Patent Publication Number: US-2009227883-A1

Title: Automated heart function classification to standardized classes

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims the benefit of U.S. Provisional Application No. 61/033,943, filed on Mar. 5, 2008, under 35 U.S.C. §119(e), which is hereby incorporated by reference. 
    
    
     BACKGROUND 
     In spite of rapid technological advances, manual New York Heart Association (NYHA) classification by a physician remains the major gauge of heart function assessment in patients with heart disease. In addition to NYHA classification, American College of Cardiology/American Heart Association (ACC/AHA) classification is another method that physicians use for assessing patient heart function status. 
     OVERVIEW 
     This document describes, among other things, a system and method that automatically classifies a patient&#39;s heart function status, such as by using an implantable medical device (IMD) to determine a physiological response to activity, and using that information to perform the classification. For example, a physical activity sensor and a physiological sensor are used to automatically classify patients into heart function status classes, such as NYHA classes or ACC/AHA classes. Changes in a patient&#39;s classification can be used to monitor heart function status over time and to monitor therapy responsiveness. 
     Example 1 describes a system. In this example, the system comprises a physical activity sensor, configured to sense an indication of physical activity of a patient; a physiological sensor, configured to sense a physiological response of a patient corresponding to the sensed indication of the physical activity of the patient; a signal processor circuit, configured to receive the indication of physical activity of the patient from the physical activity sensor, and configured to receive the physiological response of the patient from the physiological sensor, and configured to automatically classify the patient into a classification corresponding to a cardiac function status of the patient, the classification selected from a group of standard diagnostic classes describing different cardiac function statuses, the classes recognized by a medical standard-establishing organization; and a patient classification memory storage location, configured to store an indication of the classification of the patient to be provided to a user or process. 
     In Example 2, the system of Example 1 optionally includes the signal processor circuit configured to repeat the classifying over a period of time, detect a change in the classification during the period of time, and provide an indication of the change in the classification of the patient to a user or process. 
     In Example 3, the system of one or more of Examples 1-2 optionally includes the signal processor circuit configured to classify the patient into a NYHA class that is automatically selected from a group of NYHA classes using the physiological response to activity. 
     In Example 4, the system of one or more of Examples 1-3 optionally includes the signal processor circuit configured to classify the patient into an ACC/AHA class that is automatically selected from a group of ACC/AHA classes using the physiological response to activity. 
     In Example 5, the system of one or more of Examples 1-4 optionally includes the physiological sensor comprising a pH sensor configured to sense pH from the patient. 
     In Example 6, the system of one or more of Examples 1-5 optionally includes the signal processor circuit configured to use pH to determine an indication of fatigue, and to use the indication of fatigue to automatically classify the patient into a classification corresponding to a cardiac function status of the patient. 
     In Example 7, the system of one or more of Examples 1-6 optionally includes the physiological sensor comprising a heart rate sensor configured to sense a heart rate of the patient, wherein the signal processor circuit is coupled to the heart rate sensor to receive and use information about the sensed heart rate to automatically classify the patient into a classification corresponding to the cardiac function status of the patient. 
     In Example 8, the system of one or more of Examples 1-7 optionally includes the physiological sensor comprising a respiration sensor configured to sense a respiration rate of the patient, wherein the signal processor circuit is coupled to the respiration sensor to receive and use information about the sensed respiration rate to automatically classify the patient into a classification corresponding to the cardiac function status of the patient. 
     In Example 9, the system of one or more of Examples 1-8 optionally includes the physiological sensor comprising a periodic breathing sensor configured to sense a periodic breathing of the patient, wherein the signal processor circuit is coupled to the periodic breathing sensor to receive and use information about the sensed periodic breathing to automatically classify the patient into a classification corresponding to the cardiac function status of the patient. 
     In Example 10, the system of one or more of Examples 1-9 optionally includes the signal processor configured to compute an indication of the physiological response to activity by: detecting a first measurement of a physiological parameter corresponding to relatively lower degree of physical activity of the patient; detecting a second measurement of the physiological parameter at a relatively greater degree of physical activity of the patient than that corresponding to the first measurement; and determining the physiological response to activity using a change in the physiological parameter between the first and second measurements of the physiological parameter. 
     In Example 11, the system of one or more of Examples 1-10 optionally includes the signal processor configured to automatically classify the patient into a classification corresponding to a cardiac function status of a patient by processing the measurement of the physiological response to activity using at least one of: patient medication information, patient co-morbidity information, or physician-provided input. 
