Three dimensional vector cardiograph and method for detecting and monitoring ischemic events

Abstract

A method of determining an ischemic event includes the steps of: monitoring and storing an initial electrocardiogram vector signal (x, y, z) of a known non-ischemic condition over the QRS, ST and T wave intervals; calculating and storing a J-point of the vector signal and a maximum magnitude of a signal level over the T wave interval; monitoring a subsequent electrocardiogram vector signal over the QRS, ST and T wave intervals; measuring and storing the magnitude (Mag.) of the vector difference between a subsequent vector signal and the initial vector signal; measuring and storing the angle (Ang.) difference between a subsequent vector and the initial vector at points; regressing a line from points about 25 milliseconds prior to the J point and about 60 milliseconds after the J-point and determining the slope of the regression line and the deviation of the angle difference of the regression line; regressing a line from points about 100 milliseconds prior to the maximum magnitude of the signal level over the T wave interval and determining the slope of the regressing line and the deviation of the angle difference of the regression line; and comparing the slope and deviation of the lines from the J point and the T wave interval to a set of known values to determine the presence of an ischemic event.

1. A method of determining an ischemic event, said method comprising the steps of:
 monitoring and storing an initial electrocardiogram vector signal (x 1 , y 1 , z 1 ) of a known non-ischemic condition over the QRS, ST and T wave intervals;   calculating and storing a J-point of the vector signal (x 1 , y 1 , z 1 ) and a maximum magnitude of a signal level over said T wave interval;   monitoring a subsequent electrocardiogram vector signal (x 2 , y 2 , z 2 ) over the QRS, ST and T wave intervals;   measuring the magnitude (Mag.) of the vector difference between a subsequent vector signal (x 2 , y 2 , z 2 ) and the initial vector signal (x 1 , y 1 , z 1 );   measuring the angle (Ang.) difference between a subsequent vector (x 2 , y 2 , z 2 ) and said initial vector signal (x 1 , y 1 , z 1 );   regressing a line from points about 25 milliseconds prior to the J point and about 60 milliseconds after the J-point and determining the slope of the regression line and the deviation of the angle difference of said regression line;   regressing a line from points about 100 milliseconds prior to said maximum magnitude of the signal level over said T wave interval and determining the slope of the regressing line and the deviation of the angle difference of said regression line; and   comparing said slope and deviation of said lines from said J point and said T wave interval to a set of known values to determine the presence of an ischemic event.   
     
     
       2. A method according to  claim 1  wherein the step of measuring and storing the magnitude (Mag.) of the vector difference includes the steps of:
 accessing the stored initial electrocardiogram vector signal (x 1 , y 1 , z 1 ) of a known non-ischemic condition over the QRS, ST and T wave intervals; 
 measuring said subsequent electrocardiogram vector signal (x 2 , y 2 , z 2 ) over the QRS, ST and T wave intervals; 
 calculating the change (Δ) in the vector signal over the QRS, ST and T wave intervals by the following formula:
   Δx=x 2 −x 1 
 
   Δy=y 2 −y 1 
 
   Δz=z 2 −z 1 ;
 
 
 
       and
 calculating the magnitude of the vector difference (Mag vd ) over the QRS, ST and T wave intervals by the following formula:
   Mag vd =√(Δx 2 +Δy 2 +Δz 2 )
 
 
 
     
     
       3. A method according to  claim 1  wherein the step of measuring and storing the angle of the vector difference (Ang.) includes the steps of:
 accessing the stored initial electrocardiogram vector signal (x, y, z) of a known non-ischemic condition over the QRS, ST and T wave intervals; 
 measuring said subsequent electrocardiogram vector signal (x, y, z) over the QRS, ST and T wave intervals; 
 calculating the change (Δ) in the vector signal over the QRS, ST and T wave intervals by the following formula:
   Δx=x 2 −x 1 
 
   Δy=y 2 −y 1 
 
   Δz=z 2 −z 1 ,
 
 
 calculating an Azimuth angle (Az. Ang.) of said angle vector difference over the QRS, ST and T wave intervals by the following formula:
   Az. Ang.=arc tan (Δz/Δx); and
 
 
 calculating an Elevation angle (El. Ang.) of said angle vector difference over the QRS, ST and T wave intervals by the following formula:
   El. Ang.=arc tan(Δy/√(Δx 2 +Δz 2 )).
 
