Abstract:
A heart stimulator provides for a more appropriate yet simple setting of the AV-delay. The heart stimulator comprises a stimulation pulse generator adapted to generate electric stimulation pulses and connected to a ventricular stimulation electrode for delivering electric stimulation pulses. A sensing stage connected to an electrode for picking up electric potentials inside a ventricle is adapted to sense an excitation or a contraction of a heart chamber. A memory is adapted to store parameters defining a Bezier function determining the relationship between AV-delay values and heart rate and a control unit connected to said memory, said sensing stage and to said stimulation pulse generator, is adapted to determine an actual AV-delay based on an actual intrinsic heart rate or an actual stimulation rate and a non-linear smoothing interpolation between said parameters stored in said memory.

Description:
FIELD OF INVENTION 
       [0001]    The invention refers to a heart stimulator for stimulating at least one chamber of a heart by means of electrical stimulation pulses that are delivered when a delay time started by a cardiac event expires. The invention particularly refers to implantable pacemakers and implantable cardioverter/defibrillators for atrium synchronous stimulation of a ventricle of a heart. 
       BACKGROUND OF THE INVENTION 
       [0002]    Implantable heart stimulators can be used for treating a variety of heart disorders like bradycardia, tachycardia or fibrillation by way of electric stimulation pulses delivered to the heart tissue, the myocardium. Strong enough a stimulation pulse outside a heart chamber&#39;s refractory period leads excitation of the myocardium of that heart chamber, which in turn is followed by a contraction of the respective heart chamber. 
         [0003]    Depending on the disorder to be treated, such heart stimulator generates electrical stimulation pulses that are delivered to the heart tissue (myocardium) of a respective heart chamber according to an adequate timing regime. Delivery of stimulation pulses to the myocardium is usually achieved by means of an electrode lead that is electrically connected to a stimulation pulse generator inside a heart stimulator&#39;s housing and that carries a stimulation electrode in the region of its distal end. A stimulation pulse also is called a pace. Similarly, pacing a heart chamber means stimulating a heart chamber by delivery of a stimulation pulse. 
         [0004]    In order to be able to sense a contraction a heart chamber that naturally occurs without artificial stimulation and that is called intrinsic, the heart stimulator usually comprises at least one sensing stage that is connected to a sensing electrode on said electrode placed in the heart chamber. An intrinsic excitation of a heart chamber results in characteristic electrical potentials that can be picked up via the sensing electrode and that can be evaluated by the sensing stage in order to determine whether an intrinsic excitation—called: intrinsic event—has occurred. 
         [0005]    Usually, a heart stimulator features separate stimulation generators for each heart chamber to be stimulated. Therefore, in a dual chamber pacemaker, usually an atrial and a ventricular stimulation pulse generator for generating atrial and ventricular stimulation pulses are provided. Delivery of an atrial or a ventricular stimulation pulse causing an artificial excitation of the atrium or the ventricle, respectively, is called an atrial stimulation event A P  (atrial paced event) or a ventricular stimulation event V P  (ventricular paced event), respectively. 
         [0006]    Similarly, common heart stimulators feature separate sensing stages for each heart chamber to be of interest. In a dual chamber pacemaker usually two separate sensing stages, an atrial sensing stage and a ventricular sensing stage, are provided that are capable to detect intrinsic atrial events A S  (atrial sensed event) or intrinsic ventricular events V S  (ventricular sensed event), respectively. 
         [0007]    As known in the art, separate sensing and pacing stages are provided for three-chamber (right atrium RA, right ventricle RV, left ventricle LV) or four-chamber (right atrium RA, left atrium LA, right ventricle RV, left ventricle LV) pacemakers or ICDs. 
         [0008]    By means of a sensing stage for a heart chamber to be stimulated, the pacemaker is able to only trigger stimulation pulses when needed that is when no intrinsic excitation of the heart chamber occurs in time. Such mode of pacing a heart chamber is called demand mode. In the demand mode the pacemaker schedules an atrial or a ventricular escape interval that causes triggering of an atrial or ventricular stimulation pulse when the escape interval times out. Otherwise, if an intrinsic atrial or ventricular event is detected prior to time out of the respective atrial or ventricular escape interval, triggering of the atrial or ventricular stimulation pulse is inhibited. Such intrinsic (natural, non-stimulated) excitation are manifested by the occurrence of recognizable electrical signals that accompany the depolarization or excitation of a cardiac muscle tissue (myocardium). The depolarization of the myocardium is usually immediately followed by a cardiac contraction. For the purpose of the present application, depolarization and contraction may be considered as simultaneous events and the terms “depolarization” and “contraction” are used herein as synonyms. 
