Abstract:
Switchover of a filtered and unfiltered pulse oximetry sensor is provided with gain controlled amplifiers controlled by separate gain control voltages that may change in opposite directions over a period of time. The outputs of the gain controlled amplifiers may be coupled to voltage-to-current converters whose outputs may be coupled in parallel. The parallel coupled outputs of the voltage-to-current converters may produce a current signal representative of the output of the gain controlled amplifier having the highest gain/signal.

Description:
TECHNICAL FIELD 
       [0001]    The present disclosure relates generally to pulse oximetry and, more particularly to switching photodetector sensor output filtering in an oximeter. 
       BACKGROUND 
       [0002]    This section is intended to introduce the reader to various aspects of art that may be related to various aspects of the present invention, which are described and/or claimed below. This discussion is believed to be helpful in providing the reader with background information to facilitate a better understanding of the various aspects of the present invention. Accordingly, it should be understood that these statements are to be read in this light, and not as admissions of prior art. 
         [0003]    Pulse oximetry is a non-invasive method of monitoring the percentage of hemoglobin (hereinafter “Hb”) that is saturated with oxygen. A pulse oximeter may include of a pheripheral probe linked to a monitor that may be microprocessor controlled. The probe may be placed on a peripheral part of the body such as a digit (e.g., one finger or toe), ear lobe or nose. Within the probe, there are typically two light emitting diodes (LEDs), one in the visible red spectrum (e.g., 660 nm) and the other in the infrared spectrum (e.g., 940 nm). Using a transmission type sensor, these two beams of light pass through tissue to a photodetector. During passage through tissue, some light is absorbed by blood and soft tissue depending on the concentration of Hb. The amount of light absorption at each light wavelength depends on the degree of oxygenation of Hb within the tissue. By calculating the light absorption at the two wavelengths, the microprocessor of the monitor may compute the proportion of oxygenated Hb. A microprocessor of an oximeter may average oxygen saturation values over five to twenty seconds. The pulse rate may also be calculated from the number of LED cycles between successive pulsatile signals and averaged over a similar variable period of time, depending on the particular oximeter. A monitor may display the percentage of oxygen saturated Hb together with an audible signal for each pulse beat, a calculated heart rate, and in some monitors, a graphical display of the blood flow past the probe. User programmable audible alarms may also be provided. 
         [0004]    From the proportions of light absorbed at each light wavelength, the microprocessor may calculate an estimation of the patient&#39;s SpO 2  level. The monitor may then display the oxygen saturation digitally as a percentage and/or audibly as a tone of varying pitch. 
         [0005]    Reflection pulse oximetry uses reflected, rather than transmitted, light on a single-sided sensor. It can therefore be used proximally anatomically, e.g., on the forehead or bowel, although it may be difficult to secure. Other than using specific reflection spectra, the principles are generally the same as for transmission oximetry. 
         [0006]    Oximeters may be calibrated during manufacture and may automatically check internal circuits when turned on. Oximeters may be accurate in the range of oxygen saturations of about 70% to 100% (±2%), but may be less accurate under 70%. The pitch of the audible pulse signal may fall in reducing values of saturation. The size of the pulse wave (related to flow) may be displayed graphically. Some models automatically increase the gain of the display when the flow decreases, but in these models, the display may prove misleading. The alarms usually respond to a slow or fast pulse rate or an oxygen saturation below 90%. At this level, there may be a 30 marked fall in PaO 2  representing serious hypoxia. 
       SUMMARY 
       [0007]    Certain aspects commensurate in scope with the originally claimed invention are set forth below. It should be understood that these aspects are presented merely to provide the reader with a brief summary of certain forms the invention might take and that these aspects are not intended to limit the scope of the invention. Indeed, the invention may encompass a variety of aspects that may not be set forth below. 
         [0008]    According to a specific example embodiment of this disclosure, an apparatus for switchover of a pulse oximetry sensor may comprise: a pulse oximetry sensor; a first gain controlled amplifier having an input coupled to the pulse oximetry sensor; a digital filter having an analog input coupled to the pulse oximetry sensor; a second gain controlled amplifier having an input coupled to an analog output of the digital filter; a first voltage-to-current converter having a voltage input coupled to an output of the first gain controlled amplifier; a second voltage-to-current converter having a voltage input coupled to an output of the second gain controlled amplifier; the first and second voltage-to-current converters having outputs coupled together to produce a single current output; and a controller having a first gain control output coupled to the first gain controlled amplifier and a second gain control output coupled to the second gain controlled amplifier, wherein the controller may increase the gain of one of the gain controlled amplifiers over a period of time while decreasing the gain of the other gain controlled amplifier over the same period of time, so that the single current output from the first and second voltage-to-current converters represents the output of the gain controlled amplifier having the highest gain. 
