fig1 is a flow chart showing an embodiment of the signaling method according to the invention . the method is started with a step s 10 . in a subsequent step s 12 the output voltage of a battery is measured as a status parameter . the battery serves for the supply of energy for an implantable cardiac stimulation device . in a subsequent step s 14 the measured output voltage is compared to a reference or target voltage . it will be appreciated that , instead of a reference voltage , it is also possible to use a reference value range for the comparison procedure . if the measurement value coincides with the reference value , then step s 12 is carried out again . the loop formed by the steps s 12 and s 14 therefore serves for monitoring the battery voltage . prior to renewed execution of the step s 12 after a condition of coincidence has been detected between the measurement value and the reference value , it is also possible to pass through a waiting loop of predetermined duration . if it is established in step s 14 that the measured output voltage is not coincident with the reference value or the reference value range , then in a subsequent step s 16 a check is made to ascertain whether there is a magnetic triggering signal . in an alternative embodiment , that method step s 16 is additionally executed between the steps s 10 and s 12 . in other words , in this embodiment , measurement of the output voltage is only executed in response to the magnetic triggering signal . if no magnetic triggering signal can be detected the method is terminated with step s 18 . alternatively , it is also possible to provide a waiting loop of prolonged duration , after the end of which a check is again made for the presence of the magnetic triggering signal . the magnetic triggering signal is produced in known manner by applying an external programming and checking unit for the cardiac stimulation device to the chest of the patient . it will be appreciated however that implantation of the cardiac stimulation device is not a prerequisite for carrying the method into effect . the triggering signal can equally be detected if the cardiac stimulation device is not implanted . if a magnetic triggering signal is detected , then in a step s 20 the current cardiac period duration is determined as the time difference between two moments in time t ( q ′) and t ( q ) of two successive cardiac periods , at each of which the curve passes through the respective voltage minimum of the q - spike in the electrocardiogram . it will be appreciated that , instead of the q - spike of the ecg , it is also possible to refer to another phase in excitation of the myocardium in the cardiac cycle for the purposes of measuring the period duration . the electrocardiogram is recorded in known manner by means of the stimulation electrodes or measurement electrodes which are provided specifically for that purpose . in a subsequent step s 22 the moment in time t a at which a signal pulse is to be produced is calculated in advance . in this case , recourse is had to a predetermined time constant δt . in the present example δt is 100 ms . the triggering time is determined in step s 22 in accordance with the formula : in that respect the sum t ( q ′)+ t describes the hypothetical moment in time of the next occurrence of the q - spike for the situation where the cardiac period remains unchanged . it will be appreciated that the precise moment in time of the occurrence of the next q - spike basically cannot be determined in advance . the assumption of an identical period duration of two successive cardiac periods however represents a good assumption which is sufficiently accurate for the signal pulse to be externally recognizable . what is important is that the moment in time of the occurrence of a given cardiac phase is calculated in advance by means of that sum in step s 22 . then , with a step s 24 , the occurrence of the moment in time t a is awaited . a signal pulse is triggered at the time t a with a step s 26 . then , a predetermined number n of cardiac periods is awaited , with a step s 28 . in that respect the q - phases which occur are counted up to n before the procedure jumps back to step s 16 in order to again check whether a magnetic triggering signal is still present . if that is the case , steps s 20 through s 28 are then carried out again . if however a magnetic triggering signal is no longer present , the procedure is terminated at step s 18 . fig2 shows a simplified block circuit diagram of an embodiment of the cardiac stimulation device according to the invention . this involves an integrated cardioverter / defibrillator ( icd ) 10 which in addition is adapted to implement a two - chamber pacemaker therapy . the icd 10 is shown in the implanted condition . the icd 10 has an implantable pulse generator 12 and a stimulation electrode 16 which is connected thereto and which can be implanted in a heart 14 . the stimulation electrode 16 is adapted to deliver stimulation pulses in the right atrium 18 and in the right ventricle 20 . the pulse generator 12 has a central control unit 22 . it controls , co - ordinates and monitors the functional features of the pulse generator 12 . it receives and processes for that purpose the signals and data from various measurement units . thus the control unit 22 is connected on the one hand to an electrocardiography ( ecg ) unit 24 . the unit 24 records an intracardial electrocardiogram ( iegm ) by means of two measurement electrodes 26 and 28 which are provided at the stimulation electrode 16 . the measurement signal is passed to the control unit 22 for further processing . more details in this respect will be described hereinafter with reference to fig3 . at the same time the measurement signal , controlled by the control unit , can be stored in a region 36 . 