DEPENDENCE OF ANISOTROPIC MYOCARDIAL ELECTRICAL-RESISTIVITY ON CARDIAC PHASE AND EXCITATION-FREQUENCY

Knowledge of myocardial electrical resistivity is of interest because passive electrical properties govern the electrotonic spread of curren t through the myocardium and influence the shape and velocity of the e xcitation wave. In addition, measurements of myocardial resistivity ma y provide information about tissue structure and components. The aim o f the present study was to determine the excitation frequency dependen ce and the changes during the cardiac cycle of anisotropic myocardial electrical resistivity. Longitudinal and transverse myocardial resisti vity were measured using an epicardial sensor in four open-chest dogs with excitation frequencies in the range of 5-60 kHz. Mean longitudina l resistivity gradually decreased from 313+/-49 Omega.cm at 5 kHz to 2 12+/-32 Omega.cm at 60 kHz, transverse resistivity decreased from 487/-49 to 378+/-53 Omega.cm. To analyze the phasic changes, we compared mean resistivity (averaged over the full cardiac cycle) with resistivi ty during four cardiac phases: pre-ejection, ejection, early diastole and late diastole. Longitudinal resistivity was significantly higher d uring the ejection phase (+9.6+/-4.1 Omega.cm) and lower during late d iastole (-6.9+/-2.9 Omega.cm). Transverse resistivity was significantl y higher during late diastole (+4.0+/-2.3 Omega.m). The values during the other cardiac phases were not significantly different from mean re sistivity. The phasic changes in longitudinal and transverse resistivi ty during the cardiac cycle were independent of the excitation frequen cy. We speculate that these changes are related to geometrical changes , especially to changes in myocardial blood volume.