         [0009]    According to another specific example embodiment of this disclosure, a method of manufacturing a pulse oximeter may comprise providing a first gain controlled amplifier to which a pulse oximetry sensor may be coupled; coupling the pulse oximetry sensor to a digital filter; coupling the digital filter to a second gain controlled amplifier; coupling the first gain controlled amplifier to a first voltage-to-current converter; coupling the second gain controlled amplifier to a second voltage-to-current converter; and coupling the first and second voltage-to-current converter outputs together to produce a current output wherein the gain of one of the gain controlled amplifiers is adapted to increase over a period of time while the gain of the other gain controlled amplifier is adapted to decrease over the same period of time so that the current output from the first and second voltage-to-current converters represents the gain controlled amplifier having the highest gain. 
         [0010]    According to yet another specific example embodiment of this disclosure, a pulse oximeter system having a switchover between unfiltered and filtered channels coupled to a pulse oximetry sensor may comprise: a pulse oximetry sensor; an unfiltered channel comprising a first gain controlled amplifier having an input coupled to the pulse oximetry sensor, and a first voltage-to-current converter having a voltage input coupled to an output of the first gain controlled amplifier; a filtered channel comprising a digital filter having an analog input coupled to the pulse oximetry sensor, a second gain controlled amplifier having an input coupled to an analog output of the digital filter, and a second voltage-to-current converter having a voltage input coupled to an output of the second gain controlled amplifier; the first and second voltage-to-current converters having outputs coupled together to produce a single current output; and a controller having a first gain control output coupled to the first gain controlled amplifier and a second gain control output coupled to the second gain controlled amplifier, wherein when the unfiltered channel is selected the controller may increase the gain of the first gain controlled amplifier over a period of time while decreasing the gain of the second gain controlled amplifier over the same period of time so that the single current output from the first and second voltage-to-current converters represents an unfiltered and amplified signal from the pulse oximetry sensor; and when the filtered channel is selected the controller may increase the gain of the second gain controlled amplifier over a period of time while decreasing the gain of the first gain controlled amplifier over the same period of time so that the single current output from the first and second voltage-to-current converters represents a filtered and amplified signal from the pulse oximetry sensor. 
     
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0011]    A more complete understanding of the present disclosure may be acquired by referring to the following description taken in conjunction with the accompanying drawings wherein: 
           [0012]      FIG. 1  is a schematic block diagram of a switchover, two channel, gain controlled oximeter sensor amplifier having a current output, according to a specific example embodiment of the present disclosure; and 
           [0013]      FIG. 2  is a detailed schematic block diagram of a switchover, two channel, gain controlled oximeter sensor amplifier having a current output, according to another specific example embodiment of the present disclosure. 
       
    
    
       [0014]    While the present disclosure is susceptible to various modifications and alternative forms, specific example embodiments thereof have been shown in the drawings and are herein described in detail. It should be understood, however, that the description herein of specific example embodiments is not intended to limit the disclosure to the particular forms disclosed herein, but on the contrary, this disclosure is to cover all modifications and equivalents as defined by the appended claims. 
       DETAILED DESCRIPTION 
       [0015]    During testing or use of oximeters, a need exists for an improved method to switch between photodetectors and/or different signal processing paths of a photodetector. Heretofore known switching between photo detectors and/or different signal processing paths of a photodetector has introduced undesirable signal transients that may set off oximeter monitor alarms and/or require longer periods of time for the transient to settle out of the normal five to twenty seconds averaging performed by a microprocessor of an oximeter monitor. 
         [0016]    Accordingly, there is a need for improved methods, materials, and/or equipment to switch between photo detectors and/or different signal processing paths of a photo detector, e.g., unfiltered and filtered channels of a pulse oximetry sensor. 
         [0017]    Referring now to the drawings, the details of specific example embodiments are schematically illustrated. Like elements in the drawings are represented by like numbers, and similar elements are represented by like numbers with a different lower case letter suffix. 
         [0018]    Referring to  FIG. 1 , depicted is a schematic block diagram of a switchover, two channel, gain controlled oximeter sensor amplifier having a current output, according to a specific example embodiment of the present disclosure. A channel  1  input signal  102  may be applied to a gain controlled amplifier  114 , and a channel  2  input signal  108  may be applied to a gain controlled amplifier  116 . Gains of the gain controlled amplifiers  114  and  116  may be controlled by gain control signal lines  106  and  112 , respectively. The outputs  118  and  120  of the gain controlled amplifiers  114  and  116 , respectively, are coupled to voltage-to-current converters  122  and  124 , respectively. The current outputs of voltage-to-current converters  122  and  124  may be connected in parallel so as to generate a single switchover output current signal  126 . Voltage-to-current converters  122  and  124  may be, for example, but not limited to, optical isolators each having a voltage controlled light emitting diode as an input and a photodetector as an output. 
