RELATED APPLICATIONS 
     This application claims the benefit of priority under 35 U.S.C. § 119(e) of U.S. Provisional Patent App. No. 60/168,366 filed on Dec. 1, 1999 and entitled Pulse Oximeter Monitor for Expressing the Urgency of the Patient&#39;s Condition. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to pulse oximeters and methods for expressing the urgency of the patient&#39;s condition to the monitoring individuals. 
     2. Description of the Related Art 
     Pulse oximeters measure the oxygen saturation in pulsatile arterial blood by analyzing the variations in light transmission through a section of body tissue, and are well-known in the art. As disclosed by Wood in U.S. Pat. No. 2,706,927, the oxygen saturation can be calculated from two measurements of light absorption at two different wavelengths by a section of body tissue. The quantity of arterial blood in a section of body tissue varies in synchroneity with the heartbeat, so heartbeat-synchronized variations in the transmitted light intensity therefore correspond to variations of the pulsatile arterial blood. Because the absorption coefficient of the oxyhemoglobin component of blood can be determined, a comparison of the transmitted light intensities at different moments during the periodic heartbeat can be converted into a measure of the fraction of oxyhemoglobin in arterial blood, i.e., the arterial oxygen saturation. Various methods of calculating the oxygen saturation level are known in the prior art, such as, for example, the methods disclosed in Diab et al., U.S. Pat. No. 5,632,272, issued May 27, 1997. 
     SUMMARY OF THE INVENTION 
     The present inventors recognized several disadvantages inherent in the pulse oximeter monitor technology described in the prior art. First, in order to be able to interpret the information contained in the variations of the pitch of the audible sounds created by the prior art devices, a health care individual such as a nurse must be able to put the audible sounds in the correct context. To audibly detect a problem, the individual must have some experience in discerning the difference between a sound pitch which is within tolerances and a sound pitch which is indicative of a problem. Using sound pitch as the measure of oxygen saturation therefore requires cognitive thought by the individual who has been trained to recognize the sound pitches associated with low oxygen saturation conditions. Such high-level thought processes occur more slowly, and are more prone to mistakes in judgment, than are other, more instinctive, thought processes. Second, the monitors described in the prior art which use sound pitch as an indication of low oxygen saturation do not exploit the psychological response of humans to associate particular sounds with danger or trouble. Because there is nothing inherent in the pitch of a sound itself which is indicative of a problem, individuals must use high-level thought processes to translate the change of sound pitch into a measure of the patient&#39;s condition. Third, various individuals have different abilities to detect differences or changes in sound pitch. Ear defects or injuries of the individual monitoring the patient&#39;s condition can impact the individual&#39;s ability to immediately detect a problem with the patient&#39;s oxygen saturation. 
     It is an object of the present invention to provide a monitor for a pulse oximeter which emits an audible sound containing information regarding the patient&#39;s condition that requires less training and subjective judgment by the individual monitoring the patient&#39;s condition. 
     It is another object of the present invention to provide a monitor for a pulse oximeter which emits an audible sound containing information regarding the patient&#39;s condition that can be more quickly interpreted by the individual monitoring the patient&#39;s condition. 
     It is also another object of the present invention to provide a monitor for a pulse oximeter which emits an audible sound containing information regarding the patient&#39;s condition that is less affected by hearing disabilities of the individual monitoring the patient&#39;s condition. 
     One aspect of the present invention is a monitor for a pulse oximeter for use in measuring a patient&#39;s pulse rate and oxygen saturation level. The monitor comprises a generator which generates an audio signal. The audio signal contains information regarding the patient&#39;s pulse rate and oxygen saturation level. A transducer is coupled to the generator to receive the audio signal and to convert the audio signal into audible sounds containing information regarding the patient&#39;s pulse rate and oxygen saturation level. Unlike the prior devices, the information is not contained in variations of the pitch of the audible sounds. 
