referring to the drawing , an illustration of an embodiment of the present invention is applied to determining when tachyarrhythmias are pace - terminable in the ventricle . as explained above , the invention can equally well be applied to determining when tachyarrhythmias are pace - terminable in the atrium . referring to the drawing , there is depicted a flowchart showing the decision path of a routine in accordance with the invention , with entry point 1 , following the detection of an r - wave ( i . e ., a ventricular beat ). at step 2 the routine determines if the rr interval length exceeds a preset &# 34 ; slow &# 34 ; limit ( which may be programmable and is preferably approximately 600 ms ), in which case the rhythm is probably a sinus rhythm sr . ( a large interval corresponds to a slow cardiac rate ). if yes , a slow cycle counter is incremented at 3 . at 5 , the slow cycle counter count is tested , and if the count reaches a preset number y of consecutive slow cycles ( where y may be programmable and is preferably approximately 16 cycles ), then each bin of the histograms is reset at step 6 . this action is taken based on the information previously stored being no longer current . thus , in this manner , when the rr interval is greater than the slow limit ( step 2 ), the routine does not perform the time - and power - consuming steps of updating and analyzing the histograms , which it does perform as described below , when a tachyarrhythmia might be present . this reduces battery consumption which is an important advantage for an implantable antitachycardia device . if the rr interval does not exceed the slow interval limit at step 2 , then the slow cycle counter is reset at 4 . then , at step 7 the method updates an rr interval histogram and a pr interval histogram . in this regard , in the rr interval histogram , when the bin corresponding to the rr interval just elapsed is incremented , the bin that was incremented z fast cycles ago ( where z may be programmable and is preferably 16 ), if any , is decremented . similarly , in the pr interval histogram , the bin corresponding to each of the pr intervals detected in the last rr cycle , one for each detected p - wave , is incremented , and any bins which were incremented z fast cycles ago , are decremented . in this manner , the histograms store information for the z most recent cycles where the ventricular rate was faster than the slow limit . a count of the total number of rr intervals in the rr histogram is maintained , referred to as the rr total ( or auto - correlation total ), and a count of the total number of pr intervals in the pr histogram is maintained , referred to as the pr ( or cross - correlation total ). it is noted that when sorting rhythms originating in the atrium , a pp interval histogram and rp interval histogram are used in a parallel manner . the histogram for cardiac intervals also is referred to as the auto - correlation histogram , and the histogram for the conduction intervals also is referred to as the cross - correlation histogram . at step 8 , the routine determines the auto - correlation peak ( also called the rr peak ), and the cross - correlation peak ( also called the pr peak ). each peak is determined by scanning all the bins of the particular histogram with a window , whose width may be programmable and is preferably 64 ms . in this regard , the window is placed at one end of the histogram range and the number of events in the bins within the window are counted . the window is then shifted through the histogram range , one bin at a time . each time the window is moved by one bin , the total of the counts in all the bins in the window is calculated . the highest total count of all of the possible window positions is saved as the maximum or peak count . in this manner , the peak is the maximum total number of counts in any set of contiguous bins which fit inside the window . at step 9 an averaged rr interval is compared to a preset &# 34 ; slow &# 34 ; limit , which may be programmable and is preferably approximately 600 ms . if the average rr interval is longer than the &# 34 ; slow &# 34 ; limit , the routine declares an sr condition at step 10 and the routine returns to wait for the next cycle at point 24 , for subsequent cardiac event processing beginning at point 1 . at step 11 the averaged rr interval is compared to a preset &# 34 ; very fast &# 34 ; limit ( which may be programmable and is preferably approximately 375 ms ). if the average rr interval is shorter than the &# 34 ; very fast &# 34 ; limit , a vf condition is declared at step 12 . optionally , therapy to revert ventricular fibrillation is applied at step 21 in response to a declared condition vf at step 12 . the averaged rr interval is used at steps 9 and 11 to prevent isolated premature beats , unsensed beats , and bursts of electrical noise , from grossly affecting the decision . it is maintained separately from the histogram and may be an average of the last 16 determined intervals . in alternate embodiments , a moving average , low - pass filter , x out of y detector , or other conventional filter could be used . at step 13 , the rr peak is divided by rr total . if the quotient is less than a preset fraction f1 ( which may be programmable and is preferably approximately 75 %) then it is declared unlikely that there is conduction on a fixed pathway from one ventricular sensed event causing the next . accordingly , the device declares an svt condition at step 14 . at step 15 , the pr peak is divided by the rr peak . if the quotient is less than a preset fraction f2 ( which may be programmable and is preferably approximately 75 %) then it is declared less likely that there is conduction from the atrium to the ventricle than from the ventricle to