Main Findings
In contrast to studies investigating the urine storage,24 this study explored if the voiding function adapts in response to the TOT by specifically focusing on the thermodynamic efficacy of the bladder. Our results reveal the word performed by the bladder was increased post-operatively; that could be attributed to that bladder develops an elevated pressure gradient in response to the TOT-enhanced outlet resistance during urine emission.
Our conclusion is based on lines of evidence. First, in consistent with studies showing TOT adds-on outlet resistance during storage to restore adequate continence,12,25 our data demonstrated TOT post-operatively increased the Rv (mean resistance) during voiding. While no change was evidenced in the Vv (voided volume) or Tv (voiding time), Pv (mean voiding pressure) was elevated post-operatively in accompanied by the Rv increment. Considering the Rv was defined by the Pv divided by the Fv (mean voiding flow; Rv=Pv/Fv), and the Fv was further calculated as the Vv over the Tv (Fv=Vv/Tv); the relationship between the Rv, Vv, and Tv can be described as Rv=Pv/[Vv/Tv]. Based on this equation, unchanged Vv and Tv means the TOT-increased Rv is accompanied by a corresponding Pv elevation. Since TOT is well-accepted to add-on outlet resistance,12,25 these data imply bladder develops an elevated voiding pressure in response to the TOT-increased resistance.
Moreover, though the sample size is not very large, the TOT-increased Apv statistically correlated with the changes of Rv and Pv (ΔRv and ΔPv, respectively) but rather than that of Vv (ΔVv) or Tv (ΔTv), indicating the TOT-enhanced thermodynamic efficacy is ΔRv and ΔPv dependent. Notably, when examining the relationship between the TOT-induced ΔRv and ΔPv, the ΔRv was highly correlated with the ΔPv. Considering the above data showing bladder develops an elevated voiding pressure in response to the TOT-increased resistance, these findings together imply that the enhanced Apv during voiding is resulted from the ΔRv-associated ΔPv, i.e., the enhanced thermodynamic efficacy during voiding is resulted from the elevated bladder pressure in response to the TOT-increased outlet resistance.
This proposal is further supported by the PVA showing that instead of the left, right, and bottom boundaries, the upper bolder of the loop trajectory representing the pressure during the emission was markedly shifted upward post-operatively; and thereby increased the Apv. For the Pv was the mean bladder pressure during urine emission, this results imply the increased Apv is attributed to an elevated Pv. Collectively, these several lines of evidence reveal that after the TOT, the bladder compensatory develops an elevated pressure during the voiding to overcome the enhanced outlet resistance that as a result to increase Apv.
We suggest that before TOT, the bladder of SUI patients fail to develop a sufficient pressure gradient not only because the increment of bladder pressure results in leakage during urine storage but also it possibly causes premature urine flow that brings about an early pressure decline during voiding.26 Whereas, not restrictively to the storage, TOT-increased outlet resistance also prevents urine leakage during the early voiding that allows the bladder to develop an acceptable pressure gradient thereby ensures an adequate driving force for the urine flow during the whole voiding period.
An issue need to be concerned about is “Dose the elevated pressure is sufficient to overcome the TOT-enhanced outlet resistance during voiding?”. Our data reveal TOT affected neither Vv nor Tv post-operatively, indicating, at least not inferior to the pre-operative condition, the bladder is capable to void an acceptable urine volume within a satisfactory time period. Nevertheless, for we measured urodynamic parameters short after TOT, if the bladder is able to maintain sufficient contractility and thereby persistently develops adequate pressure gradient to overcome the everlasting resistance increment needs longer following-up.