Abstract:
In a medical system and a method for operating such a system, the system includes an implantable medical device of a patient, a programmer device, and an extracorporeal stress equipment adapted to exert a physiological stress on the patient, for automatically determining settings of a sensor for sensing a physiological parameter of the patient or for automatically determining a pacing setting of the device over a broad range of workloads of the equipment. The ingoing units and/or devices of the medical system, i.e. the implantable medical device of the patient, the programmer device, and the extracorporeal stress equipment, communicate bi-directionally with each other and form a closed loop.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The present invention generally relates to cardiac pacing systems and, in particular, to methods and medical systems including implantable medical devices, for automatic characterization of sensors of such implantable medical devices and automatic evaluation of settings, e.g. pacing settings such as VV-delays, of such devices. 
         [0003]    2. Description of the Prior Art 
         [0004]    Medical devices are implanted in human bodies for e.g. monitoring physiological conditions or treating diseases. One particular example of implantable medical devices is a cardiac rhythm management device including pacemakers and defibrillators implanted in a patient to treat irregular or other abnormal cardiac rhythms by delivering electrical pulses to the patient&#39;s heart. Pacemakers are often used to treat patients with bradyarrythmias, that is, hearts that beat too slow or irregularly. Defibrillators are capable of delivering higher energy electrical stimuli to the heart and are often used to treat patients with tachyarrythmias, that is, hearts that beat too quickly. 
         [0005]    Such implantable medical devices comprises a number of sensors which are used to sense different physiological parameters of the patient, and in particular of the patient&#39;s heart, in order to deliver an accurate and reliable stimuli of the heart with respect to timing, amplitude etc. For example, sensors for determining an activity level of the patient, e.g. an accelerometer, sensors for determining a breathing rate of the patient are often included in such implantable medical devices. In order to obtain a reliable and correct operation of the implantable device, the characteristics and settings of the sensors must be determined in an accurate way. 
         [0006]    Today, the normal procedure for determining the sensor characteristics and/or settings, e.g. of an activity sensor of an implanted medical device, is to start the collection of the sensor data via a programmer. The patient is then asked to lie down during a period of time and to perform a walk during a second period of time. In this way, sensor data is registered at two different activity levels; namely at rest and at normal walking. 
         [0007]    However, this mainly manual procedure for sensor characterization is associated with a number of drawbacks. For example, the sensor is characterized over a rather limited patient activity level range which may lead to an impaired function of the device due to a non-complete or limited sensor is characterization. Furthermore, it may be difficult for the operator, e.g. the physician, conducting the patient test session to obtain reliable and reproducible results from the test, for example, due to the fact that the exerted workload during the walking is subjective for the patient and the actual workload is difficult to estimate. 
         [0008]    A similar procedure is also conducted when setting up or adjusting pacing settings of an implanted medical device, e.g. setting up or adjusting a V-V timing of a pacemaker after an implantation. In this case, the physician and/or nurse measure the cardiac output and adjusts the V-V timing manually and this is repeated until satisfying results are obtained. This procedure is in some cases also combined with activity level measurements, i.e. the patient is asked to lie down during a period of time and to perform a walk during a second period of time. This manual procedure for setting up or adjusting the pacing settings of an implanted device, such as a pacemaker, may, apart from being time consuming, lead to unreliable results, for example, due to the fact that the cardiac output has to be interpreted and connected to a new V-V delay. 
         [0009]    Thus, there is a need of an improved and automatized procedure for characterization of rate-responsive sensors of implantable medical devices, such as pacemakers, and for an improved and automatic procedure for evaluation and optimizing of pacing settings of implantable medical devices, such as pacemakers. 
         [0010]    United States Patent Application Publication No. 2004/0220636 discloses a system in which an IMD (Implantable Medical Device) programming device receives hemodynamic data from a hemodynamic measurement device (for example an external device) and programs one or more pacing parameters of the IMD as a function of the received pacing data. The IMD programming device is telemetrically linked to the IMD and may read, write, or store, for example, pacing parameters of the IMD to the IMD and/or to the IMD programming device. The hemodynamic measurement device monitors the patient and generates updated hemodynamic data and the programmer may set or adjust the pacing parameters of the IMD as a function of the updated hemodynamic data. However, this system requires that a physician or nurse conducts the test and instructs the patient, for example, in case of determining sensor characteristics at different activity levels, to lie down and to walk during certain periods of time. Accordingly, the system according to United States Patent Application Publication No. 2004/0220636 does not solve all the problems associated with the prior art procedures. 
         [0011]    Hence, there remains a need within the art of an improved and automatized procedure for characterization of rate-responsive sensors of implantable medical devices, such as pacemakers, and for an improved and automatic procedure for evaluation and optimizing of pacing settings of implantable medical devices, such as pacemakers. 
       SUMMARY OF THE INVENTION 
       [0012]    Thus, an object of the present invention is to provide methods and medical systems including implantable medical devices such as pacemakers for automatic characterization of sensors of such devices and automatic evaluation of settings, e.g. pacing settings such as W-delay settings or AV-delay settings, of such devices. 
