- Email: [email protected]
The phase-plane cardiogram: preliminary findings on a healthy population
The phase-plane cardiogram: on a healthy population
preliminary
findings
Joel S. Colton, Ph.D.* D. Eugene Lovelace, B.Sc. J. Daniel Davis, H.A.B. Gary J. Anderson, M.D. Alan R. Freeman, Ph.D. Indianapolis, Xnd.
The phase-plane cardiogram (PPC), the x-y display of voltage against its corresponding time derivative (dv/dtl, was first introduced by Freeman and co-workers.3 These preliminary studies indicated significant differences between the PPC’s of healthy individuals and those of individuals with left ventricular hypertrophy. Further studies, by this laboratory, have shown that the PPC can significantly enhance the slurring (a slope change of the primary voltage excursion) and notching (a momentary directional change of the primary voltage excursion) patterns inherent in the QRS complex of the electrocardiogram (ECG) wave-form.sg4 Notching and slurring of the ECG have been correlated with myocardial infarction, myocardial fibrosis, and certain intraventricular conduction defects13 2* 5, 7-9,I1 However, little emphasis has been placed on the notching and slurring patterns during routine clinical cardiographic interpretation. One reason is that such events occur too rapidly or without sufficient magnitude to be consistently reproduced on the standard ECG. The PPC has the ability to enhance these small changes, therefore facilitating cardiographic analysis. Characterization of the notching and slurring patterns could thus increase the diagnostic capability of the ECG. From the Departments of Psychiatry, Physiology, and Medicine, Indiana University School of Medicine, Indianapolis. Received for publication July 11, 1973. Supported by The Indiana Heart Association Grant No. 5’7-669-40. Reprint requests Dr. Alan R. Freemen, Institute of Psychiatric Research, Indiana University School of Medicine, 1100 W. Michigan St., Indianapolis, Ind. 46202. *National Institutes of Mental Health Postdoctoral Research Fellow.
May, 1974, Vol. 67, No. 5, pp. 619-626
For the characterization of abnormal PPC patterns, it is essential to be able to differentiate between abnormal and normal patterns. In order to effectively separate these patterns, it is important to establish a reliable “normal” data base with which comparisons can be made. A reliable normal data base requires data collection from a large population of healthy individuals and the capability of analyzing this data. Thus, the preliminary findings on a normal population and the methods of analyzing these data are being reported Methods
The PPC technique utilized the standard 12lead configuration for signal input. The patient input signal was determined with a lead selector switch (Harvard Apparatus Co.1 coupled to the input follower stage of a low-level preamplifier (Tektronix FM 122). The FM 122 provided a gain of approximately 1,000 with upper- and lowerfrequency filters. With the upper-frequency filter set at 250 Hz the upper-frequency response was 3 db. down at 450 Hz and 6 db. down at 1,000 Hz. The lower-frequency cutoff was set to 0.07 Hz. This band width was less than that encountered in high-frequency electrocardiography (HFE) (i.e., HFE usually better than flat to 1,000 Hz). The band width selected for the PPC allowed adequate signal reproduction with an optimal signalto-noise ratio. It may be noted, parenthetically, that high-frequency interference and noise becomes a serious problem since the derivative enhances the higher frequency signals (viz. dv/ dt = V,,oCos,where W = 2w 0. The PPC and the standard scalar ECG were American Heart Journal
619
Colton et al.
Fig. 1. Overall
arrangement
displayed on channels 1 and 2, respectively, of a dual-trace oscilloscope (Tektronix RM561A). Channel 1 of the RM561A was used as an x-y plotter by utilizing two dual-trace amplifiers (Tektronix Type 3A72), i.e., one in the horizontal (x1 axis and one in the vertical (y> axis. The PPC was obtained by placing the input voltage (V) on the vertical axis and the first derivative of the voltage (dv/dt) on the horizontal axis. The first derivative was obtained by using the negative channel of a differential amplifier (Tektronix Type 2A63) as a differentiating feedback amplifier. Calibration of the PPC was performed as as previously described.3 The data was collected in several forms: (1) Standard scalar ECG recordings were made on a strip-chart recorder (Brush 220) as part of the clinical workup. (2) Both the PPC and ECG signals were recorded from the oscilloscope screen with 35 mm. high-speed film. The developed film was placed in a microfilm reader and the data were analyzed by superimposition. This procedure was used primarily for the verification of the loops reconstructed by the computer. (3) The amplified and filtered input voltage signal was stored on one channel of an FM tape recorder (Sangamo 35001. On a second channel of the FM tape recorder a 1.34 V. rectangular pulse was synchronized with the start and stop of ECG data collection. This signal was used as a lead-change indicator in the PDPS digital computer. For computer analysis, the recorded ECG was played into a hybrid computer system as shown in Fig. 1. The ECG first entered the EAI-TR20 analog computer and the signal scaled so as to be within the range of the analog-to-digital (A/D) converter ( f 1.4 V.I. The A/D converter sampled the output of the TR20 at a rate of 4,000 samples per second. These samples were converted into a 12bit digital equivalent for processing by the PDP8 digital computer. The digital signal was then
620
for computer
analysis
of data.
