Roland Wyatt

Limited Lead Selection for Estimation of Body Surface Potential Maps in Electrocardiography

Body surface potential mapping has shown promise as a technique to improve the resolution and accuracy of diagnostic electrocardiography, but the cost and effort required to obtain maps have made wide spread use impractical. As a step toward a practical system, the problems of redundancy and uniqueness of electrocardiographic signal information contained in large numbers of leads were investigated. An algorithm for optimal selection of a limited number of leads was developed. Data obtained from 132 human subjects including some with normal electrocardiograms (ECG) as well as some with abnormal ECGs, were used in the study. Estimation of body surface potentials from limited leads was evaluated using three criteria, including rms error, mean correlation coefficient between limited lead and total lead maps, and error to signal power ratio. Using 30 leads the average rms error was 32 ¿V, average correlation coefficient was .983 and noise to signal power was 3.5% in the presence of 20 ¿V rms noise. Another finding was that optimal sites are not unique, i.e., different sets of optimal sites may be found which perform equally well. This result has practical implications for the design of lead systems for estimating maps on the critically ill and on patients undergoing stress tests.

The sequence of normal ventricular recovery

Abstract Functional refractory periods (FRPs) were measured at epicardial, intramural, and septal sites in pentobarbital anesthetized dogs. The sinus node was crushed and the atria were driven at a fixed rate. Activation times at the test sites were measured from electrograms recorded from closely spaced bipolar electrodes. The test stimuli were cathodal “make” stimuli delivered to one pole of the pairs of electrodes. FRPs at the base of the free wall of the left ventricle and of the septum were shorter than FRPs at the apex. FRPs on the epicardium were shorter than those on the endocardium, and FRPs on the right side of the septum were shorter than those on the left side of the septum. The findings indicate that normal ventricular recovery properties are systematically distributed and inversely related to activation sequence. Areas of the ventricle activated early have the longest FRPs, and areas activated late have the shortest FRPs. This distribution of recovery properties tends to make all portions of the ventricles complete recovery at about the same time, and may play a protective role in the prevention of reentrant arrhythmias. This distribution of recovery properties is also applicable to an explanation of the configuration of normal T waves.

Clinically practical lead systems for improved electrocardiography: comparison with precordial grids and conventional lead systems.

The use of limited leads for estimating total body surface potential distributions was investigated as a practical solution to the problem associated with extensive electrocardiographic sampling used in surface potential mapping. Two practical, limited lead sets of 32 leads each were derived and contrasted to a set of 30 precordial leads similar to those used in ST-segment and QRS mapping for estimating infarct size, and to a set of nine leads simulating those used in conventional 12-lead examinations. The two arrays, one of which excluded posterior sites for use in recumbent patients, showed little difference in ability to estimate 192 lead measured maps (average rms voltage error of 35,V and average correlation coefficient of 0.97). The 30- and 9-lead arrays consistently showed twice the voltage (72,uV) and poorer pattern estimation (average correlation coefficient of 0.91) than either of the 32 lead arrays. These findings indicate the need for 20-35 properly located electrodes for accurate total body surface potential estimation. They also show that there is no difference in the abilities of a 30-lead precordial array and conventional leads to estimate maps

Diagnosis of old inferior myocardial infarction by body surface isopotential mapping

Body surface isopotential maps obtained from 28 patients with old inferior wall myocardial infarction were compared with maps from 120 normal subjects. The 12 lead electrocardiogram of 8 of the 28 patients (29 percent) with inferior wall infarction was normal or showed only nondiagnostic ST-T wave abnormalities at the time the isopotential maps were obtained. In all patients with inferior wall infarction the isopotential map showed a minimum (area of negative potentials) on the inferior or right thoracic surface during the early portions of the QRS complex. This finding was observed in patients with normal or nonspecific abnormalities in the 12 lead electrocardiogram as well as those with QRS abnormalities. By contrast, the minimum during the early QRS complex in normal subjects was located on the right upper back and shoulder region...

