comprehension of the present invention can be gained through reference to the drawings in conjunction with a through review of the following explanation . in order to facilitate a full appreciation of the invention , an overview of the preferred embodiment is initially provided . the overview is followed by more detailed explanation , and , subsequently , operation and use of the invention are described . the disclosure of u . s . pat . no . 5 , 312 , 443 ( issued to adams et al .) is incorporated herein by reference and addresses identification of cardiac depolarization rates from a sequence of detected r - wave events . the &# 39 ; 443 adams et al patent is also representative of the skill in the art in preparing circuits for icd devices . the problem to be addressed by the present invention can perhaps be most readily understood by reference to the prior art and shortfalls thereof . in the prior art , an r - wave sensing system of the current generation is depicted in fig1 and 2 . the electrical voltage signal 10 from sensing electrodes implanted in the heart is displayed on the ordinate and plotted over the course of time , along the abscissa . the signal 10 from the implanted electrodes indicates a low amplitude r - wave occurring at 12 with an amplitude of about 3 mv . after the low amplitude r - wave is sensed , the current generation sensing system proceeds to set an initial threshold of sensing 14 at about 2 mv ( i . e . about 67 % or 2 / 3 of the most recently detected r - wave 12 .) the sensing threshold sensitivity then decays , for example , by an exponential decay algorithm with a decay rate constant of about 1 . 5 sec as depicted by dotted line 16 until reaching a constant floor 18 . noise in the signal 10 ranges occasionally up to about 0 . 5 mv . when this exceeds the sensing threshold ( dotted line 16 ) or the floor ( dotted line 18 ) a false detection event is sensed , as shown at 20 , 22 , and 24 , prematurely to the next true r - wave 26 . thus , false data , incorrectly suggesting a highly rapid sequence of r - waves is provided to the analytical portion of the triggering mechanism for consideration in a diagnostic model , potentially resulting in inappropriate therapeutic treatment of the patient &# 39 ; s heart . with reference to fig2 this problem is compounded by the resetting of a fresh , and very low , initial sensitivity threshold 21 to about 67 % or 2 / 3 of the amplitude of the false detection event 20 . this in turn sets up the next false detection event 22 to follow the refractory period , in large part because of the very low initial sensitivity threshold and also due to the use of the same decay rate constant , 1 . 5 seconds , from that very low initial sensitivity threshold 21 . as soon as the refractory period is completed , the next false detection event 22 can occur . in a first embodiment of the present invention , as shown in fig3 a and 3b , the shortcomings of the prior art sensing systems are overcome as follows : an electrical voltage signal 40 , from sensing electrodes implanted in a patient &# 39 ; s heart , is displayed on the ordinate and plotted over the course of time along the abscissa . the signal 40 from the implanted electrodes indicates a normal amplitude r - wave occurring at 42 with an amplitude of about 10 mv and a t - wave at 43 . after the normal amplitude r - wave is sensed , the current generation sensing system proceeds to set an initial threshold of sensing 44 at about 6 . 7 mv ( i . e . about 67 % or 2 / 3 of the most recently detected r - wave 42 .) this is done by a peak detection circuit 50 , as is illustrated in fig4 . peak detector circuit 50 comprises an amplifier / filter portion 51 , a voltage divider 52 , a capacitor 53 , and a comparator 54 . voltage divider 52 comprises resistors 55 and 56 . peak detector circuit 50 creates a stored , but slowly decaying signal which represents the peak amplitude of a sensed r - wave . the information is stored as a voltage on capacitor 53 , from which the threshold level is derived . the voltage level on capacitor 53 decreases over time at a rate equal to the time constant of the capacitor . the time constant of capacitor 53 is a function of the capacitor and voltage divider 52 . while peak detector circuit 50 is shown as implemented with discrete components using a capacitor 53 and comparator 54 , it will be appreciated by those skilled in the art that the function of peak detector circuit 50 may be accomplished in a number of ways , including the use of a microconductor and a / p circuits . although the initial threshold is preferably set at a 2 / 3 value , it is possible to utilize any value of between 50 to 95 % on most recently detected r - wave 42 . the sensing threshold sensitivity then decays , for example , by an exponential decay algorithm with a decay rate constant of a little less than 1 . 