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An apparatus and a method for identifying fusion beats as part of an automatic capture routine in a cardiac stimulation device are disclosed. By requiring each post-pulse electrical response to be closely correlated with an expected morphology for either a capture condition or a loss-of-capture cond

An apparatus and a method for identifying fusion beats as part of an automatic capture routine in a cardiac stimulation device are disclosed. By requiring each post-pulse electrical response to be closely correlated with an expected morphology for either a capture condition or a loss-of-capture condition before classification of the post-pulse electrical response as either capture or loss-of-capture respectively, the present invention is able to identify fusion beats. In various embodiments, this fusion identification is performed using both template matching and assessment of particular detection features. Moreover, in one embodiment, a moving average and a standard deviation are maintained for each detection feature, thereby creating an automatically adjusting evoked response detection threshold.

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What is claimed is: 1. A method for identifying a fusion beat following delivery of a stimulating pulse in myocardial tissue, comprising: (a) sensing a post-pulse electrical response; (b) assessing a detection feature of the post-pulse electrical response with at least one capture detection functio

What is claimed is: 1. A method for identifying a fusion beat following delivery of a stimulating pulse in myocardial tissue, comprising: (a) sensing a post-pulse electrical response; (b) assessing a detection feature of the post-pulse electrical response with at least one capture detection function to determine whether the post-pulse electrical response corresponds with capture; (c) assessing the detection feature of the post-pulse electrical response with at least one loss-of-capture detection function to determine whether the post-pulse electrical response corresponds with loss-of-capture; and (d) classifying the post-pulse electrical response as a fusion beat if the post-pulse electrical response does not correspond with either capture or loss-of-capture. 2. The method of claim 1, further comprising computing a value for the detection feature for the post-pulse electrical response. 3. The method of claim 2, wherein the detection feature comprises a paced depolarization integral for the post-pulse electrical response. 4. The method of claim 2, wherein the detection feature comprises a peak negative amplitude for the post-pulse electrical response. 5. The method of claim 2, wherein the detection feature comprises one of a positive and negative peak slope for the post-pulse electrical response. 6. The method of claim 2, wherein: (a) assessing the detection feature of the post-pulse electrical response with the capture detection function comprises: (i) comparing the computed value with a first evoked response sense level threshold; and (ii) updating the first evoked response sense level threshold, if a state of capture is identified by the capture comparison; and (b) assessing the detection feature of the post-pulse electrical response with the loss-of-capture detection function comprises comparing the computed value with a second evoked response sense level threshold. 7. The method of claim 6, wherein the updating of the first evoked response sense level threshold comprises recalculating a mean value and a measure of variance around the mean value for the detection feature using the computed value for the detection feature, if the computed value falls within a predetermined measure of variance around the mean value. 8. The method of claim 7, wherein the detection feature comprises a maximum positive slope for the post-pulse electrical response, and wherein comparing the computed value with the first evoked response sense level threshold comprises checking whether the computed value falls within a predetermined measure of variance around the mean value. 9. The method of claim 8, wherein the predetermined measure of variance is programmable. 10. The method of claim 1, wherein assessing the post-pulse electrical response with the capture detection function comprises comparing the post-pulse electrical response with an evoked response signal recognition template, and wherein assessing the post-pulse electrical response with the loss-of-capture detection function comprises comparing the post-pulse electrical response with a polarization signal recognition template. 11. The method of claim 10, wherein the polarization signal recognition template comparison comprises performing a selected correlation between the post-pulse electrical response and the polarization signal recognition template to obtain a loss-of-capture correlation coefficient, and the evoked response signal recognition template comparison comprises performing a selected correlation between the post-pulse electrical response and the evoked response signal recognition template to obtain a capture correlation coefficient. 12. The method of claim 11 wherein the selected correlation comprises: aligning the post-pulse electrical response with the signal recognition template; calculating a Pearson's correlation coefficient between corresponding data points in the post-pulse electrical response and the signal recognition template; and defining the Pearson's correlation coefficient as a measure of correlation between the post-pulse electrical response and the signal recognition template. 