John W. Cox

Detection and localization of multiple epicardial electrical generators by a two-dipole ranging technique.

The ability of a numerical procedure to detect and to localize two experimentally induced, epicardial dipolar generators was tested in 24 isolated, perfused rabbit heart preparations, suspended in an electrolyte-filled spherical tank. Electrocardiograms were recorded from 32 electrodes on the surface of the test chamber before and after placement of each of two epicardial burns. The second lesion was located either 180 degrees, 90 degrees, or 45 degrees from the first. Signals were processed by iterative routines that computed the location of one or two independent dipoles that best reconstruced the observed surface potentials. The computed single dipole acounting for 99.68% of root mean sequare (RMS) surface potential recorded after the first burn was located 0.26 +/- 0.10 cm from the centroid of the lesion. Potentials recorded after the second lesions were fit with two dipoles that accounted for 99.36 +/- 1.51% of RMS surface potentials and that were located 0.42 +/- 0.26 cm and 0.57 +/- 0.49 cm from the centers of the corresponding burn. Seventy-one percent of computed dipoles were located within the visible perimeter of the burn. Thus, two simultaneously active dipolar sources can be detected and accurately localized by rigorous study of the generated electrical field.

Localization of heart vectors produced by epicardial burns and ectopic stimuli; validation of a dipole ranging method.

Location of the equivalent cardiac dipole has been estimated but not fully verified in several laboratories. To test the accuracy of such a procedure, injury vectors were produced in 14 isolated, perfused rabbit hearts by epicardial searing. Strongly dipolar excitation fronts were produced in 6 additional hearts by left ventricular pacing. Twenty computer-processed signals, derived from surface electrodes on a spherical electrolyte-filled tank containing the test preparation, were optimally fitted with a locatable cardiac dipole that accounted for over 99% of the root-mean-square surface potential. For the 14 burns (mean radius 5.0 mm), the S-T injury dipole was located 3.4 ± 0.7 (SD) mm from the burn center. For the 6 paced hearts, the dipole early in the ectopic beat was located 3.7 mm (range 2.6 to 4.6 mm) from the stimulating electrode. Phase inhomogeneities within the chamber appeared to have a small but predictable effect on dipole site determination. The study demonstrates that equivalent dipole location can be determined with acceptable accuracy from potential measurements of the external cardiac field.

Studies of the Equivalent Cardiac Generator Behavior of Isolated Turtle Hearts

Equivalent cardiac generator components were determined for a series of excised turtle hearts immersed in Ringers solution. Relatively large preparations were contained within a specially designed spherical chamber, and electric field potentials were derived from 20 evenly spaced electrodes on the chamber wall. A laboratory computer was used to acquire and store the 20 leads of signal data in digitized form in real time. An eccentric dipole was optimally fitted to the surface potentials for each 2 msec sample; the remaining voltages were used to determine a centric multipolar series through octapolar content. In addition to the purely quantitative parameters which were thus determined, sequential mapping of isopotential distribution over the spherical boundary gave valuable qualitative insights into the behavior of the equivalent generator throughout ventricular depolarization. This activity varied in complexity from predominantly dipolar to strongly nondipolar among different preparations. Peak quadripolar activity ranged from a low of 10 to a high of 60%; the corresponding figures for octapole content were 5−61%. The overall technique permits experimental exploration of several theoretical principles which have been advanced since 1954. Pilot studies on rabbit hearts indicate that the method will also be applicable to mammalian hearts.

Left precordial isopotential mapping during supine exercise.

Junctional depression is often observed during physical exercise in overtly normal subjects. To explore its pathogenesis, 15 normal volunteers were studied during supine, bicycle ergometer, submaximal stress tests. Electrocardiograms were simultaneously recorded from 42 electrodes on the left anterior precordium at two minute intervals at rest and during exercise. Data were used to construct isopotential maps throughout the P-QRS-T intervals. At rest, maps throughout the ST segment were dominated by a single maximum along the upper left sternal border. During exercise, all subjects developed junctional depression that was maximal along the lower left sternal border. Exercise maps during the early to mid-ST segment showed an intense minimum along the lower left sternal border that was continuous with terminal QRS forces in both intensity and location. Later in ST, this minimum decreased in strength and was replaced by a maximum located in the same area as that observed at rest. These observations suggest that junctional depression is the result of competition between two effects, one being normal repolarization which is obscured in the early ST segment by the second, possibly representing delayed terminal depolarization forces.

Spatial Parameters and Shape Factors of the Normal Atrial Vectorcardiogram and Its Scalar Components

New quantitative norms for clinical evaluation of the atrial vectorcardiogram are presented. Real-time computer processing digitizes atrial electrocardiograms, reduces random noise content, rectifies base-line configuration, and corrects preamplifier distortion. Axial-system leads of 106 normal persons were so treated. The basic information derived includes spatial distribution and magnitudes of P, TP, and polar P vectors; length, width, and planarity of P loops; spatial P-TP angles; and the Eulerian angles of P-loop normalization. Normal P vectors show predominant leftward, inferior, and anterior orientation, with opposing TP direction. Polar P vectors show predominant superior, anterior, and leftward clustering. The scatter of parameters in the orthogonallead frontal plane is about the same as for the tetrahedral system. Similarly, the atrial deflections of normals show equally rich notching in both systems. Resolution of the P-wave population into uncorrelated components, followed by resynthesis from these principal factor wave forms, revealed a fairly continuous “spectrum” of signal configurations. This technique, and extensive attempts by alternate means, failed to support the view that normal P waves can be separated into distinctive right and left atrial components.

