A) Anatomic image from a patient showing femoral vein location used for evaluation of flow and venous O2 saturation. B) O2 saturation images from a patient at two time points (2 sec. and 60 sec.) following exercise, and the Fick equation for calculation of VO2. C) to E) show the average recovery curves for flow, venous O2 saturation and calculated leg VO2, for HFPEF (black), HFPEF (red) and healthy controls (HC, blue). MRT = mean response time, which is the sum of the delay term (Δ), to the onset of exponential recovery, and time constant of the best-fit mono-exponential decay function (t), as shown in E). HFPEF = heart failure with preserved ejection fraction, HFREF = heart failure with reduced ejection fraction, Hgb = hemoglobin concentration.