stored on a digital tape. Whenever a lead change was indicated by channel 2, the A/D converter was automatically turned off and the digital computer signalled an end of group character on the digital tape. When the lead-change trigger returned to threshold level, the A/D converter began sampling the new lead. This process was repeated until the entire 12 leads of the patient were complete. The end of patient data was indicated by an end-of-file character on the digital tape. Additional processing was performed on a CDC 6600 computer. An average PPC loop, of 20 consecutive QRS complexes, was determined for each lead of each patient. The determination of the average PPC was accomplished by introducing a one-second segment of digital tape into the CDC 6600 digital computer. This segment was scanned for a QRS complex, Fig. 2, Step 1. The .giterion used to locate the QRS complex was the maximum negative secant slope in excess of a threshold level. The threshold level was set by the computer at 75 per cent of the maximum negative secant slope encountered in the first 3 seconds of data. The relative position of the QRS complex was noted. A window 0.25 second wide and centered about the maximum negative secant point, P,, became the reference QRS complex as indicated in Fig. 2, Step 2. The input digital signal was again read into a buffer and the point of repeatability determined based on the autocorrelation function and demonstrated in Fig. 2, Step 3. The reference QRS complex was then averaged with this isolated QRS complex. This process, Steps 3 and 4, was repeated-with appropriate weighting factors on the averaging step-until a limiting number of beats had been reached. After some experimentation, it was determined that 30 such complexes were sufficient. The time of initiation and termination (points P, and P,, respectively, Fig. 2) of the QRS com-
May, 1974, Vol. 8 7, No. 5
The phase-plane cardiogram
Digitized
Signal
Step
1: Reference Point
Step
2: Reference QRS Complex
step
3: Isolate Next Reference QR.S Complex
step
4: update Reference Complex
Less Than Limit f
Step
6: Average PPC
Step
Fig. 2. Analytical
5: Determine Initial and Final Points In Time
QRS
scheme for computer data analysis.
Fig. 3. A, PPC loop reproduced from oscilloscope screen. B, reproduction of conventional ECG. Evidence of notching and slurring can easily be seen on the PPC when the scalar ECG shows little or no change. Labels A through D repreeent corresponding points on both the PPC and scalar ECG (see text). The central heavy black dot is produced by the P- and T-waves. This is due to their relatively low velocity and voltage amplitude with respect to the QRS complex. The recording was made from Lead I.
plex was chosen so as to remove the high intensity of inner loops (low-velocity P- and T-waves) evident in the photographic techniques. The derivative was determined from the least-square quadratic function fitted through 5 data points and the average PPC was plotted using a Calcomp plotter. The average PPC data was then saved on a digital tape for patient-to-patient analysis. Results
As demonstrated from the PPC reproduced from 35 mm. film (Fig. 3, A), evidence of notching and slurring can easily be seen on the PPC when the scalar ECG (Fig. 3, B) shows little or no change. Beginning with the central heavy black American Heart Journal
dot (the isoelectric point) in Fig. 3,A, the trace proceeds in a counterclockwise fashion. The isoelectric point corresponds to the origin of the traditional algebraic X-Y coordinates. Any slurring on the scalar ECG appears as notching on the PPC and notching on the ECG appears as a secondary loop within the major PPC loop. The initial trace travels upward and to the right, thus indicating vertically a positive increase in voltage and horizontally a positive increase in velocity (dv/dt). The first derivative (dv/dt) momentarily reverses direction as the voltage signal continues in a positive direction, thus defining a notch (point A). This portion of the trace corresponds to the rising phase of the R-wave on the standard ECG. Proceeding further along the ris-
621
Colton
et al.
