System for determining the physical activity of a patient by measuring the blood oxygen saturation

Abstract: A and system for determining the physical activity of a patient senses the minimum blood oxygen saturation in the right atrium (38) of the patient's heart (36) and uses such minimum blood oxygen saturation as a control parameter for indicating the muscular activity of the patient. Because the oxygen content of the venous blood in the right atrium varies significantly as venous blood from all parts of the body is introduced therein, evidencing differing levels of oxygen demand throughout the patient's body, the minimum oxygen content of the venous blood provides an accurate and reliable measure of those portions of the patient's body experiencing the greatest oxygen demand, i.e., experiencing muscular activity. The determined minimum blood oxygen saturation can be used to control the stimulating rate of a pacemaker.

Description: Background of the Invention The present invention relates to a system for determining the physical activity of a patient by measuring the blood oxygen saturation. The determined physical activity can be used as a control parameter to adjust the rate at which an implantable pacemaker generates and delivers electrical stimulation pulses to a patient's heart, or as a control parameter to control the medication delivery of an implantable medication delivering apparatus. A pacemaker is a medical device, usually an implantable medical device, that provides electrical stimulation pulses to a patient's heart at a controlled rate for the purpose of controlling the heart rate. Most modern implantable pacemakers can be programmed to operate in several modes, as required by the needs of a particular patient. Several common modes of operation provide stimulation pulses only when the patient's heart does not beat by itself at a minimum rate. In such mode(s), the stimulation pulses are provided only when needed, or "on demand", thereby preserving the limited power source of the implanted pacemaker for the longest possible time. Typically, the manner in which such demand pacemakers operate is to define a basic pacing interval (sometimes referred to as an "escape interval") and to wait and see if the heart beats during this interval. (A heart beat is determined by sensing a "P- Wave", indicating contraction of the atria, or an "R- wave", indicating contraction of the ventricles.) If so, the basic pacing interval starts over, and no stimulation pulse is provided. If not, a stimulation pulse is provided at the end of the pacing interval. In this manner, the pacemaker's pacing interval defines the rate at which stimulation pulses are provided to the heart in the absence of naturally occurring heart beats. It is noted that pacemakers may be employed that stimulate either, or both, chambers of the heart (i.e., either the right atrium and/or the right ventricle). A rate-responsive pacemaker is a pacemaker that automatically adjusts the pacing interval, or the rate at which stimulation pulses are provided to the patient's heart, as a function of the sensed physiological needs of the patient. That is, every person has times when his or her heart needs to beat fast, and times when his or her heart should beat slow. For example, physical activity causes a person's heart rate to increase in order to compensate for the increased oxygen demands of the muscle tissue undergoing the physical activity. Similarly, physical inactivity, such as prolonged periods of sleep or rest, allow a person's heart rate to decrease because the oxygen demands of the body tissue are less. A rate-responsive pacemaker thus attempts to sense the physiological needs of a patient at a particular time, e.g., by sensing physical activity or inactivity, and adjusts the pacing interval of the pacemaker accordingly. The operation and design of pacemakers, including rate-responsive pacemakers, are known in the art. See, e.g., Furman et al., A Practice of Cardiac Pacing , (Futura Publishing Co., Mt. Kisco, N.Y. 1986); Moses, et al., A Practical Guide to Cardiac Pacing (Little, Brown & Co., Boston/Toronto 1983); U.S. Pat. No. 4,712,555 (Thornander et al.); U.S. Pat. No. 4,856,523 (Sholder et al.). U.S. Pat. 4,712,555 (Thornander et al.) is a particularly comprehensive reference explaining the general operation of a rate-responsive pacemaker, and the application of one particular type of physiological parameter (a timing interval) for controlling such pacemaker. Further, it is known in the art to sense several different physiological parameters as the control parameter of a rate-responsive pacemaker. One common type of sensor is an activity sensor that senses the physical activity level of the patient. See, e.g., U.S. Patent No. 4,140,132 issued to Dahl; and U.S. Patent No. 4,485,813 issued to Anderson. Other types of sensors used in prior art rate-responsive pacers include sensors that sense respiration rate, blood and/or body temperature, blood pressure, the length of the Q-T interval, and the length of the P-R interval. Of particular significance to the present invention, it is also known in the art to use an implantable sensor to determine the oxygen content of blood and to use such sensor in a rate-responsive pacemaker. See, e.g., U.S. Patents No. 4,202,339 to Wirtzfeld; 4,399,820 to Wirtzfeld et al.; and 4,815,469 to Cohen et al. Further, recent studies have suggested that mixed venous oxygen saturation provides one of the best indications available of physiological need, especially for low and medium levels of exercise (physical activity). It has thus been suggested that mixed venous oxygen saturation, when combined with other parameters, provides a very useful control parameter for controlling a rate-responsive pacemaker. See, Stangl, et al., "A New Multisensor Pacing System Using Stroke Volume, Respiratory Rate, mixed Venous Oxygen Saturation, and Temperature, Right Atrial Pressure, Right Ventricular Pressure and dP/dt," PACE , Vol. 11, pp 712-724 (June 1988). Unfortunately, while oxygen saturation may be one of the most sensitive parameters to indicate low and medium level exercise, the techniques heretofore used in the prior art to sense oxygen saturation have masked out the most beneficial information provided by this parameter. For example, oxygen saturation is typically sensed optically using a sensor that includes both a source of light, such as a light emitting diode (LED), and a means for detecting light, such as a phototransistor. The sensor, including both LED and phototransistor, is positioned in the right ventricle to sense venous oxygen saturation. Light energy is directed to the blood in the right ventricle from the light source. The amount of light energy reflected back to the phototransistor is a function of the properties