Pressure gradient related to energy conversion in the aorta.

In this study, we analyzed a common form of experimental investigation of blood vessels, in which measurements are obtained with branches ligated. Utilizing representative pressure and flow pulses and the full expression for the equation of motion, we calculated the axial pressure gradient, in the time domain at a plane in the descending aorta. The time function representing the ratio between axial pressure gradient and axial flow for the resulting tapering geometry was subjected to Fourier analysis. The harmonics were utilized to obtain the real and imaginary components of the longitudinal impedance as if it were a linear system. In a linear system, the real and imaginary components represent the viscous and inertial properties of the fluid, respectively. For the system studied, however, the real part contained both viscous and substantial in-phase components arising from the inertial terms of the equation of motion. The real part, therefore, cannot be interpreted as indicative solely of dissipated energy. When measurements are obtained from an adulterated system, caution must be exercised if the interpretation is to be considered that of the real system. The analysis clarifies an anomalous issue concerning resistive features of the aorta.