Retrograde shear: backwards into the future?

TO NEW STUDENTS OF vascular physiology, it sometimes comes as a surprise that blood flow in large conduit arteries such as the femoral and brachial arteries ceases to move forward and, indeed, reverses direction throughout the cardiac cycle. This is true under a variety of conditions in humans and other species. We still recall our initial reluctance in switching to the use of Doppler ultrasound (from the more-established venous occlu-sion plethysmography), an unease that was closely linked to an inability to overcome our preconception that blood should always be moving forward, in an antegrade fashion, in these “garden-hose variety ” vessels, rather than moving backward, or retrograde, during portions of diastole, as clearly indicated in our Doppler velocity tracings. Fifteen years later, as we introduce new students to the use of Doppler ultrasound in our respective laboratories, we instruct them that oscillatory flow