         [0009]    In heart cycle, an excitation of the myocardium leads to depolarization of the myocardium that causes a contraction of the heart chamber. If the myocardium is fully depolarized it is unsusceptible for further excitation and thus refractory. Thereafter, the myocardium repolarizes and thus relaxes and the heart chamber is expanding again. In a typical electrogram (EGM) depolarization of the ventricle corresponds to a signal known as “R-wave”. The repolarization of the ventricular myocardium coincides with a signal known as “T-wave”. Atrial depolarization is manifested by a signal known as “P-wave”. 
         [0010]    A natural contraction of a heart chamber thus can be detected by evaluating electrical signals sensed by the sensing channels. In the sensed electrical signal the depolarization of an atrium muscle tissue is manifested by occurrence of a P-wave. Similarly, the depolarization of ventricular muscle tissue is manifested by the occurrence of a R-wave. A P-wave or a R-wave thus leads to an atrial sense event As or a ventricular sense event Vs, respectively. 
         [0011]    Several modes of operation are available in a state of the art multi mode pacemaker. The pacing modes of a pacemaker, both single and dual or more chamber pacemakers, are classified by type according to a three letter code. In such code, the first letter identifies the chamber of the heart that is paced (i.e., that chamber where a stimulation pulse is delivered), with a “V” indicating the ventricle, an “A” indicating the atrium, and a “D” indicating both the atrium and ventricle. The second letter of the code identifies the chamber wherein cardiac activity is sensed, using the same letters, and wherein an “O” indicates no sensing occurs. The third letter of the code identifies the action or response that is taken by the pacemaker. In general, three types of action or responses are recognized: (1) an Inhibiting (“I”) response wherein a stimulation pulse is delivered to the designated chamber at the conclusion of the appropriate escape interval unless cardiac activity is sensed during the escape interval, in which case the stimulation pulse is inhibited; (2) a Trigger (“T”) response wherein a stimulation pulse to a prescribed chamber of the heart a prescribed period of time after a sensed event; or (3) a Dual (“D”) response wherein both the Inhibiting mode and Trigger mode may be evoked, e.g., with the “inhibiting” occurring in one chamber of the heart and the “triggering” in the other. 
         [0012]    To such three letter code, a fourth letter “R” may be added to designate a rate-responsive pacemaker and/or whether the rate-responsive features of such a rate-responsive pacemaker are enabled (“O” typically being used to designate that rate-responsive operation has been disabled). A rate-responsive pacemaker is one wherein a specified parameter or combination of parameters, such as physical activity, the amount of oxygen in the blood, the temperature of the blood, etc., is sensed with an appropriate sensor and is used as a physiological indicator of what the pacing rate should be. When enabled, such rate-responsive pacemaker thus provides stimulation pulses that best meet the physiological demands of the patient. 
         [0013]    A dual chamber pacemaker featuring an atrial and a ventricular sensing stage and an atrial and a ventricular stimulation pulse generator can be operated in a number of stimulation modes like VVI, wherein atrial sense events are ignored and no atrial stimulation pulses are generated, but only ventricular stimulation pulses are delivered in a demand mode, AAI, wherein ventricular sense events are ignored and no ventricular stimulation pulses are generated, but only atrial stimulation pulses are delivered in a demand mode, or DDD, wherein both, atrial and ventricular stimulation pulses are delivered in a demand mode. In such DDD mode of pacing, ventricular stimulation pulses can be generated in synchrony with sensed intrinsic atrial events and thus in synchrony with an intrinsic atrial rate, wherein a ventricular stimulation pulse is scheduled to follow an intrinsic atrial contraction after an appropriate atrioventricular delay (AV-delay; AVD), thereby maintaining the hemodynamic benefit of atrioventricular synchrony. 
         [0014]    Since an optimal AV-delay may vary for different heart rates or stimulation rates and may even vary from patient to patient, AV-delay usually is adjustable. In order to provide individual AV-delays for different heart rates, a discrete number of e.g. 5 different AV-delay values may be programmed into the heart stimulator, each of the 5 AV-delays being provided for different range of heart rates or stimulation rates. 
         [0015]    Alternatively, a linear relationship between a longest AV-delay for the lowest heart or stimulation rate and a shortest AV-delay for the highest allowable stimulation or heart rate may be established. The intermediate values of the AV-delay for intermediate heart rates can be linearly interpolated. 
         [0016]    Both prior art approaches only poorly reflect the optimum dynamic behavior of a healthy heart. 
       SUMMARY OF THE INVENTION 
       [0017]    It is an object of the invention to provide a heart stimulator that provides for a more appropriate yet simple setting of the AV-delay. 