         [0013]    Another object of the present invention is to provide an improved and automatized procedure and medical system for characterization of rate-responsive sensors of implantable medical devices, such as pacemakers, and for an improved and automatic evaluation and optimizing of pacing settings of implantable medical devices, such as pacemakers that are capable of delivering reliable and accurate results. 
         [0014]    A further object of the present invention is to provide an improved and automatized procedure and medical system for characterization of rate-responsive sensors of implantable medical devices, such as pacemakers, and for an improved and automatic evaluation and optimizing of pacing settings of implantable medical devices, such as pacemakers, for a broad range of activity levels. 
         [0015]    Yet another object of the present invention is to provide an improved method and system for automatically evaluating and optimizing pacing settings of an implantable medical device, such as pacemakers, for a broad range of activity levels. 
         [0016]    In the context of this application, the term “hemodynamic parameter” refers to a parameter that can be measured, sensed or derived, for example, cardiac output, stroke volume, ejection fraction, blood pressure that reflects or relates to actual hemodynamic function. 
         [0017]    For the purpose of clarification, the term “cardiogenic impedance” or “cardiac impedance” is defined as an impedance or resistance variation that origins from cardiac contractions, or in other words, an impedance of tissues measured between at least one electrode located within or at the heart and one or more electrodes located within, at or outside the heart. 
         [0018]    According to an aspect of the present invention, there is provided a medical system for determining settings of an implantable medical device of the system, which device includes a controller or controlling circuit, a pulse generator adapted to produce cardiac stimulating pacing pulses and a communication unit. The system further has a programmer device including a control unit and a communication unit and at least one extracorporeal stress equipment adapted to, during use of/by the patient, exert a physiological stress on the patient, the equipment including a communication unit and a control device adapted to control the stress, wherein the extracorporeal stress equipment is adapted to, during operation, exert a physiological stress on a patient according to predetermined stress equipment workload settings. The programmer is adapted to downlink instructions to start a patient test session and to initiate an automatic setting determination procedure including starting an operation of the stress equipment, the implantable medical device comprises at least one sensor adapted to sense at least one sensor signal associated with a physiological parameter of the patient, wherein the is controlling circuit is adapted to obtain at least one sensor value using the sensor signal for each workload level of the stress equipment workload settings during the determination procedure, and the controlling circuit of the implantable medical device is adapted to determine the settings and/or characteristics of the at least one sensor for each workload level using the obtained sensor values. 
         [0019]    According to a second aspect of the present invention, there is provided a medical system for optimizing pacing settings of an implantable medical device of the system, which device includes a controller or controlling circuit, a pulse generator adapted to produce cardiac stimulating pacing pulses and a communication unit. The system further has a programmer device including a control unit and a communication unit and at least one extracorporeal stress equipment adapted to, during use of/by the patient, exert a physiological stress on the patient, the equipment including a communication unit and a control device adapted to control the stress, wherein: the extracorporeal stress equipment is adapted to, during operation, exert a physiological stress on a patient according to predetermined stress equipment workload settings; the programmer is adapted to downlink instructions to start a patient test session and to initiate an automatic setting determination procedure including starting an operation of the stress equipment; a circuit for obtaining a hemodynamical parameter of the heart of the patient during successive cardiac cycles for each workload level of the stress equipment workload settings, and an evaluation circuit adapted to evaluate the at least one hemodynamical parameter, and the workload data for each workload level of the stress equipment workload settings; and wherein the controlling circuit is adapted to iteratively control a delivery of the so pacing pulses based on the evaluation and to determine optimal pacing settings for each workload level of the stress equipment workload settings. 
         [0020]    According to a third aspect of the present invention, there is provided a method for determining settings of an implantable medical device in a medical system, the device including a pulse generator adapted to produce cardiac stimulating pacing pulses and a communication unit, wherein the system further comprises a programmer device including a control unit and a communication unit and at least one extracorporeal stress equipment adapted to, during use of/by the patient, exert a physiological stress on the patient according to an applied workload, the equipment including a communication unit and a control device adapted to control the applied workload. The method includes the steps of: starting a patient test session; initiating an automatic setting determination procedure comprising the steps of: starting an operation of the stress equipment, wherein the stress equipment, during operation, exerts a physiological stress on the patient according to predetermined stress equipment workload settings; sensing at least one sensor signal associated with a physiological parameter of the patient for each workload level of the stress equipment workload settings; obtaining at least one sensor value using the sensor signal; and determining the settings and/or characteristics of the at least one sensor for each workload level using the obtained sensor values. 
