Effects of hyperkalemia on local changes of repolarization duration in canine left ventricle.

The effect of hyperkalemia on the distribution of ventricular repolarization properties was investigated using the interval between minimum QRS and maximum T derivatives of multiple electrograms as a measure of local repolarization duration. Local repolarization duration decreased, with more marked reduction at the epicardium than endocardium near the base, and greater decrease at the endocardium than epicardium near the apex. The nonhomogeneous reduction of repolarization duration helps account for T waveform changes due to hyperkalemia and may be a factor in the occurrence of re-entrant ventricular arrhythmias.

Relation of canine atrial activation sequence to anatomic landmarks.

Activation sequence in the atria was investigated in 35 dogs. The atria were studied as four regions, and activation sequence in one region was determined in each experiment. In each region 60 electrograms were recorded simultaneously from bipolar electrodes. The maximal first derivative of the electrograms was taken as activation time. Dried atrial specimens, which permitted identification of pectinate muscles, crista terminalis, and the axis of fiber direction, were prepared. Nonuniform activation was demonstrated with more rapid conduction over the long axis of fiber direction and in Bachmann's bundle, crista terminalis, and pectinate muscles. These regions of rapid conduction were the same during sinus rhythm and ectopic and retrograde activation. Findings confirm the presence of paths with relatively rapid conduction in the atria and demonstrate that these are related to gross anatomic features. Findings also demonstrate that the paths are accessible to activation from multiple sites rather than insulated conduction paths with limited sites for entry and exit of activation.

Redundancy reduction for improved display and analysis of body surface potential maps. I. Spatial compression.

The Karhunen-Loeve technique of random process representation was investigated as a method of quantitatively characterizing body surface potential maps. One hundred ninety-two lead body surface potential maps from 124 normal subjects and 97 patients with independently documented heart disease were used in the study. Each map frame in QRS and ST-T of 34 maps in a test set was represented as a linear sum of orthonormal distributions derived from the covariance matrix estimated from all QRS frames in the 221 training maps. A 16:1 reduction in spatial data of the test set was achieved with rms errors of 45 and 21 microV in QRS and ST-T, respectively. Results suggest that 12 independent waveforms, derived from the 192 measured ECGs, may be used in place of those 192 ECGs. In addition to providing a convenient and familiar method of display for map data, the technique puts the data in an appropriate form for quantitative statistical analysis.

Redundancy reduction for improved display and analysis of body surface potential maps. II. Temporal compression.

This paper describes use of the Karhunen-Loeve expansion to identify and reduce temporal redundancy in electrocardiographic body surface potential maps (192 body surface leads recorded simultaneously at 1 kHz/channel for approximately 600 msec). Temporal data compression of about 20 to 1 was obtained with accurate representation of the original data. Use of separate sets of orthonormal basis functions for QRS and ST-T provided a more accurate representation than the basis derived from QRST. Combined with the spatial compression described in the preceding paper, overall map data compression of about 320 to 1 was obtained without significant loss of accuracy of representation or map appearance. With both spatial and temporal compression the 100,000 numbers which typically comprise a single cardiac complex were accurately represented by 216 coefficients. Using basis functions derived from a single cardiac complex were accurately represented by 216 coefficients. Using basis functions derived from a training set of 221 maps, the estimated average rms error of representation was 60 microV during the ST-T. For 34 test maps which were not part of the training set, measured average errors were 64 microV during the QRS and 23 microV during the ST-T. This technique provides a basis for quantification of the diagnostic content of maps and automated classification of maps.

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.

Detailed activation sequence in the region of electrically induced ventricular fibrillation in dogs.

We determined the details of ventricular activation sequence during the onset of ventricular fibrillation in 6 dogs. Unipolar electrograms were simultaneously recorded from 192 ventricular epicardial sites evenly spaced in a 3 by 3 cm square area surrounding 2 central sites to which stimuli for inducing fibrillation were delivered. In 5 dogs, 2 to 10 msec duration pulses with amplitudes of 150 to 220 v were used to induce fibrillation, and in 1 dog, 3 successive permature stimuli of 3 v intensity were used to induce fibrillation. In 2 dogs the pattern of activation at the onset of fibrillation was compatible with local reentrant paths near the stimulus site. Fibrillation was induced with a single high intensity pulse in both of these dogs. In the other 4 dogs, the pattern of activation at the onset of fibrillation was highly suggestive of reentrant paths distant from the stimulating electrodes. In 3 of these dogs fibrillation was induced by a single high intensity pulse and in the other it was induced by 3 repetitive stimuli. Activation patterns resembling supraventricular activation occurred prior to fibrillation in the latter animal. The data demonstrate 2 distinct modes of onset of electrically induced ventricular fibrillation. The findings support local reentry in the region of the stimulus site as the mode of onset of fibrillation in some animals and possible reentry at sites distant from the stimulus site with participation of the specialized conduction system in other animals. The differences in mode of onset of electrically induced fibrillation may be responsible for instability of fibrillation threshold measurements in some animals. In addition the role of the specialized conduction system in the onset of fibrillation may be responsible for the effects of sympathetic and vagal stimulation on fibrillation threshold.

Body surface distribution of QRS deflection areas in experimental ventricular preexcitation.

The utility of QRS isoarea maps for recognition of preexcitation sites was evaluated in chronic experiments in dogs. Pacing electrodes were surgically implanted on the atrium and ventricular sites near the AV ring. Electrocardiograms from 192 torso sites were simultaneously recorded during pacing of the atrium together with each of the ventricular sites. Time phase of atrial and ventricular stimuli was varied to yield both preexcitation with clear delta waves and more subtle forms of preexcitation with sites activated earlier than normal but after onset of the normal QRS. QRS area maps were determined by integrating QRS amplitudes at 1 msec intervals over the QRS duration. Results showed a systematic relation between the body surface location of the minimum in the QRS area maps and the preexcitation site. In additon there was a linear relation between the magnitude of maximum and minima in the QRS isoarea maps over different degrees of preexcitation, and the slope of curves showing this relation differed for different sites of preexcitation. Findings suggest that a single display of the QRS isoarea map may permit identification and localization of preexcitation including subtle forms occurring after the onset of the QRS.

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 muV and average correlation coefficient of 0.97). The 30- and 9-lead arrays consistently showed twice the voltage (72 muV) 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.

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.

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...

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

Isopotential mpas 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 electrocardiography but suggest that this will require quantitative descriptions of the details of potential patterns in addition to the location of potential peaks.