1 of a storage means or memory 36 in order later if necessary to be called up for long - term evaluation . on the other hand the control unit 22 receives signals and data from the telemetry unit 30 . the telemetry unit 30 has a receiver 32 for magnetic signals of an external interrogation , programming and monitoring device ( not shown ). a comparator circuit 34 is connected to the receiver 32 and serves for identification of the detected magnetic signals . for that purpose the comparator circuit 34 accesses reference value and allocation tables stored in a storage region 36 . 2 . after identification of the received signal the comparator circuit 34 signals to the control unit 22 which functional feature of the pulse generator 12 is being interrogated from the exterior . finally the control unit 22 receives signals from an eol - indicator 38 ( eol = end of life ) which monitors the energy content of a battery 40 . the eol - indicator 38 has a volt meter 42 connected to the output of the battery 40 . the battery 40 serves for the energy supply for the pulse generator 12 . a second comparator circuit 44 serves to compare the measured output voltage of the battery 40 to a reference value range stored in a storage region 36 . 3 . if the output voltage falls below a lower threshold value which is predetermined by the threshold value range , the eol - indicator 38 produces an eol - signal which is passed to the control unit 22 . it will be appreciated that a plurality of value ranges in respect of the output voltage of the battery 40 can also be stored in the storage means in known manner and the second comparator circuit 44 can correspondingly produce differentiated signals . in that respect for example a first value range corresponds to the full functional capability of the battery , a second corresponds to an output voltage at which replacement of the battery soon is recommended ( elective replacement interval = eri ), and a third value range corresponds to an output voltage at which maintenance of the operation of the pulse generator is endangered . also connected to the control unit 22 are a pacemaker unit 46 and a cardioversion unit 48 . depending on the control signals received from the control unit 22 , those units take over in known fashion the production and delivery of cardiac stimulation pulses and cardioversion or defibrillation pulses respectively . fig3 shows further details of the structural features of the control unit 22 which are essential for representing the invention , in the form of a simplified block circuit diagram . the control unit 22 has a signal generator unit 22 . 1 with a signal generator 22 . 2 . the signal generator is adapted to deliver control pulses to the pacemaker unit 46 or the cardioversion unit 48 . a control pulse produced by the pulse generator causes the pacemaker unit 46 or the cardioversion unit 48 to directly produce and deliver a cardiac stimulation pulse . the receiver of the control pulse which is produced by the signal generator , that is to say either the pacemaker unit 46 or the cardioversion unit 48 , is determined by means of an addressing device 22 . 3 connected upstream of the signal generator 22 . 2 . connected upstream of the addressing device in parallel relationship are a trigger unit 22 . 4 and a time matching unit 22 . 5 . these two are adapted to determine the moment in time of the delivery of a control pulse by the signal generator 22 . 2 . the trigger unit 22 . 4 produces the control signals in accordance with a program which is predetermined by a therapy control unit 22 . 6 , as is known from conventional icds with pacemaker functions . the time matching unit 22 . 5 is adapted to determine the moment in time of delivery of control pulses for the production of time - displaced cardiac stimulation pulses . for that purpose , method steps which were described with reference to fig1 are implemented in the time matching unit 22 . 5 . it is to be noted however that the time matching unit is adapted not to deliver signal pulses but control signals to the signal generator 22 . 2 . it determines the time of delivery of cardiac stimulation pulse and produces a corresponding control signal . the addressing device 22 . 3 ascertains from that control signal that the pacemaker unit 46 is to be actuated by the signal generator 22 . 2 . the latter in turn ascertains from the control signal of the time matching unit 22 . 5 the moment in time at which the cardiac stimulation pulse is to be delivered , and causes same by delivery of a suitable pulse triggering signal to the pacemaker unit 46 . the time matching unit 22 . 5 accesses the output signal of a period duration measurement unit 22 . 6 which is referred to hereinafter for the sake of brevity as the t - measurement unit . the output signal of the t - measurement unit 22 . 6 indicates the spacing in respect of time between the last and penultimate q - spikes and the moment in time at which the last q - spike occurs . the output signal of the t - measurement unit 22 . 6 therefore contains near - time information about the period duration and the phase position of cardiac activity . the time matching unit 22 . 5 determines the moment in time of delivery of a stimulation pulse by addition of the measured period duration to the moment in time at which the last q - spike occurs and subsequent subtraction of a predetermined time displacement of for example 100 ms . the value of the time displacement can be predetermined by programming from the exterior and is stored in the storage device or memory 36 . a therapy control unit 22 . 7 monitors the state of the heart by means of a cardiac monitoring unit 22 . 8 and determines , by means of known diagnostic processes , what form of therapy is to be applied at the respective moment involved . the therapy control controls access to the trigger unit or the time matching unit in the manner of a change - over switching device 22 . 9 . the time matching unit is actuated by the therapy control unit only when signaling to the exterior is to be implemented . for the purposes of carrying out the usual forms of therapy , the trigger unit 22 . 