     Another aspect of the present invention is a monitor for a pulse oximeter for use in measuring a patient&#39;s pulse rate and oxygen saturation level. The monitor comprises a generator which generates an audio signal which varies in response to the patient&#39;s pulse rate and oxygen saturation level. A transducer is coupled to the generator to receive the audio signal. The transducer converts the audio signal into audible sounds having a volume responsive to the patient&#39;s pulse rate and oxygen saturation level. 
     Another aspect of the present invention is a monitor for a pulse oximeter for use in measuring a patient&#39;s pulse rate and oxygen saturation level. The monitor comprises a generator which generates an audio signal which varies in response to the patient&#39;s pulse rate and oxygen saturation level. A transducer is coupled to the generator to receive the audio signal. The transducer converts the audio signal into audible sounds with pitches of constant frequency. The audible sounds are responsive to the patient&#39;s pulse rate and oxygen saturation level. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present application is described below in connection with the attached drawing figures in which: 
     FIG. 1 schematically illustrates a pulse oximetry system utilizing one preferred embodiment of the present invention; 
     FIG. 2 illustrates a block diagram of the pulse oximeter of FIG. 1; 
     FIG. 3 illustrates a block diagram of particular elements of the pulse oximeter controller of FIG. 2; 
     FIG. 4 a  schematically illustrates the signal produced by the heartbeat algorithm corresponding to a patient&#39;s heartbeats; 
     FIG. 4 b  schematically illustrates the signal produced by the saturation algorithm corresponding to a patient&#39;s oxygen saturation level; 
     FIG. 4 c  schematically illustrates the temporal pattern and volume of the audio signal corresponding to the heartbeat and oxygen saturation level of FIGS. 4 a  and  4   b  produced by the generator of one preferred embodiment of the present invention; 
     FIGS. 5 a  and  5   b  schematically illustrate similar signals from the heartbeat algorithm and saturation algorithm as illustrated in FIGS. 4 a  and  4   b;    
     FIG. 5 c  schematically illustrates the temporal pattern and volume of the audio signal corresponding to the heartbeat and oxygen saturation level of FIGS. 5 a  and  5   b  produced by the generator of another preferred embodiment of the present invention; 
     FIGS. 6 a  and  6   b  schematically illustrate similar signals from the heartbeat algorithm and saturation algorithm as illustrated in FIGS. 4 a  and  4   b ; and 
     FIG. 6 c  schematically illustrates the temporal pattern and volume of the audio signal corresponding to the heartbeat and oxygen saturation level of FIGS. 6 a  and  6   b  produced by the generator of another preferred embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     FIG. 1 schematically illustrates a pulse oximetry system  1  utilizing one preferred embodiment of the present invention. The pulse oximetry system  1  comprises a pulse oximeter  10  connected to a sensor  20 . The sensor  20  provides to the pulse oximeter  10  a signal indicative of the transmitted light intensity through a section of the body tissue of the patient. The sensor  20  includes a light generator  22  that generates the light incident on the section of body tissue and a light detector  24  that detects the light transmitted through the section of body tissue. Such sensors in view of the disclosures herein are well-known to those skilled in the relevant art. 
     In FIG. 2, the pulse oximeter  10  comprises a controller  30  which is in communication with a sensor driver  40 , a display driver  50  coupled to a visual display  55 , and a transducer driver  60  coupled to a transducer (e.g., a speaker)  65 . The sensor driver  40  provides the sensor  20  with the power and signals required to operate the light generator  22  and the light detector  24  of the sensor  20 , based on the controlling signals from the controller  30 . The sensor driver  40  can also advantageously include a signal amplifier to amplify the signals from the sensor  20  to levels which are usable by the controller  30 . Upon analysis of the signals from the sensor driver  40 , the controller  30  generates signals sent to the display driver  50  and the transducer driver  60 . The visual display  55  and transducer  65  are driven by their respective drivers to produce visual and audible signals indicative of the patient&#39;s condition. Persons skilled in the art are aware of adequate display drivers, displays, transducer drivers, and transducers to utilize in view of the disclosures herein. 
