Pressure-flow relations in dog arteries

The propagation of the arterial pressure and flow pulses through the vascular beds is characterized by marked changes in shape and magnitude. In order to understand better the mechanisms which account for these changes, instantaneous pressures and flows were measured simultaneously at various sites in the arterial tree of 12 anesthetized dogs, using Statham strain gauges and electromagnetic flowmeters. The data were processed by on-line Fourier analysis and the frequency spectra of pressure, flow and vascular impedance at these sites were evaluated. In these dogs about 10% of the cardiac output was distributed to the head, 14% to the kidney, 35% to the gastrointestinal tract, 14% to the pelvic organs and 6% to the legs. The input impedances of the various beds were frequency dependent, the frequency-dependent components varying between 2 and 45% of the corresponding D-C impedances (peripheral vascular resistance). The viscous losses associated with pulsatile flow were larger than those for corresponding steady flows. The excess losses varied between 9 and 30% of the mean flow losses. The marked decrease in cross section down the aorta lead to considerable acceleration of the mean blood velocity and corresponding pressure losses despite the drainage through aortic branches. Comparisons of measured flows with those predicted from Womersley's theory showed satisfactory agreement for short vascular segments. However the theory underestimates the losses occurring in the nonuniform arterial tree. On the basis of anatomic measurements, the space dependence of resistance, inertance and vascular distensibility was evaluated.

Pulsatile flow patterns in distensible tubes

Pulsatile flow patterns in polyvinyl tubes were visualized by means of streaming birefringence and compared with those obtained using an electromagnetic flowmeter.

At low flow rates the frequency spectrum of the birefringence pattern is similar to that of volume flow. As the flow rates are progressively increased, irregular eddies appear, particularly during deceleration of flow, first in the central core and later spreading into the peripheral sleeve. They are characterized by a wide frequency spectrum.

The concepts of laminar and turbulent flow are reviewed and evidence is presented which indicates that the power losses associated with the observed eddies are negligible with respect to the losses due to the average flow and its pulsatile components of low frequency.

Flow disturbances observed in the arterial tree appear to be similar in character to those found in the present study. It is unlikely that they can account for significant discrepancies between experimental and theoretically predicted pressure-flow relations.