         [0021]    According to a fourth aspect of the present invention, there is provided a method for optimizing pacing settings of an implantable medical device in a medical system, the device including a pulse generator adapted to produce cardiac stimulating pacing pulses and a communication unit, wherein the system further has a programmer device including a control unit and a communication unit and at least one extracorporeal stress equipment adapted to, during use of/by the patient, exert a physiological stress on the patient according to an applied workload, the equipment including a communication unit and a control device adapted to control the applied workload, the method comprising the steps of: starting a patient test session; initiating an automatic setting determination procedure comprising the steps of: starting an operation of the stress equipment, wherein the stress equipment, during the operation, exerts a physiological stress on the patient according to predetermined stress equipment workload settings; obtaining at least one hemodynamical parameter of the heart of the patient during successive cardiac cycles for each workload level of the stress equipment workload settings; obtaining is workload data including a currently applied workload level; evaluating the at least one hemodynamical parameter, the sensor signal value and the workload data; iteratively controlling a delivery of the pacing pulses based on the evaluation; and determining optimal pacing settings for each workload level of the stress equipment workload settings. 
         [0022]    According to a fifth aspect of the present invention, there is provided a computer program product, which when executed on a computer, performs steps in accordance with the second and third aspects of the present invention. 
         [0023]    According to a further aspect of the present invention, there is provided a computer readable medium comprising instructions for bringing a computer to perform steps of methods according to the third and fourth aspects of the present invention. 
         [0024]    Thus, the present invention is based on idea of interconnecting at least one implantable medical device implanted in a patient, at least one programmer workstation, and at least one extracorporeal stress equipment adapted to exert a physiological stress on the patient in a medical system for automatically determining settings of a sensor for sensing a physiological parameter of the patient or for automatically determining a pacing setting of the device over a broad range of workloads of the equipment. The units and/or devices of the medical system, i.e. the implantable medical device of a patient, the programmer device, and the extracorporeal stress equipment, may communicate bi-directionally with each other and form a closed loop during, for example, the patient test session and the automatic setting determination procedure for automatically determining settings of a sensor for sensing a physiological parameter of the patient or automatically determining a pacing setting of the device over a broad range of workloads of the equipment and, hence, for a broad range of exertion levels of the patient. In particular, the extracorporeal stress equipment may be controlled by the implantable device or the programmer during the patient test session, wherein a workload of the stress equipment is adjusted in accordance with a predetermined workload setting such that the patient is exerted for different levels of stress. 
         [0025]    This invention provides several advantages in comparison with the prior art. For example, a sensor can be characterized over a broad spectrum of patient stress levels. Thereby, the sensor function can be improved. Furthermore, it is possible for the physician conducting the patient test session to obtain reliable and reproducible results from the test, for example, due to the fact that the applied workload during the different workload settings can be set accurately by means of the stress equipment. Furthermore, the procedure for setting up or adjusting pacing settings of an implanted medical device, e.g. setting up or adjusting a V-V timing of a pacemaker after an implantation, can be done more efficiently since the optimization is performed automatically at different activity levels of the patient. Thus, the physician and/or nurse do not have to measure the cardiac output, adjust the V-V timing manually and repeat this until satisfying results are obtained. In addition, the pacing settings can be optimized over a broad range of patient stress level in an efficient and reliable manner with respect to hemodynamical function. 
         [0026]    According to an embodiment of the present invention, the at least one sensor is an activity level sensor adapted to sense an activity level of the patient. There are a number of other physiological or hemodynamical parameters of the patient that may be used, for example, as an alternative or complement to the activity level including, without limitation, heart rate, breath rate, posture of the patient, blood temperature, etc. By using an activity sensor, a reliable and accurate measure of the extertion of the patient can be determined and, thus, the reliability and accuracy of, for example, the setting determination can be improved. 
         [0027]    In a further embodiment, the implantable medical device further includes a memory circuit adapted to store the settings and/or characteristics of the at least one sensor value for each workload level. Accordingly, a complete sensor characterization may be stored, i.e. sensor signal value vs. stress equipment setting. 
         [0028]    According to an embodiment of the present invention, the communication circuits, devices or units of the devices of the system, e.g. the programmer, the implantable medical device and the stress equipment, is RF telemetry circuits, devices, or units, which may be adapted for e.g. inductive telemetry or UHF telemetry. 
         [0029]    In embodiments, a communication link between the programmer, the implantable medical device and the stress equipment is established, wherein two-way communications are enabled between the programmer, the implantable medical device and the stress equipment, respectively. Thus, the programmer may downlink data to the implantable medical device and data may be transferred uplink to the programmer from the implantable medical device. The two-way communication between the devices of the system can be realized by means of a number of different technologies including short-range communication links including BLUETOOTH, and IEEE 802.11b, or other types of short range wireless connections such as Infrared. Another alternative is using, for example, USB connections. Further, the devices of the system may communicate wirelessly with each other using RF-technology. 
         [0030]    The devices of the medical system may communicate with each other in a network forming a part of a wireless LAN (“Local Area Network”). For a given communication method, a multitude of standard and/or proprietary communication protocols may be used. For example, and without limitation, wireless (e.g. radio frequency pulse coding, spread spectrum frequency hopping, time-hopping, etc.) and other communication protocols (e.g. SMTP, FTP, TCP/IP) may be used. Other proprietary methods and protocols may also be used. Furthermore, combination of two or more of the communication methods and protocols may also be used. 