Fig. 4. Reconstruction of the PPC loop by computer from data recorded on magnetic tape. The similarity between details seen in the PPC loop obtained with 36 mm. film (Fig. 1 A) and the computer reconstruction demonstrates the high degree of resolution that can be obtained with the computer. The Pand T-waves are not included in the computer presentation, therefore obviating the central dense region.
Fig. 5. Superimposition of four consecutive beats from a single lead (Lead I) demonstrates the beat-to-beat repeatability of loop contour.
ing phase of the PPC, the derivative rapidly passes through zero, reverses, and continues in a positive voltage and velocity direction (A’). This defines a secondary loop within the major PPC loop (notch in standard ECG). The first derivative then falls to zero as the trace passes directly above the isoelectric point (point B). This portion of the trace corresponds to the zenith of the Rwave. The trace proceeds downward and to the left producing a series of notches of different velocities. The velocity of the falling phase slows as the voltage approaches the isoelectric line (point Cl. The trace then continues toward the maximum negative voltage where dv/dt again 622
Fig. 6. Computer average of 20 consecutive beats from a single lead (same subject and lead as that shown in Fig. 31. The averaging procedure eliminates slight beat-to-beat variations without changing the major notching patterns.
equals zero
The phase-plane cardiogram
AVR
AVL
Fig. 7. PPC loops for Leads I through aVF. Each loop represents the average of 20 consecutive beats from a single individual.
averaged PPC loops from all 12 leads are presented in Figs. 7 and 8. A common beat-to-beat variation, more in loop size than in loop contour, was often notable. This variation is attributable to the effects of respiration, and is a phenomenon commonly observed in the standard ECG (Fig. 91. By normalizing the loops to a unitary magnitude it is possible to minimize the respiration effects. Thus the averaging of 30 consecutive loops, without significantly affecting the notching and sublooping patterns, is possible. PPC’s were repeated on 8 subjects after intervals ranging from 6 days to 2.5 months. All cases demonstrated consistent reproducibility for the time interval studied. This observation strongly suggested that the notching patterns’were of a real nature and were not due to day-to-day recording artifacts. Consistency of the notching pattern has also been demonstrated to some degree on a personto-person basis within 1 lead Films of Lead V, from 9 female volunteer subjects were randomly selected from the age group of 18 to 24 years. American Heart Journal
These were then put on an enlarger and superimposed tracings were made. Two groups’ were noted, and a normalized loop for each was constructed by visually averaging the superimposed PPC loops (Fig. 101. Each independent loop shows minor individual variations. Each composite represents a general pattern of the more prominent, common notches. Group I demonstrates a voltage excursion above the isoelectric point, a prominent negative dv/dt, bound by 2 common notches on the falling phase and a moderately notched rising phase of relatively low positive dv/ dt. Group I also shows, in 2 of the 5 traces, a small loop at the point of maximum negative voltage, Group II shows no voltage excursion above baseline, a highly notched falling phase of moderate velocity, and a prominent positive dv/ dt excursion with 2 characteristic notches on the rising phase. Discussion
The X-Y display of voltage against its corresponding time derivative had been previously used in this laboratory and by a number of other 623
Colton et al.
8. PPC loops for Leads VI through Vg. Each loop represents the average of 20 consecutive beats from the same individual as Fig. ‘7.
Fig.
