A system for assessing the severity of a stenosis in a vessel without 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 in communication with the first instrument, the processing unit configured to: obtain distal pressure measurements for a cardiac cycle of the patient from the first instrument (130, 152) while the first instrument is positioned within the vessel at a position distal of the stenosis; obtain proximal pressure measurements for the cardiac cycle of the patient from a second instrument (132, 175) while the second instrument is positioned within the vessel at a position (140, 142, 144, 146, 148) proximal of the stenosis; select a diagnostic window (236, 412, 422, 456, 466, 506) within the cardiac cycle (462, 472) of the patient, wherein the diagnostic window encompasses only a portion of the cardiac cycle of the patient, wherein a starting point of the diagnostic window is selected based on identifying a maximum change in pressure dPldt and wherein an ending point (386, 396) of the diagnostic window is selected based on identifying an 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. a first instrument (130, 152) sized and shaped for introduction into the vessel of the patient; a processing unit in communication with the first instrument, the processing unit configured to: obtain distal pressure measurements for a cardiac cycle of the patient from the first instrument (130, 152) while the first instrument is positioned within the vessel at a position distal of the stenosis; obtain proximal pressure measurements for the cardiac cycle of the patient from a second instrument (132, 175) while the second instrument is positioned within the vessel at a position (140, 142, 144, 146, 148) proximal of the stenosis; select a diagnostic window (236, 412, 422, 456, 466, 506) within the cardiac cycle (462, 472) of the patient, wherein the diagnostic window encompasses only a portion of the cardiac cycle of the patient, wherein a starting point of the diagnostic window is selected based on identifying a maximum change in pressure dPldt and wherein an ending point (386, 396) of the diagnostic window is selected based on identifying an 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. obtain distal pressure measurements for a cardiac cycle of the patient from the first instrument (130, 152) while the first instrument is positioned within the vessel at a position distal of the stenosis; obtain proximal pressure measurements for the cardiac cycle of the patient from a second instrument (132, 175) while the second instrument is positioned within the vessel at a position (140, 142, 144, 146, 148) proximal of the stenosis; select a diagnostic window (236, 412, 422, 456, 466, 506) within the cardiac cycle (462, 472) of the patient, wherein the diagnostic window encompasses only a portion of the cardiac cycle of the patient, wherein a starting point of the diagnostic window is selected based on identifying a maximum change in pressure dPldt and wherein an ending point (386, 396) of the diagnostic window is selected based on identifying an 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.