13. The method of claim 10, wherein: (a) the polarization signal recognition template comparison comprises performing a selected correlation between the post-pulse electrical response and the polarization signal recognition template to obtain a LOC correlation coefficient; and (b) the evoked response signal recognition template comparison comprises: (i) aligning the post-pulse electrical response with the evoked response signal recognition template; (ii) subtracting the post-pulse electrical response from the evoked response signal recognition template; and (iii) checking the resulting difference for excessive magnitude, where the resulting difference is one of the sum of the differences and the mean of the square of the differences. 14. A method for detecting a fusion beat following delivery of a stimulating pulse in myocardial tissue, comprising: (a) sensing a post-pulse electrical response; (b) computing a value for a detection feature for the post-pulse electrical response; (c) classifying the post-pulse electrical response as a fusion beat if the value falls outside a prescribed deviation and not within a loss-of-capture range specified by an evoked response sense level threshold, wherein the prescribed deviation is determined using a moving average and measure of variance for the detection feature; and (d) recalculating the moving average and the measure of variance using the value of the detection feature for the post-pulse electrical response if the value falls within the prescribed deviation. 15. The method of claim 14, further comprising: (a) selecting the detection feature; (b) determining the evoked response sense level threshold for the detection feature; and (c) generating the moving average and the measure of variance for determined values for the detection feature for a set of post-pulse electrical responses identified as evoked response signals. 16. The method of claim 15, wherein the determining step comprises setting the evoked response sense level threshold to a predetermined value between a mean value for the determined values and a maximum observed value for the detection feature for a set of post-pulse electrical responses identified as polarization signals. 17. The method of claim 16, further comprising programming the evoked response sense level threshold. 18. The method of claim 17, wherein programming the evoked response sense level threshold comprises setting the evoked response sense level threshold to a value between a mean value for the determined values and a maximum observed value for the detection feature for a set of post-pulse electrical responses identified as polarization signals. 19. The method of claim 14, wherein the detection feature comprises a maximum positive slope for the post-pulse electrical response. 20. The method of claim 19, wherein the prescribed deviation comprises a first deviation and a second deviation, and wherein the classifying step comprises: (a) first-classifying the post-pulse electrical response as a fusion beat if the value falls above the first deviation; (b) terminating the classification step if the post-pulse electrical response is classified as a fusion beat in the first-classifying step, thereby reducing an amount of time needed to carry out the method; and (c) second-classifying the post-pulse electrical response as a fusion beat if the value falls below the second deviation and above the evoked response sense level threshold. 21. The method of claim 20, wherein the first deviation and the second deviation are separately programmable. 22. The method of claim 20, wherein the detection feature is only used to detect fusion beats and not evoked responses. 23. The method of claim 14, wherein the prescribed deviation is programmable. 24. The method of claim 14, wherein the detection feature comprises a peak negative amplitude for the post-pulse electrical response. 25. The method of claim 14, wherein the detection feature comprises a paced depolarization integral for the post-pulse electrical response. 26. The method of claim 25, wherein the prescribed deviation is three standard deviations and the loss-of-capture range comprises a low-end equal to zero and a high-end equal to the evoked response sense level threshold. 27. A system for detecting fusion beats comprising: post-pulse electrical response sensing circuitry; and a control unit coupled with the sensing circuitry, the control unit being configured to: assess a detection feature of a post-pulse electrical response with at least one capture detection function to determine whether the post-pulse electrical response corresponds with capture; assess the detection feature of the post-pulse electrical response with at least one loss-of-capture detection function to determine whether the post-pulse electrical response corresponds with loss-of-capture; and classify the post-pulse electrical response as a fusion beat if the post-pulse electrical response does not correspond with either capture or loss-of-capture. 28. The stimulation device of claim 27, wherein the control unit comprises a microcontroller and a control program. 29. The stimulation device of claim 27, wherein the control unit is further configured to enter a fusion avoidance mode if the post-pulse electrical response is classified as a fusion beat. 30. The stimulation device of claim 27, wherein the control unit is further configured to compute a value for the detection feature for the post-pulse electrical response. 31. The stimulation device of claim 30, wherein: the capture detection function compares the computed value for the detection feature with a first evoked response sense level threshold and updates the first evoked response sense level threshold; and the loss-of-capture detection function compares the computed value for the detection feature with a second evoked response sense level threshold. 