Patent Publication Number: US-7909769-B2

Title: Carbon dioxide and activity monitoring

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. patent application Ser. No. 11/258,432, filed Oct. 25, 2005, now U.S. Pat. No. 7,479,107, which is a divisional of U.S. patent application Ser. No. 10/154,142, filed May 22, 2002, now U.S. Pat. No. 7,037,273, said applications are incorporated herein by reference. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to monitoring heart performance in heart failure patients. More specifically, the present invention relates to monitoring heart performance to detect the occurrence of a drop in core body temperature in heart failure patients. 
     BACKGROUND OF THE INVENTION 
     As early as 1985, medical literature documented a relationship between decreased core body temperature and increased bodily activity in heart failure patients. In a paper by Stanley A. Rubin (R UBIN , S TANLEY  A.  Core body temperature regulation of heart rate during exercise in humans . J. Appl. Physiol. 62(5): 1997-2001, 1987), a significant correlation was found between an observed change in core body temperature and heart rate changes in heart failure patients. Rubin defined core body temperature as the mixed venous blood pool found in the pulmonary artery. Using a thermistor mounted on the distal end of a Swan-Ganz catheter, Rubin found that the core body temperature of heart failure patients decreased during exercise, while the core body temperature in normal patients typically increased during exercise. 
     This relationship between core body temperature and heart activity has been observed in other studies. In a paper by Frank G. Shellock, et al. (S HELLOCK , F RANK  G., H. J. C. S WAN, AND  S TANLEY  A. R UBIN.    Muscle and femoral vein temperatures during short term maximal exercise in heart failure . J. Appl. Physiol. 58(2): 400-408, 1985), reductions in core body temperatures in heart failure patients were observed while the heart failure patients underwent exercise. Shellock, et al. concluded that the core body temperature decrease observed in heart failure patients is partially caused by the re-distribution of cooler blood from the underperfused skeletal muscles into the core blood during exercise. Another paper by Shellock and Rubin (F RANK  G. S HELLOCK AND  S TANLEY  A. R UBIN.    Mixed venous blood temperature response to exercise in heart failure patients treated with short - term vasodilators . Clinical Physiology 5, 503-514, 1985), also concluded that the core body temperature drop was due to circulatory inadequacies. This conclusion was based on Shellock&#39;s and Rubin&#39;s conclusion that heart failure patients treated with vasodialators experience an attenuation of the core body temperature response typical during exercise, as a result of vasodialator-induced improvement in circulation. 
     More recent research ascribes the drop in core body temperature to low blood pH levels. As a heart failure patient exercises, the heart failure patient experiences an increase of CO 2  in its blood as the blood attempts to eliminate the CO 2  from the exercising muscles. The heart failure patient will normally increase its breathing volume in an attempt to evacuate the increased level of CO 2  from the blood. However, in heart failure patients, the blood system cannot deliver the blood to the lungs fast enough to support an adequate CO 2  for O 2  exchange in the lungs. It is well known that higher levels of CO 2  correlate with lower levels of blood pH. As CO 2  accumulates in the heart failure patient&#39;s body, the blood&#39;s pH level drops. Because pH is a catalyst for virtually every chemical reaction in the body, the heart failure patient&#39;s chemical reactions will be slowed by the reduced pH levels. As the blood&#39;s pH level drops with the increased CO 2  levels, the heart failure patient&#39;s bodily chemical reactions responsible for heat production during exercise are slowed and the core body temperature stays flat or decreases. 
     While the core body temperature drop phenomenon has been documented in the literature described above, little has been done to apply this knowledge to benefit heart failure patients. A heart failure patient may require immediate medical attention or an alteration of medical treatment in certain conditions where an increase in heart activity and a decrease in core body temperature are coincident. 
     Therefore, there is a need in the art for a core body temperature monitoring system that can determine when the core body temperature level of a heart failure patient is outside of an acceptable range. The monitoring system also should determine whether the relationship between the core body temperature level and the activity of the heart failure patient is outside of an acceptable range. The monitoring system should be capable of generating an alarm when an unacceptable level has been detected. The monitoring system also should be able to transmit an alarm to a remote location. 