of the blood, including the level of oxygen saturation of the blood. Thus, by monitoring the ratio of emitted light energy to reflected light energy, it is possible to measure the blood oxygen saturation level of the blood in the right ventricle. However, because the return blood in the right ventricle comes from all parts of the body, it contains significantly different levels of blood oxygen saturation, reflecting the different activity levels of various parts of the body. That is, if the patient is walking, the blood returned from the legs and arms (assuming the arms are swinging as the legs are walking) will have a significantly lower oxygen content than will blood from other parts of the body. This is because the leg and arm muscle tissue is working harder (and therefore consuming more oxygen) than is muscle tissue at other body locations. Hence, while the blood oxygen saturation measured in the right atrium tends to fluctuate over a wide range, by the time it reaches the right ventricle, it is mixed fairly well at the time the measurement is made. Thus, blood oxygen saturation in the right ventricle will fluctuate to some degree, but not so much as it does in the right atrium. To compensate for these fluctuations, the prior art teaches averaging or integrating the measurement over a sufficiently long period of time to smooth out such fluctuations. Disadvantageously, such averaging or integrating masks out the most beneficial portions of the measurement--the oxygen saturation level of the blood returned from the isolated parts of the body, such as the arms and legs, or other parts of the body, that are experiencing physical activity. What is needed, therefore, is a technique for measuring the oxygen saturation of the blood as it first returns to the heart and still provides information on the portions of the body undergoing the greatest physical activity, thus isolating that portion of the fluctuating oxygen saturation measurement indicative of such physical activity. Further, when measuring blood oxygen saturation using an optical sensor that measures reflected light energy, and when such sensor is positioned in the heart, the amount of reflected light energy detected by such sensor is significantly influenced by optical reflections from the heart wall or valves. Such optical reflections disadvantageously may, during portions of the heart's activity, give erroneously high readings. Hence, what is needed is a sensing method or system that senses only those optical reflections from returned blood, not from optical reflections occurring within the heart. More particularly, what is needed is a system for sensing optical reflections from blood returned to the heart from only those body portions undergoing the most strenuous physical activity. US 4,870,968 relates to a system for controlling the stimulation frequency of heart pacemakers in which a mean value of measured blood oxygen signals is taken based on a plurality of minimum values taken during respective measurement cycles. The averaged value is subjected to a further control function to determine the final pacing rate. The present invention advantageously provides a system of blood oxygen saturation measurement that addresses the above and other needs. Summary of the Invention The disadvantages and limitations of the prior art discussed above are overcome by the present invention as defined in the claims. According to the invention there is provided a rate-responsive pacemaker comprising: rate-responsive pulse generating means for generating stimulation pulses; measurement means for measuring the oxygen content of blood; monitoring means for monitoring the blood oxygen measurements made with said measurement means, the measurement means being adapted for repeatedly measuring the oxygen content of the blood in the right atrium, the monitoring means being adapted for monitoring the blood oxygen measurements over a prescribed interval; and determining means provided for determining the minimum value of blood oxygen content measured during said prescribed interval, characterised in that: the measurement means are arranged to be located in the heart to measure the oxygen content of blood in the right atrium of the patient's heart as the blood enters the right atrium before the blood has an opportunity to become thoroughly mixed; said interval includes a plurality of consecutive cardiac cycles; and said minimum value of blood oxygen content provides a direct indicator of the blood oxygen content of the blood returning from body tissue of the patient undergoing the greatest oxygen demand, and hence a direct indicator of the body tissue experiencing physical exercise, the lower the minimum blood oxygen content the greater the physical activity; said rate-responsive pulse generating means generating stimulation pulses at a rate directly controlled by said minium value of blood oxygen content. In accordance with one aspect of the present invention, a system for determining the physical activity of a patient senses the minimum blood oxygen saturation level of the venous blood in the right atrium of a heart, and uses such minimum blood oxygen saturation level as a control parameter for indicating the muscular activity of a patient. As mentioned above, the oxygen content of the venous blood in the right atrium varies significantly as venous blood from all parts of the body returns thereto, but is not thoroughly mixed therein. Some of the venous blood exhibits a low oxygen content when returned from insulated parts of the body, such as the arms or legs, undergoing muscular activity. Other of the venous blood, from parts of the body not experiencing significant muscular activity, exhibits a higher blood oxygen content. Advantageously, the minimum oxygen content of the poorly mixed venous blood in the right atrium thus provides an accurate and reliable measure of muscular activity used to adjust the rate at which pacing pulses are provided on demand to the patient. For that purpose, a blood oxygen sensor, e.g., a sensor that optically senses the oxygen content of the blood in contact therewith, is positioned so as to sense the oxygen content of the poorly mixed venous blood in the right atrium. Preferably, this oxygen sensor forms an integral part of a pacing lead that couples a rate-responsive pacemaker with the heart. The minimum oxygen content thus sensed is used as a control parameter to automatically adjust the pacing rate of the rate-responsive pacemaker, i.e., to automatically adjust the rate at which pacing or stimulation pulses are provided on demand by the pacemaker in order to meet the physiologic needs of the patient. An optical sensor is preferably used with the present invention to sense the oxygen content of the venous blood in the right atrium. Advantageously, when