Abstract:
A dual-chamber cardiac pacemaker having a stimulation control unit for time control of the production and output of the stimulation pulses, and a method of operating the pacemaker according to the current invention are provided. The pacemaker comprising switching means for switching over from an asynchronous to a vestibule-synchronous mode, criterion memory means for storage of at least one switching-over criterion for mode switching-over, and a processing unit for processing signals from the atrium sensing means and the ventricle sensing means and the timer for checking fulfilment of the switching-over criterion, wherein the criterion memory means has a signal sequence memory for storage of a predetermined selection of signal sequence patterns reflecting an electrical activity of the heart and the processing unit has comparison means for comparison of the electrical activity detected by the atrium sensing means and the ventricle sensing means in a plurality of successive cardiac cycles to the stored signal sequence patterns and the stimulation control unit further has counting means connected to the comparison means on the input side for counting the number of cardiac cycles in which identity of the detected electrical activity was established with one of the pre-stored signal sequence patterns, within a predetermined total number of successive cardiac cycles.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims priority to German application serial number 198 59 651.0, filed Dec. 15, 1998. 
     FIELD OF INVENTION 
     The invention concerns generally to dual-chamber cardiac pacemakers, and more particularly to dual-chamber pacemakers having both atrium-synchronous and asynchronous modes of operation. 
     BACKGROUND OF THE INVENTION 
     Dual-chamber cardiac pacemakers which stimulate the ventricle synchronously with respect to atrial events are well-known in the art. Often these pacemakers are dual-mode and have a mode-switch function that switches the pacemaker from an atrium-synchronous to an asynchronous mode of operation, in the event that an upper limit rate of detected atrial cardiac actions is exceeded. 
     The reverse procedure, namely changing from an asynchronous to an atrium-synchronous mode of operation, i.e., termination of mode-switch episodes, is normally triggered by the detection of what is known as the ‘x out of y-criterion’. The ‘x out of y-criterion’ occurs when the number of atrially detected events within a predetermined total number of atrial events occur at a rate below the upper atrial limit rate. 
     However, two serious errors can occur in re-synchronization of the ventricular stimulation pulses to the atrially detected events. Firstly, the atrially detected events can result from the retrograde transfer of ventricular stimulation pulses. In that case, each ventricularly stimulated event causes a retrograde-transfer atrial event, which in turn triggers an AV-time, after which there is simulated a ventricular event which again causes a retrograde-transfer atrial event, and so forth. In such a case, re-synchronization to the atrially detected events would result in synchronization to the ventricle stimuli and directly trigger pacemaker mediated tachycardia (PMT). 
     Secondly, the atrially detected rate can result from a 2:1-detection of atrial events on the basis of atrial blanking, which makes each second atrial event ‘invisible’ for the pacemaker. In such a case, re-synchronization would then directly result in the 2:1-block behaviour which is highly undesirable from a physiological point of view. 
     Attempts have been made to design around the re-synchronization problem. For example, U.S. Pat. No. 5,549,648 describes a pacemaker system and an operating procedure for the improved detection of the end phase of a retrograde transfer. The aim of the &#39;648 device is to provide a system capable of analyzing atrial and ventricular cardiac actions on a beat-to-beat basis by reference to given criteria (referred to as ‘beat-to-beat’ analysis). On the basis of that analysis, the operation of the pacemaker is then to be re-synchronized. However, the re-synchronization errors described above are not eliminated with the procedure of the &#39;648 patent. On the contrary, the system disclosed in the &#39;648 patent is only concerned with the fastest possible detection of the condition in which the mode switch episode can be concluded. Indeed, the aim of the &#39;648 patent, to expediently initiate re-synchronization, tends to make checking the analysis result and eliminating re-synchronization errors impossible. 
