An intravascular system includes at least one pressure-sensing instrument sized and shaped for introduction into a vessel of a patient a processing unit in communication with the pressure-sensing instrument, the processing unit configured to: obtain proximal pressure measurements for at least one cardiac cycle of the patient while the pressure-sensing instrument is positioned proximal of a stenosis of the vessel obtain distal pressure measurements while the pressure-sensing instrument is positioned distal of the stenosis select a diagnostic window within a cardiac cycle by identifying a change in sign of a slope associated with the proximal and/or 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 and proximal obtained during the diagnostic window and output the calculated pressure ratio to a display device in communication with the processing unit.
