An intravascular system comprising: at least one pressure-sensing instrument sized and shaped for introduction into a vessel of a patient; a processing unit in communication with the at least one pressure-sensing instrument, the processing unit configured to: obtain proximal pressure measurements for at least one cardiac cycle of the patient from the at least one pressure-sensing instrument while the at least one pressure-sensing instrument is positioned within the vessel at a position proximal of a stenosis of the vessel; obtain distal pressure measurements for the at least one cardiac cycle of the patient from the at least one pressure-sensing instrument while the at least one pressure-sensing instrument is positioned within the vessel at a position distal of the stenosis of the vessel; select a diagnostic window within a cardiac cycle based on at least one feature of the cardiac cycle of the patient by identifying a change in sign of a segment slope waveform associated with the at least one feature of the cardiac cycle through a period-by-period or segment-by-segment analysis of slopes of segments of a pressure waveform of the proximal pressure measurements or the distal pressure measurements, wherein the diagnostic window encompasses only a portion of the cardiac cycle of the patient; calculate a pressure ratio between the distal pressure measurements obtained during the diagnostic window and the proximal pressure measurements obtained during the diagnostic window; and output the calculated pressure ratio to a display device in communication with the processing unit. at least one pressure-sensing instrument sized and shaped for introduction into a vessel of a patient; a processing unit in communication with the at least one pressure-sensing instrument, the processing unit configured to: obtain proximal pressure measurements for at least one cardiac cycle of the patient from the at least one pressure-sensing instrument while the at least one pressure-sensing instrument is positioned within the vessel at a position proximal of a stenosis of the vessel; obtain distal pressure measurements for the at least one cardiac cycle of the patient from the at least one pressure-sensing instrument while the at least one pressure-sensing instrument is positioned within the vessel at a position distal of the stenosis of the vessel; select a diagnostic window within a cardiac cycle based on at least one feature of the cardiac cycle of the patient by identifying a change in sign of a segment slope waveform associated with the at least one feature of the cardiac cycle through a period-by-period or segment-by-segment analysis of slopes of segments of a pressure waveform of the proximal pressure measurements or the distal pressure measurements, wherein the diagnostic window encompasses only a portion of the cardiac cycle of the patient; calculate a pressure ratio between the distal pressure measurements obtained during the diagnostic window and the proximal pressure measurements obtained during the diagnostic window; and output the calculated pressure ratio to a display device in communication with the processing unit. obtain proximal pressure measurements for at least one cardiac cycle of the patient from the at least one pressure-sensing instrument while the at least one pressure-sensing instrument is positioned within the vessel at a position proximal of a stenosis of the vessel; obtain distal pressure measurements for the at least one cardiac cycle of the patient from the at least one pressure-sensing instrument while the at least one pressure-sensing instrument is positioned within the vessel at a position distal of the stenosis of the vessel; select a diagnostic window within a cardiac cycle based on at least one feature of the cardiac cycle of the patient by identifying a change in sign of a segment slope waveform associated with the at least one feature of the cardiac cycle through a period-by-period or segment-by-segment analysis of slopes of segments of a pressure waveform of the proximal pressure measurements or the distal pressure measurements, wherein the diagnostic window encompasses only a portion of the cardiac cycle of the patient; calculate a pressure ratio between the distal pressure measurements obtained during the diagnostic window and the proximal pressure measurements obtained during the diagnostic window; and output the calculated pressure ratio to a display device in communication with the processing unit.