Strengths
In accompanied by the well-accepted cystometry, this study for the first
time establishes the PVA of voiding cycles in human subjects. Stepwise
analysis of the thermodynamic processes reveals that during the filling
(F; Figure 6A); accumulating fluid gradually increases bladder volume
with a slightly but progressively elevated baseline pressure; thereby an
amount of potential energy represented by the area under the trajectory
along the volume change is stored in the bladder.30Next, in the isovolumic contraction (IVC; Figure 6B), the bladder
contracts without fluid elimination. For there is no shortening in the
detrusor, the bladder performs no mechanical work; instead, it gains
potential energy caused by contraction-increased
stiffness.31 In the subsequent emission (E; Figure
6C), the bladder performs a mechanical work characterized by the
integration of the trajectory along the volume change to repulse
fluid.30 Finally, the bladder isovolumically relaxes
without fluid elimination (IVR; Figure 6D). For the detrusor length
remains unchanged, the bladder gains no mechanical work. Nevertheless,
the potential energy of the bladder is reduced by the
relaxation-decreased stiffness.31
Despite this model obviously neglects the kinetic energy of fluid moving
and the friction energy between fluid and the bladder, we suggest the
trajectory-enclosed area coarsely quantifies a simplified but neat
mechanical work performed by the bladder in a voiding cycle (net; Figure
6E);30 and in accompanied with a cystometry assaying
time-domain events, an additional PVA which represents the voiding work,
will benefit to clinicians because lasting increased work loading could
be a risk factors causing un-compensatory bladder
functions.17