Path and significance of heart vector migration during QRS and ST-T complexes of ectopic beats in isolated perfused rabbit hearts.

Heart vector location was estimated for 23 isolated, perfused rabbit hearts during paced ectopic beats. Twenty computer-processed signals, derived from the surface electrodes of a spherical electrolyte-filled tank containing the hearts, were optimally fitted with a locatable cardiac dipole every millisecond of the QRS and every 3rd msec of the ST-T interval. During the QRS, the computed heart vector location of hearts subepicardially paced from the left ventricular free wall originated very close to the stimulating electrode, traversed the heart from left to right, and terminated in the right ventricle. Daring the first portion of repolarization for the hearts paced from the left ventricle, the position of the heart vector was almost stationary within the left ventricle, whereas after the peak of the T wave, heart vector location again moved from left to right. The first quarter of the QRS interval for hearts stimulated from the right ventricular free wall was nondipolar; during the remaining three-quarters of excitation, location of the heart vector moved from right to left, terminating in the left ventricle. Throughout the entire T wave of hearts paced from the right ventricle, the position of the heart vector remained almost motionless within the left ventricle. This study demonstrates the ability of heart vector location, by its rapid motion, graphically to portray passage of an ectopic beat across the heart and, by its slower motion within the central portion of the heart, to indicate the diffuse nature of the resulting ventricular recovery.

Dipole, Quadripole, and Octapole Measurements in Isolated Beating Heart Preparations

An experimental technique is described for measuring the equivalent multipole model for isolated beating hearts. The dipole, quadripole, and octapole moments are measured from weighted sums of 20 electrode potentials on the surface of a 6.35-cm diameter sphere. An iterative procedure is used to eliminate the effects of source eccentricity for dipolar data.

Relative dipolar behavior of the equivalent T wave generator: quantitative comparison with ventricular excitation in the rabbit heart.

We studied the relative dipolar and nondipolar content of signal energy throughout ventricular excitation and recovery in 34 isolated, perfused rabbit hearts, suspended in an electrolyte-filled spherical chamber. Computer-processed signals were derived from 20 evenly spaced tank-surface electrodes, and a single, moving, equivalent cardiac dipole generator was optimally fitted to the recorded potentials for each 1-msec sampling interval. Superimposed, time-based plots of signal energy for the 34 preparations showed ventricular excitation to be strikingly more nondipolar than was recovery. In terms of the summed square ratio of nondipolar residual energies, overall nondipolarity of QRS exceeded that of ST-T by 41%. Furthermore, the maximum instantaneous ratio during QRS was considerably greater than during the ST-T. Evaluation of paired differences, comparing nondipolar behavior throughout QRS with all of ST-T, proved highly significant (P < .005). We also found that in contrast to the considerable mobility exhibited by the equivalent QRS dipole, the ST-T dipole locus remained nearly stationary during most of ventricular recovery. Presumably because repolarization is temporally and spatially a relatively diffuse process, it may generate electrical fields which are notably more dipolar than those caused by depolarization.

Experimental comparison of four inverse electrocardiographic constructs in the isolated rabbit heart.

Summary Complex models of the hearts electrical activity have been proposed as being superior to the fixed-location-dipole equivalent cardiac generator. To test and to compare the adequacy of four such proposals, i.e., a four element centric multipole series (CMS), a single moving dipole (SMD), two moving dipoles (TMD), and a moving dipole-quadripole pair (DPQP), the electrical fields generated by thirty isolated, perfused rabbit hearts placed in a spherical volume conductor were studied. Waveforms recorded from 32 surface electrodes were sampled 2500 times per second per channel, and generator parameters were computed using previously reported methods. A CMS accounted for 99.5±0.21% (mean±S.E.M.) of sum-squared surface potential recorded during ventricular depolarization. The dipole, quadripole, octapole, and hexadecapole terms fit 3.3 to 98.2%, 7.2 to 70.8%, 0.2 to 29.9% and 0.02 to 12.7% of observed potential, respectively. A SMD fit 90.9 ±0.25% of surface activity, whereas the TMD and DPQP constructs accounted for 98.4 ±0.21% and 99.2±0.21% of surface potential, respectively. The CMS accounted for significantly more (p

Examination of a multiple dipole inverse cardiac generator, based on accurately determined model data

Summary The multiple dipole array is a model of cardiac electrical activity whose elements are dipoles which are fixed in location and orientation to correspond to specific regions of the heart. The dipole moments are constrained to positive or zero values so that the dipoles always point outward from endocardium to epicardium. An array of this type was examined for its ability to reflect the dipole moments of each region of the “Coriolis” generator (a biventricular model of the electromotive forces of the canine heart). Dipole strengths of the array were calculated inversely from simulated surface potentials which would have been created by the Coriolis generator if it had been placed in a bounded, spherical, homogeneous medium. Although inversely computed dipole strengths of the original array represented dipole moments of regions of the right ventricle and septum of the Coriolis generator very poorly, a modified array indicated adequately the combined moments of these two structures. This modified array included within the same region portions of both the septum and the overlying right ventricle. The ability of the modified array to detect, locate, and quantify an increase or decrease in dipole moment of any portion of the Coriolis generator was excellent. However, when the angle between the actual dipole moment of a region and the array dipole representing that region was enlarged, the ability of the array to represent the Coriolis generator was greatly diminished.