investigator& lo, I3 as a method for observing time-dependent processes (e.g., membrane potential changes and ionic conductances) in single cells. Since the ECG is a time-dependent process it was believed that phase-plane analysis might reveal clinically significant information. Preliminary studies utilizing this technique indicated that the PPC significantly enhanced the slurring and notching patterns inherent in the QRS complex of the ECG wave-form.3, 4 Slurring and, to a lesser degree, notching have been shown to occur in the ECG’s of normal individuals.**9*12~ 14However, with an increase in the amount of pathologic myocardial involvement the amount of notching and slurring was shown to increase and thus to have diagnostic signiflcance.2.7~8~9~1Ll2814 Evans and McRae’ have noted that notching on the downstroke of the R-wave can be a lone “indicating myocardial cardiographic finding damage and cardiac infarction in a patient with chest pain.” Weinberg and co-workers14 have demonstrated that marked notching and slur624
ring of the QRS complex was evident in 31.7 per cent of the individuals shown to have myocardial fibrosis at autopsy. The authors also indicated that the notching and slurring of the QRS complex could occur normally in Lead III or near the isoelectric line in the other leads. However, when the notching or slurring was found near the apex of the QRS complex, in multiple leads, or in large numbers, it was considered abnormal and reflected intrinsic myocardial damage. With the use of high-frequency electrocardiography (HFE), Langner and co-worker& 9 observed that patients with myocardial infarction showed a significant increase in the incidence of high-frequency notching and slurring as compared to normal subjects. These investigators also noted that approximately 40 per cent of the subjects with angina pectoris showed an abnormal increase in high-frequency notching and slurring patterns, whereas the conventional RCG indicated no significant changes. In a comparative study of normal subjects and patients with myocardial disease, Reynolds and co-workers12 May, 1974, Vol. 87, No. 5
The phase-plane cardiogram
also observed that the high-frequency notching increased with pathologic myocardial involvement. The PPC has the ability to enhance the slurring and notching patterns of the standard ECG. This was consistently demonstrated by the data from 100 normal subjects. In addition, it was seen that repeatable notching patterns occur from beat-to-beat within the same subject. This, plus the fact that loop contours were consistently repeatable within subjects over a time period of at least 2% months, indicated that the notching patterns were of a physiologic origin and not due to day-to-day artifacts. The PPC notching and sublooping patterns represent electrical events such as changes in the relative magnitude and direction of the mean electrical vector resulting from the complex spread of depolarization through the myocardium. According to Langner and co-workers,8, g multiple notching and slurring in the QRS complex is probably a result of a mosaic of interspersed electrically active and inert myocardial tissue. Such small areas in the normal heart could result in the notching and sublooping patterns observed in the PPC. The ability to classify the PPC patterns of healthy individuals into groups demonstrates that the minor notching and sublooping patterns of each individual do not obscure the major loop configuration and thus allows the averaging of PPC loops from patient-to-patient. Such averaged population loops will permit the establishment of a reliable normal data base with which comparisons to abnormal populations can be made. Summary
A method for increasing the diagnostic capability of the clinical electrocardiogram has been further developed. The coordinated display of voltage against the time derivative of voltage (dv/dt), i.e., PPC, was found to be remarkably sensitive to subtle aberrations in QRS contours not easily visualized in the standard electrocardiographic portrayal of voltage against time. A standard twelve-lead electrocardiogram and phase-plane loops were displayed and photographed on an oscillographic recorder, the latter by placing voltage (V) on the vertical axis and the first time derivative of voltage on the horizontal axis. Data storage on magnetic tape and comAmerican Heart Journal
Fig. 9. Photograph of two consecutive PPC loops from Lead Vs, demonstrating respiration artifact. Outer loop inspiration, inner loop expiration.
10. Top, superimposed tracings of PPC loops, Lead VI taken from 9 subjects (female, ages 18 to 24 years) and divided into 2 groups. Bottom, visual composites of superimposed loops, illustrating major, common notching patterns. Fig.
puter analysis of the data were also carried out. Data from 100 “normal” (as determined from clinical ECG, history, and physical examination) subjects demonstrated evident, repeatable notching patterns in the PPC. The repeatability of these patterns was demonstrated from beat-tobeat in each lead of every individual, and was found to be consistent even when the readings were taken over long periods of time. The PPC’s of different individuals showed consistent common notching patterns which would indicate that anatomical and physiological bases exist to explain this phenomenon.
02s
Colton et al.
REFERENCES
1. Evans, W., and McRae, C.: The lesser electrocardiographic signs of cardiac pain, Br. Heart J. 14429, 1952. 2. Flowers, N. C., and Horan, L. G.: Diagnostic import of QR+9 notching in high-frequency electrocardiograms of living subjects with heart disease, Circulation 64:606, 1971. 3. Freeman, A. R., Berkoben, J. P., Stein, L. A., Tolbert, J., and Wilson. W. S.: A new annroach to clinical electrocardiography: the phase-blane cardiogram, AM. HEART J. 82:664,1971. 4. Freeman, A. It., Colton, J., and Grosz., H. J.: A new method of electrocardiographic recordings; namely the phase-plane cardimam, European Congress of Cardiology, 1972, Abstracts. 5. Holcroft, J. W., and Liebman, J.: Notching of the QRS complex in high-frequency electrocardiograms of normal children and in children with rheumatic fever, J. Electrocardiol. 3133, 1970. 6. Jenerick, N.: Phase-plane trajectories of the muscle snike notentials. Bionhvs. J. 3:363. 1963. 7. iangner, P. H., jr., ini Geselowit~ D. B.: First derivative of the electrocardiogram, Circulation 10:220,1962. a. Langner, P. H., Jr., Geselowitz, D. B., and Mansure, F. T.: High-frequency components in the electrocar-
826
9.
