[Urodynamic assessment of the voiding phase in childhood]

To determine the more useful parameters of urethral resistance and bladder contractile power to explain the values of maximum flow and postvoid residual urine observed in free uroflowmetry and appropriately assess the voiding dynamics in childhood.

A cross-sectional study was carried out in 68 children (46 girls and 22 boys) with a mean age of 8.9 years. These children were subjected to free uroflowmetry in order to determine the maximum flow rate and the postvoid residual urine, and to a complete urodynamic study with perineal surface electromyography in order to determine the parameters of urethral resistance and bladder contractile power. A univariate linear regression model was built from these parameters for explanatory purposes. The independent variables were the parameters of the urodynamic study and the outcome variables were the value of the maximum flow in the free uroflowmetry and the proportion of postvoid residual urine in relation to the voiding volume of the free uroflowmetry. The resulting models were validated in order to determine its loss of predicting power (shrinkage). For this purpose, we used two other independent series: one comprised 50 patients (34 girls and 16 boys) extracted from the same sample as the derived model, and the other comprised 85 patients (53 girls and 32 boys) from another population.

The multivariate model demonstrated that the maximum flow of the free uroflowmetry has a direct relationship with the square root of the voiding volume and the bladder contractile power [measured by Schafer's projected isometric pressure (PIP)] and an inverse relationship with the urethral resistance measured by means of a parameter (URA) and with the presence of electromyographic perineal activity (EMG) during voiding. Similarly, the proportion of postvoid residual urine has a direct relationship with the urethral resistance (measured by means of the parameter URA) and with the inverse function of the projected isometric pressure (I/PIP). The shrinkage of the multivariate models, when compared with other series, ranged from 26.9% and 1.3%.

The urodynamic evaluation of the lower urinary tract in childhood can be carried out appropriately by means of the determination of the urethral resistance by the parameter URA and of the bladder contractile power by the projected isometric pressure (PIP), obtaining as a limit of normal values a URA of 16 cm H2O and a PIP of 105 cm H2O.