     Accordingly, a need exists for a dual-chamber cardiac pacemaker capable switching back from an asynchronous to an atrium-synchronous mode and re-synchronization of the pacemaker to the atrium in such a way as to substantially exclude malfunctions. 
     SUMMARY OF THE INVENTION 
     The present invention is direct to a dual-chamber pacemaker designed to avoid errors in the switching-back and re-synchronization procedures. Generally, the dual-chamber pacemaker comprises: a programming device, a body sensor, an atrium electrode, and a ventricle electrode. An internal telemetry may be further provided for connecting the implanted pacemaker to the programming device and a stimulation control unit may be provided for operational control. In such an embodiment, the stimulation control unit generally comprises an internal program memory, a data memory, and a clock and timer. 
     Any suitable body sensor may be utilized in the current invention. For example, the body sensor may include one or more of the following: a piezoelectric activity sensor, a sensor for blood oxygen saturation or blood temperature, or an impedance plethysmograph. 
     In one embodiment, a ventricle sensing unit and a ventricle stimulation unit are connected to the ventricle electrode, and similarly an atrium sensing unit and an atrium stimulation unit are connected to the atrium electrode. In such an embodiment, the sensing units may be connected to data inputs of the stimulation control unit and the stimulation units may be connected to control signal outputs thereof. 
     In another embodiment, the invention is also directed to a method of utilizing the dual-chamber pacemaker. In such an embodiment, the method of operation comprised the following steps: a) a counter used to detect fulfilment of the x-out of-y-criterion (‘x-out of-y-counter’) is incremented only at the occurrence of certain known sequences of cardiac events (hereinafter also referred to as ‘signal sequence patterns’); b) if the counter condition satisfies a predetermined x-out of-y-criterion, in particular in the form of modulation or variation of a plurality of ventricularly detected or stimulated events (V), such as a V-V-interval—a test is carried out for the presence of a retrograde transfer. If the interval between the V and atrially detected events (As), or V-AS, respectively following modulation are of a constant duration, the situation entails retrograde transfer. If, in contrast, the moment in time at which atrially detected events occur is independent of the (varied) moment in time of ventricular stimulation, the events were not retrogradedly transferred. 
     In yet another alternative embodiment, the event sequences detected for include: As-V-As wherein As-As&gt;1.000 ms; V-As-As-V wherein As-As&gt;1.000 ms; V-V-As wherein V-As&gt;1.000 ms; and V-V without interposed As. 
     In still another alternative embodiment, modulation is desirably implemented in the form of a prolongation of the time intervals by a post-ventricular atrial blanking time. In such an embodiment an additional increment and re-synchronization is activated if at least three successive V-As-time intervals were constant after modulated ventricle stimuli. 
     In still yet another embodiment of the invention, the criterion memory is adapted for the storage of at least one rate threshold value concerning the time intervals between successive atrial activities—preferably for the storage of two different threshold values in different stages of the test procedure—and the processing unit has a calculating unit for ascertaining the current atrial time intervals or atrial rate and (at least) one rate comparison unit for comparison thereof with the stored rate threshold value. The result of the rate comparison is then passed to a logic stage, together with the result of the x-out of-y-counting operation. In such an embodiment, activation of the switching means for switching over into the synchronous mode and re-synchronization of ventricle stimulation is made dependent not only on the result of the logical processing operation, but presupposing attainment of the counter condition ‘x’ by the counter means, on the additional presence of an output signal from the rate comparison unit which indicates when the atrial rate falls below a predetermined threshold value (‘decision rate’). In accordance with the foregoing, in one such embodiment, the logic stage implements AND-gating of the x-out of-y-counting procedure and the additional test by modulation of the W-stimulation interval, which is preceded in particular also by OR-gating between the test result and the rate comparison. 