this is done, the minimum oxygen content thus sensed automatically rejects any erroneous high readings caused by optical reflections within the heart. Alternately, a chemical sensor measuring oxygen partial pressure or the partial pressure of carbon dioxide may be used, the consumption of oxygen produces an equivalent amount of carbon dioxide; thus, either may be measured and used as an indicator of physical activity. If a carbon dioxide sensor is used, the system will look for the maximum carbon dioxide reading (indicating minimum oxygen in the blood). This maximum will be used to control the pacemaker or a medication delivering system. The system according to the invention will eliminate the disadvantages of previously-known systems. Since the minimum signal is taken in the atrium, before the blood is mixed, it will represent the muscular activity of isolated parts of the body being exercised, such as the arms and legs. By using this minimum signal, the system becomes more sensitive both to localized exercise and to lower levels of exercise. In addition, the use of the minimum signal also eliminates the effect of high signal levels caused by reflections from the wall of the heart or the valve. Thus, artifacts caused by such reflections are eliminated from the signal utilized. It is a preferred feature of the present invention to provide an accurate and reliable system for determining the oxygen content of venous blood using optical measuring techniques, i.e., emitting light energy into the blood and sensing the amount of light energy reflected therefrom, despite reflections and other erroneous light energy sensings that may occur. It is another preferred feature of the invention to provide a reliable and accurate system for measuring the oxygen content of blood returning from body tissue undergoing the greatest oxygen demand, e.g., experiencing the most physical exercise, even though such blood is in the process of being mixed with blood returning from body tissue not experiencing a high oxygen demand. In other words, it is a feature of the present invention to provide a system that sorts relevant blood oxygen measurements (i.e., from blood returning from active body tissue) from irrelevant blood oxygen measurements (i.e., from blood returning from non-active body tissue). It is a further preferred feature of the invention to provide such a system for measuring relevant blood oxygen levels that can be used as a control parameter in a rate-responsive pacing system. It is yet another preferred feature of the invention to provide a rate-responsive pacing system wherein stimulation pulses may be provided to a patient's heart on demand at a rate that is determined by the oxygen content of blood returning from those parts of the patient's body experiencing the greatest oxygen demand. Brief Description of the Drawings The above and other aspects, features and advantages of the present invention will be more apparent from the following more particular description thereof, presented in conjunction with the following drawings wherein: FIG. 1 is a schematic diagram of an optical blood oxygen sensor of the prior art; FIG. 2 is a block diagram depicting a blood oxygen sensor used in a rate-responsive pacing system; FIG. 3A is a waveform diagram illustrating representative fluctuations in the output signal from an oxygen sensor when such sensor is placed in the right atrium of a patient's heart, as shown in FIG. 2; FIG. 3B is a waveform diagram of a small segment of the waveform in FIG. 3A, with an expanded horizontal (time) axis; FIG. 4 is a simplified flow chart depicting a method of adjusting the pacing interval of a rate-responsive pacing system in accordance with one embodiment of the present invention; FIG. 5 is a simplified functional block diagram of the sensor process circuit of FIG. 2 made in accordance with a digital embodiment of the present invention; FIG. 6A is a simplified electrical schematic diagram of the sensor process circuit of FIG. 2 made in accordance with an analog embodiment of the present invention; FIG. 6B is a timing waveform diagram illustrating the operation of the analog embodiment of FIG. 6A; FIG. 6C shows one embodiment of the circuit of FIG. 6A; and FIG. 6D is a timing diagram illustrating the relationship between the clock signals used in the peak detection circuit of FIG. 6C. Detailed Description of the Invention It is noted that in accordance with one aspect of the present invention, a rate-responsive pacing system is provided in which the oxygen content of blood is used as a physiological parameter to control the rate at which stimulation pulses are provided to a heart of a patient. As previously indicated, rate-responsive pacing systems are known and described in the art, and will not be described in any detail herein. While such systems take several forms, all employ some means for sensing one or more physiological parameters of the patient indicative of how fast or slow the patient's heart should beat. The present invention is directed primarily to the manner in which the oxygen content of blood can be accurately measured, and once measured, used as a physiological parameter for controlling a rate-responsive pacing system. In order to better appreciate the advantages associated with the use of the present invention, it will first be helpful to have a basic understanding of the manner in which oxygen content of blood is sensed. Accordingly, reference is made to FIG. 1, where there is shown a schematic diagram of an optical blood oxygen sensor of the prior art. The sensor includes two light-emitting diodes 20 and 22 connected in parallel, with the anode of diode 20 being connected to the cathode of diode 22, and the anode of diode 22 being connected to the cathode of diode 20. A phototransistor 24 is connected in parallel with a resistor 26, and the collector of the phototransistor 24 is connected to the same node as is the anode of diode 22 and the cathode of diode 20. The node comprising the anode of diode 20 and the cathode 22 comprises one input terminal 28, and the emitter of phototransistor 24 and one side of the resistor 26 comprises another terminal 30 of the sensor. In operation, a bi-phase voltage pulse is applied across terminals 28 and 30. This bi-phase voltage pulse is also illustrated in FIG. 1 and includes a positive portion, having an amplitude of +V1; followed by a negative portion, having a negative amplitude of -V2. The positive portion of the bi-phase voltage pulse causes a current I1 to flow through light-emitting diode 20, thereby causing light energy E1 to be emitted by the LED 20. The light E1 comes in contact with a desired body fluid 32, such as blood. Depending upon the properties of the fluid 32, a portion of the light energy E1 is reflected back