Abstract:
A method and apparatus for dynamically estimating the ST segment of a rate varying electrocardiogram by comparing the result yielded from a mathematical relationship between calculated beat averages and a predetermined standard. Typically a table of known heartbeats is made from which means for estimating are created. An average of beat to beat intervals is compared with the means for estimating and the length of the ST segment for any particular beat is derived making it possible to dynamically alter ST segment length for any given heartrate.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to analyzing electrocardiographic signals within stress testing systems, and more specifically, to estimating the length of ST segments within electrocardiographic signals as the rate at which the heart beats change as a result of exercise. 
     2. Description of the Prior Art 
     Numerous stress test systems exist that test electrocardiographic signals. These systems perform many functions that need to be performed within a stress test system including the construction of a digital representation of the ECG to be analyzed. Conventional stress test systems use a previously defined length for the ST segments in analysis of the digital representation of ECG signals. While this standard length gives some value for an ST segment, it does not provide a realistic estimate of the dynamically changing value of the ST segment. Changes that occur in the ST segment as a result of varying rates of heartbeats will go unnoticed without some method or apparatus for dynamically altering the ST segment to provide a realistic estimate of the length of the ST segment. 
     Is should be apparent from the foregoing discussion that there is a need for a system in which the ST segment is dynamically altered in response to varying rates of heartbeats. The present invention provides such a system. 
     SUMMARY OF THE INVENTION 
     A system is disclosed for estimating ST segments of electrocardiographs in stress test systems. The system requires that a digital representation of electrocardiographic signals be acquired from a human subject. From this digital representation the system calculates an average R to R interval within a predetermined period, usually 8 to 16 beats. The system has stored either a table of predetermined heartbeats or an equation that is derived from such a table by for example a least square fitting routine, that provides an estimated value for ST segments based on the calculated heart rate. By comparing the calculated average R to R interval with the predetermined set of varying heartbeats, an estimates ST value can be extracted from the predetermined set and be used as an estimated value on the digital representation of the electrocardiograph. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a block diagram for the Stress Test System. 
     FIG. 2 is a diagram detailing the determination of the QRS Time of Occurrence. 
     FIG. 3 is an illustration of the basic reference points used in ST measurements. 
     FIG. 4 is a diagram used for ST slope measurements. 
     FIG. 5 is a diagram used to illustrate ST integral measurements. 
     FIG. 6 is a diagram used to illustrate QRS Amplitude and Width Measurements. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     A filtering method and apparatus has been discovered that is useful in correcting the length of ST segments within time varying electrocardiographic signals. The present invention is envisioned as being used within a stress test environment. However, it is also envisioned by the inventor that the generic properties of the present invention have uses beyond the stress testing environment disclosed, herein, as the preferred embodiment. 
     Referring to FIG. 1, the basic stress test system 5 can be seen which requires some means for exercising the subject 2. The preferred embodiment employs treadmill 10 as a means for accelerating the heartbeat of subject 2. The subject 2 has a plurality of electrodes 3 attached which detect bioelectric signals of the heart of subject 2. Electrodes 3 generate analog ECG signals which are converted to digital signals by Patient Input Module (PIM) 6. PIM 6 is located close to subject 2 to avoid signal drop over a long conductor prior conversion to a digital format. 
     Serial data are output from PIM 6 to the computer 20 of the stress test system 5 through Stress Interface Board (SIB) 7 within computer 20 receives the serial data generated by PIM 6. The serial data are transmitted by PIM 6 along with clock, power and ground signals are transmitted by PIM 6 on separate conductors to SIB 20 where they these signals are used by the computer 20 of stress test system 5. The serial data are converted into a parallel format by SIB 7 which also synchronizes the parallel data with clock signals received by SIB 7. The serial data are converted into parallel data by ASIC device 26 which performs the function of a receiver gate array and takes the data and clock from the data/clock interface 22 and converts the serial data into 16 bit parallel words. These 16 bit parallel words are next sent to EPLD device 8 which is an interface circuit between the computer bus 24 and ASIC device 26. EPLD device will interface with the computer 19 to determine the exact times at which data is placed on computer bus 24. SIB 7 has an interface to the treadmill stress system via RS232 interface 34 which exchanges data with treadmill 10 via a serial interface. Patient data 32 within ASIC device 26, and treadmill 10 data are sent to the CPU 21 across computer bus 34 under interrupt control; by interrupt controller 23. 
     The remainin G  basic components of the stress test system consists of CRT 40, laser printer 45, a keyboard 50, trackball 55 and isolation transformer 60. Computer 20 interfaces with peripheral very commonly found within computer workstations. These peripherals are Generally operated under the control of CPU 21, consist of SCSI controller board 44, VGA display controller board 42, CRT 40, hard disc 46, laser printer interface board (hereinafter referred as the &#34;Lasermaster&#34;) 48 Serial/Parallel interface board 17 and stress interface board (SIB) 7. 
     Referring now to FIG. 1, the system shall record each event and assign a time of occurrence that is measured from the time at which the event occurred as measured in milliseconds from the start of the test. The system detects QRS complexes by processing several ECG leads. For each QRS detected, the system shall determine the onset of the QRS and the classification label for the beat. Referrin G  to Fi G . 1, for each QRS detected, the system shall record: 1) the time of occurrence of the beat; 2) the previous R to R interval; 3) average RR interval and 4) beat classification label. Equation 1 below shows the method of calculation used for the previous R to R interval. The previous RR interval shall be determined for all beats detected except the first beat detected after the initiation of testing. 
     
