Analysis of bladder-outlet function with the linearized passive urethral resistance relation, linPURR, and a disease-specific approach for grading obstruction: from complex to simple.

The concept of the passive urethral resistance relation (PURR) to quantify bladder outflow conditions in few parameters from the complex pressure/flow relation is generally accepted. The most simple, yet realistic, linearized format is the linear PURR (linPURR). This two-dimensional format allows clear identification of individual outflow conditions with distinction of different obstruction types. Unequivocal grading of obstruction, however, requires a one-dimensional format. Theoretical considerations show that voiding function can be completely defined by a single parameter only when detrusor strength and obstruction type are uniform. This can be achieved with a disease-specific approach such as our pressure/flow diagram, which is specific for prostatic obstruction. It allows grading of obstruction stepwise for clinical decisions making or on a continuous scale for statistical applications using the maximal flow rate with related detrusor pressure as a single data point alone. Adding the actual linPURR to the diagram offers the unique feature of inherent conceptual quality control, relevant for individual assessment. The detrusor-adjusted mean PURR factor (DAMPF) is an alternative format of reducing the PURR to a single number, excluding at least the impact of variable detrusor strength, a conceptual advantage when the obstruction type is less uniform. The voiding pressure at maximal flow is a suitable parameter for most simple obstruction grading. Its validity can be significantly enhanced only when it is used in a disease-specific format, such as our pressure/flow diagram in combination with linPURR and DAMPF. Computerization does not improve the results of manual graphical analysis. Much more important is the clear conceptual definition and transparent application. More sophisticated computer-dependent methods such the original PURR/DURR and the three-parameter model can abstract more detailed information about outflow conditions, which requires expertise in their application and perfect data quality, but this does not result in better obstruction grading.