     Example 12 describes a method. In this example, the method comprises using a medical device, detecting an indication of physical activity of a patient; using the medical device, detecting a measurement of a physiological response of the patient corresponding to the measurement of physical activity of the patient; using the measurement of the physiological response, automatically classifying the patient into a classification corresponding to a cardiac function status of a patient, the classification selected from a group of standard diagnostic classes describing different cardiac function statuses, the group of classes recognized by a medical standard-establishing organization; and providing an indication of the classification of the patient to a user or process. 
     In Example 13, the method of Example 12 optionally comprises repeating the classifying over a period of time; detecting a change in the classification during the period of time; and providing an indication of the change in the classification of the patient to a user or process. 
     In Example 14, the method of one or more of Examples 12-13 optionally comprises classifying the patient into a classification corresponding to cardiac function status of the patient by classifying the patient into a NYHA class that is automatically selected from a group of NYHA classes using the measurement of the physiological response to activity. 
     In Example 15, the method of one or more of Examples 12-14 optionally comprises classifying the patient into a classification corresponding to cardiac function status of the patient by classifying the patient into an ACC/AHA class that is automatically selected from a group of ACC/AHA classes using the measurement of the physiological response to activity. 
     In Example 16, the method of one or more of Examples 12-15 optionally comprises detecting the measurement of the physiological response corresponding to the measurement of physical activity by measuring pH. 
     In Example 17, the method of one or more of Examples 12-16 optionally comprises using measured pH for generating an indication of fatigue, and using the generated indication of fatigue for automatically classifying the patient into the classification corresponding to the cardiac function status of the patient. 
     In Example 18, the method of one or more of Examples 12-17 optionally comprises detecting the measurement of the physiological response corresponding to the measurement of physical activity by measuring heart rate, wherein classifying the patient into the classification corresponding to a cardiac function status of the patient includes using the measured heart rate. 
     In Example 19, the method of one or more of Examples 12-18 optionally comprises detecting the measurement of the physiological response corresponding to the measurement of physical activity by measuring respiration rate, wherein classifying the patient into the classification corresponding to a cardiac function status of the patient includes using the measured respiration rate. 
     In Example 20, the method of one or more of Examples 12-19 optionally comprises detecting the measurement of the physiological response corresponding to the measurement of physical activity by measuring periodic breathing, wherein classifying the patient into the classification corresponding to a cardiac function status of the patient includes using the measured periodic breathing. 
     In Example 21, the method of one or more of Examples 12-20 optionally comprises detecting the measurement of the physiological response corresponding to the measurement of physical activity by: detecting a first measurement of a physiological parameter corresponding to relatively lower degree of physical activity of the patient; detecting a second measurement of the physiological parameter at a relatively greater degree of physical activity of the patient than that corresponding to the first measurement; and determining the physiological response to activity using a change in the physiological parameter between the first and second measurements of the physiological parameter. 
     In Example 22, the method of one or more of Examples 12-21 optionally comprises determining a measurement of the physiological response to activity by determining at least one degree of physical activity of the patient using at least one of: a six-minute walk, a maximum exercise intensity level, or a maximum exercise duration. 
     In Example 23, the method of one or more of Examples 12-22 optionally comprises automatically classifying the patient into a classification corresponding to a cardiac function status of a patient by using the measurement of the physiological response to activity, including processing the measurement of the physiological response using at least one of: patient medication information, patient co-morbidity information, or physician-provided input. 
     This overview is intended to provide an overview of subject matter of the present patent application. It is not intended to provide an exclusive or exhaustive explanation of the invention. The detailed description is included to provide further information about the present patent application. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In the drawings, which are not necessarily drawn to scale, like numerals can describe substantially similar components throughout the several views. Like numerals having different letter suffixes can represent different instances of substantially similar components. The drawings illustrate generally, by way of example, but not by way of limitation, various embodiments discussed in the present document. 
         FIG. 1  is schematic diagram illustrating generally an example of a cardiac function management system, such as for use with a human or animal subject. 
         FIG. 2  is a flow chart illustrating generally an example of a technique for automatically classifying a patient into a cardiac function status class. 
         FIG. 3  is a diagram illustrating generally an example of a system for automatically classifying a patient into a cardiac function status class. 
         FIG. 4  is a diagram illustrating generally examples of inputs used in a system for classifying a patient into a cardiac function status class. 
         FIG. 5  is a diagram illustrating generally an example of a system for computing an indication of a patient&#39;s physiological response to physical activity. 
     
