Patent Publication Number: US-2010121398-A1

Title: Implantable medical device and method for monitoring valve movements of a heart

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention generally relates to the field of implantable heart stimulation devices, such as pacemakers, implantable cardioverter-defibrillators (ICD), and similar cardiac stimulation devices that also are capable of monitoring and detecting electrical activities and events within the heart. More specifically, the present invention relates to an implantable medical device for monitoring the movements of the valve planes of the heart to determine at least one hemodynamic measure reflecting a mechanical functioning of a heart of a patient. 
     2. Description of the Prior Art 
     Implantable heart stimulators that provide stimulation pulses to selected locations in the heart, e.g. selected chambers, have been developed for the treatment of cardiac diseases and dysfunctions. Heart stimulators have also been developed that affect the manner and degree to which the heart chambers contract during a cardiac cycle in order to promote the efficient pumping of blood. 
     Furthermore, the heart will pump more effectively when a coordinated contraction of both atria and ventricles can be provided. In a healthy heart, the coordinated contraction is provided through conduction pathways in both the atria and the ventricles that enable a very rapid conduction of electrical signals to contractile tissue throughout the myocardium to effectuate the atrial and ventricular contractions. If these conduction pathways do not function properly, a slight or severe delay in the propagation of electrical pulses may arise, causing asynchronous contraction of the ventricles which would greatly diminish the pumping efficiency of the heart. Patients who exhibit pathology of these conduction pathways, such as patients with bundle branch blocks, etc., can thus suffer from compromised pumping performance. For example, asynchronous movements of the valve planes of the right and left side of the heart, e.g. an asynchronous opening and/or closure of the aortic and pulmonary valves, is such an asynchrony that affects the pumping performance in a negative way. This may be caused by right bundle branch block (RBBB), left bundle branch block (LBBB), or A-V block. In a well functioning heart the left and right side of the heart contract more or less simultaneously starting with the contraction of the atria flushing down the blood through the valves separating the atria from the ventricles. In the right side of the heart through the tricuspid valve and in the left side of the heart through the mitral valve. Shortly after the atrial contraction the ventricles contract resulting in increasing blood pressure inside the ventricles that first closes the one way valves to the atria and after that forces the outflow valves to open. In the right side of the heart it is the pulmonary valves, that separates the right ventricle from the pulmonary artery that leads the blood to the lung, which is opened. In the left side of the heart the aortic valve separates the left ventricle from the aorta that transports blood to the whole body. The outflow valves, the pulmonary valve and aortic valve, open when the pressure inside the ventricle exceeds the pressure in the pulmonary artery and aorta, respectively. The ventricles are separated by the intraventricular elastic septum. Hence, for a well functioning heart a substantially synchronous operation of the left and right hand side of the heart, e.g. a synchronous opening and/or closure of the aortic and pulmonary, is of a high importance. 
     Various procedures have been developed for addressing disorders related to asynchronous function of the heart. For instance, cardiac resynchronization therapy (CRT) can be used for effectuating synchronous atrial and/or ventricular contractions. Furthermore, cardiac stimulators may be provided that deliver stimulation pulses at several locations in the heart simultaneously, such as biventricular stimulators. The stimulation pulses could also be delivered to different locations with a selected delay in an attempt to optimize the hemodynamic performance, e.g. synchronize the closure of the aortic and pulmonary valves, in relation to the specific cardiac dysfunction present at the time of implant. 
     Information about the mechanical functioning of a heart can be obtained by means electrical signals produced by the heart. In a healthy heart the sinus node, situated in the right atrium, generates electrical signals which propagates throughout the heart and control its mechanical movement. Some medical conditions, however, affect the relationship between the electrical and mechanical activity of the heart and, therefore, measurements of the electrical activity only cannot be relied upon as indicative of the true status of the heart or as suitable for triggering stimulation of the heart. 
     Consequently, there is a need within the field of methods and devices for obtaining accurate and reliable signals reflecting different aspects of mechanical functioning of the heart. 
     Impedance measurements has been shown to provide reliable information regarding the mechanical functioning of the heart. Through the impedance measurements, blood volume changes are detectable. Blood has a higher conductivity (lower impedance) than myocardial tissue and lungs. The impedance-volume relationship is inverse; the more blood—the smaller impedance. In EP 1 561 489, for example, transvalvular impedance measurements are made between an atrium and a ventricle electrode of a implanted electro-catheter to provide information indicative of the mechanical state of the heart. The information is used to control the pacing rate of a rate responsive pacemaker. In particular, the impedance between across the tricuspid valve between the atrium and the ventricle of the right hand side of the heart is measured. 
     However, in order to be able to optimize the functioning of the heart it is of interest to obtain information that provide a more complete picture of the mechanical functioning and the pumping action of the heart and that provide accurate and reliable information of the mechanical functioning and the pumping action of the heart. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to address the problem of obtaining information that reflects the mechanical functioning and the pumping action of the heart. 
     A further object of the present invention is to provide a device and method that automatically obtains information that reflects the mechanical functioning and the pumping action of the heart in an accurate and reliable way. 