to the phototransistor 24. In FIG. 1, as well as in the other figures, that portion of light energy reflected back to the phototransistor is identified as E2. Thus, in FIG. 1, the amount of current I2 that flows through phototransistor 24 is a function of the light energy E2 that is incident upon the base of the phototransistor 24. The balance of the current I1 that does not flow through phototransistor 24, therefore, flows through the resistor 26. This current is identified as I3. Thus, it is seen that I1 is equal to I2 and I3. The current I2 varies as a function of the light energy E2, thereby also affecting the amount of current I3 that flows through resistor 26. The voltage developed across terminals 28 and 30 (which voltage is a function of the forward drop across LED 20 and the voltage drop across resister 26 caused by the current flow I3) will thus vary as a function of the reflected light energy E2 that is incident upon the phototransistor 24. Hence, by monitoring the voltage across the terminals 28 and 30, it is possible to get an indication of the reflectance properties of the fluid 32. In order to determine the amount of voltage variation across terminals 28 and 30 caused by the current I2, it is necessary to isolate other variations in this voltage from the measurement. This is typically done by causing current I4 to flow through resistor 26 and LED 22 during the negative portion of the bi-phase voltage waveform. During this portion of the waveform, both the phototransistor 24 and LED 20 are back biased, and therefore no current flows through either of these devices. The value of I4 is selected to be close to the value of I1 so that the forward voltage drop across LED 20 will be approximately the same as the forward voltage drop across LED 22. Because of some of the difficulties associated with using and operating the prior art sensor shown in FIG. 1, an improved blood oxygen sensor has been proposed as disclosed in U.S. Patent No. 4,815,469 to Cohen et al. While one of the sensors described in the '469 patent, is the preferred type of sensor for use with the present invention, it is to be emphasized that any type of sensor capable of sensing the oxygen content of blood (or the carbon dioxide content of blood) may be used with the methods and systems of the present invention. Referring next to FIG. 2, there is shown a block diagram depicting a preferred manner of using a blood oxygen sensor 34 (such as the sensor described in the '469 patent), in a rate-responsive pacing system. Although FIG. 2 depicts a unipolar single-chamber pacemaker, the present invention is equally applicable to bipolar and/or dual-chamber pacemakers. The sensor 34 is positioned within an area of a patient's body where venous blood is able to come in contact with light energy E1 emitted by the sensor. The preferred placement of the sensor is within a heart 36 of the patient, and more particularly within the right atrium 38 of the heart 36. An implantable rate-responsive pacemaker 40 is implanted in the patient in conventional manner. Included within the implantable pacemaker 40 is a sensor drive circuit 42, a sensor process circuit 44, and conventional rate-responsive pacemaker circuits 46. Also included in the pacemaker 40 is a source of electrical energy, e.g., a battery 48. The drive circuit 42 provides the drive voltage necessary for operation of the sensor 34. The sensor process circuit 44, senses the returning signal from the sensor, e.g. the voltage potential at the output of the sensor 34, in response to an applied drive voltage. As is known in the art, and as described in the '469 patent, this sensor output voltage varies as a function of the oxygen content of the blood from which the emitted light energy is reflected. Hence, by monitoring changes in this output voltage, a qualitative measurement of the oxygen content of the blood may be made. By using appropriate calibration techniques, a quantitative measurement of the oxygen content of the blood may be made. The drive circuit 42 and the sensor circuit 44 are coupled to each other and to the pacemaker circuits 46. Appropriate timing signals 50 are shared between the sensor drive circuit 42 and the sensor process circuit 44. Such timing signals assure that both circuits operate only at a desired time within the cardiac cycle or other control cycle. (The "cardiac cycle" is the time required by the heart 36 to complete one beat. This cycle is typically manifest by contraction or depolarization of the atria, evidenced by the generation of a P-wave, followed by contraction or depolarization of the ventricles, evidenced by the generation of an R-wave. P-waves and R-waves are evident by examining the patient's electrocardiogram, or ECG. The cardiac cycle is frequently measured from R-wave to R-wave, as the R-wave is the predominant wave, and thus the easiest to measure, in the ECG.) Further, in order to synchronize the sensing function of the sensor 34 with other events associated with the operation of the pacemaker circuits 46, the sensor drive circuit 42 and the sensor process circuit 44 receive a clock signal 52 and a timing reference signal 54 from the pacemaker circuits 46. Thus, for example, the timing reference signal 54 may be a signal indicating a cardiac event, such as a V-pulse or an R-wave signal, which signals indicate that the ventricle of the heart has either been paced (meaning that a stimulation pulse, e.g. a ventricular stimulation pulse, or V-pulse, has been provided by the pacemaker), or that a ventricular contraction, an R-wave, has been sensed. In operation, the clock signal 52, as well as a timing reference signal, such as a V/R signal, are provided from the pacemaker circuit 46 to the sensor drive circuit 42 and the sensor process circuit 44. A pacing lead 60, connected to the pacemaker 40 by way of, e.g., a conventional bipolar pacer connector 62, allows the pacemaker to deliver stimulation pulses to the heart 36 at a distal electrode tip 66 through conductor 70. This same conductor 70 allows the pacemaker circuits 46 to sense cardiac events occurring near the lead tip 66. In a preferred embodiment, the sensor 34 is advantageously embedded within the pacemaker lead 60 at a distance from the distal tip so as to place the sensor 34 in the right atrium 38 of the heart 36. Further, when positioned properly within the heart, the lead is formed in a manner that causes the sensor 34 to face blood (and therefore measure the oxygen content of blood) just after the blood enters the atrium 38, before such blood has an opportunity to become thoroughly mixed within the atrium. One terminal of the sensor 34 is connected to a separate conductor 68 of the lead 60. The other terminal of the sensor 34 is connected within the lead to the conductor 70. The sensor process