Objective   To clarify the urodynamic mechanism of upper urinary damage due to voiding dysfunction.      Methods   The medical history

radiographic studies and urodynamic results of 216 patients with voiding dysfunction who were treated at our hospital from January 1999 to June 2008 were reviewed retrospectively. The upper urinary tracts were assessed by type-B ultrasonic and excretory urography to determine the incidence of hydronephrosis. The relationships between the urodynamic variables

including residual urine

vesical pressures to such upper tract deterioration w

ere evaluated using both univariate and multiple logistic regression analysis.    Results   Bladder outlet obstruction (89.4%

n=185) was the main pathologic change in 216 patients. Upper urinary tract damage occurred in patients with lower residual urine (50 cmH

2

O). The vesical pressures in 70.1% (n=145) of patients were higher than 40 cmH

2

O. The average volume [(438.4±81.7) ml

]

at the first desire to voiding was higher than that of safety volume [(357.9±73.3) ml

P0.01

]

which meant it would damage to upper urinary tract.     Conclusion   The 40cmH

2

O of vesical pressure is a cutoff point and safety pressure. The vesical pressure lower than 40 cmH

2

O is critical for avoiding of hydronephrosis caused by voiding dysfunction. Keeping bladder from filling after the safety pressure is the cardinal risk factor for hydronephrosis. To keep the vesical pressure under safety value and voiding before safety volume could prevent upper urinary from damaging.