ELECTROPHYSIOLOGICAL STUDY The patient must be in a fasting state and must stop taking medications that affect the study for at least 5 half-lives. It is made by puncturing the right femoral vein, electro-catheters are placed in the high right atrium, in the bundle of His and in the right ventricular apex. It is stimulated the double of the thresholds to 3 cycle lengths (600, 500 and 430 ms) and three extra cycles, starting at 350 ms, and with successive decreases of 10 ms until the refractory period is found.