    
    
     DETAILED DESCRIPTION 
     This document describes, among other things, automatic classification of a patient into a heart function status class, such as by using an implantable medical device that measures a physiological response to physical activity. Such information can be used to classify the patient into a medically recognized standardized heart function class. 
     Table 1 illustrates NYHA classification, a standardized medically-recognized schema that is typically used by doctors for classifying heart status manually, rather than automatically using physiological response to activity information obtained from an implantable medical device, as described below. Advancement to a higher-numbered NYHA class is generally accompanied by increased heart failure mortality of the subpopulation represented by that class. NYHA Class II patients generally exhibit a heart failure mortality rate of 5-10%, Class III patients generally exhibit a heart failure mortality rate of 10-15%, and Class IV patients generally exhibit a heart failure mortality rate of 30-40%. 
     
       
         
           
               
             
               
                 TABLE 1 
               
             
            
               
                   
               
               
                 NYHA classification 
               
            
           
           
               
               
            
               
                 Class 
                 Patient Symptoms 
               
               
                   
               
               
                 Class I 
                 No limitations of physical activity. Ordinary physical activity 
               
               
                   
                 does not cause undue fatigue, palpitation, or dyspnea. 
               
               
                 Class II 
                 Slight limitation of physical activity. Comfortable at rest, but 
               
               
                   
                 ordinary physical activity results in fatigue, palpitation, or 
               
               
                   
                 dyspnea. 
               
               
                 Class III 
                 Marked limitation of physical activity. Comfortable at rest, 
               
               
                   
                 but less than ordinary activity causes fatigue, palpitation, 
               
               
                   
                 or dyspnea. 
               
               
                 Class IV 
                 Unable to carry out any physical activity without discomfort. 
               
               
                   
                 Symptoms of cardiac insufficiency at rest. If any physical 
               
               
                   
                 activity is undertaken, discomfort is increased. 
               
               
                   
               
            
           
         
       
     
     Table 2 illustrates ACC/AHA classification based on a patient&#39;s symptoms and the physical condition of the patient&#39;s heart. The ACC/AHA classification schema is a standardized medically-recognized schema that is typically used by doctors for classifying heart status manually, rather than automatically using physiological response to activity information obtained from an implantable medical device, as described below. At the present time, ACC/AHA stages may be thought of as being less dynamic the NYHA classes. For example, once a patient is classified as ACC/AHA stage B, the patient generally cannot improve to stage A, even if that patient&#39;s NYHA classification improves. In the future, however, technology may allow for earlier detection and reversal of heart failure signs, which would permit patients to improve from one ACC/AHA stage to the next. In either case, long-term monitoring of ACC/AHA stages may be useful. 
     
       
         
           
               
             
               
                 TABLE 2 
               
             
            
               
                   
               
               
                 ACC/AHA Heart Failure (HF) classification schema 
               
            
           
           
               
               
               
            
               
                 Stage 
                 Description 
                 Examples 
               
               
                   
               
               
                 A 
                 Patients at high risk of developing 
                 Systemic hypertension; coronary 
               
               
                   
                 HF because of the presence of 
                 artery disease; diabetes mellitus; 
               
               
                   
                 conditions that are strongly 
                 history of cardiotoxic drug therapy 
               
               
                   
                 associated with the development 
                 or alcohol abuse; personal history of 
               
               
                   
                 of HF. Such patients have no 
                 rheumatic fever; family history of 
               
               
                   
                 identified structural or functional 
                 cardiomyopathy. 
               
               
                   
                 abnormalities of the pericardium, 
               
               
                   
                 myocardium, or cardiac valves 
               
               
                   
                 and have never shown signs or 
               
               
                   
                 symptoms of HF. 
               
               
                 B 
                 Patients who have developed 
                 Left ventricular hypertrophy or 
               
               
                   
                 structural heart disease that is 
                 fibrosis; left ventricular dilation or 
               
               
                   
                 strongly associated with the 
                 hypocontractility; asymptomatic 
               
               
                   
                 development of HF but who have 
                 valvular heart disease; previous myocardial 
               
               
                   
                 never shown signs or symptoms of 
                 infarction. 
               
               
                   
                 HF. 
               