     SUMMARY OF THE INVENTION 
     An implanted heart monitor of the present invention includes sensors that measure various aspects of the heart failure patient&#39;s heart. A remote heart monitoring system can connect the implanted heart monitor to a care provider, such as a physician. The data provided by the implanted heart monitor permits the care provider to obtain valuable data on the heart in order to make health care decisions affecting the heart failure patient&#39;s treatment. In many cases, the measurement of core body temperature and other patient data will enable the care provider to alter the patient&#39;s treatment to address the patient&#39;s condition. The implanted heart monitor can communicate over a wireless communication link with an external monitor. The implanted heart monitor may be implemented as part of a pacing device (i.e., pace maker) or may be a separate unit devoted to monitoring functions. The external monitor communicates with a monitoring station over a communication link. The monitoring station can operate as a centralized data collection unit, collecting data from multiple external monitors and multiple implanted heart monitors. Various other aspects of a heart failure patient&#39;s heart and/or body can be monitored, such as heart rate, blood pH levels, blood CO 2  levels, and any other indications of the heart failure patient&#39;s activity. Various predetermined thresholds may be set to trigger alarms and/or data reports. 
     In one aspect of the present invention, a heart monitoring system is provided. The heart monitoring system includes a temperature sensor for determining a core body temperature of a patient and a bodily activity sensor for determining a bodily activity level of the patient. A communication link is used for transmitting the core body temperature and the bodily activity level to an external monitor. 
     In another aspect of the invention, a method is provided for diagnosing a performance level of a heart in a patient. A core body temperature and a bodily activity level are determined. A relationship between the core body temperature and the bodily activity level is also determined. In response to determining that the relationship between the core body temperature and the bodily activity level is outside of a predetermined acceptable relationship range, a first alarm message is generated. 
     In yet another aspect of the invention, an implantable heart monitor is provided. The implantable heart monitor has a core body temperature sensor for determining a core body temperature of a patient. A bodily activity sensor is used to determine a bodily activity level of a patient. The implantable heart monitor also has a transmission unit for transmitting the core body temperature and the bodily activity level to an external monitor and a processing unit for calculating a relationship between the core body temperature and the bodily activity level and for generating an alarm message in response to a determination that the relationship is outside of a predefined acceptable range. The transmission unit can transmit the alarm message to the external monitor. 
     The various aspects of the present invention may be more clearly understood and appreciated from a review of the following detailed description of the disclosed embodiments and by reference to the drawings and claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a graph depicting an exemplary relationship between observed core body temperature measurements and heart rate measurements in normal patients and in heart failure patients. 
         FIG. 2  is a block diagram depicting the blood system of a heart failure patient with an implanted heart monitor that is an exemplary embodiment of the present invention. 
         FIG. 3  is a block diagram of a remote heart monitoring system that is an exemplary embodiment of the present invention. 
         FIG. 4  is a flow chart depicting an exemplary method for monitoring and reporting core body temperature and heart rate levels for a heart failure patient. 
     
    
    
     DETAILED DESCRIPTION 
     In an exemplary embodiment of the present invention, an implanted heart monitor includes sensors that measure various aspects of a heart failure patient&#39;s heart. A remote heart monitoring system connects the implanted heart monitor to a care provider, such as a physician. The data provided by the implanted heart monitor permits the care provider to obtain valuable data on the heart in order to make health care decisions affecting the heart failure patient&#39;s treatment. In many cases, the measurement of core body temperature and other patient data will enable the care provider to alter the patient&#39;s treatment to address the patient&#39;s condition. The implanted heart monitor can communicate over a wireless communication link with an external monitor. The implanted heart monitor may be implemented as part of a pacing device (i.e., pace maker) or may be a separate unit devoted to monitoring functions. The external monitor communicates with a monitoring station over a communication link. The monitoring station can operate as a centralized data collection unit, collecting data from multiple external monitors and multiple implanted heart monitors. Various other aspects of a heart failure patient&#39;s heart and/or body can be monitored, such as heart rate, blood pH levels, blood CO 2  levels, and any other indications of the heart failure patient&#39;s activity. Various predetermined thresholds may be set to trigger alarms and/or data reports. 
       FIG. 1  is a graph depicting an exemplary relationship between observed core body temperature measurements and heart rate measurements in normal patients and in heart failure patients. The graph of  FIG. 1  is a two-dimensional graph with core body temperature levels indicated on the y axis  100  in degrees Celsius and heart rate levels indicated on the x axis  102  in beats per minute. The graph of  FIG. 1  relates core body temperature measurements and heart rate temperature measurements for normal patients along normal curve  104 . The graph of  FIG. 1  relates core body temperature measurements and heart rate measurements for heart failure patients along the heart failure patient curve  106 . 
