The profile of a nonstop trans-vesical cystometrogram (CMG) pressure curve in urethane-anesthetized rats for the study of lower urinary tract physiology consists of three phases: an initial increase in intravesical pressure (Pves), a plateau period with intra-luminal pressure high-frequency oscillations (IPHFOs) accompanied by the expulsion of urine, and a rebound characterized by a further increase in Pves. The third phase has been discouraged from inclusion in the analysis of detrusor contractile dynamics due to the variability of this phenomenon. The objectives of this study were to investigate the effect of catheter implantation and urethane anesthesia on the contractile pattern of the bladder during voiding and to study conditions involved in the variability of occurrence of the rebound phase.

All animal procedures were performed according to the NIH guidelines, and protocols reviewed and approved by the locals Institutional Animal Care and Use Committees. Twenty-one female Wistar rats were used in three separate experiments. Experiment 1 (n=6): To study the effect of urethane on the bladder, a PE50-tube catheter was placed in the dome of the bladder (Standard) and tunneled under to the rat's dorsal neck. After 24 hours, a CMG on fully awake and freely moving rats was conducted, with the same rats undergoing urethane-CMG the following day for comparison. Experiment 2 (n=5): In order to determine the effect of bladder catheter implantation which requires an incision of the dome, CMG and electromyography (EMG) of the external sphincter of the urethra was done under urethane-anesthesia with placement of a polyethylene tube (PE10) catheter through the left ureter into the bladder (U1). After 10 voiding cycles, a PE60-tube catheter was implanted into the bladder dome and 10 more voiding cycles were recorded. Experiment 3 (n=10): To study the presence and variability of intra-vesical pressure in the third phase of micturition, four configurations of double urinary bladder catheterization were performed as follow: Dual Ureter (U2), PE-10 in the left ureter for filling and Pves measurement, and PE-10 in the right ureter for Pves measurement only. U1/Standard:  PE-60 for filling and Pves measurement in the dome, and PE-10 in the right ureter for measurement. U2 Urethra: PE-10 in the left ureter for filling and Pves measurement, and PE-10 in the right ureter for Pves measurement only and urethral transducer for Pves measurement. Standard Urethra: Dome catheter for filling and Pves measurement and urethral transducer for Pves measurement. In Experiments 2 and 3 bilateral electrodes were implanted in the external urethral sphincter (EUS) to perform electromyography (EMG)

In awake rats, CMG at 24-hours post-implantation as well as after 48-hours under urethane, the contractile curves were smooth (no IPHFOs or Pves spikes). Nonetheless, a decrease in the maximal bladder pressure (Mbp), and voided volume (VV), was seen with urethane-anesthesia relative to awake conditions. In rats where the bladder was filled via the ureter and the bladder remained intact, the Pves curve had a continuous ascending phase, even during the expulsion of urine, without IPHFOs in the plateau phase and rebound effects. Subsequent catheterization, via an incision of the vesical dome, resulted in the presence of IPHFOs in the plateau phase and a rebound Pves increase. Furthermore, the baseline pressure (Bp), ICI, VV, and the bursting time (Bt) was reduced by bladder catheterization. In the Experiment 3 U2 configuration, identical pressure curves were found as described for the U1 configuration. However, after an initial increase in Pves, a plateau period with IPHFOs accompanied by the expulsion of urine occurred but with a minimal or absent rebound pressure change (post-void rebound). U2/Standard configuration exhibited two different patterns of pressure waves: The first pattern (n=5) consisted of both recordings, ureteral and dome, with identical pressure patterns for post-void rebound (PvR). The second pattern (n=5) showed the post-void rebound pattern in the ureteral catheter. Meanwhile, in the recording obtained using the dome catheter, a noticeable increase of Pves, which started during bursting, was observed (early rebound). The early rebound (ER) pattern had an inverse correlation between IPHFOs time to Bt ratio (IPHFO/Burst ratio). The ER was evocated by twisting and/or pulling the bladder catheter. The PvR shape was found in records gained by ureter and urethra placement in the U2 urethra configuration. Standard urethra configuration showed the same dimorphism shown in U2/Standard, with the same relationship between IPHFO/Burst ratio and post-void Pves. Rats showing similar IPHFOs time and Bt, displayed variable PvR amplitude. The PvR amplitude has an inverse correlation with Bt.

Taken together, the present results demonstrate that an incision of the bladder wall for standard CMG assessments generates changes in detrusor contractile dynamics manifested as a decrease in the ICI, Vv, and Bt. Additionally, anesthesia has suppressive effects on the filling-voiding cycle. Furthermore, the use of a single tubing to infuse saline solution and measure the Pves modifies the shape of the pressure curve due to the resistance generated by the tubing. Previous studies have noticed a correspondence between Bt with urine flow time (Ft) in rats, as well as an equivalence between the duration of IPHFOs and Ft. However, our data shows that the duration of Bt and IPHFOs duration is not always the same in animals with a bladder dome implanted catheter. Furthermore, the standard dome catheterization induces early Pves increases in 50% of the animals, and this Pves increases do not match with the Pves measured through the ureter catheter or with a pressure sensor placed into the bladder through the urethra.  The amplitude of this ER has inverse proportion with the Bursting-IPHFOs ratio. Such data strongly suggests the artificial nature of this Pves increase, since the presence of bursting electrical activity and flow of urine demonstrates that the urethra remains open. We hypothesize that mechanical factors causes the ER observed in these animals by misplacement of the catheter, probably because the tip of the catheter is in touch with the bladder wall, increasing the resistance for saline flow. The consistency shown in all configurations regarding the presence of PvR suggests the accuracy of the Pves measured, as well as, the physiological nature of this Pves increase. Based upon the inverse relationship between BT and the amplitude of PvR, we consider that this pressure increase is the result of an early closure of the urethra while the detrusor is still contracting.

The use of the novel dual ureter configuration generates Pves recordings similar to those obtained with a single large catheter placed through the bladder dome, without any confounding variables. If standard dome placement is used for CMG studies, it will be necessary to distinguish PvR from ER by using the bursting-IPHFOs ratio. If EMG is not recorded, the Ft being longer than IPHFOs is indicative of an artificial rebound. We recommend that, if artificial rebound phenomenon is suspected, analysis of the third phase of CMG should not be performed.