A system (150) for assessing the severity of a stenosis (108) in a vessel (100) without the administration of a hyperemic agent, the system comprising: a first instrument (130, 152) sized and shaped for introduction into the vessel of the patient； a processing unit (172) in communication with the first instrument, the processing unit being configured to: obtain measurements of the distal pressure (304) for a patient's cardiac cycle from the first instrument (130, 152), while the First instrument is placed inside the vessel in a distal position (138) of the stenosis； obtain measurements of the proximal pressure (302) for the patient's cardiac cycle from the second instrument (132, 175), while the second instrument is placed inside the vessel in a proximal position (140, 142, 144, 146, 148) of stenosis； select a diagnostic window (236, 412, 422, 456, 466, 506) within the cardiac cycle (462, 472) of the patient, where the diagnostic window only covers a portion of the patient's cardiac cycle, where an initial point ( 348) of the diagnostic window is selected based on the identification of a maximum change in pressure dPldt (344) and where an end point (386, 396) of the diagnostic window is selected based on the identification of one end ( 382, 392) of the cardiac cycle； and calculate a pressure ratio between the distal pressure measurements obtained during the diagnostic window and the proximal pressure measurements obtained during the diagnostic window to thereby assess the severity of the stenosis.Un sistema (150) para evaluar la gravedad de una estenosis (108) en un vaso (100) sin la administración de un agente hiperémico, comprendiendo el sistema: un primer instrumento (130, 152) dimensionado y conformado para su introducción en el vaso del paciente； una unidad de procesamiento (172) en comunicación con el primer instrumento, estando la unidad de procesamiento configurada para: obtener mediciones de la presión distal (304) para un ciclo cardíaco del paciente desde