               
                 C 
                 Patients who have current or prior 
                 Dyspnea or fatigue due to left 
               
               
                   
                 symptoms of HF associated with 
                 ventricular systolic dysfunction; 
               
               
                   
                 underlying structural heart 
                 asymptomatic patients who are 
               
               
                   
                 disease. 
                 undergoing treatment for prior 
               
               
                   
                   
                 symptoms of HF. 
               
               
                 D 
                 Patients with advanced structural 
                 Patients who are frequently 
               
               
                   
                 heart disease and marked 
                 hospitalized for HF or cannot be 
               
               
                   
                 symptoms of HF at rest despite 
                 safely discharged from the hospital; 
               
               
                   
                 maximal medical therapy and who 
                 patients in the hospital awaiting 
               
               
                   
                 require specialized interventions. 
                 heart transplantation; patients at 
               
               
                   
                   
                 home receiving continuous 
               
               
                   
                   
                 intravenous support for symptom 
               
               
                   
                   
                 relief or being supported with 
               
               
                   
                   
                 mechanical circulatory assist device; 
               
               
                   
                   
                 patients in a hospice setting for the 
               
               
                   
                   
                 management of HF. 
               
               
                   
               
            
           
         
       
     
       FIG. 1  is schematic diagram illustrating generally an example of a cardiac function management system  100 , such as for use with a human or animal subject  101 . In this example, the system  100  includes an implantable cardiac function management device  102 , which can include or be coupled to one or more intravascular or other leads  104 . The cardiac function management device  102  can include a communication circuit, such as for establishing a bidirectional wireless communication link  105  with an external local interface  106 . In certain examples, the external local interface can further bidirectionally communicate with an external remote interface  108 , wirelessly or otherwise, such as via a shared communication or computer network  110 . An example of using such a communication network  110  can include using the Boston Scientific Corp. (Cardiac Pacemakers, Inc.) LATITUDE® Patient Monitoring System, which can provide remote patient monitoring, such as by automatically collecting information from a patient&#39;s implanted medical device and communicating the information to a secure website accessible by the patient&#39;s healthcare providers. 
       FIG. 2  is a flow chart illustrating generally an example of a technique  200  for automatically classifying a patient into a cardiac function status class based on the patient&#39;s physiological response to physical activity. Some examples of measuring a patient&#39;s physiological response to physical activity are described in Beck et al., U.S. Patent Application Serial No. US 2007/0021678 entitled “Methods and Apparatus for Monitoring Physiological Responses to Steady State Activity” (Attorney Docket No. 279.916US1), assigned to Cardiac Pacemakers, Inc., and filed on Jul. 19, 2005, which is incorporated herein by reference in its entirety, including its description of measuring a patient&#39;s physiological response to physical activity. At  202 , an indication of physical activity is detected from the patient. The indication of physical activity can be generated, for example, by using one or more implantable movement or exertion sensors, such as an accelerometer. At  204 , a measurement of physiological response corresponding to the physical activity is detected from the patient. The measurement of a physiological response to the physical activity can be generated by one or more physiological sensors, such as an implantable pH sensor, a heart rate sensor, a respiration sensor, or a periodic breathing sensor, for example. At  206 , the patient is automatically classified into a class describing cardiac function status. The classification can be based on the indication of physical activity  202  and the measurement of physiological response  204 . In certain examples, the classification can be based on baseline measurements of a patient&#39;s physiological response to physical activity. Baseline measurements are measurements of a physiological response to a physical activity at a particular point in time. Baseline measurements can later be compared to physiological responses measured at other times in order to detect relative changes. A six-minute walk test, for example, can be used to establish baseline measurements of a patient&#39;s pH, heart rate, and respiration rate. These baseline measurements can then be used to set one or more parameters used in automatically classifying a particular patient&#39;s heart status. For example, when a patient is initially classified into a cardiac function class using the baseline measurements, the parameters for later classifications can then be determined using the patient&#39;s initial classification. Other information such as co-morbidities or medications can also be used to determine the parameters used for later classifications. Cardiac function classes can include medically-recognized standard diagnostic classes, such as NYHA classes or ACC/AHA classes. At  208 , an indication of the patient&#39;s automatic heart status classification is provided to a user or process, such as through a communication network  110 . The classification indication can be stored in a memory storage location, or displayed to the user, in certain examples. In the example of  FIG. 2 , the detecting the indication of physical activity  202  and the detecting the measurement of physiological response corresponding to physical activity  204  can be performed internally within a subject&#39;s body using an implantable cardiac function management device. The automatic heart status classification of the patient  206  and the generation of an indication of classification  208  can be performed internally within the implantable device, or externally, such as within a local or remote user interface device. 