10.
11. 12.
13. 14.
diograms of normal subjects and of patients with coronary heart disease, AM. HEART J. 82:746,1961. Langner, P. H., Jr.: Further studies in high-fidelity electrocardiography: myocardial infarction, Circulation 8: 906,1953. Morelock, N. L., Benamy, D. A., and Grundfest, H.: Analysis of spike electrogenesia of eel electroplaques with phase-plane and impedance measurements, J. Gen. Physiol. 82:22, 1966. Oppenheimer, B. S., and Rothschild, M. A.: Electrocardiographic changes associated with myocardial involvement, JAMA 69:429,1917. Reynolds, E. W., Jr., Muller, B. F., Anderson, G. J., and Muller, B. T.: High-frequency components in the electrocardiogram: a comparative study of normals and patients with myocardial disease, Circulation 36:195, 1967. Sperelakis, N., and Shumaker, H. K.: Phase-plane analysis of cardiac action potentials, J. Electrccardiol. 1:31, 1968. Weinberg, S. L., Reynolds, R. W., Rosenman, R. H., and Katz, L. N.: Electrocardiographic changes associated with patchy myocardial fibrosis in the absence of confluent myocardial infarction, AM. HEART J. -745, 1950.
May, 1974, Vol. 87, No. 5
preliminary
findings
Joel S. Colton, Ph.D.* D. Eugene Lovelace, B.Sc. J. Daniel Davis, H.A.B. Gary J. Anderson, M.D. Alan R. Freeman, Ph.D. Indianapolis, Xnd.
The phase-plane cardiogram (PPC), the x-y display of voltage against its corresponding time derivative (dv/dtl, was first introduced by Freeman and co-workers.3 These preliminary studies indicated significant differences between the PPC’s of healthy individuals and those of individuals with left ventricular hypertrophy. Further studies, by this laboratory, have shown that the PPC can significantly enhance the slurring (a slope change of the primary voltage excursion) and notching (a momentary directional change of the primary voltage excursion) patterns inherent in the QRS complex of the electrocardiogram (ECG) wave-form.sg4 Notching and slurring of the ECG have been correlated with myocardial infarction, myocardial fibrosis, and certain intraventricular conduction defects13 2* 5, 7-9,I1 However, little emphasis has been placed on the notching and slurring patterns during routine clinical cardiographic interpretation. One reason is that such events occur too rapidly or without sufficient magnitude to be consistently reproduced on the standard ECG. The PPC has the ability to enhance these small changes, therefore facilitating cardiographic analysis. Characterization of the notching and slurring patterns could thus increase the diagnostic capability of the ECG. From the Departments of Psychiatry, Physiology, and Medicine, Indiana University School of Medicine, Indianapolis. Received for publication July 11, 1973. Supported by The Indiana Heart Association Grant No. 5’7-669-40. Reprint requests Dr. Alan R. Freemen, Institute of Psychiatric Research, Indiana University School of Medicine, 1100 W. Michigan St., Indianapolis, Ind. 46202. *National Institutes of Mental Health Postdoctoral Research Fellow.
May, 1974, Vol. 67, No. 5, pp. 619-626
For the characterization of abnormal PPC patterns, it is essential to be able to differentiate between abnormal and normal patterns. In order to effectively separate these patterns, it is important to establish a reliable “normal” data base with which comparisons can be made. A reliable normal data base requires data collection from a large population of healthy individuals and the capability of analyzing this data. Thus, the preliminary findings on a normal population and the methods of analyzing these data are being reported Methods
The PPC technique utilized the standard 12lead configuration for signal input. The patient input signal was determined with a lead selector switch (Harvard Apparatus Co.1 coupled to the input follower stage of a low-level preamplifier (Tektronix FM 122). The FM 122 provided a gain of approximately 1,000 with upper- and lowerfrequency filters. With the upper-frequency filter set at 250 Hz the upper-frequency response was 3 db. down at 450 Hz and 6 db. down at 1,000 Hz. The lower-frequency cutoff was set to 0.07 Hz. This band width was less than that encountered in high-frequency electrocardiography (HFE) (i.e., HFE usually better than flat to 1,000 Hz). The band width selected for the PPC allowed adequate signal reproduction with an optimal signalto-noise ratio. It may be noted, parenthetically, that high-frequency interference and noise becomes a serious problem since the derivative enhances the higher frequency signals (viz. dv/ dt = V,,oCos,where W = 2w 0. The PPC and the standard scalar ECG were American Heart Journal
619
Colton et al.