     FIG. 3 schematically illustrates particular elements of the controller  30  which comprises in part a heartbeat algorithm  70 , a saturation algorithm  80 , and a generator  90 . The heartbeat algorithm  70  and the saturation algorithm  80  are preferably embodied in a microprocessor (not shown) programmed to function as described below. Based on signals from the sensor driver  40  corresponding to the heartbeat of the patient, the heartbeat algorithm  70  generates an output signal which is communicated to the generator  90 . The saturation algorithm  80  generates an output signal based on signals from the sensor driver  40  corresponding to the patient&#39;s oxygen saturation level. This output signal from the saturation algorithm  80  is then communicated to the generator  90 . As it receives the output signals from the heartbeat algorithm  70  and the saturation algorithm  80 , the generator  90  generates an audio signal which contains information concerning both the heartbeat and the oxygen saturation level of the patient. The audio signal from the generator  90  is a function of a comparison between the computed oxygen saturation level and various preset levels. This audio signal is then communicated to the transducer driver  60  to be converted by the transducer  65  into sounds audible to the monitoring individual. 
     FIG. 4 a  illustrates one preferred embodiment of the output signals from the heartbeat algorithm  70  and FIG. 4 b  illustrates one preferred embodiment of the output signals from the saturation algorithm  80 . As shown in FIG. 4 a , the heartbeat algorithm  70  produces for each detected heartbeat a corresponding signal which is communicated to the generator  90 . Thus, heartbeats detected at times t 1 , t 2 , t 3 , etc., correspond to signal pulses from the heartbeat algorithm  70  at times t 1 , t 2 , t 3 , etc. As shown in FIG. 4 b , the saturation algorithm  80  produces a signal corresponding to the patient&#39;s oxygen saturation level, which is communicated to the generator  90 . In the embodiment shown in FIG. 4 b , the patient&#39;s oxygen saturation level has decreased below a preset satisfactory level sometime between t 3  and t 4 , dipping below a preset danger level sometime between t 5  and t 6 . Persons skilled in the art recognize that there is a wide variety of possible signals produced by the heartbeat algorithm  70  and signals from the saturation algorithm  80  which can adequately convey information regarding the patient&#39;s heartbeat and oxygen saturation level to the generator  90 . 
     Monitors accompanying such pulse oximeters have been disclosed in the prior art. For example, New et al., U.S. Pat. No. 4,653,498 discloses a display monitor which emits an audible sound pulsed in synchroneity with the heartbeat of the monitored patient, with the pitch of the audible sound varying continuously with changes in the oxygen saturation. 
     FIG. 4 c  illustrates one preferred embodiment of the resulting audio signal produced by the generator  90 . In this preferred embodiment, the audio signal produced by the generator  90  is an audio pulse train, wherein each pulse in FIG. 4 c  represents a plurality of cycles of an audio tone having a selected duration (e.g., approximately 50 cycles of a 500 Hz tone having a duration of approximately 100 milliseconds). For each heartbeat of the patient, if the patient&#39;s oxygen saturation level is above the preset satisfactory level, the generator  90  produces an audio pulse corresponding to an audible sound of a preset volume. However, if the patient&#39;s oxygen saturation level has decreased below the preset satisfactory level, the audio pulse from the generator  90  corresponds to louder sounds generated by the transducer  65 . The magnitude of the sound volume is a function of the patient&#39;s oxygen saturation level as compared to the preset satisfactory level. Additional preset levels may be utilized to produce more significant increases of the sound volume for further reductions of the patient&#39;s oxygen saturation level. As in FIG. 4 c , if the patient&#39;s oxygen saturation level dips below the preset danger level, the audio pulses from the generator  90  correspond to significantly louder sounds generated by the transducer  65 . The net effect of this preferred embodiment is to yield audible sounds, in synchroneity with the patient&#39;s heartbeat, which become louder as the patient&#39;s oxygen saturation level decreases. 