0 second as depicted by complete line 46 until reaching a constant floor ( complete line 48 .) noise in the signal 40 again might range occasionally up to about 0 . 5 mv . in this situation , the noise is unlikely to exceed the sensing threshold ( complete line 46 ) or the floor ( complete line 48 ) so a false detection event is not sensed premature to the next true r - wave 50 . thus , false data , incorrectly suggesting a highly rapid sequence of r - waves is not provided to the analytical portion of the triggering mechanism for consideration in a diagnostic model and the potential for inappropriate therapeutic treatment of the patient &# 39 ; s heart is not present . it will be understood that the first embodiment , after sensing a normal amplitude r - wave provides a sensitivity threshold above the level which might be affected by noise so as to result in a false and premature r - wave sensing . in this respect , it resembles the current generation icds . continuing on with the description of the first embodiment , the difference from the prior art will become apparent . as shown in fig3 b , the shortcomings of the prior art sensing systems may be largely overcome as follows : in fig3 b , an electrical voltage signal 60 from sensing electrodes implanted in a patient &# 39 ; s heart is displayed on the ordinate and plotted over the course of time , along the abscissa . the signal 60 from the implanted electrodes indicates a low amplitude r - wave occurring at 62 with an amplitude of about 1 mv and a t - wave at 63 . after the low amplitude r - wave is sensed , like the current generation sensing systems , the first embodiment proceeds to set an initial threshold of sensing 64 at about 0 . 67 mv ( i . e . about 67 % or 2 / 3 of the most recently detected r - wave 62 .) departing , however , from the similarity to the current generation systems , the sensing threshold sensitivity then again decays , but not by the fast time constant applied to normal amplitude r - wave as described with respect to fig3 a . instead , the sensitivity threshold decays at a different and slower rate , for example , by an exponential decay algorithm with a decay rate constant of a little more than 2 . 0 second as depicted by intermittent line 66 until reaching a constant floor ( intermittent line 68 .) noise in the electrical signal 60 again might range occasionally up to about 0 . 5 mv . in this situation , the noise is unlikely to exceed the sensing threshold ( intermittent line 66 ) or the floor ( intermittent line 68 ) so a false detection event is still not sensed , premature to the next true r - wave 70 , even though it is another low amplitude r - wave . thus , in this first embodiment when a low r - wave is encountered , false data , incorrectly suggesting a highly rapid sequence of r - waves is not provided to the analytical portion of the triggering mechanism for consideration in a diagnostic model and the potential for inappropriate therapeutic treatment of the patient &# 39 ; s heart remains greatly reduced . moreover , if the diagnostic model does classify the patient &# 39 ; s heart situation as requiring therapy , then accurate r - wave event timing information is available for precise timing of the application of therapeutic electrical stimulation . note also that t - waves 43 and 63 are avoided in both fig3 a and 3b . in a second embodiment , schematically depicted in fig5 the shortcomings of the prior art sensing systems may also be largely overcome as follows : an electrical voltage signal 80 from sensing electrodes implanted in a patient &# 39 ; s heart is displayed on the ordinate and plotted over the course of time , along the abscissa . the signal 80 from the implanted electrodes indicates a low amplitude r - wave occurring at 82 with an amplitude of about 1 mv and a t - wave 83 . after the low amplitude r - wave is sensed , like the current generation sensing systems , the second embodiment proceeds to set an initial threshold of sensing 84 at about 0 . 