the ventricle . accordingly , the device declares a vt condition at step 16 . in an alternative embodiment , at step 15 the pr peak is divided by rr total ( not shown ). the decision is then made based on absolute stability of the pr interval , rather than on the relative stability of the pr interval , as compared with the rr interval . at step 17 , the pr peak is divided by pr total . if the quotient is less than a preset fraction f3 ( which may be programmable and is preferably approximately 75 %) then conduction from atrium to ventricle is many : 1 , and there is little risk of confusing this with conduction from ventricle to atrium . accordingly , the device declares an svt condition at step 14 . as an alternative , when the number of events counted in the second chamber is divided by the number of events counted in the first chamber is greater than a predetermined fraction of , for example , 1 . 33 , or less than a fraction of , for example , 0 . 66 , an association of many : 1 is declared . at step 18 , there is rr stability and pr stability , with 1 : 1 conduction . this situation could correspond to an atrial tachyarrhythmia conducted to the ventricle , or a ventricular tachyarrhythmia conducted to the atrium . however , the process must apply another criterion to determine the origin of the tachyarrhythmia . preferably , the criterion applied is an acceleration criterion in which the ventricular rate acceleration is compared to a preset limit , which may be programmable , and is preferably approximately 25 %. if the rate does not exceed the limit , then it is declared likely that this is sinus tachycardia , and accordingly the device declares an st condition at step 19 . at step 20 , it is determined if there was atrioventricular disassociation at the onset of ventricular acceleration . if there was , then it is declared likely that the tachyarrhythmia had a ventricular origin , and the device declares a vt condition at step 16 . if there was not , then the device declares an svt condition at 14 . one method for determining association requires , for example , that the conduction intervals for the accelerated beat and its predecessor both fall within a predetermined range corresponding to physiological conduction , e . g ., 30 ms to 300 ms for atrio - ventricular conduction . as illustrated in the drawing , in accordance with a preferred embodiment of the invention , and according to the declared cardiac condition , the appropriate therapy at steps 21 , 22 , or 23 is delivered by the device . thereafter , the routine returns to step 24 and waits for the next cycle , and then resumes analysis at entry point 1 . after the histograms are reset at step 6 , the routine can suspend declaring the type of rhythm present until a preset number of cycles have been used to update the histograms . the preset number may be programmable , and is preferably approximately 8 . the system embodying the present invention includes a device capable of the creation and maintenance of the appropriate histograms in suitable data storage devices and a software program or solid state finite state machine for the automatic computation of the rr and pr intervals ( recalling that alternately and additionally the intervals may be based on atrial events , e . g ., pp intervals and rp intervals ), and the processing of the intervals to make the tests and determinations regarding stability , conduction instability , association , and acceleration , and the storage of the various predetermined parameters , constants , and fractions . more preferably , such a device also includes the detection of atrial and ventricular beats and the measuring of these beats and the noted intervals are performed by conventional devices having atrial and ventricular sense amplifiers , signal conditioning circuits , analog - to - digital conversion circuits , and suitable memory and registers for time - based digital data processing and manipulation as are now common in cardiac pacemaker , defibrillator and cardioverter devices . representative electronic circuits for acquiring the cardiac signals and determining the pr and rr intervals ( and pp and rp intervals ) are those found in the series of dual chamber pacemakers available from ela medical , montrouge , france , offered under the chorus trademark . the present invention is preferably implemented in software instructions loadable or loaded in memory for operating a microprocessor to process acquired cardiac signals , and is specifically applied following acquisition of the cardiac electric signals by conventional sense amplifiers , more preferably after the acquired signals have been conditioned and converted to a digital form . one skilled in the art will appreciate that the present invention can be practiced by other than the preferred embodiments , which are presented for purposes of illustration and not of limitation . numerous other modifications may be made and other arrangements may be devised without departing from the spirit and scope of the present invention .
methods and apparatus for classifying cardiac rhythm by analyzing sensed atrial and ventricular heartbeats , and determining if the rhythm might be pace - terminable in a given chamber . the method maintains a histogram of the sensed beat interval in the chamber of interest and a second histogram of conduction time from the other chamber . the method classifies a rhythm as pace terminable in the given chamber when the intervals in that chamber are stable , and the conduction times to that chamber are unstable . it applies further criteria when the intervals in the given chamber are stable , and the conduction times are also stable . antitachycardia pacing is deliverable to terminate tachyarrhythmias determined to be pace terminable .