 
 
     
     
       4. A method according to  claim 1  wherein the step of calculating said J point includes the steps of:
 calculating the magnitude of the initial vector signal (Mag vs ) over the QRS, ST and T wave intervals by the following formula:
   Mag vs =√(x 2 +y 2 +z 2 );
 
 
 filtering said magnitude of the vector signal (Mag vs ) over the QRS, ST and T wave intervals through a low pass filter to establish a smooth vector signal (VS sm ) and a maximum value and time of the QRS interval (QRS max  and QRS maxtime ); 
 differentiating said smooth vector signal (VS sm ) from said magnitude of the vector signal (Mag vs ) over the QRS, ST and T wave intervals and establishing a derivative vector signal (dVS sm ); 
 calculating a set of initial parameters from the QRS interval including: the magnitude of the maximum QRS signal (QRS max   1 ); the maximum of the QRS time interval (QRS maxtime ); and the end point of the QRS signal (QRS EndInit ); 
 calculating a set of initial parameters from the T wave interval including: the magnitude of the maximum T wave signal (Twave max ); and the maximum of the T wave time interval (Twave maxtime ); and 
 calculating an initial estimate of the end of the QRS interval (QRS EndInit ); 
 fitting the vector signal along a cubic polynomial curve; 
 calculating the change in the derived vector signal (dVS sm ) over a prescribed time period to establish a smooth test interval (S Test ); 
 fitting a first order polynomial curve to the initial vector signal (Mag vs ) starting at the end of the QRS interval (QRS_EndInit) to a point which is equal to the end of the QRS interval (QRS_EndInit) plus the smooth test interval (S Test ); and 
 calculating the intersection of the cubic polynomial curve and the first order polynomial curve and selecting a point of intersection that is furthest from the time of the maximum QRS value (QRS maxtime ) to establish the J point. 
 
     
     
       5. A method according to  claim 1  wherein after said step of monitoring and storing an initial electrocardiogram vector signal (x, y, z) of a known non-ischemic condition over the QRS, ST and T wave intervals, the method includes the step of estimating a magnitude and angle of said ST interval. 
     
     
       6. A method of determining and displaying the presence of an ischemic event implemented in a processor of a medical device, said method comprising the steps of:
 monitoring and storing in said processor an initial electrocardiogram vector signal (x1, y1, z1), comprising a first sequence of samples, of a known non-ischemic condition over the QRS, ST, and T wave intervals;   calculating and storing in said processor a magnitude (Mag.) and angle (Ang.) for each sample of the first sequence of samples of the initial vector signal (x1, y1, z1) including a J-point and a maximum magnitude over each of the QRS, ST and T wave intervals;   monitoring a subsequent electrocardiogram vector signal (x2, y2, z2), comprising a second sequence of samples including a J-point, and calculating a magnitude (Mag.) and angle (Ang.) for each sample of the second sequence of samples over the QRS, ST and T wave intervals;   shifting the first sequence of samples or the second sequence of samples, where necessary, in order to substantially align the initial vector signal (x1, y1, z1) and the subsequent vector signal (x2, y2, z2) in time;   calculating a difference vector (Vd) as a difference in magnitude (Mag.) and a difference in angle (Ang.) between each sequenced sample of the first sequence of samples and each corresponding sequenced sample of the second sequence of samples over the QRS, ST, and T wave intervals;   regressing a line (A1) and a line (E1) from scalar representatives of an Azimuth angle (Az. Ang.) part and Elevation angle (El. Ang.) part of the sequenced angle (Ang.) samples of the difference vector (Vd) from about 25 milliseconds prior to the J point to about 60 milliseconds after the J-point;   determining slopes of said regression line (A1) and said regression line (E1);   determining a standard deviation of an angle difference between the samples comprising said regression line (A1) and the corresponding azimuth angle (Az. An.) samples of the difference vector (Vd), and a standard deviation of an angle difference between the samples comprising said regression line (E1) and the corresponding elevation angle (El. An.) samples of the difference vector (Vd); and   comparing said slopes and said standard deviations of said angle differences of said regression line (A1) and said regression line (E1) to a set of known slope values and known standard deviation values to determine the presence of an ischemic event.   
     
