Abstract:
An artificial blood flow simulator to be used to test or calibrate a pulse oximeter has a body which is at least partially transparent to red and infrared light waves. Within the body is a light valve which is responsive to an electronic signal for varying the amount of light passing through the body. Connected to the light valve is a signal generator for generating a pulsating electronic signal which corresponds to a given blood flow. The device has the permeability of a human appendage between pulses of blood in the arterial system when the signal generator is generating an electronic pulse and has the permeability of an appendage having arterial blood flowing therethrough when the signal generator is not generating an electronic pulse.

Description:
This application is a continuation application Ser. No. 09/009,086 filed on Jan. 20, 1998 now abandoned. 
    
    
     The present invention relates to devices for testing the accuracy of a pulse oximeter, and in particular to a device for simulating a given percentage of oxygen saturation of blood flow and heart rate which can be measured by a pulse oximeter. 
     BACKGROUND OF THE INVENTION 
     In recent years, the pulse oximeter has become a useful diagnostic medical instrument for patient care which calculates the oxygen saturation of arterial blood to thereby monitor the patient&#39;s pulmonary system. The pulse oximeter measures the amount of light absorbed by arterial hemoglobin at red and infrared waveforms and establishes a ratio between the absorption rates of the two waveforms. 
     The pulse oximeter is described in many documents, including U.S. Pat. No. 4,869,254 and other references. Although there are several variations in the technology for the pulse oximeters, the current technology includes a light source for generating two given wavelengths of light, typically red and infrared, which is projected through a relatively thin appendage or body portion, such as a finger or earlobe. A light detector is positioned on the opposite side of the body portion, and the intensity of the light passing through the body portion for both wavelengths are measured. The absorption of light through a given medium, such as a portion of the human body, is an exponential factor of the distance traveled and, therefore, the theory of the pulse oximeter is based upon the mathematical relationship of the Beer-Lambert law. 
     The pulse oximeter typically produces two wavelengths of light which are capable of penetrating the thickness of the appendage around which the light source and detector have been positioned and which have a given absorption characteristic for oxygenated and de-oxygenated hemoglobin. One typical pulse oximeter employs wavelengths of light of 660 nanometers (red) and 880 nanometers (infrared) as wavelengths which are produceable from economically available sources and which have suitable absorption characteristics of oxygenated and de-oxygenated hemoglobin. The device then calculates an oxygen saturated ratio (ROS) which fluctuates in response to changes in the permeability of the media through which the light is directed. The changes in the permeability are caused by the changes in the oxygenation of the hemoglobin. When a patient&#39;s heart drives a pulse of oxygenated hemoglobin into the arterial structure of the appendage, the ROS for the appendage is different than when oxygenated blood is not being driven through the arterial system, and the differences of the ROS of the medium is caused solely by the presence of the oxygenated hemoglobin in the arterial system. An equation built into the device is used to calculate a measure of the oxygenation of the hemoglobin. 
     Since the pulse oximeter measures the light absorption for both oxygenated and unoxygenated blood, the device also calculates the patient&#39;s pulse rate and the pulse rate is displayed as an output of the device. 
     The usefulness of a diagnostic instrument such as a pulse oximeter lies only in its accuracy, and if a pulse oximeter is not properly calibrated it will not provide accurate readings, and the readings will mislead medical personnel who rely on the instrument and the benefit which would otherwise be achieved by the use of the instrument will, therefore, be lost. It is necessary, therefore, to test or calibrate a pulse oximeter. 
     An obvious method of testing or calibrating a pulse oximeter would be to provide a simulated human appendage having characteristics which duplicate the light absorptive qualities of a living appendage through which flow pulses of the arterial blood having a given percentage of oxygenation. Such a simulator must duplicate the light absorptive qualities of an appendage of the human body between pulses of blood through the arterial system and during pulses of blood through the arterial system such that the device can thereby measure the differences of light passing through the simulated appendage and calculate the simulated oxygenation of the hemoglobin. 
     Prior efforts to simulate a human appendage have resulted in a mechanically operated device such as manufactured by Nonin Medical Incorporated of Plymouth, Minn. which includes an elongate member simulating the finger of a patient to which is attached a compressible bulb which can be squeezed by the operator of the test equipment. When the bulb is not being compressed, the simulated finger has the light absorptive qualities of a human finger during intervals of time between pulses of oxygenated blood in the arterial system. When the bulb is compressed, a liquid is forced within the simulated finger which alters the light absorptive qualities of the simulated finger to that of a human finger which is receiving a pulse of blood through its arterial system. Such existing devices, however, cannot test the accuracy of the pulse rate measuring capabilities of the pulse oximeter being tested. It would be desirable, therefore, to provide an arterial blood flow simulator which can accurately simulate the pulsating changes in the absorptive qualities of a human appendage in response to given pulses rates of hemoglobin having a given oxygen saturation rate passing through the appendage. 
