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Systems and methods that improving the specificity of discriminating between a monomorphic tachyarrhythmia and a polymorphic tachyarrhythmia are provided that examine frequency content and baseline information of the EGM as discriminatory signatures. Particular algorithms of the present invention th

Systems and methods that improving the specificity of discriminating between a monomorphic tachyarrhythmia and a polymorphic tachyarrhythmia are provided that examine frequency content and baseline information of the EGM as discriminatory signatures. Particular algorithms of the present invention that are employed to determine frequency content of the QRS complexes are titled the Slope Distribution Metric (SDM) algorithm and the Slope Distribution Composite (SDC) algorithm.

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What is claimed is: 1. In an implantable medical device that provisionally detects a polymorphic tachyarrhythmia of the heart of a patient as a function of measured time intervals between sensed events in a cardiac signal, a method of improving the specificity of discriminating between a monomorphi

What is claimed is: 1. In an implantable medical device that provisionally detects a polymorphic tachyarrhythmia of the heart of a patient as a function of measured time intervals between sensed events in a cardiac signal, a method of improving the specificity of discriminating between a monomorphic tachyarrhythmia and a polymorphic tachyarrhythmia comprising the steps of: a) provisionally declaring a polymorphic tachyarrhythmia when at least a first number of the measured time intervals satisfy the polymorphic tachyarrhythmia detection criteria; b) successively sampling, processing, and temporarily storing the cardiac signal to provide sampled signal amplitudes; c) determining a baseline window between sensed events and a sensed event window encompassing the sensed event; d) determining a set of signal slopes of the baseline window; e) determining a cumulative distribution of baseline window slopes CDSB; f) determining a set of signal slopes of the event window; g) determining a cumulative distribution of event window slopes CDSE; h) calculating a maximum absolute difference KSevent between the CDSE and a sigmoid reference function; i) calculating a maximum absolute difference KSbaseline between the CDSE and a CDSB; j) calculating a slope distribution metric SDM value by SD metric=KSbaseline-(0.5-KSevent) k) comparing SDM to a slope distribution match threshold; l) accumulating a count x of SDM values that meet the slope distribution match threshold over y repetitions of steps b) through j) as long as a polymorphic tachyarrhythmia is provisionally declared in step a); m) withholding the final declaration of a polymorphic tachyarrhythmia if the count x indicates that the corresponding cardiac signals exhibit frequency content suggestive of a monomorphic tachyarrhythmia; and n) making the final declaration of a polymorphic tachyarrhythmia if the count x indicates that the corresponding cardiac signals exhibit frequency content suggestive of a polymorphic tachyarrhythmia and the polymorphic tachyarrhythmia is provisionally declared in step a). 2. The method of claim 1, further comprising repeating steps b) through n) as long as step a) is met until step n) is met. 3. The method of claim 2, wherein the implantable medical device further comprises an implantable cardioverter/defibrillator having the capability of delivering a C/D shock therapy and further comprising: o) delivering a C/D shock therapy when the final declaration of a polymorphic tachyarrhythmia is made in step n). 4. The method of claim 2, wherein step m) comprises: establishing a withhold count corresponding to z measured time intervals between sensed events of successive cardiac signals; and repeating steps a) through l) at least z times. 5. The method of claim 2, wherein the implantable medical device further comprises an implantable cardioverter/defibrillator having the capability of delivering a C/D shock therapy and wherein: step m) comprises establishing a withhold count corresponding to z measured time intervals between each detected features successive cardiac signals; and further comprising: o) repeating steps b) through n) at least z times; and p) delivering a C/D shock therapy when the final declaration of a polymorphic tachyarrhythmia is made in step n). 6. The method of claim 2, wherein the implantable medical device further comprises an implantable cardioverter/defibrillator having the capability of delivering a C/D shock therapy and delivering an anti-tachycardia therapy and further comprising: o) delivering a C/D shock therapy when the final declaration of a polymorphic tachyarrhythmia is made in step n); and p) delivering an anti-tachycardia therapy when the count x indicates that the corresponding cardiac signals exhibit frequency content suggestive of a monomorphic tachyarrhythmia in step m) and a polymorphic tachyarrhythmia is provisionally declared in step a). 