[Instantaneous pressure/flow ratio for non-permanent flows. Impact on the assessment of urethral resistance]. / Relation pression/débit instantanée pour des flux non permanents. Conséquences sur l'évaluation de la résistance uréthrale.

ABSTRACT

OBJECTIVE: To verify the validity of the hypothesis of the almost continuous flow necessary for evaluation of instantaneous urethral resistance R by the pressure/flow relation R = P/Q2. METHOD: An experimental device allowed investigation of the pressure/flow relation for discontinuous flow in a flexible tube. The tube was submitted to various collapsing pressures and opened under the effect of flow. The flow was discontinuous because of the variations of the proximal pressure or oscillations in the hydraulic diameter of the tube. RESULTS: These experiments showed that the approximation of continuous flow is justified within the limits of precision of the measuring methods used in urodynamics. CONCLUSION: The formula R = Q2 is therefore theoretically correct and validated by the experimentation. There is no hydrodynamic obstacle to the application of this formula to the calculation of instantaneous urethral resistance.