         [0031]    In one embodiment, the programmer is adapted to, upon receiving a patient test session initiation command, to establish the communication link. Further, the programmer is adapted to, upon after having established the communication link to the implantable medical device, send a command initiating the automatic determination procedure to the implantable medical device. However, master device and slave device in the medical system according to the present invention may vary during a session. 
         [0032]    According to another embodiment, the controlling circuit of the implantable medical device is adapted to obtain a list of stress equipment workload settings comprising a predetermined number of different workload levels; and wherein the controlling circuit is adapted to send running instructions to the extracorporeal stress equipment instructing the extracorporeal stress equipment to operate according to the stress equipment workload settings of the list. The list may be stored in the memory circuit of the implantable medical device or in the programmer. Alternatively, the programmer may be connected to a communication network and the list can be obtained from a database connected to the network or a computer connected to the network, which may be, for example, a LAN (“Local Area Network”) or a WAN (“Wide Area Network”). The programmer may be connected to the communication network via a network such as Internet. 
         [0033]    In yet another embodiment of the present invention, the implantable medical device further comprises a signal processing unit adapted to process the at least one sensor signal from at least one sensor of the implantable device, which signal processing unit is adapted to start a signal processing procedure of the sensor signal a predetermined period of time after an adjustment of workload according to the workload settings. Further, the signal processing circuit may be adapted to perform an averaging process of the sensor signal during the signal processing procedure. Alternatively or in addition, the signal processing circuit may be adapted to perform a filtering process of the obtained signals. 
         [0034]    In yet another embodiment of the present invention, the circuit for obtaining a hemodynamical parameter comprises a circuit for measuring an impedance of tissues between right and left side of the heart for successive cardiac cycles; a circuit for measuring a heart rate of the patient; and wherein the circuit for obtaining a hemodynamical parameter is adapted to determine a relative cardiac output (CO) based on the measured impedance and the measured heart rate for successive cardiac cycles. Of course, as the skilled person within the art realizes, there are other hemodynamical parameters that can be measured, sensed or derived, for example, stroke volume, ejection fraction, heart sounds, blood pressure and used as metric of the hemodynamic function of the heart during the optimization procedure. 
         [0035]    In a further embodiment of the present invention, the controlling circuit of the implantable medical device is adapted to: a) select an initial workload setting of 
         [0036]    the predetermined stress equipment workload settings; 
         [0037]    b) operate the stress equipment according to the selected workload setting; c) iteratively control the delivery of the pacing pulses based on the evaluation; d) determine optimal pacing settings for the initial workload level; e) store the optimal pacing setting and corresponding activity level for the initial workload level in the memory circuit of the implantable medical device; f) repeat steps b)-e) for each workload level of the stress equipment workload setting; and g) store a matrix of pacing settings and corresponding activity levels in the memory circuit. 
         [0038]    According to embodiment, the pacing settings are VV-delay settings, but in alternative embodiments, the settings may be, for example, AV-delay settings. 
         [0039]    The system according to the present invention may also comprise external diagnostic equipment, for example, an ultra sound machine connected to the patient. The ultra sound machine is adapted to communicate with the programmer and/or the implantable medical device and/or the stress equipment. For example, the programmer may control the ultra sound machine and the measurements results from the ultra sound machine can be used in the automatic setting determination procedure to characterizing a sensor and/or the optimization of pacing settings such as VV-delay or AV-delay settings can be performed using the measurements from the ultra sound machine. As the skilled man realize, there are a number of alternative or complementing device that can be used, for example, a breath monitoring device may be connected to the patient to measure or sense, for example, a composition of the expiration air of the patient. 
         [0040]    According to another embodiment of the present invention, the condition of the patient undergoing the patient test session at the stress equipment is monitored or supervised. In one embodiment, the cardiac output (CO) and the patient activity levels are monitored or observed. In case of worsening of the condition, e.g. if the cardiac output is found to exceed or fall below predetermined limits, the workload of the stress equipment may be reduced. Thereby, the safety of the patient can be enhanced during the test sessions. 
         [0041]    As realized by the person skilled in the art, steps of the methods of the present invention, as well as preferred embodiment thereof, are suitable to realize as a computer program or a computer readable medium. 
         [0042]    The features that characterize the invention, both as to organization and to method of operation, together with further objects and advantages thereof, will be better understood from the following description used in conjunction with the accompanying drawings. It is to be expressly understood that the drawings is for the purpose of illustration and description and is not intended as a definition of the limits of the invention. These and other objects attained, and advantages offered, by the present invention will become more fully apparent as the description that now follows is read in conjunction with the accompanying drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0043]      FIG. 1   a  schematically shows a medical system in accordance with an embodiment of the present invention. 
           [0044]      FIG. 1   b  schematically shows a medical system in accordance with another embodiment of the present invention. 