7. The method of claim 1, wherein the implantable medical device further comprises an implantable cardioverter/defibrillator having the capability of delivering a C/D shock therapy and delivering an anti-tachycardia therapy and further comprising: o) delivering a C/D shock therapy when the final declaration of a polymorphic tachyarrhythmia is made in step n); and p) delivering an anti-tachycardia therapy when the count x indicates that the corresponding cardiac signals exhibit frequency content suggestive of a monomorphic tachyarrhythmia in step m) and a polymorphic tachyarrhythmia is provisionally declared in step a). 8. The method of claim 7, wherein: step l) comprises comparing the count x to a frequency content count threshold; and step n) comprises making the final declaration of a polymorphic tachyarrhythmia if the count x does not meet the frequency content count threshold and a polymorphic tachyarrhythmia is provisionally declared in step a). 9. The method of claim 1, wherein the implantable medical device further comprises an implantable cardioverter/defibrillator having the capability of delivering a C/D shock therapy and further comprising: o) delivering a C/D shock therapy when the final declaration of a polymorphic tachyarrhythmia is made in step n). 10. The method of claim 9, wherein: step l) comprises comparing the count x to a frequency content count threshold; and step n) comprises making the final declaration of a polymorphic tachyarrhythmia if the count x does not meet the frequency content count threshold and a polymorphic tachyarrhythmia is provisionally declared in step a). 11. The method of claim 1, wherein step m) comprises: establishing a withhold count corresponding to z measured time intervals between sensed events of successive cardiac signals; and repeating steps a) through l) at least z times. 12. The method of claim 1, wherein the implantable medical device further comprises an implantable cardioverter/defibrillator having the capability of delivering a C/D shock therapy and wherein: step m) comprises establishing a withhold count corresponding to z measured time intervals between each detected features successive cardiac signals; and further comprising: o) repeating steps b) through l) at least z times; and p) delivering a C/D shock therapy when the final declaration of a polymorphic tachyarrhythmia is made in step n). 13. The method of claim 1, wherein step l) comprises comparing the count x to a frequency content count threshold; and step n) comprises making the final declaration of a polymorphic tachyarrhythmia if the count x does not meet the frequency content count threshold and a polymorphic tachyarrhythmia is provisionally declared in step a). 14. In an implantable medical device that provisionally detects a polymorphic tachyarrhythmia of the heart of a patient as a function of measured time intervals between sensed events in a cardiac signal, a system of improving the specificity of discriminating between a monomorphic tachyarrhythmia and a polymorphic tachyarrhythmia comprising: provisional declaring means for provisionally declaring a polymorphic tachyarrhythmia when at least a first number of the measured time intervals satisfy the polymorphic tachyarrhythmia detection criteria; signal processing means for successively sampling, processing, and temporarily storing the cardiac signal to derive a plurality y of data sets of signal amplitudes related to y sensed events; window defining means for determining a baseline window between sensed events and a sensed event window encompassing the sensed event of each of the y data sets; means for determining a set of signal slopes of the baseline window and a set of signal slopes of the sensed event window; means for determining a cumulative distribution of baseline window slopes CDSB; means for determining a cumulative distribution of event window slopes CDSE; means for calculating a maximum absolute difference KS event between the CDSE and a sigmoid reference function; means for calculating a maximum absolute difference KS baseline between the CDSE and a CDSB; means for calculating a slope distribution metric SDM value by SD metric=KSbaseline-(0.5-KSevent) means for comparing SDM to a slope distribution match threshold; counting means for accumulating a count x of SDM values that meet the slope distribution match threshold over y comparisons of SDM to a slope distribution match threshold; withholding means for withholding the final declaration of a polymorphic tachyarrhythmia if the count x indicates that the corresponding cardiac signals exhibit frequency content suggestive of a monomorphic tachyarrhythmia; and final declaring means for making the final declaration of a polymorphic tachyarrhythmia if the count x indicates that the corresponding cardiac signals exhibit frequency content suggestive of a polymorphic tachyarrhythmia and the polymorphic tachyarrhythmia is provisionally declared. 15. The system of claim 14, further comprising means for comparing the count x to a frequency content count threshold; and the final declaring means further comprises making the final declaration of a polymorphic tachyarrhythmia if the count x does not meet the frequency content count threshold and a polymorphic tachyarrhythmia is provisionally declared. 