circuit 44 develops a control signal 49 that is representative of the reflectance properties of the blood (and hence relatable to the amount of oxygen within the blood). This control signal 49 is presented to the pacemaker circuits 46 and is used as a physiological parameter to control the rate at which the pacemaker circuits deliver a stimulation pulse to the heart. Referring next to FIG. 3A, a waveform diagram illustrating representative fluctuations in the output signal from the sensor 34 of FIG. 2 (when such sensor is placed in the right atrium 38 of a patient's heart 36) is illustrated. The horizontal axis in the diagram shown in FIG. 3A represents time, while the vertical axis represents the output signal, e.g., the output voltage, obtained from the sensor 34. As this output signal represents the optical reflectance properties of the blood, which properties are relatable to the oxygen content of the blood, the waveform shown in FIG. 3A thus depicts the variations in the oxygen content of the blood as a function of time. The blood oxygen content measured in the right atrium of the patient's heart fluctuates as a function of time for two reasons: (1) there are different oxygen demands placed on the patient's body tissue at different times of the day depending upon the activities of the patient; and (2) different body tissue within the patient undergoes different oxygen demands because of the location of the body tissue. The first variation is a relatively slow variation, and may be considered as the average oxygen demand. At certain times of the day, such as when the patient is sleeping, the average oxygen demand is lowest. At other times of the day, such as when the patient is exercising, the average oxygen demand increases significantly. The second variation is a relatively fast variation, and occurs due to the fact that the blood returning to the right atrium from various body tissue locations is rather poorly mixed. Thoroughly mixed blood, from all body tissue locations, would only reflect gross changes in metabolic rate. In FIG. 3A, the first type of variation is predominantly illustrated. At time t1, for example, when the sensor output is high, the blood oxygen content is likewise high, indicating a time of relative inactivity of the patient. In contrast, at times t2 and t3, when the sensor output is low, the blood oxygen content is likewise low, indicating a time of relative activity of the patient. In FIG. 3B, the second type of variation is illustrated. That is, FIG. 3B depicts the type of variations in the blood oxygen measurement that may occur during a relatively short portion of the waveform of FIG. 3A, e.g., during the portion included within the circle B. As seen in FIG. 3B, such variations in the sensor output may be rather abrupt and sudden, evidencing the entry of blood into the right atrium from body tissue locations having markedly different oxygen content. A low sensor output, such as at the point P1, may be indicative of blood returning from a relatively active portion of the patient's body, such as an arm, where the oxygen demand of the body tissue is high. A high sensor output, such as at point P2, may be indicative of blood returning from a relatively inactive portion of the patient's body, where the oxygen demand of the body tissue is low. It is, of course, not possible to determine the origin of the blood with high or low oxygen content. Alternatively, a high sensor output, such as at point P3, may be indicative of inappropriate reflection of light energy into the phototransistor of the sensor caused, e.g., by a moving heart valve or the wall of the heart. In operation, the sensor 34 does not typically operate continuously (although it could with appropriate circuitry). That is, the sensor is typically energized during a refractory period of the heart and/or pacemaker circuits, and a "sample" of the blood oxygen content at that measurement time is made. Such sample times, i.e., those times when a measurement is made, are represented in FIG. 3B as marks equally spaced along the horizontal axis. Statistically, assuming the fast variations in the blood oxygen content are more or less random, some of these sample times occur when the blood oxygen content is low, and others occur when it is high. Hence, within a particular measurement window 71, which "window" 71 includes a plurality of sample times, there will be one sample measurement that has a lower value than the others. In FIG. 3B, this low or minimum measurement is the one made at point P1. It is a feature of the present invention, to identify the low or minimum measurement within a given measurement window 71, and to use such measurement as an indicator of the relevant blood oxygen content, i.e., to use such minimum value as an indicator of the oxygen content of the blood returning from the body tissue undergoing the highest oxygen demand. This minimum value can then be used as a reliable indicator of the physiological need to adjust the heart rate, e.g., as controlled by a rate-responsive pacemaker. It is to be noted that while FIG. 3B suggests that sample measurements made within the measurement window 71 be equally spaced in time, such equally spaced samples are not necessary. If sample measurements are taken, all that is necessary is that sufficient samples be obtained so that a statistically accurate minimum value will be obtained. (In contrast, if a continuous measurement is made, all that is required is that the minimum value of the blood oxygen content be determined for a prescribed measurement window.) For example, a plurality of discrete blood oxygen measurements could be made only during the refractory interval of a cardiac cycle. Such refractory interval may last, e.g., only 10-20 milliseconds during an 800 millisecond cardiac cycle. However, during this 10-20 milliseconds, several discrete measurements, e.g. 5-10, of the blood oxygen content can be measured. Alternatively, there is no requirement that the blood oxygen measurement be performed only during a refractory period. Thus, if desired, the blood oxygen measurement can be made at regular intervals throughout the cardiac cycle, either synchronous with the cardiac cycle, or asynchronous relative to the cardiac cycle. FIG. 4 shows a simplified flow chart depicting one method of determining the minimum blood oxygen measurement and using this minimum blood oxygen measurement to automatically adjust the pacing interval of a rate-responsive pacing system. As seen in FIG. 4, once rate-responsive (RR) pacing has been started, an initialization step is performed. Such initialization step may involve, e.g., calibrating the blood oxygen sensor against a standard. Further, such step may involve assigning nominal values of blood oxygen measurement values until actual