     According to an aspect of the present invention, there is provided an implantable medical device for determining at least one hemodynamic measure reflecting a mechanical functioning of a heart of a patient including a pace pulse generator adapted to produce cardiac stimulating pacing pulses and being connectable to at least one medical lead for delivering the pulses to cardiac tissue of the heart. The implantable medical device has an impedance measuring circuit that, during impedance measuring sessions, measures impedance between at least a first pair of electrodes of the at least one medical lead. The at least first pair includes at least one electrode located in an atrium of the heart and at least one valve plane electrode located substantially at the level of a valve plane the heart. The impedance measuring circuit also measures impedance between at least a second pair of electrodes of the at least one medical lead, the at least second pair including at least one electrode located in a ventricle of the heart and at least one valve plane electrode located substantially at the level of the valve plane. These measured impedances reflect valve plane movements. A hemodynamic parameter determining circuit determines at least one hemodynamic parameter based on the impedances, wherein the at least one hemodynamic parameter representing the mechanical functioning of a heart. 
     According to a second aspect of the present invention, there is provided a method for determining at least one hemodynamic measure reflecting a mechanical functioning of a heart of a patient using an implantable medical device including a pace pulse generator adapted to produce cardiac stimulating pacing pulses and being connectable to at least one medical lead for delivering the pulses to cardiac tissue of the heart. The method includes the steps of, during impedance measuring sessions, measuring impedance between at least a first pair of electrodes of the at least one medical lead, the at least first pair including at least one electrode located in an atrium of the heart and at least one valve plane electrode located substantially at the level of a valve plane the heart, and between at least a second pair of electrodes of the at least one medical lead, the at least second pair including at least one electrode located in a ventricle of the heart and at least one valve plane electrode located substantially at the level of the valve plane. These impedances reflect valve plane movements are obtained. At least one hemodynamic parameter based on the impedances is automatically determined, that reflects the mechanical functioning of a heart. 
     According to a third aspect of the present invention, there is provided a computer readable medium comprising instructions that cause a programmable device to perform steps of a method according to the second aspect of the present invention. 
     Thus, the present invention is based on the insight of monitoring valve movements using electrodes placed adjacent to or substantially at the level of the valve plane of the heart by measuring impedance variations between at least one electrode placed adjacent to or substantially at the level of the valve plane and at least one electrode attached in an atrium and at least one electrode attached in a ventricle, respectively. The valve plane movements are caused by the pumping action of the heart, i.e. by the increased and decreased volume of the ventricles, and by studying the valve plane movements the contraction pattern and mechanical functioning of the heart can be monitored. The measured impedances can, in turn, be used to determine hemodynamic parameters reflecting the mechanical functioning of the heart. Thereby, it is possible to automatically obtain information that accurately and reliably reflects the mechanical functioning and the pumping action of the heart. 
     The obtained information regarding the mechanical functioning of the heart may, for example, be used to optimize parameters of the implantable medical device such as the AV or VV delay. In one embodiment of the present invention, the implantable medical comprises an AV and/or VV delay determining circuit adapted to initiate an optimization procedure, wherein the pace pulse generator is controlled to, based on the hemodynamic parameter, iteratively adjust a present AV and/or VV delay to optimize an AV and/or VV delay with respect to the hemodynamic parameter. Thereby, the AV and/or VV delay can be dynamically and automatically adjusted with respect to the present pumping action of the heart. Further, the adjustments of the AV and/or VV delay can be made dynamically as a response to a changing mechanical functioning of the heart. 
     In a further embodiment of the present invention, the impedance measuring circuit, during impedance measuring sessions, measures impedance between the at least first pair of electrodes of the at least one medical lead including an electrode located in an atrium of the heart and at least one first valve plane electrode located substantially at the level of the valve plane in close proximity to the right atrium of the heart and at least one second valve plane electrode located substantially at the level of the valve plane in close proximity to the left atrium of the heart, respectively, as well as between the at least second pair of electrodes of the at least one medical lead including an electrode located in a ventricle of the heart and the valve plane electrodes located substantially at the level of the valve plane, respectively. These impedance signals reflect valve plane movements at respective sides of the heart and the hemodynamic parameter determining circuit determines a synchronicity measure based on the impedances, the synchronicity measure reflecting a synchronicity between the valve plane movements of the right hand side and the left hand side of the heart, respectively, during the measurement sessions. In embodiments of the present invention, synchronicity between a closure of the aortic valve and the pulmonary valve and/or an opening of the aortic valve and the pulmonary valve is/are determined. 
     Thereby, it is possible to monitor the parallelity or synchronicity of the left and right hand side of the heart in an accurate and reliable way as well as the operation of the aortic valve and the pulmonary valves. In a well functioning heart the left and right side of the heart contract more or less simultaneously starting with the contraction of the atria flushing down the blood through the valves separating the atria from the ventricles. In the right side of the heart through the tricuspid valve and in the left side of the heart through the mitral valve. Shortly after the atrial contraction the ventricles contract resulting in increasing blood pressure inside the ventricles that first closes the one way valves to the atria and after that forces the outflow valves to open. In the right side of the heart it is the pulmonary valves, that separates the right ventricle from the pulmonary artery that leads the blood to the lung, which is opened. In the left side of the heart the aortic valve separates the left ventricle from the aorta that transports blood to the whole body. The outflow valves, the pulmonary valve and aortic valve, open when the pressure inside the ventricle exceeds the pressure in the pulmonary artery and aorta, respectively. Hence, for a well functioning heart a substantially synchronous opening and/or closure of the aortic and pulmonary is of a high importance. 