16. The system of claim 15, wherein the implantable medical device further comprises an implantable cardioverter/defibrillator having the capability of delivering a C/D shock therapy and further comprising: means for delivering a C/D shock therapy when the final declaration of a polymorphic tachyarrhythmia is made. 17. The system of claim 15, wherein: the withholding means further comprises: means for establishing a withhold count corresponding to z measured time intervals between sensed events of successive cardiac signals if the count x indicates that the corresponding cardiac signals exhibit frequency content; and means for decrementing the withhold count each time that a count x indicates that the corresponding cardiac signals exhibit frequency content suggestive of a polymorphic tachyarrhythmia; and the final declaring means further comprises means responsive to the withhold count for declaring a polymorphic tachyarrhythmia only when the withhold count is decremented to a withhold count less than z. 18. The system of claim 17, wherein the implantable medical device further comprises an implantable cardioverter/defibrillator having the capability of delivering a C/D shock therapy and further comprising: means for delivering a C/D shock therapy when the final declaration of a polymorphic tachyarrhythmia is made. 19. The system of claim 15, wherein the implantable medical device further comprises an implantable cardioverter/defibrillator having the capability of delivering a C/D shock therapy and delivering an anti-tachycardia therapy and further comprising: means for delivering a C/D shock therapy when the final declaration of a polymorphic tachyarrhythmia is made; and means for delivering an anti-tachycardia therapy when the count x indicates that the corresponding cardiac signals exhibit frequency content suggestive of a monomorphic tachyarrhythmia and a polymorphic tachyarrhythmia is provisionally declared. 20. The system of claim 14, wherein the implantable medical device further comprises an implantable cardioverter/defibrillator having the capability of delivering a C/D shock therapy and further comprising: means for delivering a C/D shock therapy when the final declaration of a polymorphic tachyarrhythmia is made. 21. The system of claim 14, wherein: the withholding means further comprises: means for establishing a withhold count corresponding to z measured time intervals between sensed events of successive cardiac signals if the count x indicates that the corresponding cardiac signals exhibit frequency content; and means for decrementing the withhold count each time that a count x indicates that the corresponding cardiac signals exhibit frequency content suggestive of a polymorphic tachyarrhythmia; and the final declaring means further comprises means responsive to the withhold count for declaring a polymorphic tachyarrhythmia only when the withhold count is decremented to a withhold count less than z. 22. The system of claim 21, wherein the implantable medical device further comprises an implantable cardioverter/defibrillator having the capability of delivering a C/D shock therapy and further comprising: means for delivering a C/D shock therapy when the final declaration of a polymorphic tachyarrhythmia is made. 23. The system of claim 14, wherein the implantable medical device further comprises an implantable cardioverter/defibrillator having the capability of delivering a C/D shock therapy and delivering an anti-tachycardia therapy and further comprising: means for delivering a C/D shock therapy when the final declaration of a polymorphic tachyarrhythmia is made; and means for delivering an anti-tachycardia therapy when the count x indicates that the corresponding cardiac signals exhibit frequency content suggestive of a monomorphic tachyarrhythmia and a polymorphic tachyarrhythmia is provisionally declared. 24. A method of processing a cardiac signal to derive sensed events and discriminating between a monomorphic tachyarrhythmia and a polymorphic tachyarrhythmia comprising the steps of: a) successively sampling, processing, and temporarily storing the cardiac signal to provide sampled signal amplitudes; b) determining a baseline window between sensed events and a sensed event window encompassing the sensed event; c) determining a set of signal slopes of the baseline window; d) determining a cumulative distribution of baseline window slopes CDSB; e) determining a set of signal slopes of the event window; f) determining a cumulative distribution of event window slopes CDSE; g) calculating a maximum absolute difference KSevent between the CDSE and a sigmoid reference function; h) calculating a maximum absolute difference KSbaseline between the CDSE and a CDSB; i) calculating a slope distribution metric SDM value by SD metric=KSbaseline-(0.5-KSevent) j) comparing SDM to a slope distribution match threshold; k) accumulating a count x of SDM values that meet the slope distribution match threshold over y repetitions of steps b) through j) as long as a polymorphic tachyarrhythmia is provisionally declared in step a); l) withholding the final declaration of a polymorphic tachyarrhythmia if the count x indicates that the corresponding cardiac signals exhibit frequency content suggestive of a monomorphic tachyarrhythmia; and m) making the final declaration of a polymorphic tachyarrhythmia if the count x indicates that the corresponding cardiac signals exhibit frequency content suggestive of a polymorphic tachyarrhythmia and the polymorphic tachyarrhythmia is provisionally declared in step a). 