I. Karnak et al., The effects of flow rate, length and external pressure upon the pressure required for fluid to flow through a ureter, BJU INT, 88(4), 2001, pp. 335-338
Objective To determine in vitro the effects of increments of external press
ure on the pressure required to conduct fluid through ureters of various le
ngths at different flow rates, as the flow of a fluid through a collapsible
tube is influenced by various factors (e.g. external pressure, the pressur
e gradient between the ends, the length and diameter of the tube, and the v
iscosity of fluid).
Materials and methods Two in vitro systems were designed, composed of three
parts, i.e. a perfusion line, an exit line and a container of two differen
t widths in which short or long ureteric segments, obtained from cattle, co
uld be placed: the ureter was connected to the perfusion and exit lines. Ph
ysiological saline was added to the container until the desired external pr
essure was applied to the ureter. The flow pressure (height of the perfusio
n line) was recorded when producing flows through ureters of varying length
at 1.5 and 6 mL/min, and determined under various external pressures. The
intra-ureteric pressure during flow was also monitored by a pressure transd
ucer. The four combinations of long and short ureters with high and low flo
w rates were compared using analysis of variance, with the Pearson correlat
ion coefficient used to evaluate the relationships between the various pres
sures.
Results There were close relationships between flow pressure and external p
ressure (r = 0.727). intra-ureteric and external pressure (r = 0.766), and
the flow pressure and intra-ureteric pressure (r = 0.940, all P < 0.001). I
ncrements in external pressure resulted in greater flow and intra-ureteric
pressure (P < 0.05). Increases in flow pressure were more pronounced than i
ncreases in intra-ureteric pressure at the same external pressure (P < 0.05
) at high flow rates. A longer ureter and higher flow rates caused greater
intra-ureteric pressure (P < 0.05).
Conclusion External pressure increases the pressure required to conduct flu
id through a ureter and the effect is more pronounced at high flow rates. T
he length of the ureter also affects the flow pressure at high flow rates.
Therefore, flow through the ureter follows the Poiseuille equation only at
high flow rates. Thus, increases in intra-abdominal pressure may cause grea
ter intrapelvic pressure and induce ureteric obstruction, contributing to t
he pathogenesis of hydronephrosis.