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Biomedical subjects

R C Barr

Publications and source records attributed to R C Barr.

At least 55 records · Page 3Linked to original sources
PubMed1980

Accessory atrioventricular pathway in an infant: prediction of location with body surface maps and ablation with cryosurgery.

A 10-month-old infant with multiple muscular ventricular septal defects, congestive heart failure, Wolff-Parkinson-White syndrome, and supraventricular tachycardia is presented. The site of ventricular pre-excitation was predicted by analysis of ST-T wave isopotential body surface maps to be in the posterior free wall of the right ventricle. The site was confirmed by epicardial mapping of the ventricles during surgery. The pathyway was cryoblated and the ventricular defects were closed. The patient has been free of pre-excitation and supraventricular tachycardia for over two years since surgery.

Animals↗
PubMed1979

Extracellular potentials related to intracellular action potentials during impulse conduction in anisotropic canine cardiac muscle.

This paper considers a quantitative description of intracellular and transmembrane currents in anisotropic muscle, with emphasis on the factors that determine the extracellular potentials. Although Vmax of the intracellular action potential had no relation to changes in conduction velocity in anisotropic tissue with constant membrane properties, the extracellular waveforms were quite sensitive to velocity changes. Large amplitude biphasic deflection occurred in the fast areas, and in the slow areas the waveforms were of lower amplitude and triphasic in shape; i.e., negative potentials preceded the biphasic positive-negative deflection. The extracellular potentials were simulated on the bases of a model of intracellular currents, and the theoretical and measured results showed good agreement. In tissue with anisotropic conductivity, the relationship between the spatial intracellualr potential gradient and the magnitude of the extracellular potential of the excitation wave was opposite to the classical relationship in isotropic tissue. Due to the influence of the effective intracellular conductivity on the spread of intracellular currents and on conduction velocity, in anisotropic tissue the extracellular potential decreased as the intracellular potential gradient increased. The peak values of the positive and negative potentials and the spatial distribution of the potential gradients varied considerably along the activation front. These findings were accounted for by differences in the distribution and spatial extent of the transmembrane currents, which were determined by the intracellular currents. The theoretical analysis showed that intracellular and transmembrane currents were proportional to the local conduction velocities of the wavefront. Thereby, it was not possible to have a "uniform layer" of current when there were differences in conduction velocity along the length of the excitation wave. The implications of the analysis are considerable, since the gratifying agreement between the theoretical and measured results indicates that the details of the extracellular waveforms can be explained on the basis of the distribution of intracellular currents; i.e., extracellular potentials provide a sensitive index of intracellular current flow.

Action Potentials↗
PubMed1977

Sampling rates required for digital recording of intracellular and extracellular cardiac potentials.

Electrocardiograms and cardiac electrograms now frequently are measured for both clinical and experimental purposes by direct digital sampling, with no recording of the signal in analog form. This study examined the question of what sampling rates were required to measure accurately the continuous waveforms from the digital samples. Body surface waveforms and intracellular and extracellular waveforms measured directly from cardiac tissues were evaluated. Cardiac measurements included waveforms from the atrium, ventricle, atrioventricular transmission system and individual Purkinje strands. Sampling rates as high as 15,000 samples/sec were required to record accurately extracellular waveforms of the ventricular conduction system. Decreasing sampling rates were required as the recording site shifted through the ventricle to the body surface, where sampling rates as high as 1500 samples/sec were necessary.

Action Potentials↗
PubMed1977

Origin of body surface QRS and T wave potentials from epicardial potential distributions in the intact chimpanzee.

Epicardial and body surface QRS-T wave potential distributions were measured during normal and ectopic sequences in intact chimpanzees. Epicardial potential distributions were used because they provide a comprehensive picture of total cardiac electrical activity for relating heart and body surface events during both ventricular activation and repolarization. When the epicardial potential gradients existed over a distance greater than that to the recording points on the body surface, e.g., as occurred during the overlap of terminal ventricular activation and early repolarization, the epicardial events were mirrored well on the anterior chest surface. However, when the recording points were at a distance greater than that over which the epicardial potential gradients existed, the details of the epicardial events disappeared and their effect was to produce distinct changes in the low-level potentials over broad distant areas. The major manifestations on the body surface of selective epicardial events frequently were changes in the distant low-level potential areas while there was no change in the pattern near the maximum or minimum. The ST-T wave body surface distributions were as useful as the QRS patterns for localizing the ventricular ectopic foci presented. A direct experimental basis is provided for explaining T wave notches which occurred during normal and ectopic beats and resembled U waves. It should be possible to achieve as precise an understanding of ST-T waves on the basis of epicardial potential distributions as has thus far been achieved for QRS on the basis of isochrones.

Animals↗