         [0018]    According to the present invention the object of the invention is achieved by a heart stimulator featuring: 
         [0000]    a stimulation pulse generator adapted to generate electric stimulation pulses and being connected or being connectable to at least a ventricular stimulation electrode for delivering electric stimulation pulses to at least said ventricle of the heart,
 
a sensing stage connected or being connectable to an electrode for picking up electric potentials inside at least said ventricle of a heart, said sensing stage being adapted to sense an excitation or a contraction of a heart chamber,
 
a memory for an AV-delay setting
 
and
 
a control unit that is connected to said memory, said sensing stage and to said stimulation pulse generator.
 
         [0019]    The memory is adapted to store parameters defining a Bezier function determining the relationship between AV-delay values and heart rate values 
         [0020]    The control unit is adapted to determine an actual AV-delay based on an actual intrinsic heart rate or an actual stimulation rate. 
         [0021]    Preferably, the control unit is adapted to calculate intermediate AV-delay values based on a Bezier-function according to parameters stored in said memory depending on an actual heart rate or stimulation rate. 
         [0022]    Alternatively, the parameters stored in said memory may represent a look-up table calculated from a Bezier function, said look-up table comprising a plurality of AV-delay values each associated to a heart or stimulation rate. In such embodiment, the control unit is adapted to calculate intermediate AV-delay for those heart or stimulation rates that are not directly comprised in said memory based an linear interpolation. 
         [0023]    Instead of using a Bezier function as a non-linear smoothing interpolation between preset data points, any other suitable smoothing function could be used. However, using a Bezier function is preferred due to its ease of implementation. 
         [0024]    A further aspect of the invention is directed to a programming device for a heart stimulator according to the invention that comprises a graphical user interface and input means connected to said graphical user interface in order to graphically define a functional relationship between the heart or stimulation rate and the AV-delay. In a preferred embodiment, a number of 3 to 16 handles are provided to set AV delay values for dedicated heart rates. The programmer further comprises a Bezier calculation unit for calculating a Bezier function based an the settings defined by the position of the handles. The handles preferably are graphical representations that can be moved by the input means. The input means may comprise a computer mouse. Alternatively, the graphical user interface is designed as a touch screen panel and thus directly serves as input means. 
         [0025]    The programming device comprises a Bezier calculation unit in order to calculate control points of the Bezier function based on the position of the handles. Except for the first and the last control point, control points of a Bezier function are not necessary points on the curve defined by the Bezier function. The handles of the graphical user interface, however, are points on the curve defining the relationship between heart or stimulation rate and AV-delay. Therefore, the control points defining the Bezier function need to be calculated from the position of the handles. 
         [0026]    The control points of the Bezier function are defined by two coordinates in a plane. Thus each control point is a two dimensional vector. The control points define the Bezier function in a manner known as such. According to a first embodiment of the invention, the control points are directly stored in said memory of the heart stimulator. 
         [0027]    Alternatively, after calculating the control points of the Bezier curve and thus the Bezier curve, a selected number of AV-delay values from said Bezier curve corresponding to dedicated heart or stimulation rates are transformed into a look up table that is stored into the heart stimulator&#39;s memory. Transmission of the control points defining the Bezier curve or of the look up table from the programmer to the heart stimulator is achieved by means of transceivers for bidirectional wireless telemetric data transmission between the programmer and the heart stimulator. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0028]    The above and other aspects, features and advantages of the present invention will be more apparent from the following more particular description thereof, presented in conjunction with the following drawings wherein: 
           [0029]      FIG. 1  shows a dual chamber pacemaker connected to leads placed in a heart. 
           [0030]      FIG. 2  is a block diagram of a heart stimulator according to the invention. 
           [0031]      FIG. 3  is a schematic representation of programming device according to the invention. 
           [0032]      FIG. 4  is a block diagram of the programming device in  FIG. 3 . 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0033]    The following description is of the best mode presently contemplated for carrying out the invention. This description is not to be taken in a limiting sense, but is made merely for the purpose of describing the general principles of the invention. The scope of the invention should be determined with reference to the claims. 
         [0034]    In  FIG. 1  a dual chamber pacemaker  10  as heart stimulator connected to pacing/sensing leads placed in a heart  12  is illustrated. The pacemaker  10  is electrically coupled to heart  12  by way of leads  14  and  16 . Lead  14  has a pair of right atrial electrodes  18  and  20  that are in contact with the right atria  26  of the heart  12 . Lead  16  has a pair of electrodes  22  and  24  that are in contact with the right ventricle  28  of heart  12 . Electrodes  18  and  22  are tip-electrodes at the very distal end of leads  14  and  16 , respectively. Electrode  18  is a right atrial tip electrode RA-Tip and electrode  22  is a right ventricular tip electrode  22 . Electrodes  20  and  24  are ring electrodes in close proximity but electrically isolated from the respective tip electrodes  18  and  22 . Electrode  20  forms a right atrial ring electrode RA-Ring and electrode  24  forms a right ventricular ring electrode RV-Ring. 