           [0045]      FIG. 2   a  schematically shows an embodiment of an implantable medical device of the system shown in  FIG. 1 . 
           [0046]      FIG. 2   b  schematically shows a further embodiment of an implantable medical device of the system shown in  FIG. 1 . 
           [0047]      FIG. 3  is a high-level flow chart of the methods for setting or characterizing a sensor of the implantable medical device or for optimizing a pacing setting of an implantable medical device according to the present invention. 
           [0048]      FIG. 4  is a flow chart of a procedure for determining settings of a sensor or characterizing a sensor of the implantable medical device in accordance with the present invention. 
           [0049]      FIG. 5  is a flow chart describing an algorithm for determining or optimizing a VV-delay of an implantable medical device in accordance with the present invention. 
           [0050]      FIG. 6  is a flow chart showing the optimization procedure according to the present invention shown in  FIG. 5  in more detail. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0051]    In the following, the present invention will be discussed in the context of a medical system comprising at least an implantable bi-ventricular pacemaker, an external or extracorporeal programmer workstation and an extracorporeal stress equipment. However, the present invention may also be implemented in system including other implantable devices such as a CRT (Cardiac Resynchronization Therapy) device, or an ICD (Implantable Cardioverter Defibrillator). Furthermore, the system may also include other devices, units or equipment such as, for example, an ultrasound machine. 
         [0052]    With reference first to  FIGS. 1   a  and  1   b , embodiments of the medical system of the present invention will be described. In one embodiment of the present invention shown in  FIG. 1   a , the medical system  1  includes a programmer workstation  2 , an implanted medical device  20  (which will be described in more detail with reference to  FIGS. 2   a  and  2   b ) implanted in a patient (not shown) and extracorporeal stress equipment  6 , for example, a treadmill. The programmer  2  includes a control unit  4  and a communication unit  5 , i.e. an RF telemetry circuitry for providing bi-directional RF communications with, for example, the implanted medical device  20  and/or the stress equipment  6 . The programmer  2  may downlink data, commands or instructions to the implanted medical device  20  and/or the stress equipment  6  and may receive data, commands or instructions uplink from the implanted medical device  20  and/or the stress equipment  6 . 
         [0053]    Further, the programmer  2  has a memory  7  for storing, for example, predetermined stress equipment workload settings for different patients, a display unit or monitor, (not shown) for presenting information for a user by means of a graphical user interface (GUI), and input devices (not shown), for example, a keyboard and a mouse, which enable a user to, for example, input information and commands such as a command for starting or initiating a patient test session. The stress equipment  6  has a control unit  8  adapted to, for example, control a workload of the equipment for, during use of a patient, exerting a physiological stress on the patient or starting an operation of the equipment, and a communication unit  9 , i.e. an RF telemetry circuitry for providing bi-directional RF communications with, for example, the implanted medical device  20  and/or the programmer  2 . The operations of the stress equipment  6  may be controlled, via the control unit  8  of the stress equipment, by the programmer  2  or the implantable medical device  20 , for example, the operations during a patient test session for characterizing or determining the settings of a sensor of an implanted medical device  20  or determining or optimizing pacing settings of the implanted medical device  20 . 
         [0054]    The programmer  2 , the implantable medical device  20  and the stress equipment  6  or the medical system  1  may be interconnected in a telemetry communication system which allows two-way communication between the units or devices of the medical system  1 . As the skilled person realizes, the two-way communication between the devices of the system can be implemented by means of a number of different technologies including short range communication links including BLUETOOTH, and IEEE 802.11b, or other types of short-range wireless connections such as Infrared. Further, the devices of the system may communicate wirelessly with each other using RF-technology. Moreover, the devices of the medical system may communicate with each other in a network forming a part of a wireless LAN (“Local Area Network”). 
         [0055]    The programmer  2 , the implantable medical device  20  and the stress equipment  6  of the medical system  1 ′ may also communicate with each other via a communication network  12 , such as, the internet or a wireless RF communication network  12 , see  FIG. 1   b.    
         [0056]    For a given communication method, a multitude of standard and/or proprietary communication protocols may be used. For example, and without limitation, wireless (e.g. radio frequency pulse coding, spread spectrum frequency hopping, time-hopping, etc.) and other communication protocols (e.g. SMTP, FTP, TCP/IP) may be used. Other proprietary methods and protocols may also be used. Further, combination of two or more of the communication methods and protocols may also be used. 
         [0057]    The bi-directional communication between the implantable medical device  20  and the external stress equipment  6  may be a virtual bi-directional communication, i.e. the programmer  2  acts as an intermediary device between the implantable medical device  20  and the stress equipment  6 . 
         [0058]    In another embodiment of the present invention, the medical system also has external diagnostic equipment, for example, an ultrasound machine connected to the patient. Thereby, the automatic setting determination procedure to characterizing a sensor and/or the optimization of pacing settings such as VV-delay or AV-delay settings can be performed using the measurements from the ultra sound machine. 