     
       7. A method according to claim 6 wherein the step of calculating the difference vector (Vd), corresponding to the angle (Ang.) difference includes the steps of:
 calculating changes (Δ) between each sample of the second sequence of samples and each corresponding sample of the first sequence of samples over the respective QRS, ST and T wave intervals by the following formulas:
   Δx=x2−x1;
 
   Δy=y2−y1;
 
   Δz=z2−z1;
 
   
       and
 using the Δx, Δy and Δz for each sample change,
 calculating the Azimuth angle (Az. Ang.) part of said angle-(Ang.) difference by the following formula:
   Az. Ang.=arctan(Δz/Δx); and
 
 
 calculating the Elevation angle (El. Ang.) part of said angle-(Ang.) difference by the following formula:
   El. Ang.=arctan (Δy/((Δx 2 )+(Δz 2 )) 1/2 ),
 
 
 
 wherein said angle (Ang.) difference for each sample of the difference vector (Vd) comprises an AZ. Ang. and an El. Ang. 
 
     
     
       8. A method according to claim 6, further comprising, after said step of monitoring and storing the first sequence of samples and, said step of monitoring the second sequence of samples, a step of utilizing the magnitude (Mag.) and angle (Ang.) at the J point of the second sequence of samples as an estimated offset of the ST interval. 
     
     
       9. A method according to claim 8, wherein the method further comprises the step of:
 displaying, on a display in operative communication with said processor, said J point of the second sequence of samples.   
     
     
       10. A method according to claim 6, further comprising a step of:
 displaying, on a display in operative communication with said processor, said comparison of said slopes and said standard deviations to visually communicate said determination of said presence of said ischemic event.   
     
     
       11. A method according to claim 6, further comprising steps of:
 regressing a line (A2) and a line (E2) from scalar representations of Azimuth angle (Az. Ang.) and Elevation angle (El. An.) parts of the sequenced angle (Ang.) samples of the difference vectors (Vd), from about 100 milliseconds prior to said maximum magnitude of the signal level over said T wave interval;   determining slopes of said regression line (A2) and said regression line (E2);   determining a standard deviation of an angle difference between samples comprising said regression line (A2) and the corresponding azimuth angle (Az. An.) samples of the difference vector (Vd), and a standard deviation of an angle difference between the samples comprising said regression line (E2) and the corresponding elevation angle (El. An.) samples of the difference vector (Vd); and   comparing said slopes and said standard deviations of said angle differences of said regression line (A2) and said regression line (E2) to a set of known slope values and known standard deviation values to determine the presence of an ischemic event.   
     
     
       12. A method according to claim 6, wherein the initial electrocardiogram vector signal (x1, y1, z1) is (0, 0, 0). 
     
     
       13. A method for measuring and displaying a J-point of an electrocardiogram signal implemented in a processor of a medical device, the method comprising the steps of:
 monitoring and storing an electrocardiogram vector signal (x, y, z) comprising a sequence of samples over QRS, ST and T wave intervals of the electrocardiogram signal;   calculating and storing a corresponding sequence of magnitudes (Mag vs ), where each magnitude (Mag vs ) corresponds respectively to each of the sequence of samples, a J-point and a maximum magnitude of a signal level of the electrocardiogram vector signal (x, y, z) over the QRS, ST and T wave intervals;   filtering said sequence of magnitudes (Mag vs ) through a low pass filter to establish a smooth signal (VS sm ), a maximum value of the smooth signal (VS sm ) in the QRS interval (QRSVS max ) and a time of occurrence of the maximum value of the smooth signal (VS sm ) in the QRS interval (QRSVS maxtime );   differentiating with respect to time the smooth signal (VS sm ) to establish a derivative of the smooth signal (dVS sm ) with respect to time;   calculating a set of initial parameters from the smooth signal (VS sm ) in the QRS interval including:   an initial estimated time of the start of the QRS interval (QRS startInit ) by finding the time before the time of occurrence of the maximum value of the smooth signal (Vs sm ) in the QRS interval (QRSVS maxtime ) that the derivative of the smooth signal (dVS sm ) changes polarity from negative to positive; and   an initial estimated time of occurrence of the end point of the QRS interval (QRS endInit ) by finding the time of occurrence after the time of occurrence of the maximum smooth signal (VS sm ) in the QRS interval (QRSVS maxtime ) at which the derivative of the smooth signal (dVS sm ) changes polarity from negative to positive; and   displaying, on a display in operative communication with said processor, said initial estimated time of the end point of the QRS interval (QRS endInit ).   
     
