Resistance to forced opening of the resting urethra in healthy females assessed by experimental simulation of urine ingression

A finite element model validates an external mechanism for opening the urethral tube prior to micturition in the female

PURPOSE

Existing theories for micturition in the female mandate total relaxation of the pelvic floor while detrusor pressure pushes the urethra open. However, video X-ray and electromyogram data indicate that micturition is preceded by active outwards opening of the outflow tract by backward-/downward-acting muscle vectors. If the detrusor pressure alone is enough to expand the tube, why does the active opening take place? The aim was to model the urethral tube in detail to assess the relative importance of the active opening mechanism and detrusor pressure.

METHODS

Finite element methods were used to model the urethral tube in detail. Nonlinear-elastic properties similar to urethral component tissues were taken from the literature. The boundary conditions applied to the tube model included internal pressure due to detrusor contraction (60 cm H2O) and various displacements and constraints such as pubourethral and pubovesical ligament attachments.

RESULTS

In order to achieve opening dimensions similar to those in the lateral X-ray under the action of detrusor pressure alone, the pressure had to be increased by two orders of magnitude above normal levels.

CONCLUSIONS

Normal detrusor pressure alone is not sufficient to achieve opening of the urethra against the elasticity of the constituent tissues, suggesting that normal micturition requires an active mechanism provided by backward-/downward-acting pelvic floor muscles, as predicted by the integral theory.

Chronology and urodynamic characterization of micturition in neurohormonally induced experimental prostate growth in the rat

The purpose of this study was to evaluate the impact of chronic urinary tract obstruction which was produced in the rat using neurohormonally induced experimental prostate growth. In this model, we considered the chronology of changes in the micturition characteristics of awake rats relative to prostate weight and stiffness. The corresponding urodynamic characteristics of both the upper and lower tracts were evaluated in anesthetized animals relative to the development and extent of the obstruction produced. Prostate growth was produced by capitalizing on the synergistic properties afforded by the combined administration of dihydrotestosterone propionate (DHT) and the alpha1 adrenoreceptor antagonist prazosin (PRZ). DHT (1.25 mg/kg/day) was dissolved in 0.1 ml sesame oil (SO) and coadministered with PRZ 30 microg/kg/day subcutaneously for 14 days to 12 experimental rats. SO alone was given to 8 control rats. Micturition studies were first performed using all 20 awake rats, which were placed unrestrained in metabolic cages. Urodynamics of the upper and lower urinary tracts were repeated following anesthesia at the 5th, 10th, and 15th weeks after initiation of hormonal or SO treatment. Following the urodynamic studies, the rats were killed and prostates were removed and weighed, and stiffness was measured. Studies with awake rats show that hormonal treatment produces a significant and progressive increase in mean frequency of micturition, ranging from 0.63+/-0.16 in controls and reaching the maximum of 2.15+/-0.40/hr by the 10th wk. Results from urodynamic studies with anesthetized rats also show typical and progressive obstructive characteristics: maximum detrusor voiding pressure (Pdetmax) increased from 52.7+/-2.03 in controls to a maximum of 77.5+/-2.2 cm H2O by the 10th week; urethral opening pressure Puo likewise increased from 52.6+/-2.7 in controls to 73.3+/-2.1 cm H2O in experimental rats. The duration of time during which the detrusor sustains contraction during voiding also rose, from 16.8+/-1.8 sec in controls to 32.0+/-3.2 sec by the 10th week. There were no significant changes in bladder capacity, baseline filling pressures, or arterial pressures. Prostate weight increased significantly from 0.76+/-0.05 g in controls to 1.17+/-0.1 g by the 15th week. Similarly, stiffness increased from control values of 1.33+/-0.18 g/cm to a maximum of 3.59+/-0.14 g/cm by the 10th week. It is concluded that neurohormonally stimulated prostate growth in the rat is a suitable animal model for the study of the development of urinary tract obstruction. Obstructive characteristics were validated in both awake rats by the increase in the frequency of micturition and urodynamically under anesthesia in terms of elevations in maximum detrusor pressures, urethral opening pressure, detrusor contraction time, and prostatic stiffness. The effect of obstruction was further shown to be associated with vesicoureteral reflux during micturition and elevated upper tract pressures.