         [0059]    Turning now to  FIGS. 2   a  and  2   b , the configuration including the primary components of embodiments of implantable medical devices of the system  1  described in  FIG. 1  will be described. 
         [0060]    In  FIG. 2   a , one embodiment of the implantable medical device according to the present invention is shown. The implantable medical device  20 , such as a bi-ventricular pacemaker, has a housing (not shown) being hermetically sealed and biologically inert. Normally, the housing is conductive and may, thus, serve as an electrode. The pacemaker  20  is connectable to one or more pacemaker leads, where only two are shown in  FIG. 1 ; namely a ventricular lead  26   a  implanted in the right ventricle of the heart (not shown) and one lead  26   b  implanted in a coronary vein of the left side of the heart (not shown). The leads  26   a  and  26   b  can be electrically coupled to the pacemaker  20  in a conventional manner. The leads  26   a ,  26   b  carry one or more electrodes, such as a tip electrode or a ring electrode, arranged to, inter alia, measure the impedance or transmit pacing pulses for causing depolarization of cardiac tissue adjacent to the electrode (-s) generated by a pace pulse generator  25  under influence of a controller or controlling circuit  27  including a microprocessor. The controller  27  controls, inter alia, pace pulse parameters such as output voltage and pulse duration. 
         [0061]    Furthermore, the implantable medical device  20  includes at least one sensor  29  adapted to sense at least one sensor signal associated with a physiological parameter of the patient. In one embodiment, the sensor  29  is an activity level sensor adapted to sense an activity level of the patient, for example, an accelerometer. The sensor  29  is connected to a signal processing circuit  23  adapted to process sensed signals received from the sensor  29 . 
         [0062]    Moreover, a storage unit  31  is connected to the controller  27 , which storage unit  31  may include a random access memory (RAM) and/or a non-volatile memory such as a read-only memory (ROM). Storage unit  31  is connected to the controller  27  and the signal processing circuit  23 . Detected signals from the patient&#39;s heart are processed in an input circuit  33  and are forwarded to the controller  27  for use in logic timing determination in known manner. The implantable medical device  20  is powered by a battery (not shown), which supplies electrical power to all electrical active components of the implantable medical device  20 . The implantable medical device  20  further has a communication unit  34 , for example, an RF telemetry circuitry for providing RF communications. Thereby, for example, data contained in the storage unit  31  can be transferred to a programmer (see  FIG. 1 ) via the communication unit and a programmer interface (not shown) for use in analyzing system conditions, patient information, etc. 
         [0063]    In one embodiment, the controller  27  is adapted to obtain at least one sensor value using the sensor signal for each workload level of the stress equipment workload settings during the determination procedure, which will be described in more detail below. Further, the controller  27  is adapted to determine the settings and/or characteristics of the at least one sensor for each workload level using the obtained sensor values. 
         [0064]    With reference now to  FIG. 2   b , another embodiment of an implantable medical device in accordance with the present invention and that can be used in a medical system in accordance with the present invention will be described. Like or similar parts in  FIGS. 2   a  and  2   b  will be denoted with the same reference numeral and description of parts that have been described with reference to  FIG. 2   a  will be omitted. The implantable medical device  20 ′, such as a bi-ventricular pacemaker, further has a circuit  35  for obtaining a hemodynamical parameter of the heart of the patient. In one embodiment, the circuit  35  has an impedance measuring circuit  36  for measuring an impedance of tissues between right and left side of the heart, which circuit  36  may be connected to the leads  26   a  and  26   b . The impedance may be measured between electrodes placed inside or on the surface of the heart, integrated on a pacemaker lead, for example the leads  26   a ,  26   b . Further, the circuit  35  for obtaining a hemodynamical parameter includes a heart rate sensor  37  for measuring a heart rate of the patient. The heart rate sensor can be incorporated in the device in accordance with conventional practice within the art. Of course, as those skilled in the art will realize, there are other hemodynamical parameters that can be measured, sensed or derived, for example, stroke volume, ejection fraction, heart sounds, blood pressure. As used herein, a hemodynamical parameter encompasses any metric that reflects or relates to actual hemodynamic function. 
         [0065]    Moreover, the circuit  35  for obtaining a hemodynamical parameter is adapted to determine a relative cardiac output (CO) based on the measured cardiac impedance and the measured heart rate. A portion of the cardiac impedance carries information of the amount of blood in the left ventricle and thus varies during the heart cycles. The impedance variation during a heart cycle corresponds to stroke volume, i.e. the volume of blood ejected per heart beat and equals end-diastolic volume minus end-systolic value. This portion of the measured impedance or impedance information is extracted in the impedance measuring circuit  36  or the circuit  35  for obtaining a hemodynamical parameter. Thereby, the circuit  35  for obtaining a hemodynamical parameter may obtain a relative value of the cardiac output, i.e. by knowing the heart rate and a relative value of the stroke volume of the left ventricle. The cardiac output is the volume of blood, measured in liters, ejected by the heart per minute and is determined by multiplying the heart rate and the stroke volume. Accordingly, the value of the cardiac output obtained by the circuit  35  for obtaining a hemodynamical parameter is a relative value. 