         Previous RR beat.sub.i -T.sub.i -T.sub.j -1                (1) 
    
     The system uses the values calculated for previous RR intervals to determine the average RR interval as shown in equation 2. ##EQU1## 
     Beats are classified in to one of the following classes: 1 normal; 2) supraventricular premature beat; 3) premature ventricular contraction; 4) ventricular escape beat; 5) beat of unknown origin; and 6) not QRS. 
     Rhythm calculations consist of heart rate calculations and arrhythmia event detection are performed by the system as follows. The instantaneous heart rate (IHR) is determined for the previous RR interval as shown in equation 3. ##EQU2## 
     The system then uses the values calculated as previously detailed for average RR interval to calculate average heart rate as shown by equation 4. ##EQU3## 
     Analysis on the filtered data calculates and records information on a beat by beat basis. This information comprises time of occurrence of the beat, the R to R interval of previous beats and an average R to R interval. Beats are then given a classification label as being one of the following classes: (1) normal; (2) supraventricular premature beat (atrial or nodal); (3) premature ventricular contraction; (4) ventricular escape beat; (5) beat of unknown origin; or (6) not QRS. Rhythm/Event Calculation shall compute a value for instantaneous heart rate (IHR) as a function of the previous R to R interval and also calculate a value for average heart rate using the average R to R interval. Rhythm/Event Calculation will then detect and record the arrhythmia events as detailed in Table 1 below. For each of these events the system shall store the start time and the end time of the event. 
     
                       TABLE 1______________________________________Number Event Name    Meaning______________________________________1      Couplet       Two consecutive ventricular                exsystol (VE) beats2      Triplet       Three consecutive VE beats3      Pause         Two beats where the R to R                interval is longer &gt; 2 sec.4      Bigeminy      xVxVxV pattern where V is a                VE beat and x is not.5      Trigeminy     xxVxxVxxV pattern where V is                a VE beat and x is not.6      Ventricular   Four or more consecutive  Tachycardia   VE beats.7      Superventricular                Three or more consecutive  Tachycardia   beats intervals each 20%                permature with a 3 beat                avg. heat rate ≧ 80 bpm.8      Bradycardia   Three or more consecutive                beats at a rate lower than                45 bpm with the three most                recent intervals greater                than 1500 msec.9      Average VE count                A sliding window count of  per minute    VE beats equal to 1 minute                that is updated every second______________________________________ 
    
     For each lead being analyzed in the system, the system shall perform average beat calculation. The average beat is a template of the QRS that is calculated by time aligning and averaging the previous 8 QRS complexes that meet certain criteria. When the system detects a QRS, it shall compare the candidate QRS to the currently stored template. If the candidate QRS is sufficiently similar in shape (as determined by certain criteria internal to the system), the candidate ARS is then used to update the current template. The average QRS shall be stored every 8th time the internal template is updated. The system shall perform the measurements described below on each of the stored average QRS complexes. For each average beat calculated, the system shall measure the following items on all leads being processed by the system: 1) ST level measurements; 2) ST slope measurements; 3) ST integral measurements; 4) ST index measurements; 5) QRS amplitude and duration measurements; 6) average RR interval; and 7) time of average beat calculation. Each of these will be described in more detail below. 
     Referring to FIG. 3, which is an illustration of the basic reference points used for ST measurements, the ST and QRS measurements made on the QRS complexes are illustrated. The points labeled QS and J in FIG. 3 are dynamically determined from all currently active leads by the analysis software. The point labeled QS represents the start of the QRS complex while the point labeled J represents the termination of the QRS complex. The ST level is measured in volts at three different points within the ST segment. The voltage level as measured at J is indicated as STJ. A distance we shall call a from J is a delay determined from the average of the previous R to R intervals as described by equation 5, the point a milliseconds from J is referred to as STA. The value `a` in Equation 5 is an estimation that is arrived at from a least squared fitting routine. ##EQU4## The third voltage level measured along the ST segment is a point N milliseconds from the J point, that is determined by the operator. The voltage level at this point is referred to as STN. 
     A truly novel feature of the present invention is the ability to determine dynamically varying ST voltage levels. Table 2 lists the values for a as calculated for several different heart rates rounded to the nearest millisecond. 
     