measurements of blood oxygen can be made. Once initialized, the minimum value of the last n (where n is an integer) consecutive blood oxygen measurements (where the term "S" is used in FIG. 4 to signify a blood oxygen measurement) is selected. This minimum value of S is then used as the control parameter to adjust the pacing interval of the pacemaker. If RR pacing is to continue, then the next value of S is measured and the process repeats. To further illustrate the method depicted in FIG. 4, the following example is provided. Assume that twenty (n=20) consecutive measurements of S are to be made. Initialization of the method may thus involve assigning nominal values to represent the last 20 measurements. Once initialized, a first actual measurement is made. The minimum value of blood oxygen represented in this first actual measurement and 19 of the initialized nominal values is then selected as the control parameter for the RR pacing. A second blood oxygen measurement is then made. The minimum value of blood oxygen represented in the first and second actual blood oxygen measurements and 18 of the initialized nominal values is then selected as the control parameter for the RR pacing. This process continues until twenty actual measurements have been consecutively made and all twenty have been examined to determine the minimum blood oxygen measurement. Each time a new blood oxygen measurement is made, the oldest of the twenty most recent blood oxygen measurements is discarded. In this way, the minimum blood oxygen measurement is always selected from the twenty most recent measurements. The minimum blood oxygen measurement selected from the twenty most recent measurements will change only if the new blood oxygen measurement is less than the prior nineteen measurements. It is to be understood that the above example is only one possible implementation of the method shown in FIG. 4. Any value of n could be employed, from very small values (e.g., n=3) to very large values (e.g., n=100). Further, the value of n may be automatically changed by the RR pacing circuits at certain threshold levels. For example, at normal heart rates, when the patient is at rest, one value of n may be used. At higher heart rates, when the patient is exercising, a different value of n may be used. Further, it is to be emphasized that the method shown in FIG. 4 is only one of several different types of methods or algorithms that may be used to select the minimum oxygen measurement. Any method of algorithm that systematically determines the minimum oxygen content measurement may be used. Referring next to FIG. 5, a simplified functional block diagram of a digital embodiment of the sensor process circuit 44 of FIG. 2 is shown. Such embodiment can be readily incorporated into a digitally controlled rate-rate-responsive pacemaker. (It is noted that most modern pacemakers are digitally controlled, many involving the use of a microprocessor, or equivalent, to control the operation of the pacemaker in accordance with prescribed operating programs, which programs may be altered as required to suit the needs of a particular patient.) As seen in FIG. 5, the output signal from the sensor 34, E OUT , is directed to an analog-to-digital (A/D) converter 80. The resulting digital output signal is stored in a first-in first-out (FIFO) register stack 82. The number of registers included in the FIFO register stack 82 may be selected to be any desired value, e.g., 32. The contents of the various registers within the FIFO stack 82 are compared in Minimum Value Logic circuitry 84. That is, circuitry 84 compares the contents of each register in the FIFO stack 82 and determines which one has the lowest or minimum value. This value is then selected and placed in a holding register 86. Further, during a calibration mode, as explained below, the circuitry 84 may be programmed to make an adjustment to the minimum value selected in order to compensate for variations that may occur over time to the sensor 34. During normal rate-responsive operation, the value held in the holding register 86 is selected by a multiplexer (MUX) circuit 88 and placed in a control register 90. The rate-responsive pacing circuits 46 (FIG. 2) look to the control register 90 for the control parameter that sets or controls the rate-responsive pacing interval. Still referring to FIG. 5, selection logic 92 is utilized to control the MUX 88. In addition to selecting the contents of the holding register 86, the MUX 88 may also select the contents of a calibration register 94. The calibration register 94 may have a fixed value loaded therein, or a programmed value loaded therein. If a programmed value is used, such may be loaded into the register 94 using conventional programming techniques known in the art. (See, e.g., U.S. Patent No. 4,232,679 to Schulman, for a basic description of how an implantable medical device may be programmed using an external programmer.) Selection logic 92 is controlled by an appropriate select signal. This select signal may be a programmable command signal sent to the implantable pacemaker from a non-implantable programming device, in conventional manner. During calibration, i.e., when selection logic 92 selects the contents of the calibration register 94, processing circuitry 83 examines the measured blood oxygen to determine if the sensor 34 is functioning properly. As required, the processing circuity makes programmed adjustments to the measurement value passing through the minimum value logic 84 in order to add or subtract an appropriate increment therefrom. In operation, the calibration circuitry shown in FIG. 5 functions as follows. With the patient at rest, or at some other known and controlled level of activity, an attending physician or cardiologist generates an appropriate command to cause selection logic 92 to select the contents of the calibration register 94 as the control parameter that is loaded in the control register 90. In such calibration mode, the pacemaker is essentially a non-rate-responsive pacemaker. Hence, the physician has some control as to the patient's heart rate. Further, in the calibration mode, the contents of the calibration register can be programmably altered to any desired value. In the calibration mode, the blood oxygen sensor 34 measures the blood oxygen content, with the value of such measurements, after being digitized, being directed to processing circuitry 83. The processing circuitry 83 (which may be part of the processing circuitry of the pacemaker circuits 46, particularly when such circuits 46 include one or more microprocessors) then determines if, for the known level of activity and heart rate of the patient, the blood oxygen measurements are approximately where they should be. If not, the processing circuitry 83 instructs the minimum