     In another embodiment of the present invention, a synchronicity between a closure of the mitral valve and the tricuspid valve, respectively, is determined based on the impedances. 
     According to a further example of the present invention, the AV and/or VV delay determining circuit initiates an optimization procedure, wherein the pace pulse generator is controlled, based on the synchronicity measure, to iteratively adjust a present AV and/or VV delay to identify an AV and/or VV delay that causes substantially synchronized valve plane movements of the right hand side and the left hand side of the heart, respectively, during a cardiac cycle. 
     In yet another example of the present invention, the AV and/or VV delay determining circuit initiates an optimization procedure, wherein the pace pulse generator is controlled, based on the synchronicity between a closure of the aortic valve and the pulmonary valve and/or the synchronicity between an opening of the aortic valve and the pulmonary valve, to iteratively adjust a present AV and/or VV delay to identify an AV and/or VV delay that causes a substantially synchronized closure and/or opening of the aortic valve and the pulmonary valves. 
     Moreover, the AV and/or VV delay determining circuit may be adapted to initiate an optimization procedure, wherein the pace pulse generator is controlled to, based on the synchronicity between a closure of the mitral valve and the tricuspid valve, iteratively adjust a present AV and/or VV delay to identify an AV and/or VV delay that causes a substantially synchronized closure of the mitral and tricuspid valves. 
     In embodiments of the present invention, the impedance measuring circuit determines a maximum and/or minimum impedance of each respective impedance for each cardiac cycle. Moreover, the impedance measuring circuit may be adapted to determine a maximum absolute derivative of each respective impedance for each cardiac cycle. 
     In still another example, the impedance measuring circuit performs the impedance measuring sessions during successive cardiac cycles, wherein impedance signals reflecting valve plane movements during the successive cardiac cycles are obtained. 
     According to embodiments of the present invention, the at least one valve plane electrode is placed endocardially. 
     Alternatively, the at least one valve plane electrode is placed epicardially. 
     In certain embodiments of the present invention, the at least one valve plane electrode is placed intrapericardially on the surface of the heart. 
     According to further examples of the present invention, the first valve plane electrode is placed endocardially in the right atrium, or in the left atrium, or in the left ventricle, or in the right ventricle, or epicardially and the second valve plane electrode is placed endocardially in the right atrium, or in the left atrium, or in the left ventricle, or in the right ventricle or epicardially. 
     As the skilled person realizes, steps of the methods according to the present invention, as well as preferred embodiments thereof, are suitable to realize as computer program or as a computer readable medium. 
     Further objects and advantages of the present invention will be discussed below by means of exemplifying embodiments. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a simplified partly cutaway view illustrating an implantable stimulator including an electrode configuration according to the present invention. 
         FIG. 2  is a simplified partly cutaway view illustrating an electrode configuration according to an embodiment of the present invention. 
         FIG. 3  is a simplified partly cutaway view illustrating an electrode configuration according to a further embodiment of the present invention. 
         FIG. 4  is a simplified partly cutaway view illustrating an electrode configuration according to yet another embodiment of the present invention. 
         FIG. 5  is a simplified partly cutaway view illustrating an electrode configuration according to another embodiment of the present invention. 
         FIG. 6  is a simplified partly cutaway view illustrating an electrode configuration according to a still another embodiment of the present invention. 
         FIG. 7  is a simplified partly cutaway view illustrating an electrode configuration according to a further embodiment of the present invention. 
         FIG. 8  is a simplified partly cutaway view illustrating an electrode configuration according to another embodiment of the present invention. 
         FIG. 9  is a simplified partly cutaway view illustrating an electrode configuration according to a further embodiment of the present invention. 
         FIG. 10  is a simplified partly cutaway view illustrating an electrode configuration according to yet another embodiment of the present invention. 
         FIG. 11  is an illustration in a block diagram form of an implantable stimulator according to the embodiment shown in  FIG. 1 . 
         FIG. 12  is a flow chart describing the principles of the present invention according to an embodiment will be described. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The following is a description of exemplifying embodiments in accordance with the present invention. This description is not to be taken in limiting sense, but is made merely for the purposes of describing the general principles of the invention. Thus, even though particular types of implantable medical devices such as heart stimulators will be described, e.g. biventricular pacemakers, the invention is also applicable to other types of cardiac stimulators such as dual chamber stimulators, implantable cardioverter defibrillators (ICDs), etc. 
     In the following a number of different electrode configurations suitable for obtaining impedances reflecting the mechanical functioning of the heart, and in particular movements of the valve plane, will be discussed. 
     With reference first to  FIG. 1 , there is shown a implantable medical device according to an embodiment of the present invention. According to this embodiment, the invention is implemented in a stimulation device  10 . The stimulation device  10  is in electrical communication with a patient&#39;s heart  1  by two leads  20  and  30  suitable for delivering multi-chamber stimulation, which leads  20  and  30  are connectable to the stimulator  10 . The illustrated portions of the heart  1  include right atrium RA, the right ventricle RV, the left atrium LA, the left ventricle LV, cardiac walls  2 , the ventricle septum  4 , the valve plane  6 , and the apex  8 . The valve plane  6  refers to the annulus fibrosis plane separating the ventricle from the atria and containing all four heart valves, i.e. the aortic, pulmonary, mitral, and tricuspid valves. 