     In still yet another embodiment, the stimulation control unit desirably effects dynamic rate limitation, i.e., the ventricular stimulation rate, for avoiding PMT (‘dynamic PMT limit’). In connection with such an embodiment, the stimulation function is further designed such that the specific rate limit value is selected based on the result of the test steps provided: 1) if in the outcome of the tests pacemaker operation is set to atrial sine rhythm (spontaneous atrial rate&gt;sensor-indicated rate), the rate limitation should be based on the rate of spontaneous atrial activity (‘sine rate’); and 2) if, however, the preceding tachycardia leads into atrial bradycardia (spontaneous atrial rate&lt;sensor-indicated rate), the limit rate should be derived from the sensor-indicated rate. These conditions also apply where retrograde transfer was established and active re-synchronization was implemented. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Advantageous developments of the invention are also characterized in the appendant claims or are set forth hereinafter in greater detail together with the description of the preferred embodiment of the invention, with reference to the drawings in which: 
     FIG. 1 shows a functional block circuit diagram of a dual-chamber pacemaker in accordance with an embodiment of the invention; 
     FIG. 2 shows a flow chart relating to operation of an embodiment of a dual-chamber pacemaker according to the invention; 
     FIG. 2 a  shows a subroutine of the flow chart shown in FIG. 2; 
     FIG. 3 shows a timing diagram relating to a step in a test procedure according to the invention; and 
     FIG. 4 shows a functional block circuit diagram of an embodiment of the stimulation control unit of a pacemaker according to the invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     A dual-chamber pacemaker designed to avoid errors in the switching-back and re-synchronization procedures is disclosed herein. Generally, the dual-chamber pacemaker comprises: a programming device, a body sensor, an atrium electrode, and a ventricle electrode. FIG. 1 shows a schematic diagram of the components of a pacemaker apparatus comprising: a dual-chamber cardiac pacemaker  100 , a programming device  200 , a body sensor  300 , an atrium electrode EA in the atrium A and a ventricle electrode EV in the ventricle V of a heart H. An internal telemetry unit  101  is provided for connecting the implanted pacemaker  100  to the programming device  200  and a stimulation control unit  100 A is provided for operational control. 
     Associated with the stimulation control unit  100 A in the usual manner are an internal program memory  102 , a data memory  103 , as well as a clock and timer  104 . The body sensor  300  is connected to an input of the stimulation control unit  100 A. Any suitable body sensor  300  may be utilized, such as, for example, a piezoelectric activity sensor, a sensor for blood oxygen saturation or blood temperature or an impedance plethysmograph. 
     Connected to the ventricle electrode EV on the input side is a ventricle sensing unit  105 , and on the output side a ventricle stimulation unit  106 . Similarly, connected to the atrium electrode EA on the input side is an atrium sensing unit  107 , and on the output side an atrium stimulation unit  108 . The sensing units  105  and  107  are connected to data inputs of the stimulation control unit  100 A and the stimulation units  106  and  108  are connected to control signal outputs thereof. 
     The dual-chamber pacemaker  100  of the current invention is designed to operate in a rate-adaptive dual-chamber demand mode on the basis of rate control signals which—on the basis of a programmed algorithm, which may be controlled by way of the programming device  200 —are formed on the basis of signals from the body sensor  300  in the stimulation control unit  100 A and fed to the stimulation units  106  and  108 . The pacemaker  100  is further designed for switching between an atrium-synchronous mode to an asynchronous mode during episodes of vestibule flutter and for switching back into the synchronous mode upon the termination of such episodes on the basis of specific criteria or tests. 
     In one embodiment, the switching-back procedure utilized for a pacemaker  100  according to the present invention is accomplished when, for a predetermined number of cardiac cycles from a set total number of cardiac cycles, one of a predetermined set of signal sequence patterns or indices for the termination of atrial tachycardia is detected. An exemplary set of signal sequence patterns is set forth in Table 1, below. 
     