The unidentified information content of the electrocardiogram.

DEVELOPMENT of practical recording methods for the human electrocardiogram was rapidly followed by important medical applications. Electrocardiography was established as the major means of classifying disturbances of cardiac rhythm and an important aid in the recognition of myocardial disease including infarction. Applications have been sufficiently significant to result in widespread use of the method and there have been continuing technological improvements and extensions of the methods utility. At the present time and as presently used, electrocardiography is one of the major medical diagnostic methods. Despite considerable utility, it is unlikely that the full medical significance of electrocardiographic examination has been achieved. A variety of theoretic considerations together with supporting experimental and clinical observations suggest that the record may contain information of equal or even greater medical significance than that now obtained. This communication will review some of these considerations and observations. Material will be presented under the headings of regional cardiac examination, prognostic utility, and extended diagnostic applications. None of these represent totally new objectives for electrocardiographic examination but each offers substantial possibilities for improved use of the technique. The material reviewed does not include all areas in which improvements of electrocardiography are likely and should be viewed only as selected examples. Much of the material is necessarily speculative although some theoretic and/or clinical and experimental support for the speculations in each area will be furnished.

Experimental evidence for regional cardiac influence in body surface isopotential maps of dogs.

Isopotential maps based on 192–200 body surface electrocardiograms were obtained for 20 dogs during multiple patterns of ventricular activation. The purposes of the study were to determine whether the cardiac location of events responsible for surface potentials had a recognizable influence on surface potential patterns and to examine the influence of electrical events occurring simultaneously in multiple cardiac regions. Substantially different effects of electrical activity in various cardiac regions on body surface potentials were evidenced by the body surface location of potential maxima and minima and by patterns of isopotential lines during early portions of ventricular excitation initiated at different ventricular sites. Simultaneous stimulation at some sites gave surface potential distributions with multiple extrema. These were demonstrated to be due to effects of the different cardiac regions, because addition of potentials due to stimulation of the individual sites duplicated those associated with simultaneous stimulation of the same sites. It was also shown that body surface locations of maxima and minima are not related in the same manner to the cardiac location of the responsible events when these events are present in single and multiple regions. Slopes of potentials due to events in single cardiac regions were shown to combine with slopes produced by events in other regions to yield maxima or minima at new body surface locations. Results of the study support the possibility of regional cardiac examination by electrocardiograph) but suggest that this will require quantitative descriptions of the details of potential patterns in addition to the location of potential peaks.

The relation of localized myocardial warming to changes in cardiac surface electrograms in dogs.

We studied the relationship of the size and severity of alteration of recovery properties in localized areas to changes in cardiac surface electrograms in experiments on six open-chest dogs. Alterations in recovery properties were induced thermally because size and severity of the affected area could be controlled on the basis of physical principles which were modeled. We recorded unipolar electrograms from 75 cardiac surface sites simultaneously during stimulation of atria and pulmonary conus in control periods and in the presence of warmed areas of varying sizes and intensities. Size of the areas was controlled by the diameter of an aperture through which a light source was directed. Intensity was controlled by the light source excitation voltage. Myocardial temperature was monitored with a thermistor. The QRS, STT, and QRST deflection areas were determined by computer processing and displayed as isoarea maps. Difference maps also were determined by subtracting control QRST isoarea maps from those obtained in the presence of warmed areas. QRST area difference maps were related closely to the size and severity of the thermally induced changes in recovery properties. With areas of the same size and increasing myocardial temperatures, the magnitude of the change in QRST area increased, and the gradient of contour lines between the affected and unaffected areas increased. When myocardial temperature at the center of the warmed area was kept constant and the size of the warmed area was increased, the affected cardiac surface area increased, but the number of isoarea contours remained approximately the same. These findings suggest that the change in QRST isoarea maps may be a useful indicator of lesion size when combined with an index of lesion severity such as the QRST area change in the electrogram with the maximum change. QRST areas during both activation orders were similar, suggesting that the QRST area is independent of changes in activation sequence.

