Hydrodynamic aspects of bladder-outlet obstruction: consequences of functional micturition disorders

Functional disturbances of micturition lead to secondary structural changes. Both factors may change the physical properties--shape, elasticity and consistency--of the posterior urethra and of the bladder neck. In order to demonstrate their influence on urodynamics experimental studies were undertaken, elastic models, straight tubes of equal diameter throughout their length, were used. One model was a straight elastic tube of PVC material, three other models had nonelastic segments of different shape but with the same square area as the undeformed elastic parts. Perfusion was performed under equal conditions. The experiments showed firstly that nonelastic segments in an elastic system reduce the flow rate and secondly, that flow depends on cross section versus circumference, which is expressed by the so-called hydraulic diameter. Elasticity, consistency and shape of the bladder-outlet, frequently altered in patients with neurogenic bladder disease, are therefore important factors regarding loss of friction. Our results support the clinical experience, that a functional obstruction has to be eliminated before secondary structural changes occur which alter the physical properties of the bladder outlet and therefore make flow conditions even worse.