Mechanisms of female urinary continence under stress: Frequency spectrum analysis

Intravesical and urethral pressure signals during cough and Valsalva maneuvers for 15 continent women were analyzed with frequency spectrum analysis. Clear modulation of the urethral pressure changes by the intravesical pressure rise during stress maneuvers was demonstrated in the frequency bands of 14 and 7Hz for cough and Valsalva, respectively. The linearity between the urethral and intravesical pressure signals was strong for cough, but relatively weaker for Valsalva. The observed linearity lead to the formulation of a modified continence equation to mathematically quantify stress leak point pressure (sLPP): sLPP=MUCP/(1-α1)+RBP. This algebraic equation demonstrated that sLPP depends on pressure transmission, resting bladder pressure, and maximum urethral closure pressure. The equation was validated with excellent theoretical predictions for the 15 continent subjects (R²=0.98 and 0.97 for cough and Valsalva leak point pressure, respectively) and good but somewhat weaker predictions for 46 stress incontinent women (R²=0.79 and 0.48, respectively). It has been shown that pressure transmission plays the most important role in female continence function, while it may be attributable to passive structural origin as evidenced by the minimal time delay between the two pressure signals, in the order of a few milliseconds. It can be concluded that coughing seems to have a more mechanical, rather than neuromuscular basis for its signal dynamics. This study suggests that a complete assessment of female stress continence function requires comprehensive urodynamic information in terms of pressure transmission, maximum urethral closure pressure, and resting bladder pressure.