Sequences of early and late phases of repolarization on dog ventricular epicardium

Monophasic action potentials from 32 to 43 epicardial sites were recorded with concentric suction electrodes in 11 dogs during atrial pacing at a constant rate. The duration of action potentials and the sequence of repolarization referenced to the onset of ventricular activation at 50% of the highest plateau amplitude and the end of the action potential were determined. These quantities were correlated with activation sequence determined from action potential upstrokes. On the anterior ventricular surface, both 50% and 100% repolarization sequences were qualitatively similar to activation sequence and showed positive correlation coefficients. On the posterior ventricular surface, 50% and 100% repolarization sequences had little qualitative resemblance to activation sequence and showed lower correlation coefficients. Correlations between the patterns of 100% repolarization and action potential duration were significantly higher on the posterior than the anterior ventricular surface. These findings demonstrate that activation sequence has less influence on repolarization sequence on the posterior than the anterior wall and that action potential duration is the major determinant of repolarization sequence on the posterior wall. These findings extend previous descriptions of the normal sequence of ventricular repolarization and the relative roles of activation and action potential duration in determining that sequence. The study also showed differences between the sequences of 50% and 100% repolarization suggesting different slopes of action potential downstrokes at various ventricular sites with steeper slopes at the posterior basal and upper anterolateral left ventricular wall. This finding adds new detail to previous descriptions of the distribution of intrinsic ventricular recovery properties.

The expression of normal ventricular repolarization in the body surface distribution of T potentials.

SUMMARY Isopotential maps from 120 normal subjects were obtained from 192 simultaneously recorded electrocardiographic leads. Maps were plotted at 1 msec intervals during the QRS and 5 msec intervals during the ST-T deflection. Repetition of QRS features was evident during all but the first few msec of the initial half of serial T maps. This suggests similarities of the normal sequence of ventricular excitation and recovery. Such similarities have been demonstrated by direct studies but are not evident from other electrocardio-graphic examinations. Serial maps during later portions of the T wave showed decreasing intensity of potentials with little change of body surface locations. This also correlates with an established feature of ventricular repolarization, namely that potential difference boundaries with stable locations are widely distributed during part of that process.Findings suggest isopotential maps show features of ventricular recovery not apparent from less extensive examinations.

The electrocardiographic recognition of cardiac states at high risk of ventricular arrhythmias. An experimental study in dogs.

Recognition of states in which the heart is vulnerable to arrhythmia would be a helpful guide to prophylaxis. The possibility of recognizing such states from the ECG is suggested by the already established relations between abnormally disparate recovery to both vulnerability to arrhythmia and ECG waveform. In this study, canine QRS, T, and QRST isoarea maps were determined from ECGs recorded at 192 body sites during control states and conditions of enhanced susceptibility to arrhythmia. Vulnerable states were produced by ouabain intoxication, hypothermia, premature beats, and epinephrine infusion. A hypothetical series of QRST isoarea maps that would be expected to occur without increased local inequalities of recovery was derived by adding the control QRS isoarea map to a fraction (α) of the control T isoarea map and allowing the fraction to vary from α = 1 to α =−1. One QRST isoarea map selected from the derived series was subtracted from a QRST isoarea map during each state of enhanced arrhythmia vulnerability. Derived maps were selected to minimize the average amplitude of the residual maps. RMS values of the residual maps systematically increased with increasing prematurity of depolarization, with time after a toxic injection of a dose of ouabain, with increasing hypothermia, and during the first 3 minutes of epinephrine infusion. Also, the RMS values of the residual maps decreased in hypothermic dogs during rewarming. Our findings suggest that states of vulnerability to arrhythmia due to increased disparity of recovery can be identified by analysis of ECG waveforms recorded from lead systems sensitive to electrical activity in local cardiac regions.