     In order to sense right ventricular and atrium cardiac signals and impedances and to provide stimulation therapy to the right ventricle RV, the stimulation device  10  is coupled to an implantable right ventricular lead  20  having a ventricular tip electrode  22 , a ventricular annular or ring electrode  24 , and a first valve plane electrode  26 . The ring electrode  24  is arranged for sensing electrical activity, intrinsic or evoked, in the right ventricle RV. The right ventricular tip electrode  22  is arranged to be implanted in the endocardium of the right ventricle, e.g. near the apex  8  of the heart. Thereby, the tip electrode  22  becomes attached to cardiac wall. In this example, the tip electrode  22  is fixedly mounted in a distal header portion of the lead  20 . Furthermore, the first valve plane electrode  26 , which may a annular or ring electrode, is located substantially at the level of the valve plane  6 . 
     In order to sense left atrium and ventricular cardiac signals and impedances and to provide pacing therapy for the left ventricle LV, the stimulation device  10  is coupled to a “coronary sinus” lead  30  designed for placement via the coronary sinus in veins located distally thereof, so as to place a distal electrode adjacent to the left ventricle and an electrode adjacent to the right atrium RA. The coronary sinus lead  30  is designed to received ventricular cardiac signals from the cardiac stimulator  10  and to deliver left ventricular LV pacing therapy using at least a left ventricular tip electrode  32  to the heart  1 . In the illustrated example, the LV lead  30  has an annular ring electrode  34  for sensing electrical activity related to the left ventricle LV of the heart. Moreover, a second valve plane electrode  36 , which may a annular or ring electrode, is located substantially at the level of the valve plane  6  and measurement electrode  35 , which may a annular or ring electrode, is located adjacent to the right atrium RA. 
     With reference to  FIG. 1 , the impedances that can be detected by means of the illustrated embodiment will be described. At the right side of the heart  1 , the impedance Z 26-22  between right ventricular tip electrode  22  and the valve plane electrode  26  and the impedance Z 26-35  between the valve plane electrode  26  and the electrode  35  located adjacent to the right atrium RA can be detected, respectively. Furthermore, at the left hand side of the heart  1 , the impedance Z 36-32  between the left ventricular tip electrode  32  and the valve plane electrode  36  and the impedance Z 36-35  between the valve plane electrode  36  and the electrode  35  located adjacent to the right atrium RA can be detected, respectively. Since the electrode  35  located adjacent to the right atrium RA and the ventricle electrodes  22  and  32  essentially do not move during the cardiac cycle, the variation in the impedances are mainly due to movements of the valve plane  6 . 
     Thus, the impedance Z 26-35  and the impedance Z 26-22 , respectively, will vary during the cardiac cycle as a response to the movements of the valve plane  6  at the right hand side of the heart  1 . Similarly, at the left hand side of the heart  1 , the impedance Z 36-35  the impedance Z 36-32  will vary during the cardiac cycle as a response to the movements of the valve plane  6 . 
     Moreover, the impedance Z 36-22  will be substantially constant over the cardiac cycle, which also is the case for the impedance Z 36-32 . By comparing the detected impedances of the respective sides of the heart, asynchronicity or parallelity of the valve plane movements of the respective sides of the valve plane  6  can be determined. In case of an asynchronous depolarization sequence of the heart, the valve plane may move asynchronously and be bent during the heart cycle which will be reflected by an asynchronicity between the detected impedance at the right hand side and the left hand side, respectively. 
     Turning briefly to  FIGS. 2-9 , alternative embodiments for placement of cardiac leads, and cardiac electrodes are illustrated. In  FIG. 2 , an embodiment including two endocardially positioned leads  41  and  42  connected to a stimulation device (see  FIG. 1 ) comprising an atrial distal tip electrode  44  located in the right atrium RA and a ventricular distal tip electrode  43  located in the right ventricle RV, respectively. The leads  41  and  42  can be fixedly attached to the cardiac wall according to conventional practice. Thereby, the electrodes  43  and  44  will be essentially immobile during the cardiac cycle. A third electrode  45  is located epicardially and is connected to the stimulation device by means of a lead  46 . The lead  46  and the electrode  45  can be located epicardially by means of, for example, intrapericardial implantation technique. The electrode  45  is placed at the level of the valve plane  6 . Thereby, the variation in the impedance Z 45-43  between the electrode  45  placed at the level of the valve plane  6  and the electrode  43  placed in the right ventricle RV and the impedance Z 45-44  between the electrode  45  placed at the level of the valve plane  6  and the electrode  44  placed in the right atrium RA, respectively, are mainly due to movements of the valve plane. The impedance Z 44-43  between the electrode  44  placed in the right atrium RA and the electrode  43  placed in the right ventricle RV will essentially by the same over a cardiac cycle since the electrodes  43 ,  44  are essentially immobile during the cardiac cycle. 
