Daniel A. Brody

Eccentric Dipole in a Spherical Medium: Generalized Expression for Surface Potentials

A new formulation is presented for surface potentials produced on a homogeneous spherical volume conductor by an eccentric current dipole contained therein. The formulation is free of interminancies for all relevant dipole locations and leads directly to the solution of surface potentials due to an eccentric quadripole.

Application of Computer Techniques to the Detection and Analysis of Spontaneous P-Wave Variations

A small, laboratory-oriented computer, programmed to scan highly amplified electrocardiographic signals in real-time, specifically searches for spontaneous changes in P-wave configuration. The tape-recorded bipolar extremity leads of 71 normal subjects so scanned revealed sustained periods of ectopic atrial beating in 31. There were 18 with a single well-defined ectopic focus, six with double foci, and seven with atrial activity too variable for simple categorization. Random noise was removed from atrial complexes of the classifiable groups by a discriminant procedure which averaged 50 to 100 members of a preselected P-wave family and automatically bypassed all others. The unexpectedly high incidence of variability indicates that normal atrial excitation is commonly an unstable or metastable process. Actual shifts of pacemaker location, variable exit sites from sinoatrial tissue, and preferential conduction pathways through the atria are postulated as possible explanations of the observed behavior. The richly notched configuration of normal P waves suggests that atrial depolarization proceeds through specialized conductive tissue rather than by uniform centrifugal spread.

The Inverse Determination of Simple Generator Configurations from Equivalent Dipole and Multipole Information

Mathematical methods are described for approximating electrocardiographic behavior by a pair of current dipoles. The approximation fully accounts for all dipolar and quadripolar componenets of the equivalent cardiac generator and gives a least-squares fit of octapolar content. Components of still higher order are not included in the treatment.

Microelectrode Study of Delayed Conduction in the Canine Right Bundle Branch

The electrophysiology of conduction delay was investigated in the semi-isolated right bundle branch of 30 large mongrel dogs. Delay was produced by the external application of an electric blocking current to the bundle branch. Multiple recordings with glass capillary microelectrodes revealed two basic deflections associated with the delay phenomenon. An initial, or leading, deflection originated from the proximal or leading edge of the block and was rapidly transmitted through the block with progressive decay of voltage and rise velocity. A second, or trailing, deflection originated from the regenerative response at the distal or trailing edge of the block and was rapidly transmitted retrograde into the block, with progressive decay of voltage and rise velocity. The transmitted potentials appeared to be electrotonic in nature. The leading deflection appeared to be the transmitted event which maintained a conduction ratio of 1:1 through the block. The magnitude of delay was then related to the time required for this deflection to reach the threshold potential and initiate a regenerative response in the trailing portion of the block where the cells were less affected by the blocking current.

Distribution of heart potentials on the body surface in five normal young men: With a critical commentary on technic

Abstract The present study was designed to facilitate the speed and accuracy of obtaining isopotential maps from body surface potentials. The chests of 5 healthy young men were marked using a cage-shaped protractor at 9 equally spaced levels of 20 equally spaced points each. The electrocardiographic waveform at each site was recorded for 100 heart cycles from which an averaged QRS complex in digitized form was taken for study. Digital computation permitted time alignment of the data and the construction of equipotential maps. Great similarity in the surface pattern of potential distribution among individuals was found early in ventricular repolarization with increasing disparity late in the QRS complex. This is compatible with the concept that as ventricular activation spreads outward from the cavity, the factors that enhance uniformity in surface effect become dissipated.

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.

A hardware trigger for temporal indexing of the electrocardiographic signal

Abstract A hardware trigger has been constructed to aid in the alignment of successive waveforms in real-time averaging of electrocardiographic signals on a general purpose digital computer. The device minimizes demands on central processor time and core storage while providing accurate time alignment to within plus or minus one sample for analog-to-digital conversion rates in excess of 500 samples per second. The performance of the hardware trigger is analyzed in detail for various types of electrocardiographic signals. The stability of the electrocardiographic signal is also examined.

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.

Symptomatic swallowing-induced paroxysmal supraventricular tachycardia

A 55 year old woman with palpitations during ingestion of food or drink was evaluated with surface, esophageal and dynamic electrocardiographic techniques. Recordings documented the consistent induction of supraventricular tachycardia by swallowing; no definable gastrointestinal or other cardiac abnormality was present. Nine previously reported cases are reviewed, with special reference to the role of vagovagal reflexes in the pathogenesis of this unusual clinical syndrome.