     As can be seen from the graph, in normal patients, core body temperature increases in approximately a direct relationship with an increase in heart rate. In heart failure patients, on the other hand, core body temperature stays constant or is reduced as the heart failure patient&#39;s heart rate increases. As described above, this phenomenon has been well documented in medical literature. As mentioned above, recent research ascribes the drop in core body temperature to low blood pH levels. As a heart failure patient exercises, the heart failure patient experiences an increase of CO 2  in its blood as the blood attempts to eliminate the CO 2  from the exercising muscles. The heart failure patient will normally increase its breathing volume in an attempt to evacuate the increased level of CO 2  from the blood. However, in heart failure patients, the blood system cannot deliver the blood to the lungs fast enough to support an adequate CO 2  for O 2  exchange in the lungs. It is well known that higher levels of CO 2  correlate with lower levels of blood pH. As CO 2  accumulates in the heart failure patient&#39;s body, the blood&#39;s pH level drops. Because pH is a catalyst for virtually every chemical reaction in the body, the heart failure patient&#39;s chemical reactions will be slowed by the reduced pH levels. As the blood&#39;s pH level drops with the increased CO 2  levels, the heart failure patient&#39;s bodily chemical reactions responsible for heat production during exercise are slowed and the core body temperature stays flat or decreases. 
     In the graph of  FIG. 1 , a critical zone  108  has been defined. Although the critical zone  108  could be tailored for any diagnostic and/or treatment purposes, it represents the occurrence of a critical event in the relationship between the core body temperature and the heart rate. In the case of  FIG. 1 , the negative slope of heart failure patient curve  106  indicates that the core body temperature is dropping rapidly as the patient&#39;s heart rate increases. By monitoring the slope of the heart failure patient curve  106 , a care provider (e.g., attending physician) could determine that a heart failure patient is experiencing adverse heart conditions. In response to this determination, the care provider could alter the patient&#39;s medical treatment or instruct the patient to seek medical attention. In the example of  FIG. 1 , heart rate is used as an indicator of a heart failure patient&#39;s bodily activity. Typically, increased bodily activity results in increased heart rate. However, as is well known in the art, a heart failure patient&#39;s heart rate can increase for various reasons. Thus, monitoring heart rate alone is not necessarily an indication that the heart failure patient is experiencing increased bodily activity. Those skilled in the art will also appreciate that other aspects of the heart failure patient&#39;s diagnostics can be used to indicate increased bodily activity. Such data may include breathing volume, patient motion, and CO 2  levels. 
     Core body temperature is indicative of the temperature of blood returning to the heart from the rest of patient&#39;s body. Of course, core body temperature can increase and decrease for various reasons. Consequently, the increase or decrease in core body temperature alone is not necessarily indicative of an adverse heart condition. A concurrent decrease in core body temperature and increase in patient bodily activity is indicative of an adverse heart condition. Consequently, exemplary embodiments of the present invention enable more effective treatment of a heart failure patient by monitoring the relationship between core body temperature and heart rate (or other indications of bodily activity). 
       FIG. 2  is a block diagram depicting the blood system of a heart failure patient with a heart monitor that is an exemplary embodiment of the present invention. The typical heart  200  includes a right atrium  202 , a left atrium  204 , a right ventricle  208 , and a left ventricle  206 . The left atrium  204  brings in arterialized blood from the lungs through the pulmonary veins  210 . The pumping action of the heart  200  forces blood from the left atrium  204  to the left ventricle  206  through a one-way left heart valve  212  in the direction of arrow A. The pumping action of the heart  200  forces blood from the left ventricle  206  through the aorta  214  to the body&#39;s systemic circulation system  216 . The systemic circulation system  216  is a short-hand reference to the network of blood vessels that distribute blood to all parts of the body. Blood is returned to the heart  200  from the systemic circulation system  216  through the venae cavae  218 . The returning blood is characterized by having a higher CO 2  content. The blood transports oxygen to the systemic circulation system  216  and eliminates CO 2  from the systemic circulation system. 
     Blood with high CO 2  content enters the right atrium  202  through the venae cavae  218 . The pumping action of the heart forces blood from the right atrium  202  to the right ventricle  208  through a one-way right heart valve  220  in the direction of arrow B. The pumping action of the heart then forces the blood from the right ventricle  208  through the pulmonary artery  222  to the lungs  224 . The blood is ultimately forced into capillaries in the lungs that expose the blood to air sacs. The air sacs of the lung  224  provide the architecture for exchanging CO 2  in the blood for oxygen in the lungs. Once the exchange has been made, the blood is forced from the lungs back into the heart  200  through the pulmonary veins  210 . 
