according to the invention , the pacemaker investigates the presence of a pmt by measuring the correlation between an interval t 3 , for signals generated by the heart and an interval d 2 , for signals generated by the pacemaker . reference is now made to fig4 where a non - pmt situation is illustrated to the left and a pmt situation is illustrated to the right . the lower part of the figure illustrates the atrial ( a ) and the ventricular ( v ) conduction levels and the dashed and the continuous lines indicate antegrade and retrograde conduction , respectively . the cycle time of a pmt comprises two delay intervals , one ( d 1 ) related to the retrograde heart tissue conduction , and the other one ( d 2 ) related to the pacemaker introduced interval or time delay between atrial and ventricular activity ( av - interval ). d 1 can be stable , vary regularly or stocastically . however , for short time periods , d 1 is limited and substantially constant . the av - interval , d 2 , is in this invention made to vary stocastically or in accordance with a predetermined pattern . by varying the interval d 2 in a known way , the correlation between d 2 and t 3 is calculated for a short time period . typically 2 to 6 heart cycles are needed to reach a reasonably safe decision that the value of the correlation exceeds a predetermined value and , consequently , that a pmt is present . the high correlation for d 2 and t 3 when a pmt is present is explained by way of example below . let the interval t 3 be the interval ( pp ) between two consecutive p - waves . if no pmt is present , the pp - interval is the interval between two spontaneous atrial beats , and the correlation between d 2 and t 3 is low as the pp - interval variation is independent of d 2 . if , in contrast , a pmt is present , the pp - interval is the sum of the retrograde conduction interval d 1 and the pacemaker generated interval d 2 . consequently , during a pmt , there is a very high correlation between d 2 and t 3 ( the pp - interval ). alternatively , instead of using the pp - interval as t 3 , quantities related thereto , for instance the atrial frequency or the interval between a ventricular stimulus and a p - wave , could be employed . in the last case , it is obvious that the expression &# 34 ; signals generated by the heart &# 34 ;, previously used in connection with t 3 , is intended to comprise signals related to ventricular stimuli , and further , that the correlation between d 2 and t 3 is contrary to where t 3 denotes the pp - interval . in fig1 the heart is designated 11 and the pacemaker is generally designated 10 . the pacemaker 10 is connected to the heart through an electrode lead 112 for the atrium and an electrode lead 113 for the ventricle . the electrode leads 112 and 113 are respectively connected to signal sensors and amplifiers 12 and 13 for the atrium and the ventricle . the electrode lead ( or possibly a further , separate electrode lead ), not shown for the ventricle 113 , is connected to a stimulating unit 14 . the stimulating unit 14 delivers stimuli to the heart . a further stimulating unit ( not shown ), could possibly be provided for stimulating the atrium through electrode lead 112 , or , anther through separate electrode lead ( not shown ). the basic pacemaker timing and logic unit 15 is controlled by the sensed signals applied thereto through leads 121 and 131 . the sensed signals are related to spontaneous heart activities , viz . atrial p - wave or ventricular r - wave . if the heart fails to beat normally , stimulation pulses are emitted by the pacemaker in order to maintain the normal heart function . it is also possible to deliver correctly timed stimulation pulses even if spontaneous heart beats exist . the stimulation unit 14 is connected to the basic unit 15 through lead 156 . the basic logic and timing unit 15 ( to be described later ) is connected to the calculating unit 16 ( to be described later ) through data bus 161 . in response to e . g . a sensed atrial signal on lead 121 , the basic timing and logic unit 15 ( fig2 ) generates basic pacer escape intervals ( pp - intervals ) in the time base generator and time base register 151 . a control signal on lead 152 triggers the av - interval counter 153 simultaneously with the triggering of the time base generator 151 . after the av - interval has been timed out , a control signal on lead 154 triggers the stimulation pulse width generator and time register 155 , generating a control signal on lead 156 , which controls the stimulating unit 14 . a further control circuit , similar to the one just described and also starting from the time base generator 151 but delivering stimuli for the atrium through electrode lead 112 or another separate ( not indicated ) electrode lead for the atrium can also be provided . in order to prevent incorrect control due to false signal sensing after , for instance , a stimulation pulse has been delivered or a heart signal just sensed , the refractory period registers 122 and 132 and their respective leads 123 and 133 are provided in connection with atrial and ventricular signal sensing , respectively . a communication and data register unit 171 is provided for the pacemaker programming and functional control . the communication is preferably carried out by telemetry means 172 . the data bus 161 provides for the internal pacemaker transmission of programmed parameter values , control signals and time register values . the calculating unit 16 ( fig3 ) comprises time registers , logic circuits and arithmetic processing circuits . preferably , a microprocessor is employed , and the microprocessor is operated in accordance with a correlation calculating program . arithmetic processing , time measurement and control are then carried out from a ram and rom program memory 163 connected to a central processing unit ( cpu ) 162 . measured time intervals ( d 2 , t 3 ) and calculation results are stored in read - write memory 164 . detected atrial and ventricular signals are received via interrupt registers 166 and 167 , respectively . data from the microprocessor can be placed in the input - output register 168 via the internal bus 165 for subsequent communication with the basic timing and logic unit 15 . it should be noted that , for explanatory purposes , the timing and logic unit 15 and the calculating unit 16 have been disclosed as separate units . it is , however , possible to arrange unit 16 to carry out the functions of timing and logic unit 15 . the calculation of the correlation is carried out in accordance with well - known mathematical theory in the cpu 162 , with interaction with the program memory 163 and the read / write memory 164 , as needed , and can be made in different ways . as an example , the mean value and the deviation from the mean value is calculated for each interval t 3 and each interval d 2 , respectively . the deviation for each interval t 3 and for each corresponding interval d 2 are multiplied and the products added for all t 3 , d 2 intervals . the resulting sum is then divided by the sum of the absolute values of the deviation for the d 2 intervals . if the result of this division exceeds a predetermined value , a pmt is present . the pmt decision can also be referred to two correlation levels . below the lowest level , there is no pmt and above the highest , a pmt is present . a value between the two levels indicates that a decision cannot be made with the intervals available , and therefore further intervals d 2 , t 3 should be included until a decision can be reached . another way of calculating the correlation would be to subtract d 2 from t 3 for each corresponding d 2 , t 3 interval , calculate the mean value for the resulting differences and the deviation therefrom for each resulting difference . correspondingly , the mean value and the deviations therefrom for the intervals d 2 are calculated . these last deviations are compared to the corresponding difference deviations , and , if substantially equal , the correlation is high . finally , for a reliable detection of a pmt , the variation in d 2 should be of the same magnitude or greater than the variation in d 1 . however , the number of intervals are also of importance for the reliability , and should the variation in d 1 be greater than that in d 2 , the number of intervals should be correspondingly increased . although modifications and changes may be suggested by those skilled in the art , it is the intention of the inventor to embody within the patent warranted hereon all changes and modifications as reasonably and properly come within the scope of his contribution to the art .
a pacemaker - mediated tachycardia is detected by calculating the correlations for typically two to six heart cycles between , for example , spontaneous atrial beats and the pacemaker generated av delays , which are varied in a predictable manner . the correlation is low when no pmt is present because the possible variation in the interval between spontaneous atrial beats is independent of the variation of the av delay . correlation is high when a pmt is present , because the interval between spontaneous atrial beats will then equal the pmt cycle time , i . e ., the av delay plus the retrograde conduction time in the heart tissue .