value logic 84 to make whatever adjustments are needed to bring the blood oxygen measurements to an appropriate level before such measurements are placed in the holding register 86. In this manner, the measured value of blood oxygen may be adjusted, as required, in order to compensate for changes that may occur in the sensor over time. Initially, the contents of the registers in the FIFO register stack 82 are all loaded with a nominal value. However, as actual measurements of blood oxygen are made, the digital values corresponding to such measurements supplant the nominal values initially loaded. After a short time, all values held in the register stack 82 represent the most recent n measurements of blood oxygen, where n is number of registers in the FIFO register stack 82 that are used. In this manner, the minimum value logic 84 always looks to the most recent n measurements in order to determine the minimum value that is to be used in accordance with the present invention. It is to be emphasized that that which is shown in FIG. 5 is functional, and numerous variations in the manner of calibration and operation could be readily performed by those skilled in the art. Referring next to FIG. 6A, a simplified electrical schematic diagram of an analog embodiment of the sensor process circuit 44 of FIG. 2 is shown, while FIG. 6B shows a timing waveform diagram illustrating the operation of the analog embodiment of FIG. 6A. In accordance with this embodiment, the output signal from the sensor 34 (FIG. 2) is directed to the inverting input of an input buffer amplifier 100. A peak detection circuit 102 then determines the peak of the input signal over a prescribed period of time, defined by a clock signal, and holds this value until the next period of time. The inverse of the input waveform directed to the amplifier 100 is shown as the signal waveform 104 in FIG. 6B. Thus, the waveform 104 represents the input signal, V IN , directed to the peak detection circuit 102. While this signal is shown as a continuous signal (suggesting continuous operation of the sensor 34), it is noted that it need not be continuous. Rather, the sensor may be continuous for only a small portion of a cardiac cycle, with the signal waveform 104 representing that small portion; or, the sensor may be sampled at an appropriate rate, with the signal waveform 104 representing an extrapolated representation of the sampled signal. In either event, a clock signal, or equivalent, defines a time period T during which the waveform 104 is to be examined for a peak signal. (Note that a peak signal in the waveform 104 corresponds to a minimum signal in the output signal from the sensor 34, or a minimum blood oxygen measurement.) Once this peak signal has been found, it is held until the next period T. To illustrate, in FIG. 6B, the waveform 104 is generally increasing during the first period T. The peak value of the waveform 104 occurs at the end of this first period. Thus, at the conclusion of T, the output signal, shown as the waveform 106 in FIG. 6B, assumes a value corresponding to the value of the waveform 104 at the end of period T. Similarly, during the second period which ends at 2T, the peak value of the waveform 104 occurs at the end of the second period. Thus, at the conclusion of 2T, the output signal 106 assumes a value corresponding to the value of the waveform 104 at the end of period 2T. During the third period, however, the peak of the signal 104 occurs somewhere near the start of the period. Thus, at the conclusion of the period 3T, the output waveform 106 assumes a value corresponding to this peak value. In a similar manner, the output waveform 106 assumes a value at the conclusion of each period corresponding to the peak value of the waveform 104 during that period. The output waveform 106 can then be used directly or indirectly (e.g., converted to a digital value) as a measure of the minimum oxygen content of the blood. As desired, such measure may also be used as the control parameter of a rate-responsive pacemaker. Those skilled in the art will recognize that the circuit shown in FIG. 6A and described in connection with FIG. 6B is simply a peak detection and hold circuit, sometimes referred to as a "boxcar" circuit (because the shape of the output waveform 106 resembles the profile of boxcars of a passing train, each loaded with cargo of differing heights). Such circuit, or equivalent, may advantageously be used as part of the present invention in order to readily determine the minimum value of blood oxygen during a particular time interval. FIG. 6C shows an exemplary embodiment of the peak detection circuit of FIG. 6A; and FIG. 6D is a timing diagram illustrating the relationship between the clock signals used in the peak detection circuit of FIG. 6C. As seen in these figures, the peak detection circuit 102 includes an amplifier U1 driving an emitter follower Q1. The output of the emitter follower Q1 is coupled to a holding capacitor C1. As long as the output voltage of the amplifier U1 is rising, the voltage applied to the capacitor C1 follows. However, as soon as the output voltage of the amplifier U1 falls below the voltage on the capacitor C1, the emitter-base junction of the follower Q1 becomes back biased, thereby maintaining the prior voltage level on C1. At the appropriate time, e.g., at the end of the sample period, a switch Q3 is turned on by clock signal, CLK A, allowing the voltage on C1 to be passed, through output amplifier U2, to the output signal line 106. A short time thereafter, another clock signal, CLK B, turns on switch Q2, causing the voltage held on capacitor C1 to be discharged to ground. This action thus clears capacitor C1 of is previous voltage, thereby allowing the voltage on capacitor C1 to seek the peak value of the input voltage for the next period. FIG. 6D illustrates a preferred relationship between the clock signals CLK A and CLK B. CLK A defines the measuring period T. This clock may be obtained from the pacemaker circuits 46 (FIG. 2), or derived from an appropriate oscillator. For purposes of the present invention, the period T need not be precise. CLK B includes the same period as CLK A, but is delayed a slight amount, t, therefrom. The length of delay may be small, on the order of microseconds. Its purpose is simply to ensure that switch Q3 is turned OFF, after having been turned ON to present the output voltage to the amplifier U2, before the switch Q2 is turned ON to discharge capacitor C1. As described above, it is thus seen that the present invention provides an accurate and reliable system for determining the oxygen content of venous blood using optical measuring techniques, i.e., emitting light energy into the blood and sensing the amount of light energy reflected therefrom, regardless of