How to measure urethral elastance in a simple way. Elastance: definition, determination and implications.

The elastance of a biological tube describes the resistance of the latter to dilatation. It is defined as dP/dV, where dP is the pressure increase caused by the volume increase dV. Elastance is the reciprocal of compliance. Elastance in the female urethra can be estimated from the slope of the regression line of related values of pressure and cross-sectional area. In the present study, urethral elastance was calculated by measurement of the related pressures and cross-sectional areas during stepwise dilatation by a balloon and by determination of the pressure at which inflow through side holes in catheters with increasing diameters began. There was no difference between the elastance values obtained by the two methods. Due to the linear correlation found between pressure and cross-sectional area we conclude that urethral elastance can be estimated from measurements of urethral pressure at two or more related cross-sectional areas by a simple technique using e.g., 8F, 14F, and 20F catheters.