     BRIEF DESCRIPTION OF THE INVENTION 
     Briefly, the present invention is embodied in an artificial blood flow simulator to be used to test or calibrate a pulse oximeter. Briefly, the arterial blood flow simulator has a body comprising material which is at least partially transparent to red and infrared light waves. The body is shaped and sized like that of a human appendage which can be received between the light sources and the light sensors of a pulse oximeter. Within the body is a light valve which is responsive to an electronic signal for varying the amount of light passing through the body. Connected to the light valve is a signal generator for generating a pulsating electronic signal which corresponds to a given blood flow amplitude, that is, corresponds to blood flow having a given pulse rate and a given oxygenation of the hemoglobin in the arterial blood flow. 
     The device will have the permeability of a human appendage between pulses of blood in the arterial system when the signal generator is generating an electronic pulse and will have the permeability of an appendage having arterial blood flowing therethrough when the signal generator is not generating an electronic pulse. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     A better understanding of the present invention will be had after a reading of the following detailed description taken in conjunction with the following drawings wherein: 
     FIG. 1 is a cross-sectional view of the jaws of a pulse oximeter with the finger of a patient within its jaws; 
     FIG. 2 is a cross-sectional view of an artificial finger for use with an arterial blood flow simulator in accordance with the present invention; 
     FIG. 3 is an enlarged cross sectional view of the light valve for use in an arterial blood flow simulator; 
     FIG. 4 is a block diagram of an arterial blood flow simulator in accordance with the present invention; and 
     FIG. 5 is a schematic diagram of the signal generating circuit for the device shown in FIG.  4 . 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring to FIG. 1, a pulse oximeter  10  has a pair of opposing jaws  12 ,  14  which are movable about a pin  16 . A spring  18  urges the jaws  12 ,  14  towards each other such that the distal ends of the jaws will contact each other unless a force is applied to open the jaws  12 ,  14  or an obstruction is positioned between the jaws. Extending from the rearward end of the jaws  12 ,  14  are elongated projections  20 ,  22 , respectively, which can be grasped by the fingers of an operator and squeezed to thereby open the jaws  12 ,  14 . On one jaw  12  is a first LED  24  which generates light at a first given light wave, such as red light having a wave length of 660 nanometers, and a second LED  26  which generates light at a second given wavelength, such as infrared light having a wavelength of 880 nanometers. Positioned on the second jaw  14  is a sensor  28  for detecting the intensity of the light received from either the first LED  24  or the second LED  26 . The sensor  28  is electrically connected into the circuit  32  which includes a microcomputer and a clock as described below. The circuit alternately illuminates the first LED  24  and second LED  26  such that the intensity of both light frequencies passing through an appendage  34  can be measured by the single sensor  28 . The circuit  32  also measures the rate of the changes in the intensity of light passing through the appendage  34  to thereby determine a pulse rate. The changes in intensity of the lights received by the sensor  28  as a result of a pulse of oxygenated blood passing through the appendage  34  are compared in a formula within the device  10  to calculate the oxygen saturation rate. 
     Referring to FIG. 2, to test the accuracy of the pulse oximeter  10 , an artificial blood flow simulator  36  is provided having an artificial appendage  38  which may be inserted between the jaws  12 ,  14  of the pulse oximeter in place of the finger  34  of a patient. In accordance with the present invention, the body of the artificial appendage  38  is made of a suitable material such as a plastic. The body has an upper body portion  40  and a lower body portion  42  and sandwiched between the two portions is a light valve  44  and adjacent the light valve  44  is a red filter  45 , as further described below. 
     The light valve  44  is a polymer dispersed liquid crystal, as further described below, which has a first, de-energized, state, or inactive state, in which the valve has a given light conductivity, and a second, energized, state, or active state, in which the valve has a second given light conductivity. When the light valve  44  is sandwiched between the upper body portion  40  and lower body portion  42 , and the valve  44  is in the de-energized or inactive state, the artificial appendage  38  has a combined light conductivity of the portions  40 ,  42 ,  44  which is comparable to the light conductivity of the simulated human appendage which is receiving a pulse of oxygenated blood. When the light valve  44  is in the energized, or active state, the portions  40 ,  42 ,  44  of the simulated appendage  38  have a combined light conductivity comparable to the simulated human appendage is not receiving a pulse of oxygenated blood. 