32. The stimulation device of claim 31, wherein the update performed by the capture detection function comprises recalculating a mean value and a measure of variance around the mean value for the detection feature using the current computed value for the detection feature, if the current computed value falls within a predetermined measure of variance around the mean value. 33. The stimulation device of claim 30, wherein the detection feature comprises a paced depolarization integral for the post-pulse electrical response. 34. The stimulation device of claim 30, wherein the detection feature comprises a peak negative amplitude for the post-pulse electrical response. 35. The stimulation device of claim 30, wherein the detection feature comprises a maximum positive slope for the post-pulse electrical response. 36. The stimulation device of claim 27, wherein; the capture detection function compares the post-pulse electrical response with an evoked response signal recognition template; and the loss-of-capture detection function compares the post-pulse electrical response with a polarization signal recognition template. 37. The stimulation device of claim 36, wherein: the evoked response signal recognition template comparison comprises performing a selected correlation between the post-pulse electrical response and the evoked response signal recognition template to obtain a capture correlation coefficient; and the polarization signal recognition template comparison comprises performing a selected correlation between the post-pulse electrical response and the polarization signal recognition template to obtain a LOC correlation coefficient. 38. A stimulation device comprising: a pulse generator; post-pulse electrical response sensing circuitry; and a control unit coupled with the pulse generator and the sensing circuitry, the control unit being configured to: compute a value for a detection feature for a post-pulse electrical response; classify the post-pulse electrical response as a fusion beat if the value falls outside a prescribed deviation and not within a loss-of-capture range specified by an evoked response sense level threshold, wherein the prescribed deviation is determined using a moving average and a measure of variance around the moving average for the detection feature; and recalculate the moving average and the measure of variance using the value of the detection feature for the post-pulse electrical response if the value falls within the prescribed deviation. 39. The stimulation device of claim 38, wherein the control unit comprises a microcontroller and a control program. 40. The stimulation device of claim 38, wherein the control unit is further configured to enter a fusion avoidance mode if the post-pulse electrical response is classified as a fusion beat. 41. The stimulation device of claim 38, wherein the detection feature comprises a maximum positive slope for the post-pulse electrical response. 42. The stimulation device of claim 41, wherein the detection feature is only used to detect fusion beats and not evoked responses. 43. The stimulation device of claim 41, wherein the prescribed deviation comprises a first deviation and a second deviation, and wherein the control unit is further configured to: classify the post-pulse electrical response as a fusion beat, without further checking of the value, if the value falls above the first deviation; and classify the post-pulse electrical response as a fusion beat if the value falls below the second deviation and above the evoked response sense level threshold. 44. The stimulation device of claim 43, wherein the first deviation and the second deviation are separately programmable. 45. The stimulation device of claim 38, wherein the detection feature comprises a peak negative amplitude for the post-pulse electrical response. 46. The stimulation device of claim 38, wherein the detection feature comprises a paced depolarization integral for the post-pulse electrical response. 47. The stimulation device of claim 46, wherein the prescribed deviation is three standard deviations and the loss-of-capture range comprises a low-end equal to zero and a high-end equal to the evoked response sense level threshold. 48. The stimulation device of claim 46, wherein the prescribed deviation is programmable. 49. A stimulation device comprising: sensing means for sensing a post-pulse electrical response; and control means coupled with the sensing means, the control means comprising means for comparing a value of a detection feature of the post-pulse electrical response to a first threshold to identify capture, means for comparing the value of the detection feature of the post-pulse electrical response to a second threshold to identity loss-of-capture and means for identifying a fusion beat, wherein the fusion beat identification is the default identification resulting from a failure to confirm capture or loss-of-capture. 50. A stimulation device comprising: sensing means for sensing a post-pulse electrical response; and control means coupled with the sensing means, the control means comprising: means for computing a value for a detection feature for a post-pulse electrical response; means for classifying the post-pulse electrical response as a fusion beat if the value falls outside a prescribed deviation and not within a loss-of-capture range specified by an evoked response sense level threshold, wherein the prescribed deviation is determined using a moving average and a measure of variance around the moving average for the detection feature; and means for recalculating the moving average and the measure of variance using the value of the detection feature for the post-pulse electrical response if the value falls within the prescribed deviation.