     Such audible sounds for which the volume is correlated with the patient&#39;s oxygen saturation level provide additional benefits not found in the pulse oximeter monitors described in the prior art. First, interpretation of an increasingly louder sound requires less high-level thought processes since it is a less subjective judgment than is the relative pitch of a tone. By judiciously setting the preset oxygen saturation levels and the corresponding sound volumes, the pulse oximeter can be configured to produce unambiguous audible sounds indicative of a patient&#39;s oxygen saturation level. Second, individuals monitoring the patient&#39;s condition are more psychologically prone to react to increases of sound volume as indications of the patient&#39;s deteriorating condition than they are to changes of pitch. Therefore, it is expected that reaction times and urgency will be better correlated with the severity of the patient&#39;s condition once the pulse oximeter is properly configured. Third, by utilizing the sound volume as an indication of the patient&#39;s oxygen saturation level, the influence of the monitoring individual&#39;s hearing ability is lessened, as compared to the prior art systems which utilize pitch to indicate oxygen saturation levels. While the audible sounds in both types of systems must be loud enough for the monitoring individual to hear, the present invention is not vulnerable to any hearing deficiencies of the monitoring individual with respect to pitch. If a sound corresponding to a satisfactory oxygen saturation level is sufficiently loud to be heard, it is probable that the monitoring individual can readily hear increases of the sound&#39;s volume associated with reductions of the patient&#39;s oxygen saturation level. 
     FIGS. 5 a  and  5   b  illustrate similar output signals from the heartbeat algorithm  70  and the saturation algorithm  80  as illustrated in FIGS. 4 a  and  4   b  and described above. FIG. 5 c  illustrates another preferred embodiment of the audio signal produced by the generator  90 . In this preferred embodiment, the audio signal produced by the generator  90  is an audio pulse train. For each heartbeat of the patient, if the patient&#39;s oxygen saturation level is above the preset satisfactory level, the generator  90  produces a single audio pulse corresponding to a single audible sound of a preset volume. However, if the patient&#39;s oxygen saturation level has decreased below the preset satisfactory level, the audio signal from the generator  90  causes additional sounds to be generated by the transducer  65 . As in the embodiment illustrated in FIG. 5 c , once the patient&#39;s oxygen saturation level dips below the preset satisfactory level, the generator  90  produces two or more audio pulses per heartbeat instead of the single audio pulse per heartbeat indicative of satisfactory oxygen saturation levels. Each audio pulse is then communicated to the transducer  65 , resulting in two or more audible sounds in rapid succession. The number of audio pulses per heartbeat is a function of the magnitude of the patient&#39;s oxygen saturation level as compared to the preset satisfactory level. Additional preset levels may be utilized to produce more numerous audio pulses for further reductions of the patient&#39;s oxygen saturation level. As in FIG. 5 c , if the patient&#39;s oxygen saturation level dips below the additional preset danger level, the generator  90  produces six audio pulses per heartbeat which are communicated to the transducer  65 . The net effect of this preferred embodiment is to yield sets of audible sounds in synchroneity with the patient&#39;s heartbeat, with the number of sounds in each set becoming larger as the patient&#39;s oxygen saturation level decreases. 
     In FIG. 5 c , the audible signal is illustrated as having zero volume between heartbeats and between each pulse in a set of pulses and having maximum volume for each pulse. It should be understood that the volume between heartbeats and between the pulses can be non-zero. Thus, it should be understood that the embodiment of FIG. 5 c  also illustrates the control of the volume of the audible signal in response to the patient&#39;s oxygen saturation level. 
     Such audible sounds for which the number of sounds per heartbeat is correlated with the patient&#39;s oxygen saturation level provide additional benefits not found in the pulse oximeter monitors described in the prior art or in the embodiment illustrated in FIG. 4 c . First, interpretation of the number of audible sounds per heartbeat is a less subjective judgment than either the interpretation of relative pitches or relative volumes. By judiciously setting the preset oxygen saturation levels and the corresponding number of tones per heartbeat, the pulse oximeter can be configured to produce unambiguous audible sounds indicative of a patient&#39;s oxygen saturation level. Second, individuals monitoring the patient&#39;s condition are psychologically prone to react to rapidly repeating sounds as an indication of the patient&#39;s deteriorating condition. Once the monitoring individual is informed that numerous sounds in rapid succession means that the patient&#39;s condition is deteriorating, it is expected that reaction times and urgency will be better correlated with the severity of the patient&#39;s condition once the pulse oximeter is properly configured. Third, by utilizing the number of sounds per heartbeat as an indication of the patient&#39;s oxygen saturation level, the influence of the monitoring individual&#39;s hearing ability is practically eliminated. As long as the single sound per heartbeat for satisfactory oxygen saturation levels is audible, it is assured that the additional sounds associated with less than satisfactory levels will also be audible. 