                       TABLE 2______________________________________ST Measurements For Several HeartratesHeart Rate   ARR Interval                    Value of &#34;a&#34;(in BPM)     (in msec)   (in msec)______________________________________ 60          1000        76 80          750         60120          500         41160          375         30200          300         23240          250         17______________________________________ 
    
     Table 3 illustrates the measurements made for ST level by the system. In addition to ST level, the system calculates the corrected ST level as indicated. The term QRSA is the amplitude of the QRS complex in millivolts divided by one millivolt, that serves as a correction factor for ECG signal strength. In addition to ST level and corrected ST level for each point, the system shall store the measurement points &#34;a&#34; and &#34;N&#34;. 
     
                       TABLE 3______________________________________ST Level Measurements     MeasurementMeasurement     PointPoint     Determination                 ST Level   Corrected ST______________________________________STJ       J + 0 msec. V.sub.J - V.sub.QS                             ##STR1##STJA      See Equation                 V.sub.STA - V.sub.QS                             ##STR2##STJN      J + N msec. V.sub.STN - V.sub.QS                             ##STR3##______________________________________ 
    
     Referring now to FIG. 4, an illustration of the method used to calculate the slope of the ST segment. There are two points of interest in calculating the slope of the ST segment. The first point is the J point. The voltage value (in millivolts) at the J point is then subtracted from the voltage value at the measurement point (MP). The difference is then divided by the distance between the two points (in seconds) to arrive at the slope. This relationship is shown by equation 6 below in which the ST slop is represented in millivolts per second. ##EQU5## 
     Referring to FIG. 5, which is an illustration of the ST Integral measurement, the ST Integral is measured at three points. The first integral measurement occurs between the horizontal isoelectric line established at the onset of the QRS and the area enclosed by the ECG waveform between the J point and the point labeled C on FIG. 5 Point C is established as either the intersection between the ECG and the isoelectric line or as a predetermined amount of time after the J point if earlier than the intersection with the isoelectric line. 
     The second ST integral is measured between point J and the point &#34;a&#34; as established by equation 5 above. 
     The third measurement is predetermined point &#34;N&#34; which is a fixed value in milliseconds determined by the operator. 
     The ST integral is defined by equation 7 below. ##EQU6## 
     The ST index is defined as the algebraic sum of the ST level in millimeters and the ST slope in millivolts per second. The system shall measure the ST index at two points as indicated in Table 4. 
     
                       TABLE 4______________________________________ST Index MeasurementsMeasurementPoint    Calculation          Meaning______________________________________ST Index A    STA × 10 mm/mv + ST Slope A                         ST Level                         at point AST Index N    STN × 10 mm/mv + ST Slope N                         ST Level                         at point N______________________________________ 
    
     Referring to FIG. 6, Average QRS Amplitude and Duration Measurements are performed by the system as indicated. Table 5 below illustrates the meaning for each the QRSA and the ARSD. 
     
                       TABLE 5______________________________________ST QRS MeasurementsMeasurement     UNITS         MEANING______________________________________QRSA      MILLIVOLTS    AMPLITUDE OF                   THE QRSQRSD      MILLISECONDS  DURATION OF THE                   QRS COMPLEX______________________________________ 
    
     The inventor envisions that the generic properties of the present invention have numerous applications and, while the best mode known to the inventor is disclosed in detail herein, sufficient disclosure is made to enable persons skilled within the arts of stress testing and digital signal processing to employ the invention towards other embodiments which are but obvious improvement over the disclosure of the present invention. These obvious variations would not, therefore, depart from the spirit and scope of the invention as defined in the appended claims.