     
       14. A method according to claim 13, further comprising the steps of:
 fitting said sequence of magnitudes (Mag vs ) along a cubic polynomial curve extending from a sample midway between the time of occurrence of the maximum value of the smooth signal in the QRS interval (QRSVS maxtime ) and the initial estimated time of occurrence of the end point of the QRS interval (QRS endInit ), to a sample at the initial estimated time of occurrence of the end point of the QRS interval (QRS endInit );   calculating a change in the derivative of the smooth signal (dVS sm ) for a period extending in a range of 50 to 100 ms, beyond the initial estimated time of occurrence of the end point of the QRS interval (QRS endInit ) to establish a smooth test interval (S Test );   fitting a first order polynomial curve to said sequence of magnitudes (Mag vs ) starting at the initial estimated time of occurrence of the end point of the QRS interval (QRSendinit) and an end point of the smooth test interval (S Test );   calculating points of intersection of the cubic polynomial curve and the first order polynomial curve; and   selecting a point of intersection that is furthest in time from the time of occurrence of the maximum value of the smooth signal (VS sm ) in the QRS interval (QRSVS maxtime ), which intersection point is a true end of the QRS interval (QRS trueend ), and identifying the true end as the J point.   
     
     
       15. A method according to claim 14, further comprising the step of displaying, on a display in operative communication with said processor, said J point. 
     
     
       16. A method for determining and displaying from an electrocardiogram signal at least one of a beginning and an end of the P-wave interval, a duration of the P-wave interval, a start of the QRS interval, an end of the T-wave interval, a QT interval, a PR interval, and a duration of the QRS interval, the method implemented in a processor within a medical device and comprising the steps of:
 monitoring and storing an electrocardiogram vector signal (x, y, z), comprising a sequence of samples, from the electrocardiogram signal over an entire heart beat cycle including the P-wave, QRS, and T-wave intervals;   calculating a magnitude for each of the samples of the electrocardiogram vector signal (x, y, z) over the P-wave, QRS and T-wave intervals, to realize a corresponding sequence of magnitude values comprising a magnitude signal (Mag), by the following formula:
   Mag=((x 2 )+(y 2 )+(z 2 )) 1/2    
   filtering the calculated magnitude signal (Mag) over the P-wave, QRS, and T-wave intervals to establish:   a smooth signal (VS sm );   a time of occurrence of a maximum value of the smooth signal (VS sm ) during the P-wave interval (Pwmaxtime);   a magnitude of the maximum value of the smooth signal (VS sm ) during the P-wave interval (Pwmax);   a time of occurrence of the maximum value of the smooth signal (VS sm ) during the QRS interval (QRSVSmaxtime);   a magnitude of the maximum value of the smooth signal (VS sm ) during the QRS interval (QRSVSmaxtime);   a time of occurrence of the maximum value of the smooth signal (VS sm ) during the T-wave interval (Twmax); and   a magnitude of the maximum value of the smooth signal (VS sm ) during the T-wave interval (Twmaxtime);   establishing a derivative vector signal (dVS sm ) with respect to time of the smooth signal (VS sm );   calculating from the smooth signal (VS sm ) over the P-wave interval an initial estimated time of the start of the P-wave interval (PwStInit) at a point where the derivative signal (dVS sm ) changes from negative to positive, before the time of occurrence of the maximum value of the smooth signal (VS sm ) during the P-wave interval (Pwmaxtime) and the initial estimated time of the end point of the P-wave interval (PwEndInit) where the derivative signal changes from negative to positive after the time of occurrence of the maximum value of the smooth signal (VS sm ) during the P-wave interval (Pwmaxtime); and   displaying, on a display in operative communication with said processor, the magnitude of the maximum value of the smooth signal (VS sm ) during the P-wave interval (Pwmax), the time of occurrence of the maximum value of the smooth signal (VS sm ) during the P-wave interval (Pwmaxtime), the initial estimated time of the start of the P-wave interval (PwTimeStInit), and the initial estimated time of the end of the P-wave interval (PwTimeEndInit).   
     
     
       17. A method according to claim 16, further comprising the steps of:
 calculating from the smooth signal (VS sm ) over the QRS interval an initial estimated time of the start of the QRS interval (QRSStInit) at a point where the derivative signal (dVS sm ) has a first zero value, before the time of occurrence of the maximum value of the smooth vector signal (VS sm ) of the electrocardiogram vector signal (x, y, z) during the QRS interval (QRSVSmaxtime).   
     
