Our objective is to evaluate whether it is possible to characterize the passive electrical properties of myocardial tissue in contact with the electrocatheters used in arrhythmia diagnosis or radio frequency ablation techniques. To characterize the tissue, we propose the use of electrical impedance spectroscopy to measure the impedance between the catheter tip and an external electrode, assuming a three-electrode method. We constructed a 3D finite-element model of the thorax to estimate the impedance as measured in different situations. We defined an area on the anterior wall of the left ventricle in which we simulated three tissue states: healthy, acute ischaemic and scar. We studied the effect of the following parameters on the measured impedance spectrum: the position of the external electrode, the position and orientation of the catheter tip and the overall effect of the subject's respiration. Results show that the highest frequency phase (around 300 kHz) yields the best differentiation of tissue states and that it is less sensitive to respiration than the impedance magnitude. The phase is also less influenced by the catheter tip position (either touching the wall or floating) and the orientation of the catheter inside the left ventricle. The best position for the external electrode is on the chest; this position is less affected by breathing and is more sensitive to tissue changes. One can still distinguish between tissue states if the external electrode is placed on the back, but the effect of respiration is higher.