The single compartment model applied to the large renal pelvis: a preliminary study

Diuresis renography should exclude or confirm pelvi-ureteric junction obstruction. Numerous equivocal cases seen here had large renal pelves with slow emptying on frusemide diuresis. An attempt was made to relate the rate of emptying to the volume of the renal pelvis. The fully distended renal pelvis during frusemide diuresis appears to satisfy the criteria of the single compartmental model which predicts a linear relationship between volume and flow rate, i.e. F = [0.693 V0]/t1/2 where F is the flow rate, V0 is the volume and t1/2 the time at which half the tracer has disappeared. Clearance half-time was measured from the diuresis renogram curve. Volume was estimated from a longitudinal ultrasound image at the end of the procedure, using a volumetric version of Simpson's rule as a sequential slice technique, given a roughly circular transverse image. Of 18 kidneys in 14 adult patients, three had large volume pelves and short emptying times. The remaining 15 showed a roughly linear relationship between volume as estimated and clearance half-time. This appears to validate both the theory and the method and permits calculation of the pelvic outflow rate. An hypothesis to explain the large unobstructed renal pelvis is advanced.