Taper: An important feature of y-bifurcations in porcine renal arteries and human cerebral arteries

Abstract

The geometry of arterial bifurcations has been shown to alter fluid flow and the propagation of both pressure and flow waves. Here we provide a more complete description of the renal artery bifurcation geometry and show that the geometry of the bifurcation is more complex than was believed previously. The objective of this study was to quantify changes in cross-sectional luminal area in systemic arterial bifurcations using the method developed by Macfarlane [Ph.D. thesis, University of Western Ontario, Ontario (1985)] to study the geometry of human cerebral bifurcations. Porcine renal arterial bifurcations from seven young (6–14 weeks) and six old (>52 weeks) animals were pressue-fixed (P = 140 mmHg) for 3 h with 10% formalin. Bifurcations were embedded in a block of frozen latex paint. Serial sections were cut at 20 μm (±0.01 μm) using a sledge microtome while the block face was scanned with a video camera and the images were stored on a videotape. The luminal area was measured digitally upon playback. Bifurcations from the two age groups changed in cross-sectional area not only at the flow divider but also along the socalled straight regions. Proximal linear increases in cross-sectional area were observed proximal to the apex of the bifurcation in both young and old vessels, while linear luminal area decreases were measured in the daughter branches of both young and old porcine renals. Taper was defined as the change in luminal area per unit length of parent and daughter branches, respectively. All tapers were significantly different from zero and were used to compare luminal area changes in young and old renal arteries to human cerebral bifurcations. All vessels showed the same trend: negative taper, or expansion in luminal area, was present in the parent branch proximal to the apex of the flow divider while positive taper was present in the daughter branches. The magnitude of the taper present in both the parent and daughter branches from the two age groups of porcine renals and human cerebral bifurcations were not significantly different (P>0.05). The area ratio, defined as the ratio of the summed luminal area of the daughter branches to that of the parent branch, was not significantly different in the young and the old porcine vessels as well as from the optimum value (1.2). Taper provides a better representation of the changes in luminal cross-sectional area present at bifurcations since area ratios do not take continuous luminal area changes into account and are very dependent on the location of the area measurement. These large luminal area changes, which have been overlooked previously, have significant implications for systemic flow patterns as well as for the propagation of flow and pressure waves.