Bladder Strain Softening Quantification

A UD protocol was designed to directly translate our preclinical methodology established to quantify adjustable preload tension in isolated DSM strips^{4 (link)–11 (link)} to human UD studies. To accomplish this, an extended repeat fill protocol with passive emptying and active voiding for specific fills was developed (Fig. 1A). An initial UD study was performed per best practice guidelines for clinical purposes and to determine maximum cystometric capacity (CCap).^{13 (link)} For the initial fill, the fill rate was set at 10% of patient-reported maximum voided volume per minute on a 3-day void diary completed prior to the study. When the patient acknowledged that they had reached their maximum capacity (defined as inability to tolerate further filling or the presence of an involuntary bladder contraction), filling was stopped and the patient voided. Any post void residual was removed through the catheter via syringe aspiration, and the bladder was confirmed to be empty by an ultrasound technologist with transverse and sagittal midline suprapubic images taken at the onset and conclusion of each fill-and-empty cycle with a Philips Epiq 7 system with a 1–5 MHz abdominal probe (Amsterdam, The Netherlands). For all subsequent fills, CCap was defined as the sum of the voided volume and post void residual from the initial fill. Multichannel digital pressure and flow data acquisition was performed at 10 Hz via an Aquarius TT^™ system (Laborie, Toronto). Four repeat fills were then initiated at a rate 10% CCap/min as follows: (i) fill to 30% CCap and passively empty; (ii) fill to 60% CCap and passively empty; (iii) fill to CCap and void (voluntary or involuntary); and (iv) fill to 60% CCap and void (Fig. 1A). Fills 1 and 2 were set to 30% and 60% CCap, respectively, to avoid triggering an active contraction which would limit the ability to identify the process of strain softening. Also, previous studies have used 33% stretches to effectively strain softening DSM strips.^{9 (link)}Passive emptying at the end of Fills 1 and 2 was performed via syringe aspiration through a three-way stopcock in series with the infusion catheter so as not to alter calibrated pressures. The bladder was confirmed to be empty by ultrasound as previously described. Repeat Fills 1–2 were performed to progressively strain soften the bladder with incrementally increasing volumes (Fig. 1A). In a fluid-filled thin-walled vessel, pressure is directly related to wall tension by the Law of Laplace (tension ∝ pressure × radius); therefore, strain softening reflected by decreased wall tension should be identified by comparing the change in luminal pressure between fills. For the present protocol, the degree of strain softening was determined by comparing average intravesical pressure (p_ves) from 0% to 30% CCap in subsequent passive fills. The expected result was a progressive decrease in pressure (p_ves1 > p_ves2 > p_ves3) consistent with strain softening (Fig. 1B). The active void after Fill 3 was expected to reestablish the lost tension and demonstrate the reversibility of strain softening because preclinical studies show that active contraction reverses strain softening in DSM strips.^{4 (link)–11 (link)} Thus, Fill 4 was expected to demonstrate p_ves similar to that observed in Fill 1.