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