Variations in Upper Urinary Tract Outflow Resistance

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Abstract
The upper urinary tract of the pig was subjected to acute standardized obstructions, size 10-F, 8-F and 6-F, in both the proximal and the distal part of the ureter Each obstruction was followed by measurements of the renal pelvic pressure–flow relationship between 0 and 20 ml/ureter/min. The study showed that the mean baseline pressure in the renal pelvis changed due to the obstruction with an increase related to the degree of obstruction. When the flow was increased a pressure–flow relationship in essence similar to the normal pressure–flow relationship was revealed. Thus phase 1, which is the pressure–flow relationship at low flow rate with urine flowing into a relaxed ureter, was practically eliminated and phase 2 was shifted to the left. The linear pressure–flow relationship in phase 3 was preserved but there was tendency for higher increments with increasing degree of obstruction. Even in cases of severe obstruction both baseline measurements and perfusion pressure measurements showed results well within the normal accepted limits of renal pelvic pressure. It was furthermore shown that the overlap was smallest at the low flow rates between 0 and 2 ml/ureter/min. All degrees of distal obstruction had a lower pressure at flow rates below 10 ml/ureter/min than proximal obstruction, a pressure difference which could be attributed to the peristaltic activity of the ureter. The study stresses the importance of full pressure–flow relationship investigation when the upper urinary tract is to be characterized in terms of its transport capacity.