Women sometimes void with high flow rate and low detrusor pressure (Qp) during urodynamic studies. A presumed mechanism is complete pelvic floor relaxation. However, during a normal void, forces possibly affecting the urethral flow include not only pelvic floor muscle relaxation but also urethral wall elasticity, sphincters tone, and possible abdominal straining. The flow governing zone is the location of the main sonic transition and may be deduced from pressure-flow studies (PFs) [1]. Our hypothesis was that the primary mechanism resulting in a Qp void was an increased urethral wall expansibility, with an amplitude inversely proportional to the density of urethral elastic fibers.

From urodynamic recordings of 222 women investigated for various lower urinary tract symptoms and no obstruction, 27 women exhibited a high flow/low (Qp) voiding pattern during their PFs study. Of note, those urodynamic studies were carried out with a 7 Fr triple lumen catheter allowing for urethral pressure recording.  
More expansible urethra was described in the VBN mathematical model of micturition [2] by multiplying the standard elasticity by a parameter called urethral resistance to dilatation (URD) whom value was lower than one in a Qp void.

In the 27 women, mean age was 66±11 years, range [42-88], predominant urinary complaint was stress urinary incontinence (9), mixed incontinence (9), frequency (5), urgency (2) and recurrent urinary tract infection (2). Urodynamic diagnoses were detrusor hyperactivity with impaired contractility (5), intrinsic sphincter deficiency (11), voiding with only relaxation of the urethra (9) and “normal” (2).  Mean pdet.Qmax was 7.5±4.7 cm H2O with an associated high flow rate: Qmax = 27±6 mL/s. Bladder voiding efficiency (BVE) was > 90% and no abdominal straining was observed during the intubated flow.
Urodynamic diagnoses were detrusor hyperactivity with impaired contractility (5), intrinsic sphincter deficiency (11), voiding with only relaxation of the urethra (9) and “normal” (2).  Introducing URD in the VBN computations allowed a good fitting with the recorded curves; figure shows computations with 2 values of URD for a Qp voiding. Mean value of URD for the 27 women was 0.36 ± 0.27.

In the absence of obstruction, the sonic transition in women is located at the meatus (outside of the abdominal compartment) [3], so the driving pressure is the bladder pressure in addition to an altitude component, which is due to the difference in height between the bladder and the urethral meatus when the urethra is well-supported. That last component cannot be negligible in Qp voidings because the detrusor pressure is very low. If the sonic transition remains at the meatus, variations in flow are due to local changes in urethral wall elasticity. Modeling allows an explanation of this intriguing high flow/low pressure voiding phenomenon, which involves a marked change in the elasticity of the urethral wall. Future studies will focus on histological urethral wall changes to possibly confirm these modifications in urethral elasticity.

Mathematical modeling of micturition allows proposing an explanation of voidings with high flow rate and low detrusor pressure in women which would be an increased expansibility of urethral wall.

Legend of the figure: A good fitting between recorded and computed curves is only obtained with introduction of an increased urethral expansibility.

Griffiths DJ. The mechanics of the urethra and of micturition. BJU 1973; 45: 497-507
Valentini FA, Besson GR, Nelson PP, Zimmern PE. Clinically relevant modelling of urodynamics function: The VBN model. NAU 2014; 33(3): 361-66. doi 10.1002/nau.22409
Besson G, Valentini FA Nelson PP. Progress in the theory of flow through the urethra during micturition. in ICS 26th Annual Meeting 1996; 39-43. G.Barbalias ed.