25. A system of processing a cardiac signal to derive sensed events and discriminating between a monomorphic tachyarrhythmia and a polymorphic tachyarrhythmia comprising: signal processing means for successively sampling, processing, and temporarily storing the cardiac signal to derive a plurality y of data sets of signal amplitudes related to y sensed events; window defining means for determining a baseline window between sensed events and a sensed event window encompassing the sensed event of each of the y data sets; means for determining a set of signal slopes of the baseline window and a set of signal slopes of the sensed event window; means for determining a cumulative distribution of baseline window slopes CDSB; means for determining a cumulative distribution of event window slopes CDSE; means for calculating a maximum absolute difference KS event between the CDSE and a sigmoid reference function; means for calculating a maximum absolute difference KS baseline between the CDSE and a CDSB; means for calculating a slope distribution metric SDM value by SD metric=KSbaseline-(0.5-KSevent) means for comparing SDM to a slope distribution match threshold; counting means for accumulating a count x of SDM values that meet the slope distribution match threshold over y comparisons of SDM to a slope distribution match threshold; withholding means for withholding the final declaration of a polymorphic tachyarrhythmia if the count x indicates that the corresponding cardiac signals exhibit frequency content suggestive of a monomorphic tachyarrhythmia; and final declaring means for making the final declaration of a polymorphic tachyarrhythmia if the count x indicates that the corresponding cardiac signals exhibit frequency content suggestive of a polymorphic tachyarrhythmia and the polymorphic tachyarrhythmia is provisionally declared. 26. In an implantable medical device that provisionally detects a polymorphic tachyarrhythmia of the heart of a patient as a function of measured time intervals between sensed events in a cardiac signal, a method of improving the specificity of discriminating between a monomorphic tachyarrhythmia and a polymorphic tachyarrhythmia comprising the steps of: a) provisionally declaring a polymorphic tachyarrhythmia when at least a first number of the measured time intervals satisfy the polymorphic tachyarrhythmia detection criteria; b) successively sampling, processing, and temporarily storing the cardiac signal to provide sampled signal amplitudes; c) determining a baseline window between sensed events and a sensed event window encompassing the sensed event; d) determining a set of signal slopes of the baseline window; e) determining a cumulative distribution of baseline window slopes CDSB; f) determining a set of signal slopes of the event window; g) determining a cumulative distribution of event window slopes CDSE; h) storing the cumulative distribution of event window slopes CDSE in a register comprising one or more previous cumulative distribution of event window slopes CDSPE; i) calculating a maximum absolute difference KSevent between the CDSE and a sigmoid reference function; j) storing the maximum absolute difference KSevent in a register comprising one or more previous maximum absolute difference KSpreviousevent; k) calculating a maximum absolute difference KSbaseline between the CDSE and a CDSB; l) calculating a slope distribution metric SDmetric value by: SD metric=KSbaseline-(0.5-KSevent) m) calculating a maximum absolute difference KSmatch between the current CDSE and a stored previous CDSPE; n) calculating a slope distribution match SDmatch value by: SD match=1.0-KSmatch/((1.0-(KSevent -KSpreviousevent)) o) comparing SDmetric and SDmatch to a slope distribution match threshold; p) accumulating a count x of SDmetric or SDmatch values that meet a slope distribution match threshold over y repetitions of steps b) through p) as long as a polymorphic tachyarrhythmia is provisionally declared in step a); q) withholding the final declaration of a polymorphic tachyarrhythmia if the count x indicates that the corresponding cardiac signals exhibit frequency content suggestive of a monomorphic tachyarrhythmia; and r) making the final declaration of a polymorphic tachyarrhythmia if the count x indicates that the corresponding cardiac signals exhibit frequency content suggestive of a polymorphic tachyarrhythmia and the polymorphic tachyarrhythmia is provisionally declared in step a). 27. The method of claim 26, further comprising repeating steps b) through q) as long as step a) is met until step r) is met. 28. The method of claim 27, wherein the implantable medical device further comprises an implantable cardioverter/defibrillator having the capability of delivering a C/D shock therapy and further comprising: s) delivering a C/D shock therapy when the final declaration of a polymorphic tachyarrhythmia is made in step r). 