     With reference now to  FIG. 3 , an embodiment in which the impedances are measured by means of three endocardially placed electrodes. Similar to the embodiment shown in  FIG. 2 , two endocardially positioned leads  51  and  52  connected to a stimulation device (see  FIG. 1 ) having an atrial distal tip electrode  54  located in the right atrium RA and a ventricular distal tip electrode  53  located in the right ventricle RV, respectively. The leads  51  and  52  can be fixedly attached to the cardiac wall according to conventional practice. Thereby, the electrodes  53  and  54  will be essentially immobile during the cardiac cycle. The third electrode  55  is also located endocardially in the right atrium RA and is connected to the stimulation device by means of a lead  46 , transvenously advanced through a vein to the inside of the heart  1 . The electrode  55  is placed at the level of the valve plane  6 . Thereby, the variation in the impedance Z 55-53  between the electrode  55  placed at the level of the valve plane  6  in the right atrium RA and the electrode  53  placed in the right ventricle RV and the impedance Z 55-54  between the electrode  55  placed at the level of the valve plane  6  in the right atrium RA and the electrode  54  placed in the upper part of the right atrium RA, respectively, is mainly due to movements of the valve plane  6 . The impedance Z 54-53  between the electrode  54  placed in the upper part of the right atrium RA and the electrode  53  placed in the right ventricle RV will essentially be the same over a cardiac cycle since the electrodes  53 ,  54  are essentially immobile during the cardiac cycle. 
     Referring now to  FIG. 4 , yet another electrode configuration for measuring impedances reflecting the valve plane movements will be described. According to this embodiment, two endocardially positioned leads  61  and  62  connected to a stimulation device (see  FIG. 1 ) comprising an atrial distal tip electrode  64  located in the right atrium RA and a ventricular distal tip electrode  63  located in the right ventricle RV, respectively. The leads  61  and  62  can be fixedly attached to the cardiac wall according to conventional practice. Thereby, the electrodes  63  and  64  will be essentially immobile during the cardiac cycle. Further, two electrodes are placed at the level of the valve plane  6  endocardially in this configuration. A first valve plane electrode  65  is placed endocardially at the level of the valve plane  6  in the right atrium RA adjacent to atrial septum  5  and a second valve plane electrode  67  is placed endocardially at the level of the valve plane in the right atrium RA adjacent to the cardiac wall  2 . The first and second valve plane electrodes  65 ,  67  are arranged at leads  66 ,  68  transvenously advanced through a vein to the inside of the heart  1  and connected to the stimulation device (see  FIG. 1 ). The variations in the impedance Z 65-63  between the electrode  65  placed at the level of the valve plane  6  in the right atrium RA adjacent to the atrial septum  5  and the electrode  63  placed in the right ventricle RV and the impedance Z 65-64  between the electrode  65  placed at the level of the valve plane  6  in the right atrium RA adjacent to the atrial septum  5  and the electrode  64  placed in the upper part of the right atrium RA, respectively, are mainly due to movements of the valve plane  6 . Similarly, the variations in the impedance Z 67-63  between the electrode  67  placed at the level of the valve plane  6  in the right atrium RA adjacent to the cardiac wall  2  and the electrode  63  placed in the right ventricle RV and the impedance Z 67-64  between the electrode  67  placed at the level of the valve plane  6  in the right atrium RA adjacent to the cardiac wall  2  and the electrode  64  placed in the upper part of the right atrium RA, respectively, are mainly due to movements of the valve plane  6 . The impedance Z 64-63  between the electrode  64  placed in the upper part of the right atrium RA and the electrode  63  placed in the right ventricle RV will essentially by the same over a cardiac cycle since the electrodes  63 ,  64  are essentially immobile during the cardiac cycle. 
     In  FIG. 5 , a further embodiment is shown. Two leads  71  and  72  are endocardially positioned and connected to a stimulation device (see  FIG. 1 ) comprising an atrial distal tip electrode  74  located in the right atrium RA and a ventricular distal tip electrode  73  located in the right ventricle RV, respectively. The leads  71  and  72  can be fixedly attached to the cardiac wall according to conventional practice. Thereby, the electrodes  73  and  74  will be essentially immobile during the cardiac cycle. Two electrodes are placed at the level of the valve plane  6  in this configuration. A first valve plane electrode  75  is placed epicardially at the level of the valve plane  6  at the left hand side of the heart  1  and a second valve plane electrode  77  is placed endocardially at the level of the valve plane in the right atrium RA adjacent to the cardiac wall  2 . The first valve plane  75  is arranged on a lead  76  located epicardially by means of, for example, intrapercardial implantation technique, and connected to the stimulation device (see  FIG. 1 ). The second valve plane electrode  77  is arranged at a lead  78  transvenously advanced through a vein to the inside of the heart  1  and connected to the stimulation device (see  FIG. 1 ). The variations in the impedance Z 75-73  between the electrode  65  placed at the level of the valve plane  6  and the electrode  73  placed in the right ventricle RV and in the impedance Z 75-74  between the electrode  75  placed at the level of the valve plane  6  on the left hand side of the heart  1  and the electrode  74  placed in the upper part of the right atrium RA, respectively, are mainly due to movements of the valve plane  6  on the left hand side of the heart  1 . Similarly, the variations in the impedance Z 77-73  between the electrode  77  placed at the level of the valve plane  6  in the right atrium RA adjacent to the cardiac wall  2  and the electrode  73  placed in the right ventricle RV and the impedance Z 77-74  between the electrode  77  placed at the level of the valve plane  6  in the right atrium RA adjacent to the cardiac wall  2  and the electrode  74  placed in the upper part of the right atrium RA, respectively, are mainly due to movements of the valve plane  6  at the right hand side of the heart  1 . By comparing the impedance Z 77-74  and the Z 77-73  it is possible to monitor and detect the parallelity or synchronism between the valve plane movements at the respective sides of the heart. The impedance Z 74-73  between the electrode  74  placed in the upper part of the right atrium RA and the electrode  73  placed in the right ventricle RV will essentially by the same over a cardiac cycle since the electrodes  73 ,  74  are essentially immobile during the cardiac cycle. 