     Polymer dispersion liquid crystals (PDLCS) consist of liquid crystal droplets dispersed in a polymer material. The molecules in the droplets of liquid crystal are responsive to an electric field. When the PDLC is not being subjected to an electric field, the liquid crystal droplets have a first conductivity to light, and when the PDLC is being subjected to an electric field, liquid crystal droplets have a second conductivity to light. Potentially, there are many configurations in which a PDLC can be employed to provide a light valve as disclosed in numerous references such as Drzaic, Polymer Dispersed Liquid Crystal for Large Area Displays and Light Values, J. Appl. Phys., vol. 60, pp. 2142-2148, 1986. 
     One embodiment of a light valve  44  is shown in FIG. 3 in which a PDLC film  46  is sandwiched between two plastic planar outer members  48 ,  50 , respectively. The surfaces of the plastic members  48 ,  50 , which abut against the PDLC film  46  are coated with thin layers  52 ,  54 , respectively, of conductive material such as indium tin oxide. When a voltage is applied across the conductive layers  52 ,  54 , an electric field will extend through the PDLC film and the liquid crystal molecules inside the droplets will align with the field. In this configuration when a voltage is applied to the layers  52 ,  54 , the index of refraction of the PDLC layer is substantially equal to that of the polymer matrix and light passing through the PDLC  46  is not scattered causing the valve  44  to be substantially transparent to light. When the voltage is removed from the conductive films  52 ,  54 , the orientation of the nematic liquid crystal molecules within the droplets of the PDLC becomes random and light directed toward the valve  44  will be backscattered and the valve will no longer be as transparent to light. In the preferred embodiment, the light valve  44  can reduce the transparency of the artificial appendage  38  to red light by approximately 15 percent. 
     Referring further to FIG. 4, the blood flow simulator  36  of the present invention includes a power source  56 , such as a nine volt battery for operating the various components of the device, an on/off switch  57 , a microprocessor  58  including a programmed memory  60  for retaining a wave form that corresponds to a precise arterial blood flow amplitude and heart rate. The wave form is stored in a digitalized format and is directed by the microprocessor  58  to a digital to analog converter  62  which converts the digitalized wave form to a pulsating direct current. The pulsating direct current simulates human arterial blood flow rates through an artificial appendage  38 . The direct current wave form is directed through an amplifier  63  and into a DC to AC converter  64  which converts the DC wave to an AC wave form needed to drive the light valve  44 . 
     In the preferred embodiment, the simulator  36  includes a first selector knob  66  which can be moved through a plurality of settings each of which configures the microprocessor to modify the frequency of the wave form stored in the memory  60  to correspond to a different pulse rate of a patient. In the configuration depicted, the first selector knob  66  can be set to one of three simulated pulse rates shown as 60, 80 and 100 pulses per minute. The device  36  includes a second selector knob  68  in which the amplitude of the wave form emitted by the microprocessor  58  and, therefore, the amplitude of the wave form directed to the light valve  44  is adjusted to one of a plurality of settings, each of which correspond to given saturation percentages of hemoglobin of oxygenated blood. In the embodiment depicted, the second selector knob  68  can be set to one of three percentages of oxygenation, namely, 85 percent, 95 percent, and 100 percent. 
     Referring to FIG. 5, the power source  56  includes a voltage regulator  70 , such as provided by National Semiconductor as No. LM78MO5, is connected to capacitors  72 ,  74  as prescribed by the manufacturer for converting the output of a nine volt battery  76  to five volts which is required to operate the other elements of the device. The on/off switch  57  connects or disconnects the battery  76  into the circuit for the power source  56 . 
     The heart of the device  36  is a microprocessor  58  which may be an Intel Part No. 87C51 and includes the memory  60 . Attached to the microprocessor  58  are a crystal  78 , capacitors  80 ,  82  and a resistor  84  all of which are prescribed by the manufacturer to provide an internal clock. A fifth capacitor  86  is also prescribed by the manufacturer to initialize the microcomputer when the unit is turned on. Three contacts  88 ,  90 ,  92  are connected to the contact points  94 ,  96 ,  98 , respectively, of a first rotary switch  99  the shaft of which is attached to the first selector knob  66  and the rotatable contact  100  thereof is connected to ground. Similarly, three other contacts  102 ,  104 ,  106  of the microprocessor  58  are connected to the contacts  108 ,  110 ,  112  of a second rotary switch  113 , the shaft of which is attached to the second selector knob  68 , and the rotatable contact  114  of which is also connected to ground. The position of the selector switches  66 ,  68  is determined by the microprocessor by sensing when the contacts  88 ,  90 ,  92  and  102 ,  104 ,  106 , respectively, are connected to ground. 