     Core body temperature is normally measured in the return path of the blood circulation system. Because all of the body&#39;s blood returns through the venae cavae  218  to the right atrium  202 , the blood entering the right atrium is referred to as the mixed venous blood pool. Notably, the blood in the venous blood pool has dissipated as much heat as possible in the systemic circulation system  216  prior to re-entering the heart  200 . A temperature sensor, such as a thermistor, can be placed in or near the right atrium  202  to measure the core body temperature in the venous blood pool. In a one embodiment as shown in  FIG. 2 , a core body temperature sensor  226  is located inside the right atrium  202 . Those skilled in the art will appreciate the core body temperature may be measured in various locations in addition to a location in the right atrium  202 . For example, measurements of skeletal muscle temperature, myocardial muscle temperature, or even skin temperature can be measured and correlated to a corresponding core body temperature to determine the measured core body temperature. In a practical application, a core body temperature sensor could be integrated into the outer casing of a conventional implanted pacing device. A temperature correlation table could be used to approximate the core body temperature, based on the temperature detected at the pacing device casing. 
     The right atrium  202  includes a sino-atrial (SA) node  228 . The SA node  228  signals an electrical impulse to propagate through the heart  200  to cause the heart to beat. The SA node receives an electrical impulse from the nervous system and converts the electrical impulse to a heartbeat. An atrio-ventricular (AV) node  230  is a cluster of muscle cells located in the wall between the right atrium  202  and the left atrium  204 . The AV node  230  is part of the heart&#39;s  200  electrical system. The AV node  230  helps deliver heartbeat signals from the atria to the ventricles. Heart rate measurements are often taken at the SA node  228 , because the SA node is the contact point from which the body&#39;s natural heart pacing signals emanate. Even in heart failure patients, the SA node  228  will usually receive a detectable signal from the nervous system. The natural heart-pacing signal can be detected by means of a heart rate sensing lead  232 , attached to the heart  200  at the SA node  228 . 
     The core body temperature sensor  226  and the heart rate sensor  232  can be connected to an implanted monitor  234  by a core body temperature sensing lead  236  and a heart rate sensing lead  238 , respectively. The implanted monitor  234  can be an integrated component of a pacing device or may simply be an implanted monitor with no pacing functionality. In an exemplary embodiment of the present invention, the implanted monitor  234  can communicate with an external monitor (not shown) to communicate various data collected from the patient and/or analysis performed on collected data. In any event, the implanted monitor  234  can measure heart rate and core body temperature data and can transmit that data to an external entity such as a care provider, an external monitor/alarm unit, or an external data collection center. 
     The implanted monitor  234  also can determine the relationship between collected data such as core body temperature and heart rate data. For example, the implanted monitor  234  could be used to calculate the slope of a graph of the relationship between a heart failure patient&#39;s heart rate and core body temperature measurements. As described above, the data collected and/or analyzed by the implanted monitor  234  may include various indications of bodily activity and/or heart performance. 
       FIG. 3  is a block diagram of a remote heart monitoring system  300  that is an exemplary embodiment of the present invention. The remote heart monitoring system  300  connects a heart monitor  302 , implanted in a heart failure patient  304 , to a care provider (e.g., a physician), for example, a physician at a hospital  306 . The implanted heart monitor  302  includes sensors that measure various aspects of the heart failure patient&#39;s heart  308 . The implanted heart monitor  302  can include a processing unit  303  for processing the measured data and for analyzing the data to, for example, determine a relationship between the measured data. In one example, the processing unit  303  is configured for calculating or otherwise determining a relationship between blood pH level and bodily activity level. In a further example, the processing unit  303  is configured for generating an alarm message in response to a determination that the relationship is outside of a predefined acceptable range. The implanted heart monitor  302  communicates over a wireless communication link  310  with an external monitor  312 . The implanted heart monitor  302  may include a transmission unit  305  for communicating with the external monitor over the wireless communication link  310 . In one example, the implanted heart monitor is operative to transmit the core body temperature and the bodily activity level to the external monitor, such as, for instance, a remote monitoring station. 