The urethral resistance to rapid dilation: an analysis of the effect of autonomic receptor stimulation and blockade and of pudendal nerve blockade in healthy females

The urethral closure function is based upon permanently as well as adjunctively acting closure forces during rest and stress episodes, respectively. During urine ingression intra- and peri-urethral structures are suddenly stretched resulting in a pressure response which strengthens the closure function by sustaining the resistance to dilatation of the urethra. A method for measurement of the resistance to rapid urethral dilatation was used to evaluate the influence of noradrenaline, prazosin, terbutaline, propranolol, carbachol, and atropine, as well as bilateral pudendal nerve blockades in 40 healthy women. The drugs caused no significant change in the urethral resistance to dilatation, whereas the pudendal blockade produced a significant (p < 0.05) reduction at the bladder neck and in the high pressure zone. Although a quantitative assessment of their contribution cannot be obtained from this study, it may be concluded that the striated muscles innervated by the pudendal nerve are of major importance for the urethral resistance to dilatation.

In vivo evaluation of visco-elasticity in a biological tube. Part 2. Application of a mechanical model

Based on a theoretical model, the urethral stress-relaxation response following a sudden forced dilatation was studied in ten healthy female volunteers. The energy dissipation following a sudden urethral dilatation proved to follow the function Y = Z + C alpha e-t/tau alpha + C beta e-t/tau beta. Hence, a mechanical model was chosen, consisting of two Maxwell elements and one Hooke element coupled in parallel. The decay in force following deformation may be described as F = F(o) exp (-t.E/eta) for each Maxwell element. Thus, it was possible to determine the elastic E and viscous eta coefficients for the participating mechanical equivalents in the model, and thereby quantitatively describe the visco-elastic properties in the urethra. The reproducibility of the elastic and the viscous coefficients proved to be fairly high, and they seemed to be unaffected by the size of dilatation. On the other hand, the rate of dilatation clearly influenced the computed parameters. However, this phenomenon was easily explained by the duration of the deformation, which allowed the viscous elements to move before any measurements were performed. The present method permits in vivo evaluation of the elastic and viscous properties of the urethra, as well as other accessible biological tubes, and may be of value in the description of normal physiological and pathophysiological behaviour of the structures studied.

The dynamic pressure response to rapid dilatation of the resting urethra in healthy women: an in vivo evaluation of visco-elastic properties

The urethral pressure response to a sudden forced dilatation was studied at the bladder neck, in the high-pressure zone and in the distal urethra in ten healthy female volunteers. The pressure response was fitted with a double exponential function of the form Pt = Pequ + P alpha e-t/tau alpha + P beta e-t/tau beta, where Pequ, P alpha and P beta are constants, and tau alpha and tau beta are time constants; this equation has previously been demonstrated to describe the pressure decay following dilatation. On the basis of a theoretical model the elastic and viscous constants for the urethral tissues were computed. The results showed significant differences along the urethra, with the high-pressure zone showing the highest maximum and equilibrium pressures, fastest pressure decay and highest elastic coefficient. The pressure response represents an integrated stress response from the surrounding structures, which reflects the visco-elastic properties of the tissues involved. The findings seem therefore to correlate well with the anatomical findings, which have shown a high fibre density of the horseshoe-shaped rhabdosphincter in the mid-portion of the urethra. The method permits a detailed assessment of static and dynamic urethral responses to dilatation which can be applied as an experimental simulation of urine ingression, and is therefore presumed to be of value in the evaluation of normal and pathological urethral sphincter function.

The dynamic urethral sphincter function in healthy females assessed by rapid dilatations in the resting state: evaluation of reproducibility

Recording the pressure response to rapid dilatations enables one to assess urethral sphincter function. The rapid dilatations of the urethra experimentally simulate the ingression of urine. A method that enables standardized dilatations of 2-mm long urethral segments was urine (USED).A method that enables standardized dilatations of 2-mm long urethral segments was used, and the subsequent pressure response was measured. The reproducibility of the urethral pressure response to rapid and standardized dilatations was evaluated by repeating the measurements after 1 week in 10 healthy females. The pressure response showed an acceptable reproducibility. The urethral pressure increase as a response to dilatation is considered to rely on stretching of intra- and periurethral fibers. Calculations of strain (change in length divided by the original length) of circularly arranged fibers when the urethra is dilated indicate that the tension (and pressure) bearing layers must be located outside a circular zone with a radius of 4 mm perpendicular to the longitudinal axis of the urethra.