reflections and other erroneous light energy that may be present at the time a particular measurement is made. Such an accurate and reliable measuring system is realized by making many measurements, e.g., spaced close together in time, and discarding all the measurements but the one(s) evidencing the lowest oxygen content. Thus, while the sensing of any erroneous light energy, caused by reflections or otherwise, may adversely affect one or more individual measurements (causing such measurements to indicate a higher than actual oxygen content), such affected measurements are discarded and not considered. As further described above, it is also seen that the present invention provides a reliable and accurate system for measuring the oxygen content of blood returning from body tissue undergoing the greatest oxygen demand, e.g., experiencing the most physical exercise, even though such blood is in the process of being mixed, e.g., in the right atrium, with blood returning from body tissue not experiencing a high oxygen demand. Advantageously, such system is again realized by making many measurements, e.g., spaced close together in time, and responding only to the minimum blood oxygen content measurement(s), and discarding or ignoring the non-minimum blood oxygen content measurements. The minimum blood oxygen measurement represents that portion of the poorly mixed blood having the lowest oxygen content, i.e., that blood from body tissue undergoing the greatest oxygen demand. In this way, the present invention advantageously provides a system that has improved sensitivity at low levels of physical exercise, and also responds faster than a system based on measurements of well-mixed venous blood. Moreover, as also described above, it is seen that the present invention provides a system for easily and accurately measuring relevant blood oxygen levels and using such measurements as a control parameter in a rate-responsive pacing system. Finally, as further indicated above, the present invention provides a rate-responsive pacing system and/or method wherein stimulation pulses are provided to a patient's heart on demand at a rate that is determined by the oxygen content of blood returning from those parts of the patient's body experiencing the greatest oxygen demand.

Claims: A rate-responsive pacemaker comprising: rate-responsive pulse generating means (46) for generating stimulation pulses; measurement means (34) for measuring the oxygen content of blood; monitoring means (80; 100) for monitoring the blood oxygen measurements made with said measurement means (34), the measurement means (34) being adapted for repeatedly measuring the oxygen content of the blood in the right atrium, the monitoring means (80; 100) being adapted for monitoring the blood oxygen measurements over a prescribed interval (71); and determining means (82; 84; 102) provided for determining the minimum value of blood oxygen content measured during said prescribed interval (71), characterised in that: the measurement means (34) are arranged to be located in the heart to measure the oxygen content of blood in the right atrium (38) of the patient's heart (36) as the blood enters the right atrium (38) before the blood has an opportunity to become thoroughly mixed; said interval (71) includes a plurality of consecutive cardiac cycles; and said minimum value of blood oxygen content provides a direct indicator of the blood oxygen content of the blood returning from body tissue of the patient undergoing the greatest oxygen demand, and hence a direct indicator of the body tissue experiencing physical exercise, the lower the minimum blood oxygen content the greater the physical activity; said rate-responsive pulse generating means generating stimulation pulses at a rate directly controlled by said minium value of blood oxygen content. A pacemaker as set forth in claim 1, characterised by means for measuring the blood oxygen content at a plurality of discrete times (nT) within said prescribed time interval (71), and means for comparing the values of blood oxygen content evidenced by each of said plurality of discrete time measurements and selecting the one that evidences a minimum value of blood oxygen saturation. A pacemaker as set forth in claim 2, characterised in that said determining means (82,84) includes a first-in first-out (FIFO) register stack (82) into which said n discrete measurements are placed, and further includes means (84) for examining the contents of said FIFO register stack (82) to determine which measurement of all the measurements in said FIFO register stack (82) represents said minimum blood oxygen content, and means for updating the content of the FIFO register stack (82) at selected measuring times. A pacemaker as set forth in claim 3, characterised by means for making said updated measurements at sample times which are determined relative to a cardiac cycle of said patient. A pacemaker as set forth in claim 1, characterised by means for measuring the blood oxygen content as a continuous analog signal (104) wherein a peak value of a first polarity represents a minimum oxygen content, and detecting the maximum peak value (106) of said analog signal of said first polarity during said prescribed time period. A pacemaker as set forth in any one of the preceding claims, characterised in that rate-responsive pacing means (46) are provided for generating stimulating pulses and delivering them to the patient's heart (36) at a rate which is determined by the minimum value of the blood oxygen content. A pacemaker as set forth in any one of the preceding claims, characterised in that said measuring means (34) comprise a blood oxygen sensor with light emitting means (20,22) for emitting light energy (E1) from said blood oxygen sensor (34), light sensing means (24) for sensing light energy (E2) directed to said blood oxygen sensor (34) and a sensor drive circuit (42) for selectively energising said light energy emitting means (20,22) and causing said light emitting means (20,22) to emit light energy (E1) and in that said monitoring and detecting means (80,82,84;100,102) comprise a sensor process circuit (44) for determining the amount of light energy (E2) sensed by said light sensing means (24) corresponding to a given amount of light (E1) emitted by said light emitting means (20,22) and for converting said determined amount of light energy (E2) into a first measurement representative of the minimum amount of said light energy (E2) sensed during said prescribed interval (71), said prescribed interval comprising a plurality of consecutive cardiac cycles.

Inventors: HEINZE ROLAND, LEKHOLM ANDERS DR, HEINZE, ROLAND, LEKHOLM, ANDERS, DR.
Applicants: PACESETTER INC, PACESETTER, INC.
Application year: 1991
Main classification: A61B5
IPC codes: A61B5, A61N1