     Turning to  FIG. 6 , a further embodiment of the present invention will be described. Two leads  81  and  82  are endocardially positioned, as in the embodiment shown in  FIG. 5 , comprising an atrial distal tip electrode  84  located in the right atrium RA and a ventricular distal tip electrode  83  located in the right ventricle RV, respectively. The leads  81  and  82  can be fixedly attached to the cardiac wall according to conventional practice. Thereby, the electrodes  83  and  84  will be essentially immobile during the cardiac cycle. Further, a first valve plane electrode  85  is placed endocardially at the level of the valve plane in the right atrium RA adjacent to the atrial septum  5  and a second valve plane electrode  87  is placed epicardially at the level of the valve plane  6  adjacent to the cardiac wall  2 . The first valve plane electrode  85  is arranged at a lead  86  transvenously advanced through a vein to the inside of the heart  1  and connected to the stimulation device (see  FIG. 1 ). The second valve plane  87  is arranged on a lead  88  located epicardially by means of, for example, intrapercardial implantation technique, and connected to the stimulation device (see  FIG. 1 ). The electrode configurations of this embodiment is similar to the electrode configurations shown in  FIG. 4  and thus the measured impedances will be similar to the impedances measured using the configuration shown in  FIG. 4  for what reason a detailed description thereof is omitted. 
     In  FIG. 7 , another configuration of electrodes for measuring the valve plane movements by means of impedances will be shown. As can be seen, two leads  91  and  92  are endocardially positioned, as in the embodiment shown in  FIG. 5 , having an atrial distal tip electrode  94  located in the right atrium RA and a ventricular distal tip electrode  93  located in the right ventricle RV, respectively. The leads  91  and  92  can be fixedly attached to the cardiac wall according to conventional practice. Thereby, the electrodes  93  and  94  will be essentially immobile during the cardiac cycle. Further, a first valve plane electrode  95  is placed epicardially at the level of the valve plane  6  at the left hand side of the heart  1  and a second valve plane electrode  97  is placed epicardially at the level of the valve plane  6  at the right hand side of the heart  1 . The first valve plane electrode  95  and the second valve plane  97 , respectively, are arranged on a leads  96 ,  98  located epicardially by means of, for example, intrapercardial implantation technique, and are connected to the stimulation device (see  FIG. 1 ), respectively. The electrode configurations of this embodiment is similar to the electrode configurations shown in  FIG. 5  and thus the measured impedances will be similar to the impedances measured using the configuration shown in  FIG. 5  for what reason the description thereof is omitted. 
     Referring now to  FIG. 8 , yet another embodiment of the present invention will be discussed. Two leads  101  and  92  are endocardially positioned comprising an atrial distal tip electrode  104  located in the right atrium RA and a ventricular distal tip electrode  103  located in the right ventricle RV, respectively. The leads  101  and  102  can be fixedly attached to the cardiac wall according to conventional practice. Thereby, the electrodes  103  and  104  will be essentially immobile during the cardiac cycle. In addition, a left ventricle lead  106  is placed in a left lateral coronary vein, advanced from the right atrium RA through the coronary sinus. The left ventricle lead  106  comprises a left ventricle tip electrode  109  and a valve plane electrode  105 , e.g. an annular or ring electrode, located adjacent to the valve plane  6 . The left ventricle lead  106  may be fixated at the cardiac wall using conventional practice. In this configuration, the valve plane movements detected will be substantially the valve plane movements at the left hand side of the heart  1 . The impedance Z 105-103  between the electrode  105  placed at the level of the valve plane  6  and the electrode  103  placed in the right ventricle RV and in the impedance Z 105-104  between the electrode  105  placed at the level of the valve plane  6  on the left hand side of the heart  1  and the electrode  104  placed in the upper part of the right atrium RA, respectively, are measured and the variations are mainly due to movements of the valve plane  6  on the left hand side of the heart  1 . Alternatively, the impedances Z 105-109  between the electrode  105  placed at the level of the valve plane  6  and the left ventricle electrode  109  and the impedance Z 105-103  between the electrode  105  placed at the level of the valve plane  6  and the electrode  103  placed in the right ventricle RV, respectively, can be measured. 