     FIGS. 6 a  and  6   b  illustrate similar output signals from the heartbeat algorithm  70  and the saturation algorithm  80  as illustrated in FIGS. 4 a  and  4   b  and described above. FIG. 6 c  illustrates another preferred embodiment of the audio signal produced by the generator  90 . In this preferred embodiment, the audio signal produced by the generator  90  is an audio pulse train comprising pulses which have a volume defined by an envelope. The envelope has a shape which varies in response to changes in the patient&#39;s oxygen saturation. For each heartbeat of the patient, if the patient&#39;s oxygen saturation level is above the preset satisfactory level, the generator  90  produces an audio pulse corresponding to an audible sound with a preset volume shape. As used herein, the term volume shape means the shape of the volume envelope of the audible sound at each moment from the start of one heartbeat to the start of the subsequent heartbeat. In the preferred embodiment illustrated in FIG. 6 c , this audible sound has a preset square volume shape (i.e., a single volume level during a time interval). However, if the patient&#39;s oxygen saturation level has decreased below the preset satisfactory level, the audio pulses from the generator  90  correspond to audible sounds with different volume shapes. As in the embodiment illustrated in FIG. 6 c , once the patient&#39;s oxygen saturation level dips below the preset satisfactory level, the audio pulses communicated to the transducer  65  result in audible sounds with stepped volume shapes (i.e., multiple volume levels during a time interval) with two volume levels. The volume shape of each audible sound is a function of the magnitude of the patient&#39;s oxygen saturation level as compared to the preset satisfactory level. Additional preset levels may be utilized to produce audible sounds with other volume shapes for further reductions of the patient&#39;s oxygen saturation level. As in FIG. 6 c , if the patient&#39;s oxygen saturation level dips below the additional preset danger level, the audio pulses from the generator  90  result in audible sounds with a stepped volume shape with three volume levels. Persons skilled in the art recognize that a wide variety of volume shapes (e.g., square, stepped, triangular, sinusoidal) can be utilized when practicing the present invention. The net effect of this preferred embodiment is to yield sets of audible sounds in synchroneity with the patient&#39;s heartbeat, with the volume shape of the sounds in each set varying as the patient&#39;s oxygen saturation level decreases. 
     For some persons, the variations of the volume shape of the audible sound per heartbeat may be more recognizable than either the differing volumes among audible sounds corresponding to different heartbeats or the number of sounds per heartbeat. In particularly preferred embodiments, the embodiment to be used at any given time may be selected by the setting of an output mode selector (e.g., a switch, knob, or program) of the pulse oximeter. 
     This invention may be embodied in other specific forms without departing from the essential characteristics as described herein. The embodiments described above are to be considered in all respects as illustrative only and not restrictive in any manner. The scope of the invention is indicated by the following claims rather than by the foregoing description. Any and all changes which come within the meaning and range of equivalency of the claims are to be considered within their scope.
A monitor for a pulse oximeter emits audible sounds containing information regarding the patient's pulse rate and oxygen saturation level wherein the information is not contained in variations of the pitch of the audible sounds. By utilizing sets of audible sounds pulsed in synchroneity with the heartbeat of the patient in which the sound volume is a function of the patient's oxygen saturation level, the urgency of the patient's condition is effectively expressed to the monitoring individual. Alternatively, the urgency of the patient's condition is effectively expressed by varying the number of audible sounds in each set of audible sounds, or by varying the volume shape of each audible sound. The present invention represents an improvement over the monitors described in the prior art which utilize variations of pitch with oxygen saturation level because the present invention requires less training and less subjective judgment, can be more quickly interpreted, and is less affected by hearing disabilities of the monitoring individuals.