     The implanted heart monitor  302  may be implemented as part of a pacing device (i.e., pace maker) or may be a separate unit devoted to monitoring bodily functions. The external monitor can be located within the heart failure patient&#39;s home  314  or may be mobile. Those skilled in the art will appreciate that various implementations of the external monitor  312  are possible within the scope of the present invention. It should also be appreciated that the connection between the implanted heart monitor  302  and the external monitor  312  could be a hardwire connection (i.e., not wireless), for example, when the remote heart monitoring system  300  is used in a hospital room context. 
     The external monitor  312  communicates with a base station  314  over a wireless communication link  316 . Again, those of ordinary skilled in the art will appreciate that a hardwire connection could be used between the external monitor  312  and the base station  314 . However, as with the wireless communication link  310  between the implanted heart monitor  302  and the external monitor  312 , the wireless communication link  316  provides the most versatility and convenience to the patient. 
     The base station  314  is connected to a monitoring station  318  over a communication link  320 . The monitoring station can be used to collect data from the implanted heart monitor  302 , via the external monitor  312 . Notably, the monitoring station  318  can collect data from multiple external monitors and multiple implanted heart monitors. That is, the monitoring station  318  can operate as a centralized data collection unit. The monitoring station  318  can communicate with a hospital  306  over a communication link  322 . The monitoring station  318  can communicate with multiple hospitals and/or care providers to deliver monitored and/or stored data. 
     In the remote heart monitoring system  300  of  FIG. 3 , various aspects of a heart failure patient&#39;s heart and/or body can be monitored. For example, the implanted heart monitor  302  may monitor core body temperature, ventilation rate, oxygen consumption rate, heart rate, blood pH levels, blood CO 2  levels, bodily movement, and any other indication of the heart failure patient&#39;s bodily activity. In one example, a pH level sensor determines a core body temperature by determining a blood pH level and correlating a blood pH level into a core body temperature. In a further example, the implanted heart monitor  302  is operatively connected to a blood pH level sensor and a bodily activity sensor and is configured to receive and process data representative of the blood pH level and the bodily activity level. In this example, the implanted heart monitor  302  processes data representative of blood pH level and correlates the blood pH data into a core body temperature. Various predetermined thresholds may be set to trigger alarms and/or data reports. For example, if the heart monitor  302  determines that the measured core body temperature is outside a predefined acceptable range, the heart monitor may trigger an alarm. Alternatively, the heart monitor  302  may simply deliver the data to the external monitor  312 . 
     The external monitor  312  may compare the measured levels to predetermined acceptable ranges and trigger alarms when a variance is detected. Similarly, predetermined thresholds may be set for the heart failure patient&#39;s heart rate and for the relationship between measured indications. For example, an alarm may be triggered when the slope of a graph of core body temperature versus heart rate is too low. 
     While the remote heart monitoring system  300  of  FIG. 3  has been described in the context of monitoring the conditions of a heart failure patient, it will be appreciated by those skilled in the art that the remote monitoring system could be implemented to monitor virtually any aspect of a patient&#39;s health or activity. 
     The implanted heart monitor  302  periodically collects data and reports the collected data to the external monitor  312 . The external monitor  312  can process the data and relay the data to the monitoring station  318  at predetermined intervals, via the base station  314 . The external monitor  312  may make a determination that the collected data indicates a change in patient wellness. In response to a determination that the patient&#39;s wellness has improved or declined, the external monitor  312  may generate a wellness report separate from the routinized collected data reporting. The external monitor  312  may determine that the collected data justifies the transmission of an alarm. In this case, the external monitor  312  can transmit an alarm request to the monitoring station that can then generate an alarm and convey the alarm to the appropriate care provider at the hospital  306 . Those skilled in the art will appreciate that the functionality of the implanted monitor and of the external monitor could be integrated into a single unit. It will also be appreciated that the implanted monitor could detect and transmit virtually any heart condition and/or body condition data. 
     The heart monitoring system  300  of  FIG. 3  could be used in conjunction with a pacing device or any other heart failure treatment regimen. When the implanted heart monitor  302  detects an out-of-range condition, an alarm may be generated for the care provider, as described above. If, for example, the core body temperature measurement is outside of a predefined acceptable range, the care provider may access the patient&#39;s historical records to determine potential causes. The care provider also may cause the monitoring station  318  to initiate special communications with the external monitor  312  to gather more recent data on the patient&#39;s heart or other diagnostic information. If the care provider receives an alarm indicating a relationship between the core body temperature and the bodily activity level, the care provider may contact the patient and urge the patient to seek immediate treatment. Of course, in a hospital context, the care provider may simply go to the patient&#39;s hospital room and attend to the patient. 