       FIG. 3  is a diagram illustrating generally an example of a system  300  for automatically classifying a patient into a cardiac function status class, such as based on the patient&#39;s physiological response to physical activity. In this example, a physical activity sensor  302  is configured to sense an indication of physical activity of a patient. The indication of physical activity can be sensed, for example, using an accelerometer or an exertion or movement sensor. A physiological sensor  304  can be configured to sense a physiological response of the patient corresponding to the sensed indication of the patient&#39;s physical activity. The measurement of physiological response can be generated by one or more physiological sensors, such as an implantable pH sensor, a heart rate sensor, a respiration sensor, or a periodic breathing sensor. Information from the physical activity sensor and from the physiological sensor is communicated to the classification circuit  308  within the signal processor circuit  306 . The classification circuit  308  automatically classifies the patient into a class corresponding to the cardiac function status of the patient. For example, the classification circuit  308  can classify the patient into one or more of a NYHA class or an ACC/AHA class. In addition to the classification circuit  308 , the signal processor circuit  306  can be configured to repeat the classification process over an acute or chronic period of time  310  such as to detect a change in heart status classification  312 . Changes in heart status classification automatically detected using the signal processor circuit  306  can be communicated to a classification memory storage location  314  configured to store such heart status classifications of the patient, such as for determining an indication of a change in the heart status classification over an acute or chronic period of time. Such changes in heart status classification over time can be used to monitor heart function status or to monitor therapy effectiveness or responsiveness. Detection of frequent changes in heart status classification or of worsening heart status classification can be communicated to a patient or caregiver through the generation of local or remote alerts or alarms. Automatic therapy changes can be made in response to a detected worsening, improvement, or other change in heart function status classification. In the example of  FIG. 3 , the physical activity sensor  302  and the physiological sensor  304  can be implantable, for example, included within or implantably coupled to an implantable cardiac function management device. The signal processor circuit  306  and the classification memory storage location  314  can be implantably located, such as within the implantable cardiac management device, or externally located. 
       FIG. 4  is a diagram illustrating generally an example of a system  400  in which a patient can be classified, such as according to heart status using information from the physical activity sensor  302  and the physiological sensor  304 , although additional inputs can also be used. In this example, the physiological sensor  304  can include one or more different sensors of respective physiological parameters, such as a pH sensor  402 , a heart rate sensor  404 , a respiration sensor  406 , or a periodic breathing sensor  410 . The pH sensor  402  can be configured to detect pH or other measure of acidity or alkalinity in the blood stream or in muscle tissue, such as pectoral muscle tissue or at skeletal muscle tissue of the lower limb. The pH sensor  402  can be configured to detect pH using one or more of pH electrodes or optical pH sensors, for example. A decrease in pH generally accompanies muscle fatigue, which can signal worsening heart function status, particularly when the muscle fatigue generally increases during a period of time in which the patient&#39;s physical activity level has not shown any increase. The heart rate sensor  404  can detect increased heart rate and arrhythmias, both of which can be indications of worsening cardiac function status, particularly when the patient&#39;s physical activity level has not increased. The respiration sensor  406  can detect increased respiration rate, another indication of worsening heart function status, particularly when the patient&#39;s physical activity level has not increased. The periodic breathing sensor  410  can be used to detect one or more signs of dyspnea, such as a periodically decreased tidal volume. An increasing degree of dyspnea can provide another indication of worsening cardiac function status. 
     Information about one or more of the physiological parameters measured by one or more of the various sensors can be communicated from the physiological sensor  304  to the signal processor circuit  306 . Using the information about the patient&#39;s physiological response to physical activity, the signal processor  306  can be configured to automatically classify the patient into a class corresponding to cardiac function status  420 . In addition to physiological response to physical activity data from the physiological sensor  306 , the signal processor circuit  306  can use patient co-morbidity information  414 , patient medication information  416 , and physician-provided input  418  to automatically classify the patient into a class corresponding to cardiac function status  420 . For example, from the outset, a patient who has chronic obstructive pulmonary disease (COPD), in addition to a heart failure condition, may exhibit, in response to an increase in physical activity, a bigger increase in respiration or heart rate, or a bigger decrease in pH relative to a patient having a heart failure condition without the accompanying COPD co-morbidity. These COPD-related effects can be taken into account by the signal processor circuit  306  in classifying the patient according to heart function status. Furthermore, certain medications can affect a patient&#39;s physiologic response to physical activity. For example, patients taking beta blockers generally exhibit a lesser increase in heart rate in response to physical activity compared to patients who are not on beta blockers. Thus, for a patient taking beta-blockers, the signal processor circuit  306  can be programmed to allow for a lower heart rate threshold for placing a patient into a “more compromised” heart status class when classifying the patient according to cardiac function status. In certain examples, a physician can independently classify a patient into a heart status class based on one or more of the patient&#39;s symptoms and response to a six-minute walk test, without using the patient&#39;s implanted automatic heart function status classification device. In certain examples, the physician&#39;s independent classification can be used as an input signal for the signal processor circuit  306 , and the automatic classification can be compared to the physician&#39;s classification. The physician&#39;s independent classification or the results of a patient&#39;s six-minute walk test can be used to adjust the automatic classification system for a particular patient, such as to calibrate the automatic classification system or to make the automatic classification system adaptive via a machine learning process, for example. Physician calibration can be performed recurrently or periodically. 
       FIG. 5  is a diagram illustrating generally an example of a system  500  in which the signal processor circuit  306  is configured to compute an indication of the physiological response to activity  508 . At  502 , the signal processor circuit  306  detects a physiological parameter corresponding to a lower degree of physical activity. At  504 , the signal processor circuit  306  detects the physiological parameter corresponding to a higher degree of physical activity. At  506 , the physiological parameter corresponding to the lower degree of physical activity  502  is compared to the physiological parameter corresponding to the higher degree of physical activity  504 , and the change in the physiological parameter is determined. At  508 , the physiological response to activity is determined using the change in the physiological parameter  506  between the lower and higher physical activity measurements. In certain examples, the physiological response is measured at steady-state values of physical activity, for example, such as described in the above-incorporated Beck et al. patent application. The corresponding physiological response to activity can then be used to classify the patient into a heart status class, such as described above. 
     Table 3 is an example of an automatic machine-implemented NYHA classification based on patient respiration rate, such as described above. In certain examples, a patient can be automatically classified into one of the four NYHA classes depending on that patient&#39;s measured respiration rate during various levels of physical activity. Both the respiration rate and the physical activity level can be measured using an implantable medical device, such as described below. The automatic heart status classification can then be performed using the implantable or an external device, such as described above. The numbers provided in this table are non-limiting illustrative examples. 
     
