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.