         [0019]    For example, at the beginning of a time period T, gain control signal line  106  has a control voltage  104  at a maximum V 1  thereby setting the gain of gain controlled amplifier  114  to a maximum. As the time period T proceeds, control voltage  104  decreases until it is at a minimum V 1 , thereby reducing the gain of gain controlled amplifier  114 . In a similar, but opposite fashion, gain control signal line  112  has a control voltage  110  that starts at the beginning of the time period T at a minimum V 2  which may set the gain of gain controlled amplifier  116  to a minimum, and as the time period T proceeds, control voltage  110  increases until it is at a maximum V 2 , thereby, according to an embodiment, increasing the gain of gain controlled amplifier  116 . Output current  126 , according to an embodiment, represents the dominate gain controlled amplifier output signal  118  or  120 , e.g., the one having the highest gain and signal input will block the other one. Thus, input signal transfer may switch from one of the input channels to the other without introducing a transient in the output current signal  126 . 
         [0020]    Referring to  FIG. 2 , depicted is a detailed schematic block diagram of a switchover, two channel, gain controlled oximeter sensor amplifier having a current output, according to another specific example embodiment of the present disclosure. A pulse oximetry peripheral probe  202  may comprise two light emitting diodes (LEDs), one in the visible red spectrum (e.g., 660 nm), and the other in the infrared spectrum (e.g., 940 nm). The sources of light from the two LEDs  230  pass through patient tissues to photodetector  232 . Light wavelengths not absorbed by the tissues and blood supply are detected by photodetector  232 . A current-to-voltage converter  204  receives the current source signal from detector  232  and produces a voltage on signal line  102  that represents the amplitudes of the detected light wavelengths. A signal line  102  is coupled to an input of the gain controlled amplifier  114  and an input of an analog-to-digital converter  226 . 
         [0021]    The output of analog-to-digital converter  226  may be coupled to a digital filter  224  that may be used to enhance the signal information from the photo-detector  232 . A digital-to-analog converter  222  may be coupled to the output of a digital filter  224  so as to convert this output to an analog voltage that may be coupled to an input of gain controlled amplifier  116 . According to this specific example embodiment, the photodetector  232  may have an output that may be coupled directly (or indirectly) to the input of the gain controlled amplifier  114 . In addition, the output of the photodetector  232  may be coupled (e.g., indirectly) to the gain controlled amplifier  116  through the digital filter  224 . This particular embodiment is directed, in one aspect, to providing a switchover between signal line  102  having the unprocessed information from the photodetector  232  and the signal line  108  having the digitally filtered (enhanced) information from the photodetector  232 . 
         [0022]    The controller  216 , e.g., a digital processor, a microprocessor, a microcontroller, a digital signal processor (DSP), an application specific integrated circuit (ASIC) and/or programmable logic array (PLA), in combination with digital-to-analog converters  218  and  220  may be used for controlling gains of the gain controlled amplifiers  114  and  116 . When a selection change, e.g., nurse or doctor initiated, is made between the unfiltered and filtered signal information from the photodetector  232 , the gain control signal line  106  may have a control signal from the digital-to-analog converter  220  that is at a maximum voltage at the beginning of a time period T, thereby, according to this embodiment, setting the gain of the gain controlled amplifier  114  to a maximum. As the time period T proceeds, the control signal on the gain control signal line  106  decreases until it is at a minimum, thereby, according to this embodiment, reducing the gain of gain controlled amplifier  114 . In a similar, but opposite fashion, the gain control signal line  112  may have a control signal that starts at the beginning of the time period T at a minimum voltage thereby, according to this embodiment, setting the gain of the gain controlled amplifier  116  to a minimum, and as the time period T proceeds, the control signal on the gain control signal line  112  increases until it is at a maximum voltage, thereby, according to this embodiment, increasing the gain of the gain controlled amplifier  116 . 
         [0023]    Since the outputs of the voltage-to-current converters  122  and  124  may be in parallel, the output current  126  may represent the dominant gain controlled amplifier output signal  118  or  120 , e.g., the one having the highest gain and signal input will block the other one. Thus, according to this embodiment, input signals  102  and  108  may be switched without introducing a transient in the output current signal  126 . The output current signal  126  may be used to supply sensor information to an oximeter  212 , e.g., oximeter display monitor. The timing and clocking circuit  228  may be used for clock signals and timing signals for the light emitting diodes  230  and the controller  216 . 
         [0024]    While embodiments of this disclosure have been depicted, described, and are defined by reference to example embodiments of the disclosure, such references do not imply a limitation on the disclosure, and no such limitation is to be inferred. The subject matter disclosed is capable of considerable modification, alteration, and equivalents in form and function, as will occur to those ordinarily skilled in the pertinent art and having the benefit of this disclosure. The depicted and described embodiments of this disclosure are examples only, and are not exhaustive of the scope of the disclosure.