       
         
               
             
               
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                 Exemplary Signal Sequence Patterns 
               
             
          
           
               
                   
                 Sequence 
                 Conditions 
               
               
                   
                   
               
               
                   
                 As—V—As 
                 As—As &gt; 1.000 ms 
               
               
                   
                 V—As—As—V 
                 As—As &gt; 1.000 ms 
               
               
                   
                 V—V—As wherein 
                 V—As &gt; 1.000 ms 
               
               
                   
                 V—V 
                 without interposed As 
               
               
                   
                   
               
             
          
         
       
     
     A flow chart outlining the operation of the dual-chamber pacemaker  100  is provided in FIG.  2 . First, in step S 1 , a signal sequence pattern or index for the termination of atrial tachycardia is detected. Thereupon it is established in a step S 2  whether the atrial rate is below an arbitrary first rate limit value (here 100 bpm). If the atrial rate is above the first rate limit value, the procedure at branch point ‘A’ transfers to the subroutine outlined in FIG. 2 a.  In contrast, of the atrial rate is below the first rate limit value, then for a predetermined number of ventricle stimuli the spacing interval or rate thereof is altered in a step S 3 . More specifically, in the present example the W-interval is reduced by the post-ventricular atrial blanking period (PVAB) and an additional margin of 25 ms. In step S 4  spacing interval is monitored to determine if three VA-intervals have remained constant after the variation in the W-interval. 
     If three VA-intervals have remained constant, then the detected atrial rate is attributed to a retrograde transfer, the switching-over condition, which is adopted by virtue of vestibule flutter, is deactivated in a step S 5  and the pacemaker is actively re-synchronized to atrial activity. In this case, dynamic rate limitation (‘dyn. PMT’) is effected, which is oriented to the sensor-indicated rate. 
     In contrast, if the VA-intervals have not remained constant, then the detected atrial rate is compared to a second limit value (‘decision rate’—in accordance with the criteria listed hereinbefore, 60 bpm) in step S 6 . If the atrial rate is equal to or higher than the decision rate, then the circuit assumes that the preceding test involved a 2:1-detection of an atrial rate, which was actually above the decision rate. Accordingly, the pacemaker remains in the asynchronous switched-over condition and at the same time the switching-over or re-synchronization counter is reset to zero in step S 7 . 
     Alternatively, if the detected atrial rate is below the decision rate limit value, the pacemaker is switched-over and returns to the synchronous mode in step S 8 , in which case dynamic ventricular rate limitation is oriented to the higher rate of the sine rate and the sensor-indicated rate. 
     FIG. 2 a  outlines the high atrial rate subroutine according to one embodiment of the present invention. Starting from the field ‘A’ in FIG. 2, in steps S 9  to S 14 , a subroutine similar to the procedure shown in FIG. 2 is implemented. However, the subroutine starts in step S 9  with a different kind of variation in the VV-intervals, namely with an increase in the length of the intervals by the PVAB plus an additional interval of 25 ms. Steps S 10  to S 14  are then identical to steps S 4  to S 8 , described above. 
     FIG. 3 depicts a schematic timing diagram of the above-described procedure. Shown in the left-hand part of the diagram is the end phase of a conventional x-out of-y-test for switching back from the asynchronous mode (VDIR) into the synchronous mode in which the count values  5  and  6  of the cardiac cycles are reached, and in which apparently a switching-back criterion is satisfied, i.e., the atrial rate is below the decision rate. However, in reality each second one of the atrial activities P is blanked shortly after the ventricle stimulus Vp such that the actions detected by the atrial sensing unit, on the basis of the 2:1-perception, have an apparent spacing PPapp, which simulates termination of vestibule flutter. 
     In contrast, in the operation of a pacemaker according to the present invention, after attainment of the counter condition  6 , which is predetermined as ‘x’, re-synchronization is not effected straightaway, but firstly the above-discussed W-modulation test is effected and—as this (presumably) shows the continuance of the atrial tachycardia—stimulation is continued in the VDIR-mode. That avoids a 2:1 block behaviour which would inevitably occur without the additional test. Although this additional checking operation requires at most an additional test cycle in comparison with the conventional x-out of-y-rate criterion before re-synchronization can be effected, there is a substantial decrease in the likelihood that the pacemaker will be inappropriately switched back into the synchronous mode, consequently decreasing the likelihood of a PMT. 
     FIG. 4 shows a schematic functional block circuit diagram of the stimulation control unit  100 A according to one embodiment of the current invention. It will be understood that the structure of the unit  100 A is simplified in the interests of greater ease of understanding of the stimulation control unit  100 A in an embodiment of the invention. Although the functional components of the pacemaker  100  of the current invention are shown structurally in FIGS. 1 and 4, it should be understood that the 13  functional components serving to carry the invention into effect are in practice implemented at least in part in software terms and are inseparably interwoven with the rest of the pacemaker structure. 
     The stimulation control unit  100 A includes a PP-interval or rate calculation stage  109  connected to the output of the atrium sensing unit  107 , a VA-interval calculation stage  110  connected to the outputs of the atrium sensing unit  107  and the ventricle sensing unit  105 , and a sequence determination stage  111  also connected on the input side to the sensing units  105  and  107  for registration of the succession of cardiac activities in the atrium and the ventricle. The stages  109  and  110  are each also connected to the timer  104  (FIG.  1 ), which supplies the time signals required for the interval or rate calculations and signal sequence determination. 