     The memory  60  is incorporated into the microprocessor  58 , and the wave form is emitted by the microprocessor  58  through output connectors  116 ,  117 ,  118  which are connected to contacts  120 ,  119 ,  121 , respectively, of the digital to analog converter  62 . Where the digital to analog converter is a Sipex No. SP9600, the contacts  119 ,  120 ,  121  are labelled SCLK, DIN and CS, respectively. The digital to analog converter  62  has diodes  122 ,  124 , resistor  126  and a capacitor  128  which are specified by the manufacturer for the proper operation of the IC. The converter  64  converts the digital wave form from the microprocessor to an analog wave form which simulates the human arterial blood flow rate, and also adds a DC bias voltage to the wave form. 
     The output from the digital to analog converter  62  is directed though an amplifier  63  such as made by Analog Devices. The output from the amplifier  63  is then directed through a DC to AC converter  64  such as made by Elstar of Japan. This component produces a two KHz carrier wave which can be varied between 60 and 120 volts rms depending on the DC voltage input into the amplifier  64  and modulated onto the carrier wave is the analog wave from the computer  58 . The output of the DC to AC converter  64  is then applied across the contacts  52 ,  54  of the light valve  44 . When an AC voltage signal is applied across the valve  44 , the PDLC film thereof will become clear and allow most of the light that strikes the surface thereof to pass through. When the AC voltage is reduced, the PDLC will reflect or backscatter some of the light striking the PDLC film  46  and reduce the amount of light being received by the sensor  28  thereby simulating arterial blood flowing through the appendage. 
     As can be seen, the first rotary switch  99  which is connected with selector knob  66  alters the frequency rate of the wave emitted from the microprocessor  58  and thereby simulate a plurality of human heart rates, such as 60, 80 and 100 pulses per minute. Similarly, the second rotary switch  113  which is connected to the second selector knob  68  alters the amplitude of the wave emitted by the microprocessor  58  to thereby regulate the light conductivity range of the valve  44 . The second rotary switch  113 , therefore, regulates the light conductivity of the appendage  38  to simulate the conductivity of a human appendage which is receiving arterial blood flow having any of a plurality of selected percentages of oxygenation. In the embodiment depicted, the device may be used to simulate an 85 percent oxygenation of arterial blood flow, 95 percent, and 100 percent. 
     Within the simulated appendage  38 , a 30 percent reabsorbing filter  45  constructed of four adjacent layers of cellophane  134 ,  135 ,  136 ,  137  is placed parallel to the PDLC film  46  to lower the overall percentage of red light that passes through the simulated appendage  38  and modifies the reflection path by backscattered red light without significantly affecting the infrared light. The interfaces between the layers  134 ,  135 ,  136 ,  137  of cellophane provide refraction surfaces which contribute to the scattering of red light. When the PDLC  46  is not energized, and has a minimum of transparency, approximately  30  percent of the red light and infrared light is backscattered into the red absorbing filter. The backscattering lengthens the path of the light and thereby reduces the light passing through the appendage. In contrast, when the PDLC  46  is energized it becomes nearly transparent and a minimum amount of red light is backscattered. Infrared light is not significantly affected by the filter  45 , therefore, the PDLC changes the ratio of red light to infrared light. 
     To use the device  36 , the artificial appendage  38  is inserted between the jaws  12 ,  14  of a pulse oximeter  10  and the switch  57  turned on. The selector knob  66  is adjusted such that the microprocessor  58  generates a wave form corresponding to a heart beat rate of 60, 80 or 100 which is then directed to the artificial appendage  38 . The second selector knob  68  is adjusted such that the amplitude of the wave emitted from the microprocessor  58  changes the light permeability of the valve  44  such that the artificial appendage  38  has a pulsating permeability which corresponds to the pulsating permeability of a human appendage receiving arterial blood flow having the selected percent of oxygenation. In the embodiment depicted, the second selector knob  68  can be used to select oxygenation percentages of 85 percent, 95 percent and 100 percent. By comparing the pulse rate of the device  36  to the output reading of the pulse oximeter  10 , the accuracy of the pulse oximeter  10  to determine the pulse rate of a patient can be tested. Similarly, by comparing the oxygenation percentage as selected by the second selector knob  68  of the simulator  36  with the output reading of the pulse oximeter  10  for the percentage of oxygenation of the arterial blood flow, the accuracy of the pulse oximeter to read the oxygenation percentages can be tested. 
     While one embodiment of the present invention has been shown and described, it will be apparent to those skilled in the art that many changes and modifications may be made without departing from the true spirit and scope of the present invention. It is the intent of the appended claims to cover all such changes and modifications which fall within the true spirit and scope of the invention.