Flow rates are dependent upon bladder volume and age. In carrying out a urinary flow rate, particular attention needs to be paid to certain factors as they can influence the result obtained:

a.Voided volumes of less than 150–200 mL can lead to erroneous results and should be repeated. High voided volumes > 600 mL may lower flow rates by over stretching the bladder resulting in decreased detrusor contractility.

b.If possible the patient should be in favorable surroundings and should not be stressed unduly.

c.Whether the patient is voiding standing, or bending backward, forward, to the side, or half-standing.

d.Whether the flow rate is a so-called free flow rate occurring after natural filling or after mechanical filling at the end of a urodynamic testing or following for example a retrograde bladder filling as part of a stress test or an office cystoscopy

Normal Flow: As discussed normal flow of the human lower urinary tract depends upon integrated coordination of the neural control of the bladder and outflow tract, for which an intact spinal cord is essential (Fig. 20.3a).

Under normal circumstances in adults:

•Bladder capacity is approximately 500 mL and the bladder empties, leaving no residual urine;

•Males void at a pressure of 40–50 cm H2O and a maximum flow rate of 30–40 mL/s; and

•Females void at a pressure of 30–40 cm H2O and a maximum flow rate of 40–50 mL/s.

The difference between males and females is a consequence of the higher outflow resistance exerted by the male urethra. Thus women have a lower pressure system as less pressure is required to overcome the urethral outflow resistance. One must also take account of a man’s age when interpreting these values, as men over 60 years of age with no urinary obstruction will have flow rates of just over 15 mL/s.

Abnormal Flow: Abnormalities of flow as demonstrated by uroflowmetry involve certain characteristic flow patterns;

•Fast bladder – This is an exaggeration of the normal curve and may be due to a raised end fill bladder pressure associated with detrusor overactivity or due to a significantly low outflow resistance (Fig. 20.3b).

•Prolonged flow – This is a slow flow rate over a prolonged period of time requiring a long time to reach a maximum flow. This is frequently seen with Bladder outlet obstruction (BOO), although a poorly contractile detrusor may give this picture (Fig. 20.3c).

•Intermittent flow – This irregular spiking pattern is frequently due to abdominal straining to augment the pressure required to overcome a bladder outlet obstruction or to add intra-abdominal pressure to a poorly contractile detrusor. Rarely sporadic sphincter contractions may cause this pattern (Fig. 20.3d).

•Flat Plateau – A low maximum flow rate which plateaus throughout the majority of the void, like a “box,” is characteristic of a urethral stricture (Fig. 20.3e).

Post Voiding Residual

Ultrasound is combined with a flow rate to provide more detailed information on bladder function. This is a routine investigation for all patients with voiding disorders seen as outpatients; an alternative is to measure the residual by catheterization.

The full bladder is scanned, the patient voids into flow meter in private and a post voiding scan is carried out to assess bladder residual. Interpretation of the flow rate takes into account the factors mentioned above. Any form of ultrasound probe allowing adequate visualization of the bladder is used. The patient should be scanned at the time that they feel “full” thereby providing an idea of the functional bladder capacity.Similarly the patient should be scanned as soon after voiding as possible in order to provide accurate assessment of the true bladder residual.

Figure 20.3. (a) normal – rapid change before and after peak flow, (b)“Fast Bladder” – exaggeration of normal, (c) Prolonged flow – delayed time to maximum flow and delayed overall void time, (d) intermittent flow – due to abdominal straining, (e)“Box” – showing a long plateau.22

a

Flow rate (Q)

(ml/s)

Ensure that the patient has a subjectively full bladder prior to carrying out the study to provide a representative result. Make sure that the study is carried out in circumstances where the patient is relaxed, so as not to introduce error into the results obtained.

This test provides data on bladder capacity,flow rate, and post voiding residual producing a more detailed assessment of the lower urinary tract function than a flow rate alone.9 It can be carried out easily with little specialized equipment, is noninvasive and does not use ionizing radiation.

It is of particular value in the follow up of patients attending clinics. For instance with a hypocontractile detrusor following surgery for the relief of obstruction or where it is suspected that voiding efficiency may have been compromised e.g.after a repair procedure for stress incontinence.

Patients awaiting an intravenous urogram for imaging of the upper urinary tracts, for another clinical indication, may undergo a complete intravenous urodynamogram, where the addition of uroflowmetry provides a comprehensive assessment of the lower urinary tract, detailing