Fig. 1. Overall
arrangement
displayed on channels 1 and 2, respectively, of a dual-trace oscilloscope (Tektronix RM561A). Channel 1 of the RM561A was used as an x-y plotter by utilizing two dual-trace amplifiers (Tektronix Type 3A72), i.e., one in the horizontal (x1 axis and one in the vertical (y> axis. The PPC was obtained by placing the input voltage (V) on the vertical axis and the first derivative of the voltage (dv/dt) on the horizontal axis. The first derivative was obtained by using the negative channel of a differential amplifier (Tektronix Type 2A63) as a differentiating feedback amplifier. Calibration of the PPC was performed as as previously described.3 The data was collected in several forms: (1) Standard scalar ECG recordings were made on a strip-chart recorder (Brush 220) as part of the clinical workup. (2) Both the PPC and ECG signals were recorded from the oscilloscope screen with 35 mm. high-speed film. The developed film was placed in a microfilm reader and the data were analyzed by superimposition. This procedure was used primarily for the verification of the loops reconstructed by the computer. (3) The amplified and filtered input voltage signal was stored on one channel of an FM tape recorder (Sangamo 35001. On a second channel of the FM tape recorder a 1.34 V. rectangular pulse was synchronized with the start and stop of ECG data collection. This signal was used as a lead-change indicator in the PDPS digital computer. For computer analysis, the recorded ECG was played into a hybrid computer system as shown in Fig. 1. The ECG first entered the EAI-TR20 analog computer and the signal scaled so as to be within the range of the analog-to-digital (A/D) converter ( f 1.4 V.I. The A/D converter sampled the output of the TR20 at a rate of 4,000 samples per second. These samples were converted into a 12bit digital equivalent for processing by the PDP8 digital computer. The digital signal was then
620
for computer
analysis
of data.
stored on a digital tape. Whenever a lead change was indicated by channel 2, the A/D converter was automatically turned off and the digital computer signalled an end of group character on the digital tape. When the lead-change trigger returned to threshold level, the A/D converter began sampling the new lead. This process was repeated until the entire 12 leads of the patient were complete. The end of patient data was indicated by an end-of-file character on the digital tape. Additional processing was performed on a CDC 6600 computer. An average PPC loop, of 20 consecutive QRS complexes, was determined for each lead of each patient. The determination of the average PPC was accomplished by introducing a one-second segment of digital tape into the CDC 6600 digital computer. This segment was scanned for a QRS complex, Fig. 2, Step 1. The .giterion used to locate the QRS complex was the maximum negative secant slope in excess of a threshold level. The threshold level was set by the computer at 75 per cent of the maximum negative secant slope encountered in the first 3 seconds of data. The relative position of the QRS complex was noted. A window 0.25 second wide and centered about the maximum negative secant point, P,, became the reference QRS complex as indicated in Fig. 2, Step 2. The input digital signal was again read into a buffer and the point of repeatability determined based on the autocorrelation function and demonstrated in Fig. 2, Step 3. The reference QRS complex was then averaged with this isolated QRS complex. This process, Steps 3 and 4, was repeated-with appropriate weighting factors on the averaging step-until a limiting number of beats had been reached. After some experimentation, it was determined that 30 such complexes were sufficient. The time of initiation and termination (points P, and P,, respectively, Fig. 2) of the QRS com-
May, 1974, Vol. 8 7, No. 5
The phase-plane cardiogram
Digitized
Signal
Step
1: Reference Point
Step
2: Reference QRS Complex
step
3: Isolate Next Reference QR.S Complex
step
4: update Reference Complex
Less Than Limit f
Step
6: Average PPC
Step
Fig. 2. Analytical
5: Determine Initial and Final Points In Time
QRS
scheme for computer data analysis.