       
         
           
               
             
               
                 TABLE 3 
               
             
            
               
                   
               
               
                 Automatic classification into NYHA classes using respiration as the 
               
               
                 physiological response to physical activity. 
               
            
           
           
               
               
            
               
                   
                 Physical Activity and Symptom (Dyspnea) Relationship 
               
            
           
           
               
               
               
               
            
               
                   
                 Ordinary physical 
                 Less than ordinary 
                   
               
               
                   
                 activity 
                 physical activity 
                 Rest 
               
               
                   
                 (accelerometer 
                 (accelerometer scale 
                 (accelerometer 
               
               
                 Class 
                 scale &gt;80 mg) 
                 15-80 mg) 
                 scale &lt;15 mg) 
               
               
                   
               
               
                 Class I 
                 RR ≦20 bpm 
                 RR ≦20 bpm 
                 RR ≦20 bpm 
               
               
                 Class II 
                 RR 21-25 bpm 
                 RR ≦20 bpm 
                 RR ≦20 bpm 
               
               
                 Class III 
                 RR 26-30 bpm 
                 RR 21-25 bpm 
                 RR ≦20 bpm 
               
               
                 Class IV 
                 RR &gt;30 bpm 
                 RR &gt;25 bpm 
                 RR &gt;20 bpm 
               
               
                   
               
            
           
         
       
     
     Table 4 is an example of an automatic machine-implementable NYHA classification based on patient heart rate. In certain examples, a patient can be automatically classified into one of the four NYHA classes depending on that patient&#39;s measured heart rate during various levels of physical activity. Both the heart rate and the physical activity level can be measured using an implantable medical device, such as described above. The automatic heart status classification can then be performed using the implantable or an external device, such as described above. The numbers provided in this table are non-limiting illustrative examples. 
     
