Allan C. Young

Restricted Diffusion in Biophysical Systems: Experiment

The pulsed-gradient spin echo nuclear magnetic resonance (PGSENMR) technique was used to measure restricted diffusion of water in three types of animal tissue: human blood plasma and red cells; rat and rabbit heart; rat and rabbit liver. Characteristic lengths (L) for restriction of diffusion are estimated from dependence on the measuring time. Limitations on the range of observable restrictive lengths (1.5-15 mum) are discussed.The decrease in diffusivity due to 1 mum alumina powder (volume fraction = 0.18) in glycerin/water mixtures agrees with the Wang theory assuming spherical particles and no hydration. The characteristic length (L approximately 4 mum) is larger than the particle size (1 mum) or separation (1.8 mum). Comparison of the diffusivities in tissues at short diffusion times with the Wang theory indicates some bound or trapped water.For packed red blood cells, a restriction (L approximately 2.3 mum) was attributed tothe red cell membrane. A permeability p approximately 0.014 cm/s may be estimated from the decrease in diffusivity. Average values of diffusivity ratio in heart were: 0.36 +/- 0.02 for rat; and 0.26 +/- 0.03 for rabbit; and in liver: 0.25 +/- 0.01 for rat; 0.25 +/- .04 for 10-day old rabbit; and 0.195 +/- 0.03 for 2-yr old rabbit. A restriction (L approximately 2.7 mum) in rat liver probably results from the mitochondria.

The Pathway of Ventricular Depolarization in the Dog

Studies of ventricular depolarization with multichannel recording technics permit detailed three dimensional analysis of the process. Activity commences on the mid left septal surface, followed by activity on the right septum. Rapid endocardial excitation follows and leads to endoepicardial activation of the walls. The latest areas activated are in the basal septum. These findings are related to the genesis of the normal QRS. Theories of simultaneous activation and electrocardiographic silent areas are discussed.

Spread of Electrical Activity Through the Wall of the Ventricle

Multipolar recording technics have been employed to record the spread of excitation through the wall of the ventricle. The construction of isochronous planes in a block of tissue aids in visualizing the direction of spread. The results give direct support to the idea of fast endocardial spread followed by syncytial spread through the wall at a slower rate. No evidence is found that the impulse spreads through the individual cardiac muscle bundles.

Factor Analysis of the Electrocardiogram: Test of Electrocardiographic Theory: Normal Hearts

The factor analysis of electrocardiograms from 17 normal individuals indicates that over 95 per cent of all the electrocardiographic “information” is accounted for by 3 factors. In the entire series, all individual leads were more than 93 per cent accounted for by 3 factors. These results indicate that there are no significant voltages at the body surface in normal individuals which can be ascribed to more than 3 internal generators. While these results indicate a 3-function system, they do not indicate a dipolar system since a dipole is a special case of a more general 3-function system.

Ventricular depolarization and the genesis of QRS.

An understanding of electrocardiographic complexes will be achieved when the potential a t a given body-surface point can be predicted from a knowledge of ventricular depolarization and repolarization pathways. To do this for QRS we must have exact information on three factors. First, we must know the time course and magnitude of potential changes across the membranes of the ventricular syncytium as depolarization takes place.’ Second, we must know the pathway of ventricular depolarization. Third, we must understand the basic principles of current flow in volume conductors2 and the modifications of these principles necessitated by the resistive inhomogeneity of the tissues and the irregular shape of the body. The potential Ep a t a given point P in a homogeneous conducting medium is a product (1) of the solid angle fl subtended a t P by the boundary between resting and active tissue, and (2) of the dipole moment per unit area CP across the boundary between resting and active tissue. To determine the solid angle 3, a sphere of radius R is drawn with origin a t P and intersecting all lines from P to the boundary. A is then the area of the sphere within the lines from P to the boundary, and the solid angle is equal to A/RZ. The dipole moment per unit area, CP, is determined by dividing the voltage V across the “cell” membrane by 4 ~ . Then 9 = V / h

The Mechanism of Atrioventricular Conduction

Potentials recorded at various sites in the atrioventricular (A-V) conduction system indicate that conduction is continuously electrical in nature and involves no synapse-like (i.e., chemical) conduction. The region between atrium and atrioventricular node has the slowest conduction velocity (.05 M./sec.) and lowest safety factor. Conduction through the A-V node is at about .12 M./sec. Results demonstrate shapes of potentials recorded extracellularly at various sites within the A-V node, first degree and complete block during rapid atrial stimulation, and echo-like phenomena.

Frequency Analysis of the Electrocardiogram

The frequency analysis of the electrocardiogram reveals that contributions by frequencies of 100 cycles, or higher, per second are less than 10 per cent of the amplitude of the fundamental of the QRS comples, and it thus appears that transistor-driven amplifier-pen recorders presently available are adequate to record all the information contained in the electrocardiogram, if the standard paper speed is doubled, or quadrupled.

Spread of Excitation During Premature Ventricular Systoles

Premature systoles were elicited at many points in the dog heart and the resultant pattern of excitation plotted in detail with a multipolar electrode and a 16-channel oscilloscope. In premature systoles, most of the mural endocardium is excited by a wave travelling at about 1 meter per second. These results indicate that in a normally originating beat the mural endocardium is excited by branched Purkinje tissue, the elements of which conduct at about 1 meter per second. The transmural velocity of the wave of depolarization in premature systoles is about 0.3 meter per second. No evidence was found for intramural penetration of Purkinje fibers. Intramural potentials recorded on stimulation give some insight into normal intramural potentials.

Van der Waals attraction between two conducting chains

Abstract The Van der Waals force between two conducting chains is shown from the zero point energies of the strongly spatially dispersive plasmon modes to vary at small separations L as L −3 instead of L −6 as for nonconducting chains.

Effects of Body Surface Boundary and of Tissue Inhomogenelty on the Electrocardiogram of the Dog

The voltage-current relationship between intrathoracic ‘dipoles’ and body surface locations was examined in anesthetized dogs. Controlled currents were applied at body surface points, and potentials were recorded at the intrathoracic dipoles. A technique of fastening the electrodes to a cast made on the animals torso permitted exact measurement of the torso and electrode geometry. The potentials recorded in the animal were compared with (1) an infinite-medium simulation performed on a digital computer, (2) a bounded- medium simulation performed in the plaster cast of each torso studied, and (3) a simulation involving a torso model containing heart and lungs. The correlation between recorded and simulated potentials was fair for the infinite-medium simulation, improved by the boundary, and further improved by the addition of inhomogeneity. Implications for electrocardiography are discussed.