Fig. 3. A, PPC loop reproduced from oscilloscope screen. B, reproduction of conventional ECG. Evidence of notching and slurring can easily be seen on the PPC when the scalar ECG shows little or no change. Labels A through D repreeent corresponding points on both the PPC and scalar ECG (see text). The central heavy black dot is produced by the P- and T-waves. This is due to their relatively low velocity and voltage amplitude with respect to the QRS complex. The recording was made from Lead I.
plex was chosen so as to remove the high intensity of inner loops (low-velocity P- and T-waves) evident in the photographic techniques. The derivative was determined from the least-square quadratic function fitted through 5 data points and the average PPC was plotted using a Calcomp plotter. The average PPC data was then saved on a digital tape for patient-to-patient analysis. Results
As demonstrated from the PPC reproduced from 35 mm. film (Fig. 3, A), evidence of notching and slurring can easily be seen on the PPC when the scalar ECG (Fig. 3, B) shows little or no change. Beginning with the central heavy black American Heart Journal
dot (the isoelectric point) in Fig. 3,A, the trace proceeds in a counterclockwise fashion. The isoelectric point corresponds to the origin of the traditional algebraic X-Y coordinates. Any slurring on the scalar ECG appears as notching on the PPC and notching on the ECG appears as a secondary loop within the major PPC loop. The initial trace travels upward and to the right, thus indicating vertically a positive increase in voltage and horizontally a positive increase in velocity (dv/dt). The first derivative (dv/dt) momentarily reverses direction as the voltage signal continues in a positive direction, thus defining a notch (point A). This portion of the trace corresponds to the rising phase of the R-wave on the standard ECG. Proceeding further along the ris-
621
Colton
et al.
Fig. 4. Reconstruction of the PPC loop by computer from data recorded on magnetic tape. The similarity between details seen in the PPC loop obtained with 36 mm. film (Fig. 1 A) and the computer reconstruction demonstrates the high degree of resolution that can be obtained with the computer. The Pand T-waves are not included in the computer presentation, therefore obviating the central dense region.
Fig. 5. Superimposition of four consecutive beats from a single lead (Lead I) demonstrates the beat-to-beat repeatability of loop contour.
ing phase of the PPC, the derivative rapidly passes through zero, reverses, and continues in a positive voltage and velocity direction (A’). This defines a secondary loop within the major PPC loop (notch in standard ECG). The first derivative then falls to zero as the trace passes directly above the isoelectric point (point B). This portion of the trace corresponds to the zenith of the Rwave. The trace proceeds downward and to the left producing a series of notches of different velocities. The velocity of the falling phase slows as the voltage approaches the isoelectric line (point Cl. The trace then continues toward the maximum negative voltage where dv/dt again 622
Fig. 6. Computer average of 20 consecutive beats from a single lead (same subject and lead as that shown in Fig. 31. The averaging procedure eliminates slight beat-to-beat variations without changing the major notching patterns.
equals zero
The phase-plane cardiogram
AVR
AVL
Fig. 7. PPC loops for Leads I through aVF. Each loop represents the average of 20 consecutive beats from a single individual.
averaged PPC loops from all 12 leads are presented in Figs. 7 and 8. A common beat-to-beat variation, more in loop size than in loop contour, was often notable. This variation is attributable to the effects of respiration, and is a phenomenon commonly observed in the standard ECG (Fig. 91. By normalizing the loops to a unitary magnitude it is possible to minimize the respiration effects. Thus the averaging of 30 consecutive loops, without significantly affecting the notching and sublooping patterns, is possible. PPC’s were repeated on 8 subjects after intervals ranging from 6 days to 2.5 months. All cases demonstrated consistent reproducibility for the time interval studied. This observation strongly suggested that the notching patterns’were of a real nature and were not due to day-to-day recording artifacts. Consistency of the notching pattern has also been demonstrated to some degree on a personto-person basis within 1 lead Films of Lead V, from 9 female volunteer subjects were randomly selected from the age group of 18 to 24 years. American Heart Journal
These were then put on an enlarger and superimposed tracings were made. Two groups’ were noted, and a normalized loop for each was constructed by visually averaging the superimposed PPC loops (Fig. 101. Each independent loop shows minor individual variations. Each composite represents a general pattern of the more prominent, common notches. Group I demonstrates a voltage excursion above the isoelectric point, a prominent negative dv/dt, bound by 2 common notches on the falling phase and a moderately notched rising phase of relatively low positive dv/ dt. Group I also shows, in 2 of the 5 traces, a small loop at the point of maximum negative voltage, Group II shows no voltage excursion above baseline, a highly notched falling phase of moderate velocity, and a prominent positive dv/ dt excursion with 2 characteristic notches on the rising phase. Discussion
The X-Y display of voltage against its corresponding time derivative had been previously used in this laboratory and by a number of other 623
Colton et al.
8. PPC loops for Leads VI through Vg. Each loop represents the average of 20 consecutive beats from the same individual as Fig. ‘7.