     Generally, a significant decrease in a heart failure patient&#39;s core body temperature without a coincident increase in bodily activity may signal an imminent heart failure episode, such as cardiac arrest. A decrease in a heart failure patient&#39;s core body temperature that is coincident with an increase in bodily activity is an expected relationship. However, if the decrease in core body temperature during increased bodily activity is too rapid, it too may signal an imminent heart failure episode, such as cardiac arrest. In any event, the data provided by the implanted heart monitor permits the care provider to obtain valuable data on the heart in order to make health care decisions affecting the heart failure patient&#39;s treatment. In many cases, the measurement of core body temperature and other patient data will enable the care provider to alter the patient&#39;s treatment to address the patient&#39;s condition (i.e., make the patient&#39;s condition less pathological). This treatment alteration may be as simple as alerting the patient to cease vigorous exercise, or as complex as modifying the operational parameters of the patient&#39;s pacing device (i.e., pacemaker). 
       FIG. 4  is a flow chart depicting an exemplary method for monitoring and reporting core body temperature and heart rate levels for a heart failure patient. The method of  FIG. 4  begins at start block  400  and proceeds to step  402 . At step  402 , the heart performance and core body temperature of a heart failure patient is monitored. As described in connection with  FIG. 2 , the step of monitoring heart performance may be performed by an implanted heart monitor with a heart rate sensor operative to detect a heart rate. The core body temperature monitoring may be performed by and implanted heart monitor that uses a core body temperature sensor to determine the temperature of a venous blood pool. 
     The method proceeds from step  402  to decision block  404 . At decision block  404 , a determination is made as to whether the core body temperature is within a predetermined range. If the core body temperature is within the predetermined range, the method branches to step  408 . On the other hand, if the monitored core body temperature is not within the predetermined range, the method branches to step  416  through connector A. In the exemplary method of  FIG. 4 , the predetermined range for core body temperatures may represent a range of core body temperature determined by a care provider to be a safe range for the patient. Those skilled in the art will appreciate that the range may differ from one heart failure patient to another and could, therefore, be configurable on a per-patient basis. 
     At step  416 , an alarm message is generated to indicate that the measured core body temperature is outside of an acceptable range. The method proceeds from step  416  to step  418 , wherein the alarm message is transmitted to the care provider. Those skilled in the art will appreciate that a single alarm may be used for any out-of-range condition. Alternatively, a separate alarm may be used for each out of range condition. The alarm message may include an indication of the measured data that has been determined to be out of the acceptable range. The method proceeds from step  418  to step  420  and heart performance and core body temperature monitoring is continued. 
     Returning now to decision block  404 , if a determination is made that the core body temperature is within the predefined range, the method branches from decision block  404  to step  408 . At step  408 , the core body temperature data is transmitted for storage. In the remote heart monitoring system  300  of  FIG. 3 , the core body temperature data may be stored in the external monitor  312  for later transmission or may be stored in the monitoring station  318  in a centralized patient database. The method proceeds from step  408  to decision block  410 . 
     At decision block  410 , a determination is made as to whether the heart failure patient&#39;s heart rate is within a predefined acceptable range. If the patient&#39;s heart rate is outside of the acceptable range, the method branches to step  416 , via connector A. The method proceeds from step  416  to generate an alarm indicating the out-of-range condition, as described above. If, at decision block  410 , a determination is made that the heart rate is within the predefined acceptable range, the method branches from decision block  410  to step  412 . 
     At step  412 , the heart rate data is sent for storage. As described above, the heart rate data may be stored in an external monitor, a monitoring station, or any other acceptable storage means for subsequent retrieval. A method proceeds from step  412  to decision block  414 . At decision block  414 , a determination is made as to whether the core body temperature versus heart rate slope is within a predefined acceptable range. As described above in connection with  FIG. 1 , the relationship between the core body temperature and the heart rate (or any other indication of bodily activity) is a useful measurement for diagnosing a heart failure patient. If the slope is outside of the acceptable range, the method branches to step  416  and an alarm is generated as described above. If, on the other hand, the slope is within the range, the method branches from decision block  414  to step  402  and core body temperature monitoring is continued. 
     Although the present invention has been described in connection with various exemplary embodiments, those of ordinary skill in the art will understand that many modifications can be made thereto within the scope of the claims that follow. Accordingly, it is not intended that the scope of the invention in any way be limited by the above description, but instead be determined entirely by reference to the claims that follow.