Volume conductor effects

The body surface potential distribution is determined by both the cardiac sources and the torso volume conductor. To study the volume conductor effects on the potential distribution, an eccentric spheres model of the heart - torso system was developed (Rudy and Plonsey, 1979 Rudy etal., 1979). This idealized forward model includes all important torso inhomogeneities, and yet is simple enough to permit analytic solutions for both body surface and epicardial potentials. In the model (Figure 6), the heart is represented as a sphere consisting of a central blood volume bounded by a spherical heart-muscle shell and pericardium the heart, in turn, is located eccentrically within a spherical torso, where the latter consists of a lung region bounded by muscle and fat layers. The eccentric location of the heart... 

Effects of Volume Conductor Inhomogeneities Secondary Sources and Images... 

Eddy currents will set up their own magnetic fields, opposing the external field. The magnetic field will therefore be attenuated as function of depth (skin effect). The skin depth (depth of penetration) 6 in the case of a uniform, plane electromagnetic wave propagating in a volume conductor with a magnetic permeability p. is ... 

Heat effects are certainly related to current density in volume conductors, but this is not necessarily so for nerve and muscle excitation. Excitation under a plate electrode on the skin is more highly correlated to current than current density (see Section 10.16.1). The stimulus summation in the nerve system may reduce the current density dependence if the same current is spread out over a larger volume of the same organ. Therefore, and for practical reasons, safe and hazard levels are more often quoted as current, energy or quantity of current in the external circuit, and not current density in the tissue concerned. 

The effects of the torso volume conductor on the electrocardiographic potential distribution were examined utilizing an eccentric spheres model. The effects of the blood cavity, lung region, and the surface muscle layer are described. The importance of interactions between the various torso compartments in determining the potential distribution is demonstrated. 

Body surface and epicardial potentials are shown in Figure 6 as a function of skeletal-muscle conductivity. The muscle layer attenuates surface potentials (by 25%), and is a major contributor to the smoothing effect of the torso volume conductor (see below). The surface potential decreases with increasing muscle conductivity, and a 5-fold increase in conductivity (from 0.0005 to 0.0025 mho/ cm) causes the potential to decrease by 40.5%. In contrast, epicardial potentials are not strongly affected by the presence of the muscle layer. A 5-fold increase in muscle conductivity causes a slight increase of only 8% in epicardial potential (see Figure 8). 

ANS The sensitivity of the inverse problem to uncertainties in heart position and in conductivities of the volume conductor can be studied using the model presented here. Althoug results presented here were only for the ideal, noisefree situation, we are continuing our studies of the effects of uncertainties on the epicarial potentials. 

ANS Yes, that is correct. The potentials on both the epicardium and the surface are determined by the sources and by the effects of the inhomogeneities throughout the entire volume conductor. 

We shall now prove that P, for fixed values of 7r and the temperature, is definite for a given solution. For this purpose we have first of all to show that the dilution or concentration of the solution can be effected isothermally and reversibly. If the above apparatus is constructed of some good conductor of heat, placed in a large constant-temperature reservoir, and if all processes are carried out very slowly, the isothermal condition is satisfied. Further, suppose the end pistons fixed, and then apply to the septum an additional small pressure SP towards the solution. There will be a slight motion of the septum, through a small volume SV, and work... 

With the U-Type systems (i.e. with the low chain alcohols) the trends in the conductivity - curve are consistent with percolative conduction originally proposed to explain the behaviour of conductance of conductor-insulator composite materials (27). In the latter model, the effective conductivity is practically zero as long as the conductive volume fraction is smaller than a critical value called the percolation threshold, beyond which k suddenly takes a non-zero value and rapidly increases with increase of Under these conditions. 

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