Correlation between negative near-wall shear stress in human aorta and various stages of congestive heart failure.

The critical effect of advanced congestive heart failure is reduced blood flow in descending aorta resulting from mild to severe reduction in cardiac output, usually accompanying low ejection fraction. In these patients the heart tries to compensate by beating faster, but reduced blood flow combined with increased heart rate can lead to retrograde flow and negative shear stress along the vessel walls during each cardiac cycle. Our studies show that near-wall negative shear stress can result from an entire-retrograde flow at normal heart rates or a Womersley-type phase delayed near-wall retrograde flow at high heart rate and low ejection fraction conditions. In our experiments, a compliant aortic loop with appropriate pressure and flow instrumentation was used, running on either various aqueous glycerin solutions or property filtered, anticoagulated diluted bovine blood. The flow field was mapped using a General Electric Vingmed System 5 platform. The resulting images were analyzed with Caltech's digital ultrasound speckle image velocimetry technique. We showed the occurrence of near-wall retrograde flow under certain aortic flow rates and frequencies, charted via an empirical relationship between Reynolds and Womersley numbers. Also, we demonstrated a strong correlation between retrograde flow level and transition from preliminary to advanced congestive heart failure patients.