29. The method of claim 27, wherein step q) comprises: establishing a withhold count corresponding to z measured time intervals between sensed events of successive cardiac signals; and repeating steps a) through p) at least z times. 30. The method of claim 27, wherein the implantable medical device further comprises an implantable cardioverter/defibrillator having the capability of delivering a C/D shock therapy and wherein: step q) comprises establishing a withhold count corresponding to z measured time intervals between each detected features successive cardiac signals; and further comprising: s) repeating steps b) through p) at least z times; and t) delivering a C/D shock therapy when the final declaration of a polymorphic tachyarrhythmia is made in step r). 31. The method of claim 27, wherein the implantable medical device further comprises an implantable cardioverter/defibrillator having the capability of delivering a C/D shock therapy and delivering an anti-tachycardia therapy and further comprising: s) delivering a C/D shock therapy when the final declaration of a polymorphic tachyarrhythmia is made in step r); and t) delivering an anti-tachycardia therapy when the count x indicates that the corresponding cardiac signals exhibit frequency content suggestive of a monomorphic tachyarrhythmia in step q) and a polymorphic tachyarrhythmia is provisionally declared in step a). 32. The method of claim 26, wherein the implantable medical device further comprises an implantable cardioverter/defibrillator having the capability of delivering a C/D shock therapy and delivering an anti-tachycardia therapy and further comprising: s) delivering a C/D shock therapy when the final declaration of a polymorphic tachyarrhythmia is made in step r); and t) delivering an anti-tachycardia therapy when the count x indicates that the corresponding cardiac signals exhibit frequency content suggestive of a monomorphic tachyarrhythmia in step q) and a polymorphic tachyarrhythmia is provisionally declared in step a). 33. The method of claim 32, wherein: step p) comprises comparing the count x to a frequency content count threshold; and step r) comprises making the final declaration of a polymorphic tachyarrhythmia if the count x does not meet the frequency content count threshold and a polymorphic tachyarrhythmia is provisionally declared in step a). 34. The method of claim 26, wherein the implantable medical device further comprises an implantable cardioverter/defibrillator having the capability of delivering a C/D shock therapy and further comprising: s) delivering a C/D shock therapy when the final declaration of a polymorphic tachyarrhythmia is made in step r). 35. The method of claim 34, wherein: step p) comprises comparing the count x to a frequency content count threshold; and step r) comprises making the final declaration of a polymorphic tachyarrhythmia if the count x does not meet the frequency content count threshold and a polymorphic tachyarrhythmia is provisionally declared in step a). 36. The method of claim 26, wherein step q) comprises: establishing a withhold count corresponding to z measured time intervals between sensed events of successive cardiac signals; and repeating steps a) through p) at least z times. 37. The method of claim 26, wherein the implantable medical device further comprises an implantable cardioverter/defibrillator having the capability of delivering a C/D shock therapy and wherein: step q) comprises establishing a withhold count corresponding to z measured time intervals between each detected features successive cardiac signals; and further comprising: s) repeating steps b) through q) at least z times; and t) delivering a C/D shock therapy when the final declaration of a polymorphic tachyarrhythmia is made in step r). 38. The method of claim 26, wherein step p) comprises comparing the count x to a frequency content count threshold; and step r) comprises making the final declaration of a polymorphic tachyarrhythmia if the count x does not meet the frequency content count threshold and a polymorphic tachyarrhythmia is provisionally declared in step a). 39. In an implantable medical device that provisionally detects a polymorphic tachyarrhythmia of the heart of a patient as a function of measured time intervals between sensed events in a cardiac signal, a system of improving the specificity of discriminating between a monomorphic tachyarrhythmia and a polymorphic tachyarrhythmia comprising: provisional declaring means for provisionally declaring a polymorphic tachyarrhythmia when at least a first number of the measured time intervals satisfy the polymorphic tachyarrhythmia detection criteria; signal processing means for successively sampling, processing, and temporarily storing the cardiac signal to derive a plurality y of data sets of signal amplitudes related to y sensed events; window defining means for determining a baseline window between sensed events and a