Hypothesis / aims of study
In a fluid with high Reynolds number, Newton’s law of resistance can define resistance of the fluid passing through a narrow duct. Flow resistance of fluid with high velocity can be reduced by vorticity. Indeed, vorticity is observed in the draining of bathtubs, water bottles, toilets, etc. 1, 2 Thus, vorticity is a key physical phenomenon for fluid dynamics when a fluid passes through a narrow duct including urethra of human. Although pressure flow urodynamic study is the main method of assessing voiding mechanisms in the lower urinary tract (LUT), there have been few studies regarding fluid dynamics of the LUT. We hypothesize that vorticity, flow direction and velocity, may have important role to evaluate LUT function, and be a new urodynamic parameter. As the first step to establish a method for fluid dynamic assessment of LUT, high-speed video-camera was used to observe voided urine from external urethral orifice. The aim of this study is to develop a method to evaluate fluid dynamics of urine flow, especially vorticity.
Study design, materials and methods
The inclusion and exclusion criteria of the third sub-study is the same as the first sub-study. Evacuated urine stream in the air was observed by high-speed video-camera (FASTCAM MiniAX, Photoron, Japan). The frame rate is 1000fps, and the shatter speed is 1/100000s to 1/5000s. And, the observation was done with direct lighting. Urine micturition was done in standing position under monitoring urine flow using uroflowmetry synchronously. The evacuated urine was observed in all of the process of urine micturition. We focused on the shape of evacuated urine stream at Qmax and terminal voiding to compare the evacuated urine shape and velocity of urine.
Two healthy volunteers with 0 of international prostate symptom score participated in the third sub-study. The evacuation of urine was observed in 4 times in total. The mean of Qmax and voided volume is 20.1 mL/sec and 365.0 mL, respectively. The anti-crock vorticity of evacuated urine flow was observed in all of observation. The representative findings high-speed camera images at Qmas and the terminal voiding were shown synchronizing with uroflowmetry in Figure. The vorticity was clearly observed in all of the process of urine evacuation. The first twist, the beginning of vorticity was detected at 6 cm from the external orifice approximately, when the urine flow reached at Qmax (fig. A). The distance from the external urethral orifice to the first twist (arrow head) changed in accordance with the urine flow rate (mL/sec). The voided urine generated vorticity closing to urethral orifice, and the vorticity changed to spiral shape increasing distance from urethral orifice, especially at the end of urine micturition (fig. B-F).
Interpretation of results
This is the first report to detect of urine vorticity and to investigate the relationship between vorticity and LUT function. As shown in the obtained results, high-speed video-camera is useful assessment to evaluate urine vorticity in voided urine dynamically and safely. Newton’s low of resistance, according to high Reynolds number may be the main cause of vorticity formation. Moreover, residual vorticity for body movement and jet urine stream from ureter, Coriolis force affected by the earth rotation, shape of organs such as bladder and urethra may affect vorticity formation. Further investigation for mechanism and roles of vorticity formation should be done.
Flow dynamical assessment is an undeveloped approach to LUT function. Although we have to, of course, consider the limitations and uncertain roles of this approach, the possibility to understand LUT function in a new perspective has been opened. By measuring “volticity” of urine flow, we may be able to predict urine resistance against urethra, to diagnose deformity and dysfunction in LUT, to establish new concepts of pathological condition inducing LUT symptoms, and to develop new treatment option for urine micturition. The results of the present study are a meaningful first step to investigate LUT in the aspect of flow dynamics.