     In  FIGS. 9 and 10 , alternative electrode configurations to the configuration illustrated in  FIG. 8  are shown. As can be seen in  FIG. 8 , a further lead  110  including a valve plane electrode  111  is located epicardially at the right hand side of the heart  1  by means of, for example, intrapercardial implantation technique, and connected to the stimulation device (see  FIG. 1 ). Thereby, the impedances can be measured in a similar way as in the embodiment described with reference to  FIG. 5  and it is possible to monitor and detect the degree of parallel operation or synchronism between the valve plane movements at the respective sides of the heart  1 . In the embodiment shown in  FIG. 9 , a further lead  114  is transvenously advanced through a vein to the inside of the heart  1  and connected to the stimulation device (see FIG.  1 ), which lead  114  includes a second valve plane electrode  112  placed endocardially at the level of the valve plane in the right atrium RA adjacent to the cardiac wall  2 . Thereby, the impedances can be measured in a similar way as in the embodiment described with reference to  FIG. 5  and it is possible to monitor and detect the parallelity or synchronism between the valve plane movements at the respective sides of the heart  1 . Thereby, the impedances can be measured in a similar way as in the embodiment described with reference to  FIG. 5  and it is possible to monitor and detect the parallelity or synchronism between the valve plane movements at the respective sides of the heart  1 . 
     Even though a number of examples have been illustrated in  FIGS. 1-10 , the invention is not restricted the illustrated lead and electrode configurations and placements. For example, more epicardial electrodes can be placed at the level of the valve plane. Furthermore, different types of sensors can also be arranged in the leads to obtain other types of information. For example, cardiac wall motion sensors of accelerometer type can be arranged in a right ventricle and/or a left ventricle lead. 
     Turning now to  FIG. 11 , the heart stimulator  10  of  FIG. 1  is shown in a block diagram form. For illustrative purposes, reference is made to  FIG. 1  for the elements of the leads that are intended for positioning in or at the heart. The heart stimulator  10  is connected to a heart  1  order to sense heart signals and emit stimulation pulses to the heart  1 . Electrodes located within and at the heart, for example, any one of the electrode configurations illustrated in  FIGS. 1-10  and outside the heart, for example, an indifferent electrode  12  (which, in this instance, is formed by the enclosure of the heart stimulator  10  but can also is formed by a separate electrode located somewhere in the body) are connected to a pulse generator  126  in the heart stimulator  10 . The electrodes located within and/or at the heart are connected to the stimulator  10  via leads, for example, the leads  20  and  30  shown in  FIG. 1 . A detector  128  is also connected to the electrodes in order to sense activity of the heart. 
     The pulse generator  126  and the detector  128  are controlled by a control unit  140  which regulates the stimulation pulses with respect to amplitude, duration and stimulation interval, the sensitivity of the detector  128  etc. 
     A physician using an extracorporeal programmer  144  can communicate, via a telemetry unit  142 , with the heart stimulator  10  and thereby obtain information on identified conditions and also reprogram the different functions of the heart stimulator  10 . 
     Furthermore, the heart stimulator  10  has an impedance measuring circuit  146  adapted to, during impedance measuring sessions, measure impedance signals between at least a first pair of electrodes, which at least first pair includes at least one electrode located in an atrium of the heart and at least one valve plane electrode located substantially at the level of a valve plane the heart. Further, the impedance measuring circuit  146  is adapted to, during the impedance measuring sessions, measure impedance signals between at least a second pair of electrodes, which at least second pair includes at least one electrode located in at least one ventricle of the heart and at least one valve plane electrode located substantially at the level of the valve plane. In  FIGS. 1-10 , a number of different electrode configurations by which impedance signals reflecting the valve plane movements can be obtained are shown. The impedance measuring circuit  146  includes a current generating circuit  147  adapted to apply excitation current pulses between the respective electrode pairs and a measuring circuit  148  adapted to measure the resulting voltage over the respective electrode pairs and determined resulting impedance signals. An impedance signal processor  150  is connected to the measuring circuit  148  and is adapted to process the impedance signals to determine respective impedances over each measurement session for the respective electrode pairs. The impedance signal processor  150  may be adapted to determine maximum or minimum impedances over a cardiac cycle, for example, for respective atrium and/or respective ventricle and/or to determine maximum absolute derivative of the impedance for respective atrium and/or respective ventricle. 
     Moreover, the heart stimulator  10  has a hemodynamic parameter determining circuit  152  adapted to determine at least one hemodynamic parameter based on impedances received from the impedance measuring circuit  146 . The hemodynamic parameter determining circuit  152  includes a microprocessor, which may, for example, control the impedance measuring circuit  146  to, inter alia, initiate an impedance measuring session, the length and/or amplitude of the generated current pulses. The at least one hemodynamic parameter based on the measured impedances reflects the mechanical functioning of the heart. A number of different parameters may be extracted from the measured impedances and monitored including pre-ejection period, a contraction patter, mitral regurgitation, a synchronicity between the left and right hand sides of the heart, etc. 
