[Urodynamic consequences of urethral stenosis. Hydrodynamic study with a theoretical model].

An urethral model was designed to assess the hydrodynamic consequences of stenosis of the bulbar urethra. This model was based on the geometric and hydrodynamic analysis of micturing urethrograms in a patient whose maximum flow rate was 24 ml/s for a bladder pressure of 40 cm of water. It corresponds to the conditions of flow observed wit maximum bladder pressure and flow rate. During this short period, the shape of the urethra is regarded as stable. Calculation took account of the characteristics of a turbulent flow of urine, of the head loss due to friction of the urine on the walls and of the head loss caused by the geometrical changes of the urethra from the neck of the bladder to the meatus. In these conditions, applying Bernouilli's equation allowed plotting of bladder pressure for various levels of urethral resistance. If there is no stenosis, the theoretical maximum flow rate of the model with a bladder pressure of 40 to 80 cm of water should be 19 to 26 ml/s. Calculation showed that a sudden reduction of the diameter to less than 2.5 mm occurring in a segment of the urethra with a diameter of 4 mm accounts for a flox rate lower than 15 ml/s, except when bladder pressure can exceed 100 cm of water. The decrease in the maximum flow rate caused by urethral stenosis is all the greater as bladder pressure remains low or normal, with other causes of head loss or with associated upstream dilatation. Lastly, progressive narrowing leads to a smaller head loss and will therefore be better tolerated than sudden narrowing to the same caliber. Inversely, a long stenosis will be less well tolerated as the head loss due to friction in a long and narrow passage is increased. The diameter of a stenosis accounting for a maximum flow rate of less than 15 ml/s cannot be determined in the absolute. Analyzing the hydrodynamic consequences of stenosis also requires knowing the associated urodynamic and geometric parameters.