67 mv ( i . e . about 67 % or 2 / 3 of the most recently detected r - wave 82 .) departing , however , from the similarity to the current generation systems , the sensing threshold sensitivity then again decays , first by a relatively fast decay , such as is used with a normal amplitude r - wave . a most preferred first decay is a reverse exponential . however , unlike the single decay current generation systems , once the first decay 86 drops to a predetermined sensitivity level 87 , a second decay 88 begins . the second decay 88 is at a slower rate . in a most preferred embodiment , the second decay is by a common exponential decay . a floor 89 is preferably also present . note also that t - wave 83 is avoided in this embodiment . to provide for multiple decay rates , a slightly modified peak detector circuit 90 is provided , as is illustrated in fig6 . since peak detector circuit 90 is very similar to peak detection circuit 50 from fig4 like elements will have the same reference numerals . peak detection circuit 90 has all of the elements of circuit 50 and adds a resistor 91 and a switch 92 . as stated above , the time constant of capacitor 53 is a function of the capacitor and voltage divider 52 . at a specified time , resistor 91 is switched into the circuit by switch 92 and voltage divider 52 is then changed . this changes the time constant which causes a change in the decay rate . in a third embodiment , schematically depicted in fig7 the shortcomings of the prior art sensing systems may also be largely overcome as follows : an electrical voltage signal 100 from sensing electrodes implanted in a patient &# 39 ; s heart is displayed on the ordinate and plotted over the course of time , along the abscissa . the signal 100 from the implanted electrodes indicates a low amplitude r - wave occurring at 102 with an amplitude of about 1 mv and a t - wave 103 . after the low amplitude r - wave is sensed , like the current generation sensing systems , the second embodiment proceeds to set an initial threshold of sensing 104 at about 0 . 67 mv ( i . e . about 67 % or 2 / 3 of the most recently detected r - wave 102 .) departing , however , from the similarity to the current generation systems , the sensing threshold sensitivity then again decays , first by a relatively slow decay , such as is used with a normal amplitude r - wave . however , unlike the single decay current generation systems , once the first decay 106 is allowed to drop for a predetermined time period , such as for example 300 milliseconds , and preferably in a range of 275 to 325 milliseconds , a second decay 108 is begun . the second decay 108 drops until , preferably , it reaches a floor 109 . in this situation , the noise is unlikely to exceed the sensing threshold . note also that t - wave 103 is avoided in this embodiment . a fourth alternative embodiment is schematically depicted in fig8 . the fourth alternate embodiment is similar to the third alternate embodiment yet is applicable to r - waves having amplitudes ranging from ± 1 mv to ± 20 mv . an electrical voltage signal 120 from sensing electrodes implanted in a patient &# 39 ; s heart is displayed on the ordinate and plotted over the course of time , along the abscissa . as stated above , the amplitude of the r - wave occurring at 122 may range from ± 1 mv -± 20 mv . also shown as part of signal 120 is a t - wave 123 . for all amplitudes of r - waves , the same two threshold levels and the same two decay constants are used . specifically , in the preferred alternative embodiment , a first threshold sensing level of 58 % of the first peak level , shown at 124 , using a first time constant of 1 . 75 s , shown at 126 , for a period of 325 ms after peak detector , time constant of 1 . 15 , shown at 130 , and a second threshold sensing level of 33 %, shown at 132 is used thereafter . preparation of electrical components and provision of software and hardware control of miniaturized computer systems for implementing the above - described embodiments of this invention are well within the present level of skill of this art . because numerous modifications may be made of this invention without departing from the spirit thereof , the scope of the invention is not to be limited to the single embodiment illustrated and described . rather , the scope of the invention is to be determined by appended claims and their equivalents .
an icd detection method for sensing the occurrence of an r - wave improves the ability to distinguish r - waves from noise through the use of variable declining sensitivity thresholds . the method includes the consideration of the amplitude of at least the previous most recent r - wave to determine a declining threshold of sensitivity used to recognize a subsequent electrical signal as an r - wave . in the method , the amplitude of the previous r - wave may be classified , based upon amplitude , and based upon the classification , a desirable time constant for the declining threshold of sensitivity is provided as an exponential or reverse exponential decay . alternatively , a piece wise use of various decay formulas may be combined and used to avoid false recognition of noise as an r - wave .