Electrical Resistances of Interstitial and Microvascular Space as Determinants of the Extracellular Electrical Field and Velocity of Propagation in Ventricular Myocardium

Abstract

In myocardial ventricular tissue, extracellular electrical resistance (ro) is an important determinant of propagation velocity (theta) and the magnitude of the extracellular bipolar electrogram (delta Vo). The extracellular space is composed of two compartments, the vascular space and the interstitial space. To assess the electrical equivalent of this compartmentation in the ventricular myocardium and its effect on ro, theta, and delta Vo, electrical cable analysis was performed in an arterially perfused rabbit papillary muscle. Vascular resistivity was changed from 75 to 86 to 143 and to 221 omega/cm by variation of hematocrit in the perfusate from 0% to 10% to 40% and to 60%. As a means to vary the volume of the interstitial space and with this as its resistivity, colloid osmotic pressure (COP) in the perfusate was changed from 9 to 36 and to 94 mm Hg by altering the dextran concentration in the perfusate from 10 to 40 to 80 g/L. Decreasing COP had a marked effect on ro (56% decrease), delta Vo (decrease from 61 to 42 mV), theta (increase from 48 to 59 cm/s), and the diameter of the muscle fiber (increase of 12%). If COP was increased from 36 to 94 mm Hg, ro (by 35%) and delta Vo (from 62 to 75 mV) increased; theta and diameter showed no significant changes. In contrast, alterations of intravascular electrical resistivity in a range from 75 to 221 omega/cm did not induce any significant changes in ro, delta Vo, theta, and diameter of the preparations. We conclude from our data that (1) the microvascular tree in ventricular myocardium is electrically insulated to a large degree from the interstitial space and that (2) electrical current flow in the extracellular space during excitation is confined to the narrow, anisotropic interstitial space.