Early detection of acute transmural myocardial ischemia by the phasic systolic-diastolic changes of local tissue electrical impedance

Abstract

Myocardial electrical impedance is influenced by the mechanical activity of the heart. Therefore, the ischemia-induced mechanical dysfunction may cause specific changes in the systolic-diastolic pattern of myocardial impedance, but this is not known. This study aimed to analyze the phasic changes of myocardial resistivity in normal and ischemic conditions. Myocardial resistivity was measured continuously during the cardiac cycle using 26 different simultaneous excitation frequencies (1 kHz-1 MHz) in 7 anesthetized open-chest pigs. Animals were submitted to 30 min regional ischemia by acute left anterior descending coronary artery occlusion. The electrocardiogram, left ventricular (LV) pressure, LV dP/dt, and aortic blood flow were recorded simultaneously. Baseline myocardial resistivity depicted a phasic pattern during the cardiac cycle with higher values at the preejection period (4.19 ± 1.09% increase above the mean, P < 0.001) and lower values during relaxation phase (5.01 ± 0.85% below the mean, P < 0.001). Acute coronary occlusion induced two effects on the phasic resistivity curve: 1) a prompt (5 min ischemia) holosystolic resistivity rise leading to a bell-shaped waveform and to a reduction of the area under the LV pressure-impedance curve (1,427 ± 335 vs. 757 ± 266 Ω·cm·mmHg, P < 0.01, 41 kHz) and 2) a subsequent (5-10 min ischemia) progressive mean resistivity rise (325 ± 23 vs. 438 ± 37 Ω·cm at 30 min, P < 0.01, 1 kHz). The structural and mechanical myocardial dysfunction induced by acute coronary occlusion can be recognized by specific changes in the systolic-diastolic myocardial resistivity curve. Therefore these changes may become a new indicator (surrogate) of evolving acute myocardial ischemia.

Recognition of Fibrotic Infarct Density by the Pattern of Local Systolic-Diastolic Myocardial Electrical Impedance

“…We recently proposed that the amplitude of the impedance curve is modulated by the mechanical cardiac activity thus the flat resistivity curve of the dense scar could be in part due to the local severe dyskinesis (Helle-Valle et al, 2009; Garcia-Sanchez et al, 2015). In addition, the drop of the AUC supports the hypothesis that the LVPI curves would allow differentiating the segments able to generate active force (healthy tissue) from those that are dyskinetic (necrotic tissue) and therefore mechanically passive (Jorge et al, 2015). The AUC reduction is caused by two main factors: (1) the low-amplitude of the impedance phasic changes and (2) the delayed occurrence of the resistivity peaks.…”

“…However, the prolonged time required for measuring the impedance spectrum in these studies prevented knowing the time course of myocardial impedance during the whole cardiac cycle. A recent refinement of the bioimpedance method based on fast broadband EIS permitted to characterize the changes in myocardial impedance induced during the cardiac cycle under normal and acute ischemic conditions in the in situ porcine heart (Sanchez et al, 2011; Jorge et al, 2015). In that model the normal myocardium depicted a biphasic resistivity pattern that turned to a bell-shaped morphology with an increased mean value following coronary occlusion.…”

“…Tissue impedance spectroscopy was measured as reported previously (Jorge et al, 2015). Briefly, a 4-needle electrode probe connected to a signal generation and acquisition system (PXI system, National Instruments) was inserted into the LV wall.…”

“…Early studies conducted in experimental animal models have consistently shown that acute myocardial ischemia induced by coronary artery ligation increased dramatically the myocardial impedance (Cinca et al, 1998; Padilla et al, 2003; Rodriguez-Sinovas et al, 2005). In contrast, during the healing over process the necrotic scar turns to lower than normal impedance values (Fallert et al, 1993; Wolf et al, 2001; Salazar et al, 2004).…”

“…In these studies, the technique of measuring the changes in myocardial impedance did not allow to delineate the sequential impedance variations elicited during the systole and diastole phases of the cardiac cycle because of the long time required for the whole impedance spectrum acquisition (Gersing, 1998; Casas et al, 1999; Warren et al, 2000; Cinca et al, 2008). Quite recently, the use of refined fast broadband electrical impedance spectroscopy (EIS; Sanchez et al, 2011) allowed to describe the cyclic pattern of systolic-diastolic impedance changes in a swine model of acute myocardial ischemia (Jorge et al, 2015). As compared with the classical technique, the new EIS-based method permitted a prompt detection of local myocardial ischemia based on the occurrence of specific changes in the phasic systolic-diastolic impedance curve.…”