sensed event window encompassing the sensed event of each of the y data sets; means for determining a set of signal slopes of the baseline window and a set of signal slopes of the sensed event window; means for determining a cumulative distribution of baseline window slopes CDSB; means for determining a cumulative distribution of event window slopes CDSE; means for storing the cumulative distribution of event window slopes CDSE in a register comprising one or more previous cumulative distribution of event window slopes CDSPE; means for calculating a maximum absolute difference KS event between the CDSE and a sigmoid reference function; means for calculating a maximum absolute difference KS baseline between the CDSE and a CDSB; means for storing the maximum absolute difference KSevent in a register comprising one or more previous maximum absolute difference KSpreviousevent; means for calculating a slope distribution metric SDM value by SD metric=KSbaseline-(0.5-KSevent) means for calculating a maximum absolute difference KS match between the current CDSE and a stored previous CDS PE; means for calculating a slope distribution match SDmatch value by: SD match=1.0-KSmatch/((1.0-(KSevent -KSpreviousevent)) means for comparing SDmetric and SDmatch to a slope distribution match threshold; means for accumulating a count x of SDmetric or SDmatch values that meet a slope distribution match threshold over y comparisons of SDmetric and SDmatch to a slope distribution match threshold; repetitions as a polymorphic tachyarrhythmia is provisionally declared; withholding means for withholding the final declaration of a polymorphic tachyarrhythmia if the count x indicates that the corresponding cardiac signals exhibit frequency content suggestive of a monomorphic tachyarrhythmia; and final declaring means for making the final declaration of a polymorphic tachyarrhythmia if the count x indicates that the corresponding cardiac signals exhibit frequency content suggestive of a polymorphic tachyarrhythmia and the polymorphic tachyarrhythmia is provisionally declared. 40. The system of claim 39, further comprising means for comparing the count x to a frequency content count threshold; and the final declaring means further comprises making the final declaration of a polymorphic tachyarrhythmia if the count x does not meet the frequency content count threshold and a polymorphic tachyarrhythmia is provisionally declared. 41. The system of claim 40, wherein the implantable medical device further comprises an implantable cardioverter/defibrillator having the capability of delivering a C/D shock therapy and further comprising: means for delivering a C/D shock therapy when the final declaration of a polymorphic tachyarrhythmia is made. 42. The system of claim 40, wherein: the withholding means further comprises: means for establishing a withhold count corresponding to z measured time intervals between sensed events of successive cardiac signals if the count x indicates that the corresponding cardiac signals exhibit frequency content; and means for decrementing the withhold count each time that a count x indicates that the corresponding cardiac signals exhibit frequency content suggestive of a polymorphic tachyarrhythmia; and the final declaring means further comprises means responsive to the withhold count for declaring a polymorphic tachyarrhythmia only when the withhold count is decremented to a withhold count less than z. 43. The system of claim 42, wherein the implantable medical device further comprises an implantable cardioverter/defibrillator having the capability of delivering a C/D shock therapy and further comprising: means for delivering a C/D shock therapy when the final declaration of a polymorphic tachyarrhythmia is made. 44. The system of claim 40, wherein the implantable medical device further comprises an implantable cardioverter/defibrillator having the capability of delivering a C/D shock therapy and delivering an anti-tachycardia therapy and further comprising: means for delivering a C/D shock therapy when the final declaration of a polymorphic tachyarrhythmia is made; and means for delivering an anti-tachycardia therapy when the count x indicates that the corresponding cardiac signals exhibit frequency content suggestive of a monomorphic tachyarrhythmia and a polymorphic tachyarrhythmia is provisionally declared. 45. The system of claim 39, wherein the implantable medical device further comprises an implantable cardioverter/defibrillator having the capability of delivering a C/D shock therapy and further comprising: means for delivering a C/D shock therapy when the final declaration of a polymorphic tachyarrhythmia is made. 46. The system of claim 39, wherein: the withholding means further comprises: means for establishing a withhold count corresponding to z measured time intervals between sensed events of successive cardiac signals if the count x indicates that the corresponding cardiac signals exhibit frequency content; and means for decrementing the withhold count each time that a count x indicates that the corresponding cardiac signals exhibit frequency content suggestive of a polymorphic