Femoral Doppler Blood Flow and Heart Rate (t = 0, 2, 5, 6, 7) Subjects will be evaluated via Doppler ultrasound after placement of three ECG leads for heart rate and R wave monitoring. For the two-dimensional (2-D) and Doppler ultrasound measurements, an ultrasound system (HDI-5000; Philips Medical Systems, Bothell, WA) with a linear array transducer (L7-4) will be used with a transmit frequency of 12MHz. 2-D imaging of the common femoral artery will be performed in the long axis. Images will be triggered to the R wave of the cardiac cycle, and the femoral artery diameter will be measured using online video calipers. A pulsed-wave Doppler sample blood volume will be placed at the same location in the center of the artery, and the mean blood velocity will be measured using online angle correction and analysis software. Femoral artery mean blood flow will be calculated from 2-D and Doppler ultrasound data using the equation: Q = vπ ∙ (d/2)2, where Q is femoral blood flow, v is mean femoral artery blood flow velocity, and d is femoral artery diameter. The investigators have used this technique extensively in the first cycle of this grant.