         [0035]    Referring to  FIG. 2  a simplified block diagram of a dual chamber pacemaker  10  is illustrated. During operation of the pacemaker leads  14  and  16  are connected to respective output/input terminals of pacemaker  10  as indicated in  FIG. 1  and carry stimulating pulses to the tip electrodes  18  and  22  from an atrial stimulation pulse generator A-STIM  32  and a ventricular pulse generator V-STIM  34 , respectively. Further, electrical signals from the atrium are carried from the electrode pair  18  and  20 , through the lead  14 , to the input terminal of an atrial channel sensing stage A-SENS  36 ; and electrical signals from the ventricles are carried from the electrode pair  22  and  24 , through the lead  16 , to the input terminal of a ventricular sensing stage V-SENS  38 . 
         [0036]    Controlling the dual chamber pacer  10  is a control unit CTRL  40  that is connected to sensing stages A-SENS  36  and V-SENS  38  and to stimulation pulse generators A-STIM  32  and V-STIM  34 . Control unit CTRL  40  receives the output signals from the atrial sensing stage A-SENS  32  and from the ventricular sensing stage V-SENS  34 . The output signals of sensing stages A-SENS  32  and V-SENS  34  are generated each time that a P-wave representing an intrinsic atrial event or an R-wave representing an intrinsic ventricular event, respectively, is sensed within the heart  12 . An As-signal is generated, when the atrial sensing stage A-SENS  32  detects a P-wave and a Vs-signal is generated, when the ventricular sensing stage V-SENS  34  detects an R-wave. 
         [0037]    Control unit CTRL  40  also generates trigger signals that are sent to the atrial stimulation pulse generator A-STIM  36  and the ventricular stimulation pulse generator V-STIM  38 , respectively. These trigger signals are generated each time that a stimulation pulse is to be generated by the respective pulse generator A-STIM  36  or V-STIM  38 . The atrial trigger signal is referred to simply as the “A-pulse”, and the ventricular trigger signal is referred to as the “V-pulse”. During the time that either an atrial stimulation pulse or ventricular stimulation pulse is being delivered to the heart, the corresponding sensing stage, A-SENS  32  and/or V-SENS  34 , is typically disabled by way of a blanking signal presented to these amplifiers from the control unit CTRL  40 , respectively. This blanking action prevents the sensing stages A-SENS  32  and V-SENS  34  from becoming saturated from the relatively large stimulation pulses that are present at their input terminals during this time. This blanking action also helps prevent residual electrical signals present in the muscle tissue as a result of the pacer stimulation from being interpreted as P-waves or R-waves. 
         [0038]    Furthermore, atrial sense events As recorded shortly after delivery of a ventricular stimulation pulses during a preset time interval called post ventricular atrial refractory period (PVARP) are generally recorded as atrial refractory sense event A rs  but ignored. 
         [0039]    Control unit CTRL  40  comprises circuitry for timing ventricular and/or atrial stimulation pulses according to an adequate stimulation rate that can be adapted to a patient&#39;s hemodynamic need as pointed out below. 
         [0040]    Still referring to  FIG. 2 , the pacer  10  may also include a memory circuit MEM  42  that is coupled to the control unit CTRL  40  over a suitable data/address bus ADR  44 . This memory circuit MEM  42  allows certain control parameters, used by the control unit CTRL  40  in controlling the operation of the pacemaker  10 , to be programmably stored and modified, as required, in order to customize the pacemaker&#39;s operation to suit the needs of a particular patient. Such data includes the basic timing intervals used during operation of the pacemaker. Further, data sensed during the operation of the pacer may be stored in the memory MEM  42  for later retrieval and analysis. 
         [0041]    Memory MEM  42  is adapted to store parameters determining the functional relationship between a heart rate or a stimulation rate as determined by control unit CTRL  40 . According to a preferred embodiment, these parameters defining determining the functional relationship between a heart rate or a stimulation rate are control points of a Bezier function defining a Bezier curve that in turn defines the functional relationship between a heart rate or a stimulation rate. Control unit CTRL  40  comprises a Bezier calculation unit that is adapted to determine an AV-delay for an actual heart rate or stimulation rate based on the Bezier function that is defined by the control points stored in memory MEM  42 . 