Fig.
investigator& lo, I3 as a method for observing time-dependent processes (e.g., membrane potential changes and ionic conductances) in single cells. Since the ECG is a time-dependent process it was believed that phase-plane analysis might reveal clinically significant information. Preliminary studies utilizing this technique indicated that the PPC significantly enhanced the slurring and notching patterns inherent in the QRS complex of the ECG wave-form.3, 4 Slurring and, to a lesser degree, notching have been shown to occur in the ECG’s of normal individuals.**9*12~ 14However, with an increase in the amount of pathologic myocardial involvement the amount of notching and slurring was shown to increase and thus to have diagnostic signiflcance.2.7~8~9~1Ll2814 Evans and McRae’ have noted that notching on the downstroke of the R-wave can be a lone “indicating myocardial cardiographic finding damage and cardiac infarction in a patient with chest pain.” Weinberg and co-workers14 have demonstrated that marked notching and slur624
ring of the QRS complex was evident in 31.7 per cent of the individuals shown to have myocardial fibrosis at autopsy. The authors also indicated that the notching and slurring of the QRS complex could occur normally in Lead III or near the isoelectric line in the other leads. However, when the notching or slurring was found near the apex of the QRS complex, in multiple leads, or in large numbers, it was considered abnormal and reflected intrinsic myocardial damage. With the use of high-frequency electrocardiography (HFE), Langner and co-worker& 9 observed that patients with myocardial infarction showed a significant increase in the incidence of high-frequency notching and slurring as compared to normal subjects. These investigators also noted that approximately 40 per cent of the subjects with angina pectoris showed an abnormal increase in high-frequency notching and slurring patterns, whereas the conventional RCG indicated no significant changes. In a comparative study of normal subjects and patients with myocardial disease, Reynolds and co-workers12 May, 1974, Vol. 87, No. 5
The phase-plane cardiogram
also observed that the high-frequency notching increased with pathologic myocardial involvement. The PPC has the ability to enhance the slurring and notching patterns of the standard ECG. This was consistently demonstrated by the data from 100 normal subjects. In addition, it was seen that repeatable notching patterns occur from beat-to-beat within the same subject. This, plus the fact that loop contours were consistently repeatable within subjects over a time period of at least 2% months, indicated that the notching patterns were of a physiologic origin and not due to day-to-day artifacts. The PPC notching and sublooping patterns represent electrical events such as changes in the relative magnitude and direction of the mean electrical vector resulting from the complex spread of depolarization through the myocardium. According to Langner and co-workers,8, g multiple notching and slurring in the QRS complex is probably a result of a mosaic of interspersed electrically active and inert myocardial tissue. Such small areas in the normal heart could result in the notching and sublooping patterns observed in the PPC. The ability to classify the PPC patterns of healthy individuals into groups demonstrates that the minor notching and sublooping patterns of each individual do not obscure the major loop configuration and thus allows the averaging of PPC loops from patient-to-patient. Such averaged population loops will permit the establishment of a reliable normal data base with which comparisons to abnormal populations can be made. Summary
A method for increasing the diagnostic capability of the clinical electrocardiogram has been further developed. The coordinated display of voltage against the time derivative of voltage (dv/dt), i.e., PPC, was found to be remarkably sensitive to subtle aberrations in QRS contours not easily visualized in the standard electrocardiographic portrayal of voltage against time. A standard twelve-lead electrocardiogram and phase-plane loops were displayed and photographed on an oscillographic recorder, the latter by placing voltage (V) on the vertical axis and the first time derivative of voltage on the horizontal axis. Data storage on magnetic tape and comAmerican Heart Journal
Fig. 9. Photograph of two consecutive PPC loops from Lead Vs, demonstrating respiration artifact. Outer loop inspiration, inner loop expiration.
10. Top, superimposed tracings of PPC loops, Lead VI taken from 9 subjects (female, ages 18 to 24 years) and divided into 2 groups. Bottom, visual composites of superimposed loops, illustrating major, common notching patterns. Fig.
puter analysis of the data were also carried out. Data from 100 “normal” (as determined from clinical ECG, history, and physical examination) subjects demonstrated evident, repeatable notching patterns in the PPC. The repeatability of these patterns was demonstrated from beat-tobeat in each lead of every individual, and was found to be consistent even when the readings were taken over long periods of time. The PPC’s of different individuals showed consistent common notching patterns which would indicate that anatomical and physiological bases exist to explain this phenomenon.
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Colton et al.
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May, 1974, Vol. 87, No. 5