     
       18. A method according to claim 17, further comprising the steps of:
 calculating from the smooth signal (V sm ) over the T-wave interval an initial estimated time of occurrence of the end point of the T-wave interval (TwTimeEndInit) at a point in time where the derivative of the smooth signal (dVS sm ) changes from negative to positive after the time of occurrence of the maximum smooth signal (VS sm ) during the T-wave interval (Twmaxtime).   
     
     
       19. A method according to claim 18, further comprising the steps:
 fitting a first cubic polynomial curve to the smooth signal (VS sm ), from the initial estimated time of the start of the P-wave interval (PwaveTimeStInit) to the time of occurrence of the maximum smooth signal (VS sm ) during the P-wave interval (Pwmaxtime); and   finding a point (1) within the first cubic polynomial curve displaying a magnitude equivalent to an average of the signal magnitude (Mag vs ) averaged over a region that starts 20 ms before the initial estimated time of the start of the P-wave interval (PwaveTimeStInit) and ends at the initial estimated time of the start of the P-wave interval (PwaveTimeStInit), wherein said point (1) comprises a refined estimate of the start of the P-wave interval (PwStart).   
     
     
       20. A method according to claim 19, further comprising the steps:
 fitting a second cubic polynomial curve to the smooth signal (VS sm ), from the time of occurrence of the maximum smooth signal (VS sm ) during the P-wave interval (Pwmaxtime) to the initial estimated time of the end of the P-wave interval (PwaveTimeEndInit); and   finding a point (2) within the second cubic polynomial curve equal to a magnitude equivalent to an average of the magnitude signal (Mag vs ) averaged over a region that begins at the initial estimated time of the end of the P-wave interval (PwaveTimeEndInit) and ends at the initial start time of the QRS interval (QRStime StartInit), wherein said point (2) comprises a refined estimate of the time of occurrence of the end of the P-wave interval (PwEnd).   
     
     
       21. A method according to claim 20, further comprising the steps:
 fitting a third cubic polynomial curve to the smooth signal (VS sm ), from the initial start time of the QRS interval (QRStimeStartInit) to the time of occurrence of the maximum value of the smooth signal (VS sm ) during the QRS interval (QRSVSmaxtime); and   finding a point (3) within the third cubic polynomial curve equal to a magnitude equivalent to an average of the magnitude signal (Mag vs ) averaged over a region that starts from the initial estimated time of the end of the P-wave interval (PwTimeEndInit) and extends to the initial estimated time of the start of the QRS interval (QRSStInit), wherein said point (3) comprises a refined estimate of the start time of the QRS interval (QRSStart).   
     
     
       22. A method according to claim 21, further comprising the step of:
 calculating the PR interval, using the following formula:
   PR interval=QRSStart−PwStart.
 
   
     
     
       23. A method according to claim 21, further comprising the step of:
 fitting a fourth cubic polynomial curve to a portion of the smooth signal (VS sm ) that extends from a point midway between the time of occurrence of the maximum smooth signal (VS sm ) in the Twave interval (Twmaxtime) and the initial estimated time of occurrence of the end point of the Twave interval (TwTimeEndInit).   
     
     
       24. A method according to claim 23, further comprising the steps of:
 calculating a change in the derivative of the smooth signal (dVS sm ) over a prescribed time period after the initial estimated time of occurrence of the end point of the T wave interval (TwTimeEndInit) when the derivative of the smooth signal (dVS sm ) exhibits a substantially constant magnitude to establish a post T wave smooth test interval (Twtime endInit+Post Twsmoothtest);   fitting a first order polynomial curve to a portion of the smooth signal (VS sm ) that starts at the initial estimated time of occurrence of the end point of the T wave interval (TwTimeEndInit) and extends through the initial estimated time of occurrence of the end point of the T wave interval (TwTimeEndInit) for about another 20 ms;   calculating points of intersection of the fourth cubic polynomial curve and the first order polynomial curve; and   selecting one of the points of intersection that is furthest in time from the time occurrence of the maximum smooth signal (VS sm ) during the Twave interval (Twavemaxtime), which said one of the points comprises the time of occurrence of the end point of the T wave interval (TwaveEnd).   
     
     
       25. A method according to claim 24, further comprising the step of displaying, on a display in operative communication with said processor, said time of occurrence of the end point of the T wave interval (TwaveEnd).