     In one embodiment, the hemodynamic parameter determining circuit  152  is adapted to determine a synchronicity measure based on the impedances reflecting the synchronicity between the valve plane movements of the right hand side and the left hand side of the heart, respectively, during impedance measurement sessions. Through the impedance measurements, blood volume changes are detected. Blood has a higher conductivity (lower impedance) than myocardial tissue and lungs. The impedance-volume relationship is inverse; the more blood—the smaller impedance. Accordingly, the impedance will vary over the cardiac cycle in connection with the contraction and filling of the atria and ventricles, respectively, in, hence, in connection with the pressure variations during the cycle. For example, the ventricle volume is at a maximum level at the onset of the systolic phase of the ventricles, which corresponds to a minimum impedance measured over the ventricles, and the ventricle volume is at a minimum level at onset of diastolic phase of the ventricles, which corresponds to a maximum impedance measured over the ventricles. 
     In one embodiment, the synchronicity between a closure of the aortic valve and the pulmonary valve and/or a synchronicity between an opening of the aortic valve and the pulmonary valve is determined using the measured impedances. For example, if the electrode configuration illustrated in  FIG. 5  is used, the impedance Z 75-73  between the electrode  75  placed at the level of the valve plane  6  and the electrode  73  placed in the right ventricle RV reflects the movements of the valve plane  6  on the left hand side of the heart  1  and the variations of the blood volume of the right ventricle RV. Similarly, the variations in the impedance Z 77-73  between the electrode  77  placed at the level of the valve plane  6  in the right atrium RA adjacent to the cardiac wall  2  and the electrode  73  placed in the right ventricle RV reflects the movements of the valve plane  6  at the right side of the heart  1  and the variations of the blood volume of the left ventricle LV. By comparing the impedance Z 75-73  and the impedance Z 77-73  it is possible to detect an asynchronism between the valve plane movements at the respective sides of the heart, for example, the opening and/or closure of the aortic and pulmonary valves. For example, peak amplitudes of the impedances, maximum absolute derivative, or morphology of the impedance curves over a cardiac cycle can be studied to detect such an asynchronism. 
     The heart stimulator  10  further has an AV and/or VV delay determining circuit  154  adapted to determine a AV and/or VV delay with respect to a determined hemodynamic parameter, for example, a synchronicity between an opening and/or a closure of the pulmonary and aortic valves. In one embodiment, the AV and/or VV delay determining circuit  154  is integrated in the control circuit  140 . The AV and/or VV delay determining circuit  154  is adapted to initiate an optimization procedure (e.g. via the control circuit  140 ), wherein the pace pulse generator  126  is controlled to iteratively adjust a present AV and/or VV delay to optimize an AV and/or VV delay with respect to the determined hemodynamic parameter starting from the determined AV and/or VV delay. For example, if it is determined that the movements of the valve plane at the right hand side of the heart is ahead the movements of the left hand side in the cardiac cycle, the VV delay may be adjusted such that the left ventricle is stimulated first and vice versa. However, it is important that AV delay of the left side has a sufficient length, i.e. if the different between the right side and the left side is large there is a risk that the effective AV delay on the left side becomes too short. In such a case, the AV delay should also be lengthened. 
     Turning now to  FIG. 12 , the principles of the present invention according to an embodiment will be described. First, at step  200 , a request for an initiation of impedance measuring session is received by the hemodynamic parameter determining circuit  152 . This request may be received from the control circuit  140  or from an external device via the telemetry unit  142 . Alternatively, the hemodynamic parameter determining circuit  152  may be adapted to automatically initiate the impedance measuring sessions at regular intervals. The hemodynamic parameter determining circuit  152  sends an initiation signal to the impedance measuring circuit  146 . This step may be preceded by a check of the measuring conditions, for example, which posture the patient is in, or the activity level. This measuring condition information may be used in the optimization of the AV and/or VV delay. Upon receipt of the initiation signal, which may include information regarding, for example, current pulse width or current amplitude, the impedance measuring circuit  146  measures, during impedance measuring sessions, impedances between at least a first pair of electrodes of the at least one medical lead, the at least first pair including at least one electrode located in an atrium of the heart and at least one valve plane electrode located substantially at the level of a valve plane the heart, and between at least a second pair of electrodes of the at least one medical lead, the at least second pair including at least one electrode located in a ventricle of the heart and at least one valve plane electrode located substantially at the level of the valve plane, wherein impedances reflecting valve plane movements are obtained, and in  FIGS. 1-10 , a number of conceivable electrode configurations are illustrated. Thereafter, at step  202 , at least one hemodynamic parameter based on the impedances, wherein the at least one hemodynamic parameter reflects the mechanical functioning of a heart, is determined. According to embodiments, a synchronicity measure is determined, which reflects a parallelity or synchronicity between the left and right side of the heart. In particular, a parallelity or synchronicity between the valve planes at the left and right side of the heart can be monitored by means of the impedances, e.g. the synchronicity between an opening and/or closure of the aortic and pulmonary valves. Then, at step  204 , an optimization procedure is initiated including adjusting present AV and/or VV delay to optimize an AV and/or VV delay with respect to the hemodynamic parameter. 
     In further embodiments, the indication of an asynchronicity between valve plane movements of the right and left side of the heart, for example, between the opening and/or closing of the aortic and pulmonary valves, could be used for triggering an alarm signal to the patient. This alarm signal could be intended for prompting the patient to seek medical assistance for care of follow-up. The alarm signal may alternatively, or as a compliment, be transferred to an extracorporeal unit  144  or a care institution via the telemetry unit  142 . 
     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 hereon all changes and modifications as reasonably and properly come within the scope of their contribution to the art.