     Connected to the output of the PP-interval calculation stage  109  is a rate comparison stage  112  whose second input is connected to the first memory region of a dual-region limit rate memory  113 , and in which a comparison is made between the ascertained current atrial rate and a first limit rate (‘decision rate’). In the result thereof, when the rate is below a limit rate, there appears an output signal which is passed to an input of an AND-gate  114 . 
     The output of the sequence determination stage  111  is connected to an input of a signal sequence comparison unit  115  whose second input is connected to a signal sequence pattern memory  116  and whose output goes to the second input of the AND-gate  114 . The output of the AND-gate clocks an x-counter  117 . The counter  117  has two reset inputs of which one is connected to the output of a y-counter stage  118 , which in turn is connected at its input side to the sensing units  105  and  107 . The assignment of the second reset input is described hereinafter. 
     The so-called ‘x-out of-y-counting procedure’ is implemented with the above-mentioned functional blocks  109  and  111  through  118 , with parallel application of a PP-rate criterion and a signal sequence criterion corresponding to the first two of the criteria set forth in Table 1, with regard to the termination of an atrial tachyarhythmia (to which the description herein is to be limited in simplifying fashion). In each of y successive cardiac cycles, which are counted in the counter stage  118 , a check is made comparing the identity of the detected signal sequence with a signal sequence pattern previously stored in the memory  116 , and comparing the atrial rate to a decision rate stored in the memory  113 , and if both criteria are satisfied, the x-counter  117  is incremented. As soon as it has reached the value x, an output signal indicating the positive result of the checking procedure is outputted. In the embodiment illustrated in FIG. 4, it is assumed by way of simplification that the x-counter  117  is reset to zero when the y-counter  118  is running before attaining the count value x and a new checking cycle is started. In practice, however, a sliding x-out of y-counting procedure with rejection of the ‘respective ‘oldest’ cardiac cycle based on the FIFO-principle will be more desirable. as it permits a faster reaction on the part of the pacemaker. 
     The output of the x-counter  117  is connected to a stimulation test stage  119  for influencing the spacing of a pre-programmed number of ventricle stimuli. The precise mode of influencing that spacing is set by way of a control input of the stage  119 , described below. In addition to the ventricle stimulation unit  106 , the output of the stimulation test stage is connected to a control input of the VA-calculation stage  110 , which triggers the determination of the current VA-intervals and the transfer of those intervals into a VA-interval memory  120  shortly before initiation of the above-mentioned modulation test. The memory  120  is connected to the input of a VA-evaluation unit  121  in which the VA-intervals stored during the modulation test are checked for constancy within a pre-programmed range of fluctuation. If constancy of the VA-intervals is found, a corresponding output signal one (‘1’) is outputted, otherwise the output remains at zero (‘0’). 
     If constancy is found, an activation signal is passed to a switching-over and re-synchronization stage  124  by way of an OR-gate  122  connected on the output side of the VA-evaluation unit  121 , and an AND-gate  123  whose second input is connected to the output of the x-counter  117 . The stage  124  in turn controls in the atrium-synchronous mode a V-pulse timing stage  125 , which in addition is connected on the input side to the atrium sensing unit  107  and the body sensor  300 . 
     In contrast, in the case of a negative outcome of the modulation test, a second PP-rate comparison unit  127  is activated by a test-terminated signal from the stimulation test stage  119  and the ‘0’-signal at the output of the VA-evaluation stage  121  by way of an EXOR-gate  126 , which is connected to those stages. The unit  127  is connected on the input side to the PP-rate calculation stage  109  and the second memory region of the limit rate memory  113 , and it checks whether the current atrial rate is below a second pre-programmed limit rate (as shown in FIG.  2 :100 bpm). In addition, the unit  127  is also connected to the second input of the above-mentioned OR-gate  122  and by way of same and the AND-gate  123  implements activation of the switching-over and re-synchronization stage  124  when that additional criterion is met. The stage  127  is further connected to a EXOR-gate  128  by way of which, in the presence of the test-terminated signal from the stimulation test stage  119  and a negative outcome in respect of rate checking (output ‘0’ of the stage  127  after a negative outcome of the previous checking for VA-constance), the x-counter  117  is reset to zero by way of its second reset input. Finally, the second rate comparison stage  127  is also connected on its output side to a control input of the stimulation test stage  119 , by way of which the modulated interval between the ventricle stimuli is set in dependence on the outcome of the rate checking operation; these steps are summarized in flowchart form as steps S 3  and S 9  in FIGS. 2 and 2 a.    
     The VA-evaluation stage  121  and the second rate comparison stage  127  are finally connected to control inputs of a rate limiting stage  129 , which is connected by way of further inputs to the body sensor  300  and the output of the PP-rate calculation stage  109 , and which on the basis of the current atrial rate and the signals from the body sensor and in dependence on the test results (as described above) implements rate limitation for ventricular stimulation. 
     The invention is not limited in terms of carrying it into effect to the preferred embodiments set forth hereinbefore. On the contrary a number of alternative configurations are possible, which make use of the illustrated solution even in a design configuration of a different kind.