         [0042]    A telemetry circuit TEL  46  is further included in the pacemaker  10 . This telemetry circuit TEL  46  is connected to the control unit CTRL  40  by way of a suitable command/data bus. Telemetry circuit TEL  46  allows for wireless data exchange between the pacemaker  10  and some remote programming or analyzing device which can be part of a centralized service center serving multiple pacemakers. 
         [0043]    In particular, by means of telemetry circuit TEL  46 , control points for the Bezier function defining the functional relationship between a heart rate or a stimulation rate can be remotely received from a programming device as illustrated in  FIGS. 3 and 4 . 
         [0044]    The pacemaker  10  in  FIG. 1  is referred to as a dual chamber pacemaker because it interfaces with both the right atrium  26  and the right ventricle  28  of the heart  12 . Those portions of the pacemaker  10  that interface with the right atrium, e.g., the lead  14 , the P-wave sensing stage A-SENSE  32 , the atrial stimulation pulse generator A-STIM  36  and corresponding portions of the control unit CTRL  40 , are commonly referred to as the atrial channel. Similarly, those portions of the pacemaker  10  that interface with the right ventricle  28 , e.g., the lead  16 , the R-wave sensing stage V-SENSE  34 , the ventricular stimulation pulse generator V-STIM  38 , and corresponding portions of the control unit CTRL  40 , are commonly referred to as the ventricular channel. 
         [0045]    In order to allow rate adaptive pacing in a DDDR or a DDIR mode, the pacemaker  10  further includes a physiological sensor ACT  48  that is connected to the control unit CTRL  40  of the pacemaker  10 . While this sensor ACT  48  is illustrated in  FIG. 2  as being included within the pacemaker  10 , it is to be understood that the sensor may also be external to the pacemaker  10 , yet still be implanted within or carried by the patient. A common type of sensor is an activity sensor, such as a piezoelectric crystal, mounted to the case of the pacemaker. Other types of physiologic sensors are also known, such as sensors that sense the oxygen content of blood, respiration rate, pH of blood, body motion, and the like. The type of sensor used is not critical to the present invention. Any sensor capable of sensing some physiological parameter relatable to the rate at which the heart should be beating can be used. Such sensors are commonly used with “rate-responsive” pacemakers in order to adjust the rate of the pacemaker in a manner that tracks the physiological needs of the patient. 
         [0046]    The control unit CTRL  40  is adapted to determine an adequate heart rate or stimulation rate in any manner known as such. The rate thus determined is used for determining an adequate AV-delay by the Bezier calculation unit based on the control points stored in memory MEM  42 . 
         [0047]    In  FIG. 3 , a programming device  50  for remotely programming pacemaker  10 . Programming device  50  features a touch sensitive graphical display (touch screen)  52  that serves as graphical user interface and as input means. Of course, programming device  50  may comprise more further input means like buttons or scroll wheels. 
         [0048]    Programming device  50  is displayed in its operating mode for defining the functional relationship between a heart rate or a stimulation rate by means of a Bezier curve. 
         [0049]    Six handles  54  are provided to set an AV-delay for a dedicated heart rate. Each handle is a virtual representation on the graphical display  52  and can be each moved along an axis that is indicated by means of a dashed line. Moving of the handles  54  is effected by touching the graphical display in the are of a handle  54  to moved and moving the handle  54  to the desired position representing the AV delay for the heart rate the handle is associated to. By moving handles  54 , a curve  56  is defined that represents the relationship between the heart rates or the stimulation rates and the AV-delay. 
         [0050]    In the embodiment shown in  FIG. 3 , AV delays are represented in fractions of a heart cycle that is the reciprocal values of the heart rate. Therefore, curve  56  is ascending although the absolute value of the AV delay is descending when the heart rate ascends. 
         [0051]    In an alternative embodiment, duration of the AV-delay may directly be displayed resulting in a descending curve on the graphical display  52 . 
         [0052]    It is to be noted, that the position of handles  54  define points of curve  56  itself and thus are not necessarily control points of a Bezier curve corresponding to curve  56 . Therefore, the control points for Bezier curve  56  need to be calculated by means of a Bezier calculation unit  60 , that is connected to the graphical display  52  and to a memory  62  and a telemetry unit  64  of the programming device  50 ; see  FIG. 4 . 
         [0053]    Once calculated, the control points of the Bezier curve representing the functional relationship between the heart rate and the AV-delay, the control points are stored in memory  62  and can be transmitted to pacemaker  10  whenever a remote data transmission is established between programming device  50  and pacemaker  10 .