       
         
           
               
             
               
                 TABLE 4 
               
             
            
               
                   
               
               
                 Automated classification to NYHA classes using heart rate as the 
               
               
                 physiological response to physical activity. 
               
            
           
           
               
               
            
               
                   
                 Physical Activity and Symptom (Palpitation) Relationship 
               
            
           
           
               
               
               
               
            
               
                   
                 Ordinary physical 
                 Less than ordinary 
                   
               
               
                   
                 activity 
                 physical activity 
                 Rest 
               
               
                   
                 (accelerometer 
                 (accelerometer scale 
                 (accelerometer 
               
               
                 Class 
                 scale &gt;80 mg) 
                 15-80 mg) 
                 scale &lt;15 mg) 
               
               
                   
               
               
                 Class I 
                 HR ≦90 bpm 
                 HR ≦90 bpm 
                 HR ≦90 bpm 
               
               
                 Class II 
                 HR 91-100 bpm 
                 HR ≦90 bpm 
                 HR ≦90 bpm 
               
               
                 Class III 
                 HR 101-120 bpm 
                 HR 91-100 bpm 
                 HR ≦90 bpm 
               
               
                 Class IV 
                 HR &gt;120 bpm 
                 HR &gt;100 bpm 
                 HR &gt;100 bpm 
               
               
                   
               
            
           
         
       
     
     Additional Notes 
     In this document, certain examples have been described with respect to using a “respiration rate measurement,” for illustrative clarity. However, such examples can also be performed using a “respiration interval measurement” rather than a “respiration rate measurement,” without departing from the scope of the described systems and methods. 
     The above detailed description includes references to the accompanying drawings, which form a part of the detailed description. The drawings show, by way of illustration, specific embodiments in which the invention can be practiced. These embodiments are also referred to herein as “examples.” All publications, patents, and patent documents referred to in this document are incorporated by reference herein in their entirety, as though individually incorporated by reference. In the event of inconsistent usages between this document and those documents so incorporated by reference, the usage in the incorporated reference(s) should be considered supplementary to that of this document; for irreconcilable inconsistencies, the usage in this document controls. 
     In this document, the terms “a” or “an” are used, as is common in patent documents, to include one or more than one, independent of any other instances or usages of “at least one” or “one or more.” In this document, the term “or” is used to refer to a nonexclusive or, such that “A or B” includes “A but not B.” “B but not A,” and “A and B,” unless otherwise indicated. In the appended claims, the terms “including” and “in which” are used as the plain-English equivalents of the respective terms “comprising” and “wherein.” Also, in the following claims, the terms “including” and “comprising” are open-ended, that is, a system, device, article, or process that includes elements in addition to those listed after such a term in a claim are still deemed to fall within the scope of that claim. Moreover, in the following claims, the terms “first,” “second,” and “third,” etc. are used merely as labels, and are not intended to impose numerical requirements on their objects. 
     Method examples described herein can be machine or computer-implemented at least in part. Some examples can include a computer-readable medium or machine-readable medium encoded with instructions operable to configure an electronic device to perform methods as described in the above examples. An implementation of such methods can include code, such as microcode, assembly language code, a higher-level language code, or the like. Such code can include computer readable instructions for performing various methods. The code may form portions of computer program products. Further, the code may be tangibly stored on one or more volatile or non-volatile computer-readable media during execution or at other times. These computer-readable media may include, but are not limited to, hard disks, removable magnetic disks, removable optical disks (e.g., compact disks and digital video disks), magnetic cassettes, memory cards or sticks, random access memories (RAM&#39;s), read only memories (ROM&#39;s), and the like. 
     The above description is intended to be illustrative, and not restrictive. For example, the above-described examples (or one or more aspects thereof) may be used in combination with each other. Other embodiments can be used, such as by one of ordinary skill in the art upon reviewing the above description. The Abstract is provided to comply with 37 C.F.R. §1.72(b), to allow the reader to quickly ascertain the nature of the technical disclosure. It is submitted with the understanding that it will not be used to interpret or limit the scope or meaning of the claims. Also, in the above Detailed Description, various features may be grouped together to streamline the disclosure. This should not be interpreted as intending that an unclaimed disclosed feature is essential to any claim. Rather, inventive subject matter may lie in less than all features of a particular disclosed embodiment. Thus, the following claims are hereby incorporated into the Detailed Description, with each claim standing on its own as a separate embodiment. The scope of the invention should be determined with reference to the appended claims, along with the full scope of equivalents to which such claims are entitled.