         [0066]    Furthermore, the implantable medical device  20 ′ has an evaluation circuit  38  adapted to evaluate the obtained hemodynamical parameter (in one embodiment the cardiac output), the sensor signal (in one embodiment the activity level of the patient) and workload data for each workload level of the stress equipment workload settings, which may be stored in the memory circuit  31  of the implantable device  20 ′ or in the programmer  2  (see  FIG. 1 ), in which case the stress equipment workload settings can be obtained by the implantable device by means of a data transfer using the communication unit  34 . 
         [0067]    In one embodiment, the evaluation circuit  38  is adapted to evaluate the at least one hemodynamical parameter, e.g. a relative value of the cardiac output as described above, and the workload data for each workload level of the stress equipment workload settings. Further, the controller  27  may be adapted to iteratively control a delivery of the pacing pulses based on the evaluation and to determine optimal pacing settings for each workload level of the stress equipment workload settings. In addition, the evaluation circuit  38  may be adapted use at least one sensor signal from a sensor, wherein the at least one sensor is an activity level sensor adapted to sense an activity level of the patient, for each workload level of the stress equipment workload settings during the determination procedure in the evaluation. 
         [0068]    With reference now to  FIG. 3 , a high-level description of the steps of the methods for setting or characterizing a sensor of the implantable medical device or for optimizing a pacing setting of an implantable medical device according to the present invention will be given. First, in step  40 , a communication link between the devices of a medical system, e.g. the devices of the system  1  in  FIG. 1 , is established. For example, this can be performed by an operator of the programmer, e.g. a physician, using the input devices of the programmer  2 . Thereafter, at step  42 , the operator of the programmer initiates the patient test session and the automatic setting determination procedure, e.g. for characterizing the activity sensor of the implantable medical device  20  or for determining a pacing setting of the implantable medical device  20  such as a VV-delay. Subsequently, at step  44 , the programmer  2  sends a command or instruction to the implantable medical device to initiate the automatic setting determination procedure. Then, at step  46 , the automatic setting determination procedure is initiated and executed as will be described in detail hereinafter. Finally, at step  48 , the operator of the programmer is presented for a message, e.g. on the display unit informing the operator of the finalization of the automatic setting determination procedure. 
         [0069]    Referring to  FIG. 4 , a procedure for determining settings of a sensor or characterizing a sensor of the implantable medical device  20  will be described. First, at step  50 , the implantable medical device  20  receives an instruction to start the automatic setting determination procedure from the programmer. That is, after the communication link between the devices of a medical system, e.g. the devices of the system  1  in  FIG. 1 , has been established the patient test session and the automatic setting determination procedure have been initiated by the operator of the programmer  2 . Then, at step  52 , the controller  27  of the device  20  obtains a stress equipment workload setting list, which can be stored in the memory circuit  31  or in the memory circuit  7  of the programmer  2  and the device  20  sends an instruction to the stress equipment  6  to start operate at an initial workload, for example, at zero workload. Thus, the patient is exterted for an initial exertion or stress, for example, on a treadmill. The stress equipment workload setting list may be a predetermined protocol for the patient including a sequence of workload level settings, i.a. equipment speed, equipment inclination in case of a treadmill and the duration of each workload level setting. The controller  27  of the device  20  may control the stress equipment in accordance with the predetermined workload setting or the protocol including the predetermined setting may be transferred to the stress equipment. At step  54 , the activity sensor response a predetermined time after a new equipment setting is stored and associated with the present equipment setting. Then, at step  56 , it is checked whether all equipment settings of the list have been used. If no, the procedure returns to step  58  and the next equipment setting is read from the list and the stress equipment is instructed to adjust the workload in accordance to the new settings. On the other hand, if all settings of the list have been used, the procedure proceeds to step  60  where all sensor responses and corresponding stress equipment settings, i.e. the sensor characterization (measured sensor signal vs. stress equipment setting) is stored in the memory circuit  31  of the implantable medical device  20 . Thereafter, a stop command is sent to the stress equipment  6  and to the programmer  2  informing them that the procedure is completed. The operator, e.g. a physician, may be informed of the results from the characterization procedure on the display unit via the graphical user interface of the programmer  2 . 
         [0070]    Turning now to  FIG. 5 , an algorithm for determining or optimizing a pacing setting such as a VV-delay of an implantable medical device will be described. First, at step  70 , the implantable medical device  20  receives an instruction to start the automatic setting determination procedure from the programmer  2 . That is, after the communication link between the units and/or devices of a medical system, e.g. the devices of the system  1  or  1 ′ in  FIGS. 1   a  and  1   b , respectively, has been established, the patient test session and the automatic setting determination procedure have been initiated by the operator of the programmer  2 , the implantable medical device  20  receives the instruction to start the automatic setting determination procedure from the programmer  2 . Then, at step  72 , the controller  27  of the device  20  obtains a stress equipment workload setting list, which can be stored in the memory circuit  31  or in the memory circuit  7  of the programmer  2  and the device  20  sends an instruction to the stress equipment  6  to start operate at an initial workload, for example, at zero workload. Thus, the patient is exterted for a initial stress, for example, on a treadmill. The stress equipment workload setting list may be a predetermined protocol for the patient including a sequence of workload level settings, i.a. equipment speed, equipment inclination in case of a treadmill and the duration of each workload level setting. The controller  27  of the device  20  may control the stress equipment in accordance with the predetermined workload setting or the protocol including the predetermined setting may be transferred to the stress equipment. 