tachyarrhythmia; and the final declaring means further comprises means responsive to the withhold count for declaring a polymorphic tachyarrhythmia only when the withhold count is decremented to a withhold count less than z. 47. The system of claim 46, wherein the implantable medical device further comprises an implantable cardioverter/defibrillator having the capability of delivering a C/D shock therapy and further comprising: means for delivering a C/D shock therapy when the final declaration of a polymorphic tachyarrhythmia is made. 48. The system of claim 39, wherein the implantable medical device further comprises an implantable cardioverter/defibrillator having the capability of delivering a C/D shock therapy and delivering an anti-tachycardia therapy and further comprising: means for delivering a C/D shock therapy when the final declaration of a polymorphic tachyarrhythmia is made; and means for delivering an anti-tachycardia therapy when the count x indicates that the corresponding cardiac signals exhibit frequency content suggestive of a monomorphic tachyarrhythmia and a polymorphic tachyarrhythmia is provisionally declared. 49. A method of processing a cardiac signal to derive sensed events and discriminating between a monomorphic tachyarrhythmia and a polymorphic tachyarrhythmia comprising the steps of: a) successively sampling, processing, and temporarily storing the cardiac signal to provide sampled signal amplitudes; b) determining a baseline window between sensed events and a sensed event window encompassing the sensed event; c) determining a set of signal slopes of the baseline window; d) determining a cumulative distribution of baseline window slopes CDSB; e) determining a set of signal slopes of the event window; f) determining a cumulative distribution of event window slopes CDE; g) storing the cumulative distribution of event window slopes CDSE in a register comprising one or more previous cumulative distribution of event window slopes CDSPE; h) calculating a maximum absolute difference KSevent between the CDSE and a sigmoid reference function; i) storing the maximum absolute difference KSevent in a register comprising one or more previous maximum absolute difference KSpreviousevent; j) calculating a maximum absolute difference KSbaseline between the CDSE and a CDSB; k) calculating a slope distribution metric SDM1 value by: SD metric=KSbaseline-(0.5KSevent) l) calculating a maximum absolute difference KSmatch between the current CDSE and a stored previous CDSPE; m) calculating a slope distribution match SDM1 value by: SD match=1.0-KSmatch/((1.0-(KSevent -KSpreviousevent)) n) comparing SDmetric and SDmatch to a slope distribution match threshold; o) accumulating a count x of SDmetric or SDmatch values that meet a slope distribution match threshold over y repetitions of steps b) through o); and p) declaring a polymorphic tachyarrhythmia if the count x indicates that the corresponding cardiac signals exhibit frequency content suggestive of a polymorphic tachyarrhythmia. 50. A system of processing a cardiac signal to derive sensed events and discriminating between a monomorphic tachyarrhythmia and a polymorphic tachyarrhythmia comprising: signal processing means for successively sampling, processing, and temporarily storing the cardiac signal to derive a plurality y of data sets of signal amplitudes related to y sensed events; window defining means for determining a baseline window between sensed events and a sensed event window encompassing the sensed event of each of the y data sets; means for determining a set of signal slopes of the baseline window and a set of signal slopes of the sensed event window; means for determining a cumulative distribution of baseline window slopes CDSB; means for determining a cumulative distribution of event window slopes CDSE; means for storing the cumulative distribution of event window slopes CDSE in a register comprising one or more previous cumulative distribution of event window slopes CDSPE; means for calculating a maximum absolute difference KS event between the CDSE and a sigmoid reference function; means for calculating a maximum absolute difference KS baseline between the CDSE and a CDSB; means for storing the maximum absolute difference KSevent in a register comprising one or more previous maximum absolute difference KSpreviousevent; means for calculating a slope distribution metric SDM value by SD metric=KSbaseline-(0.5-KSevent) means for calculating a maximum absolute difference KS match between the current CDSE and a stored previous CDS PE; means for calculating a slope distribution match SDmatch value by: SD match=1.0-KSmatch/((1.0-(KSevent-KSpreviousevent)) means for comparing SDmetric and SDmatch to a slope distribution match threshold; means for accumulating a count x of SDmetric or SDmatch values that meet a slope distribution match threshold over y comparisons of SDmetric and SDmatch to a slope distribution match threshold; means for making the final declaration of a polymorphic tachyarrhythmia if the count x indicates that the corresponding cardiac signals exhibit frequency content suggestive of a polymorphic tachyarrhythmia.