4 is operated by means of known therapy control procedures . the therapy control unit 22 . 6 is additionally connected on the input side to an apparatus monitoring unit 22 . 10 . the apparatus monitoring unit in turn receives the output signals of the first and second comparator circuits 34 and 44 . if the second comparator circuit 44 indicates that the battery voltage is below the reference value range and at the same time the first comparator circuit 34 of the telemetry unit 30 indicates that the status of the battery is being interrogated from externally , the apparatus monitoring circuit 22 . 10 transmits the eol - or eri - signal from the comparator circuit 44 to the therapy control unit 22 . 7 . upon reception of an eol - or eri - signal the therapy control unit 22 . 7 interrogates the heart monitoring unit 22 . 8 for the presence of a pathological state of the heart . if such a state is present , suitable therapy is initiated . when the heart is working normally — stimulated or in a natural rhythm — the time matching unit is activated by means of the change - over switching device 22 . 9 . it will be appreciated that in the same manner other internal statuses of the cardiac stimulation device can be signaled . fig4 is a diagrammatic view showing an electrocardiogram 50 which was externally recorded during a signaling procedure by a cardiac stimulation device according to the invention . the time t is plotted on an abscissa 52 in units which are not specified in greater detail , while a measurement voltage u is plotted on an ordinate 54 , also in undefined units . a resulting ecg - curve 52 shows the known variation over the duration of five cardiac periods p 1 through p 5 . the ecg - curve 52 reproduces the ecg of a natural cardiac activity . the period duration of the periods p 1 , p 3 and p 4 is characterized by arrow t 1 , t 3 , t 4 and t 6 , the beginning and end points of which respectively coincide with the occurrence of a q - spike in the ecg - curve 52 . the period durations of the periods p 1 through p 5 are not identical by virtue of the natural fluctuations in cardiac activity . in the present case t 4 is somewhat shorter than t 1 , t 3 and t 6 . arrows characterized by the same references denote periods of time of equal length . the beginning and end times of the periods p 1 through p 5 are identified by vertical broken lines . the beginning time of the period p 1 is identified by t ( q ) beneath the abscissa 56 . that is the time of the occurrence of the q - spike of the cardiac period p 1 . the time of the occurrence of the subsequent q - spike is identified by t ( q ′). when carrying out the method according to the invention by means of a cardiac stimulation device according to the invention , firstly the period of time t 1 is determined in accordance with step s 20 in fig1 by the t - measurement unit 22 . 6 in fig3 . in the course of the initial phase of the cardiac period p 2 the time matching unit 22 . 5 determines the triggering time t a for a signal pulse . this is additionally identified in fig4 by a dash - dotted vertical line . the figure shows a displacement of that dash - dotted line t a relative to the right - hand end of the arrow t 1 which extends over the cardiac period p 2 . this therefore indicates when the period of time t 1 , that is to say the duration of the cardiac period p 1 , would expire again after the beginning of the period p 2 . a strong deflection 58 towards negative voltage values can be seen in the ecg - curve 52 , occurring at the time t a . that deflection is the indication , which is visible in the ecg , of a cardiac stimulation pulse which is delivered by the pacemaker unit 46 . that cardiac stimulation pulse can be distinguished from a regular , therapeutic cardiac stimulation pulse and can be identified as a signal pulse , by virtue of its time displacement relative to the time t ( q ′)+ t 1 . this therefore indicates that the battery voltage of the cardiac stimulation device 10 has fallen below a reference value . as the identification of only one signal pulse is not always unambiguous , the occurrence is repeated after two cardiac periods p 4 and p 5 in the cardiac period p 6 . waiting for two periods without signaling corresponds to the step s 28 of the method as shown in fig1 wherein the number of two periods to be waited is stored in accordance with programming from the exterior in the storage device 36 ( fig2 ). for the purposes of determining the triggering time t a in the period p 6 the time matching unit 22 . 5 ( fig3 ) accesses the period duration t 4 and the time t ( q ′) of the beginning of the period p 5 , which are determined by the t - measurement unit 22 . 6 . the ecg - curve 52 correspondingly exhibits a second negative deflection 60 . that time pattern can also be seen in the ecg - curve 52 repeated for later times ( not shown here ) as long as the telemetry unit indicates external reception readiness for the signal and cardiac activity is normal .
the invention concerns a method of signaling an internal status of an implantable cardiac stimulation device , which status is defined by a value of a status parameter , including the steps : a ) recording a measurement value representing the status parameter , b ) ascertaining the existence of the status by comparison of the measurement value with a comparative value , c ) delivering a signal pulse with a predetermined , measurable pulse parameter , wherein the pulse parameter is uniquely associated with the status if the measurement value corresponds to the comparative value within predetermined limits . in accordance with the invention the time difference of equal , predetermined phases of two successive , periodically recurring events prior to delivery of the signal pulse is measured , and then the signal pulse is delivered prior to or after the next expiry of the measured time difference by a predeterminable period of time uniquely associated with the status . the invention further concerns a cardiac stimulation device for carrying out that method .