         [0071]    Subsequently, at step  74 , an optimization of the pacing settings of the device, for example, the VV-delay at the current workload is performed or executed. The optimization procedure will be described in more detail with reference to  FIG. 6 . Thereafter, at step  76 , the optimal VV-delay and the corresponding sensor value, i.e. in this embodiment the activity level, are returned from the optimization subroutine and are stored. This is repeated for the sequence of workloads of the list and, hence, a matrix of optimal VV-delays and corresponding activity levels are created and stored, for example, in the memory circuit  31  of the implantable medical device  20 . Then, the automatic setting determination procedure is terminated and the operator may be presented for the results on the display unit of the programmer  2 . 
         [0072]    Referring now to  FIG. 6 , the optimization procedure according to the present invention will be described in detail. The procedure is described with reference to an optimization of a VV-delay but, however, as the skilled person realizes, the present optimization procedure may also be performed to optimize other pacing settings of an implantable medical device, such as, the AV-delay. 
         [0073]    First, at step  80 , an initialization is executed, the VV-delay is set to zero and the stress equipment is instructed to operate according to the workload settings list, i.e. equipment speed and inclination is set according to the list in case of a treadmill. The controller  27  of the implantable medical device  20  may be adapted to obtain a VV-delay optimization protocol including the initial or start W-delay, the increments or step increase to use during the optimization, maximum VV-delay, maximum negative VV-delay, and step decrease to use during the optimization. This protocol may be stored in the memory circuit  31  of the implantable device or it may be obtained or transferred from the programmer workstation  2  at initialization of the optimization procedure. 
         [0074]    Then, at step  82 , the heart is stimulated bi-ventricularly during a number of successive cardiac cycles and a relative value of the cardiac output (CO) is obtained in accordance with the description given above. At step  84 , the activity level is obtained from the activity sensor (or sensors) and stored. As the skilled person realizes, there are other conceivable physiological parameters that can be used instead of, or as a complement to the activity level to improve the optimization, for example, the posture of the patient or the breath rate of the patient. 
         [0075]    At step  86 , the obtained CO-value is stored and linked to the present VV-delay and the obtained activity level. Subsequently, at step  88 , the VV-delay is increased with a predetermined step in accordance with the VV-delay protocol. At step  90 , the heart is stimulated bi-ventricularly during a number of successive cardiac cycles and a relative value of the cardiac output (CO) is obtained in accordance with the description given above. Then, at step  92 , the obtained CO-value is stored and linked to the present VV-delay and the obtained activity level. Thereafter, at step  94 , it is checked whether the maximal VV-delay has been reached. If no, the algorithm returns to step  88  and, on the other hand, if yes, the algorithm proceeds to step  96  where the VV-delay is decreased with a predetermined step. Thereafter, at step  98 , it is checked if the present VV-delay already has been processed. If yes, the algorithm returns to the previous step, step  98 , and the VV-delay is decreased yet another step. If it is verified in step  96  that the present VV-delay not has been processed, the algorithm proceeds to step  100  where the heart is stimulated bi-ventricularly during a number of successive cardiac cycles and a relative value of the cardiac output (CO) is obtained in accordance with the description given above. Subsequently, at step  102 , the obtained CO-value is stored and linked to the present VV-delay and the obtained patient activity level. After this, at step  104 , it is checked whether the maximum negative VV-delay has been reached. If no, the algorithm returns to step  96  and the present VV-delay is decreased with a predetermined step. If it is verified that the maximum negative VV-delay has been reached, the algorithm instead proceeds to step  106  where the VV-delay which is correlated with the maximum CO value is identified and stored as the optimal VV-delay for the present activity level. 
         [0076]    According to another embodiment of the present invention, the controller  27  of the implantable medical device is adapted to supervise the condition of the patient undergoing the patient test session at the stress equipment. This can be performed, for example, by monitoring or observing the cardiac output (CO) and the patient activity levels. In case of worsening of the condition, e.g. if the cardiac output is found to exceed or fall below predetermined limits, the controller may be adapted to, for example, reduce the speed or inclination of the stress equipment and thus reduce the workload of the patient. Thereby, the safety of the patient can be enhanced during the test sessions. 
         [0077]    Although modifications and changes may be suggested by those skilled in the art, it is the intention of the inventors to embody within the patent warranted heron all changes and modifications as reasonably and properly come within the scope of their contribution to the art.