmit oxygen saturation values. The process begins at step  501  in which the programmable system on a chip  120  determines whether or not contact with the skin has been detected by means of measurements obtained from skin contact detector  105 . If skin contact has not been detected, step  501  is returned to until skin contact is detected. When skin contact has been detected, the process proceeds to step  502  in which the intensity of first light source  102  and second light source  103  are automatically increased or decreased as needed to produce detector signals that are within the operating range of photodetector  104 . The process then proceeds to step  503  in which the tissue oxygen saturation value is calculated based on readings obtained from photodetector  102 . The process finally proceeds to step  504  in which the tissue oxygen saturation results are transmitted to the wireless transceiver. 
         [0051]      FIG. 6  is a time-course plot of tissue oxygen saturation obtained from a prototype of one embodiment of the present invention that has been reduced to practice. Sensor  101  was placed on the thenar eminence of a human hand. The vertical axis represents calculated tissue oxygen saturation in percent units, and the horizontal axis represents elapsed time in seconds. Baseline measurements obtained during the first 20 seconds demonstrate initial tissue oxygen saturation readings between 95% to 100%. Pressure was then applied to the tissue of the hand, thereby reducing blood perfusion and was maintained for 40 seconds. During this period of applied pressure, the measured tissue oxygen saturation values steadily declined to under 60%. When the applied pressure was removed, circulation in the tissue under the sensor was therefore restored and a corresponding rise in tissue oxygen saturation was measured reaching 100%.  FIG. 5  therefore demonstrates that the present invention is sensitive to changes in tissue perfusion. 
         [0052]      FIG. 7  is a plot of blood lactate as a function of running speed before and after physical training in accordance with an embodiment of the present invention. In fitness training, runners may be placed on a bicycle ergometer where they are presented with increasing levels of work in stages. As the level of work increases, a point is reached in which the tissue oxygen saturation begins to drop below an established baseline. This point represents the “breakpoint” beyond which the muscle becomes increasingly hypoxic and transitions from aerobic metabolism to anaerobic metabolism. This is also the point at which the lactate begins to rise above its established baseline and is known by those skilled in the art as the “Lactate Breakpoint” or “Lactate Threshold (LT)”. 
         [0053]    Studies show that endurance athletes achieve the highest performance when they do not exceed their “Lactate Threshold” during their many weeks of training. For an ordinary person interested in fitness, it would therefore be useful to be alerted to when their muscles are becoming hypoxic during exercise so that they can adjust their level of exertion to match their own threshold. Also, as athletes become more physically fit from training, this increase in fitness can be detected by means of an increase in their LT. 
         [0054]      FIG. 8  is a plot of breakpoint workload derived from tissue oxygen saturation versus breakpoint workload derived from lactate in accordance with an embodiment of the present invention. This plot was obtained from the literature, and demonstrates that the breakpoint workload as measured by blood lactate measurements correlates with the breakpoint workload as obtained by tissue oxygen saturation measurements. 
         [0055]    An embodiment of the present invention provides an easy to use consumer-friendly index for exercise intensity level based on non-invasive tissue oxygen saturation measurements rather than invasive blood lactate measurements. Let StO 2  be the current value of tissue oxygen saturation provided by a tissue oximeter in the unit of percentage (%) and StO 2 | At rest  be the StO 2  reading at rest before exercise, also called baseline for StO 2 . Then we define the following five parameters as candidates for exercise indices based on StO 2 . All these exercise indices are unitless and range from 0 to 100, as follows: 
       Exercise Index Ox-1 (or “Index for Muscle Oxygen Level”) 
       [0056]      Ox-1=100−StO 2 .  (1)
 
       Exercise Index Ox-2 (or “Index for Muscle Oxygen Drop Rate”) 
       [0057]      Ox-2=[Drop Rate of StO 2 ]×5.  (2)
 
         [0000]    Here the unit of StO 2  drop rate is percentage per hour. We apply the multiplying factor 5 because StO 2  drop rates ≧20%/hour are observed when blood supply to a tissue flap is blocked. 
       Exercise Index Ox-3 (or “Index for Oxygen Ratio”, or “Anaerobic Index 1”) 
       [0058]      Ox-3=(StO 2 | At rest /StO 2 −1)×100.  (3)
 
       Exercise Index Ox-4 (or “Index for Oxygen Difference”, or “Anaerobic Index 2”) 
       [0059]      Ox-4=StO 2 | At rest −StO 2 .  (4)
 
       Exercise Index Ox-5 (or “Combined Exercise Index”) 
       [0060]      Ox-5=Maximum of Indices Ox-1,Ox-2,Ox-3 and Ox-4.  (5)
 
         [0061]      FIGS. 9A ,  9 B,  9 C,  9 D, and  9 E are plots of exercise index Ox-1, Ox-2, Ox-3, Ox-4, and Ox-5 respectively versus exercise stage in accordance with an embodiment of the present invention. Values of these five exercise indices are graphically shown at different stages of exercise. In the calculation of StO 2  drop rate the time duration employed for each stage is 30 minutes. Statistical parameters for time series of the indices are listed in Table 1. All exercise indices at rest before exercise are of a small value close to zero. As the oximeter user exercises, the StO 2  of the muscle decreases, and all the exercise indices, as well as the lactate value, rise as a trend. Therefore the exercise indices are correlated to the Lactate value, and both remain near their own baselines until the muscle becomes hypoxic, transitioning from aerobic metabolism to anaerobic metabolism. In addition, both parameters rise as a trend when exercise intensity increases. 
         [0000]                                          TABLE 1                   Comparison between the five exercise indices.                Exercise Index   Baseline required   R 2     p-value                       Ox-1   No   0.92   0.22           Ox-2   No   0.61   0.21           Ox-3   Yes   0.92   0.20           Ox-4   Yes   0.92   0.13           Ox-5   Yes   0.98   0.22                        
Consider the following linear calibration for the five exercise indices”
 
         [0000]      Ox- i =Ox- i×k+b, i= 1,2,3,4,5.  (6)
 
         [0000]    Here k and b are linear calibration factors. When the calibration factors are as listed in Table 2 below, the calibrated exercise indices are graphically shown in  FIG. 9 . 
         [0000]    
       
         
               
             
               
               
               
             
               
               
               
             
           
               
                 TABLE 2 
               
             
             
               
                   
               
               
                 Factors for linear calibration. 
               
             
          
           
               
                 Exercise Index 
                 k 
                 b 
               
               
                   
               
             
          
           
               
                 Ox-1 
                 6 
                 −180 
               
               
                 Ox-2 
                 1 
                 0 
               
               
                 Ox-3 
                 3 
                 −20 
               
               
                 Ox-4 
                 4.5 
                 −10 
               
               
                 Ox-5 
                 1 
                 0 
               
               
                   
               
             
          
         
       
     
         [0062]      FIGS. 10A ,  10 B,  10 C,  10 D, and  10 E are plots of exercise index Ox-1, Ox-2, Ox-3, Ox-4, and Ox-5 respectively versus lactate in accordance with an embodiment of the present invention. 
         [0063]    In an embodiment of the present invention, an exercise index value can be displayed numerically or in the form of an easy to understand red, yellow, and green light in which green indicates the intensity of exercise is in the aerobic range and exercise may continue, yellow indicates a transition from an aerobic to an anaerobic state and therefore exercise should be slowed, and red indicates the anaerobic range has been reached and that exercise should stop. 
         [0064]    The present examples are to be considered as illustrative and not restrictive, and the invention is not to be limited to the details given herein, but may be modified within the scope of the appended claims.