Application of Heated-Film Velocity and Shear Probes to Hemodynamic Studies

A constant-temperature heated-film anemometer system has been adapted for the detailed study of in-vivo aortic velocity fields. Two types of sensing probes were developed: a velocity probe and a velocity-gradient or fluid shear stress probe. These probes were evaluated for steady and pulsatile flow in rigid circular tubes using both a glycerin-water mixture and blood. Measurements using both devices agreed closely with the values predicted by well established theory. Moreover, the integrated velocity profiles that were measured correlated well with the simultaneously recorded flow values using orifice meter and electromagnetic flowmeter techniques. In-vivo studies were made along the thoracic aortas of anesthetized dogs and pigs. Velocity measurements along the aorta indicated that the velocity profiles are blunt. The flow-pulse forms obtained by the heated-film technique in vivo were also similar in magnitude and contour to those obtained simultaneously from an electromagnetic flowmeter. Fully developed turbulent flow was not observed; however, occasional "eddy" turbulence occurred in the aortic arch of dogs weighing less than 30 kg. Preliminary measurements indicate that peak wall-shear stresses reach values that are approximately one-third that of the endothelial yield stress.