(1)
Functional Urology Unit, Casa Madre Fortunata Toniolo, Bologna, Italy
Classically, noninvasive urodynamics include uroflowmetry and post-void residual assessment.
6.1 Uroflowmetry
6.1.1 Definition
Flow is defined by the fluid expelled from the urethra per unit of time.
The test is a simple, safe, inexpensive, and noninvasive way of measuring and recording the urinary flow rate during micturition. The aim is to recreate a patient’s natural voiding pattern.
ICS terminology (Fig. 6.1):
Figure 6.1
Normal flow curve and pattern depicting the terminology of ICS
Flow rate (Q): Volume of fluid expelled via the urethra per unit time (ml/s)
Voided volume (V void): Total volume expelled via the urethra (ml)
Maximum flow rate (Q max): Maximum measured value of the flow rate after correction for artifacts
Voiding time: Total duration of micturition (s)
Flow time: Time over which measurable flow actually occurs
Average flow rate (Q ave): Voided volume divided by the flow time
Time to maximum flow: Elapsed time from onset of flow to maximum flow
6.1.2 Techniques of Measurement
There are two different types of flow transducer:
Weight transducer: It operates by measuring the weight of the hydrostatic pressure at the base of collecting cylinder. Weight variations of collecting device during micturition generate the flow curve.
Rotating disk: Urine stream is directed onto a rotating disk. The power needed to keep the disk rotating at a constant speed is proportional to the flow rate of urine.
Uroflowmetry does not require an elaborated setup. The instrument should be located in a clean, quiet room preferably lockable. In some case, it can be actually set up in a toilet.
In women, the examination is performed in a sitting position. In some cases, this also applies to man. Most often, however, the man voids in standing position through the funnel connected directly to the baker.
The chair ranges from simple commode-like chair to the most advanced electromechanical chair which is part of a multichannel urodynamic equipment.
The base plate containing the transducer can be connected to a single recorder unit (portable flowmeter) or to a multichannel recording device (integrated flowmeter). There are also “wireless” transducers that can be used with the PC (Fig. 6.2).
Figure 6.2
Wireless uroflowmeter. (a) recording unit, (b) micturition chair, (c) male voiding stand, both height adjustable. (Courtesy of Albyn Medical)
The ICS recommends specific standards for the equipment:
A range of 0–50 ml/s for Q max
A range of 0–1000 ml for voided volume
Maximum time constant of 0.75 s
An accuracy of +5 % relative to full scale
6.1.3 How to Read a Flow Curve
A voided volume of at least 150 ml is necessary to ensure a reliable interpretation of the curve. In reading a flow curve, two aspects should be considered:
Morphology of the curve
Numerical parameters
6.1.3.1 Morphology of the Curve
The flow can be continuous or intermittent. Flow patterns can be a useful mean for a presumptive diagnosis. The following samples are described:
- (a)
Normal: It is a bell-shaped curve with Q max reached in the initial one third of the void (usually 3–10 s) (Fig. 6.3).
Figure 6.3
Normal “bell-shaped” flow curve with Q max reached in the initial one third of void
- (b)
“Compressive” obstruction, e.g., BPH: Pattern of flow seems normal til Q max (lower than normal) with a terminal prolongation. Average flow is typically lower than normal.
- (c)
“Constrictive” obstruction, e.g., urethral stricture: A low Q max is rapidly reached, and the flow rate remains relatively constant, giving to the curve a plateau-shaped appearance.
- (d)
“Staccato” curve, e.g., dysfunctional voiding: Fluctuations in the flow curve due to burst of involuntary external sphincter contractions during voiding.
- (e)
Intermittent flow, e.g., abdominal straining or neuropathic sphincter dyssynergia: A flow that stops and starts several times during voiding (Fig. 6.4).
Figure 6.4
Flow curves: (a) continuous including compressive (BPH) and constrictive (urethral stricture) patterns (b) interrupted including “staccato” (dysfunctional voiding) and intermittent (abdominal straining and/or detrusor sphincter dyssynergia) patterns
- (f)
“Supervoider”: Very high Q max with very rapid upstroke and downstroke. Not diagnostic, but people (mostly females) with detrusor overactivity or stress urinary incontinence may have a flow rate at the top of the range (the so-called fast bladder) (Fig. 6.5).
Figure 6.5
“Supervoider” female flow curve
6.1.3.2 Numerical Parameters
Among the different parameters of a flow curve is the value of Q max to be commonly used in clinical practice.
Normal Values of Q max
They are different in males and females.
Footnote
Urodynamic evaluation of lower urinary tract is not a physiological test. Even if normality in UDS can be defined, tests must always be interpreted against patient characteristics, complaints, and symptoms.
In males with no bladder outlet obstruction, the value of Q max tends to decrease with age:
Under 40 years the value is usually over 25 ml/s.
Over 60 years the value should be over 15 ml/s.
In females, the flow rate is higher than in males of the order of 5–10 ml/s for a given bladder volume due to the simplified anatomy of the female urethra.
As previously said, abnormally high flow can be observed in women with stress urinary incontinence where the outlet resistance is reduced and also in patients which have a significant detrusor overactivity.
Flow Nomograms
The value of Q max is highly dependent on the volume voided, because the contractile efficiency of the detrusor increases in relation to the filling up to a maximum over the which decreases (Starling law). In practice it is believed that below 150 ml, the value of Q max is erroneously low as well as above 550 ml. The optimum for flow analysis should be between 200–400 ml of filling.
Since the value of Q max is highly dependent on the volume voided, the study of micturition in men and women has been hampered by the lack of a normal reference range covering urinary flow rates over a wide range of voided volumes.
The nomogram charts have been constructed both for the maximum and average urinary flow rate using statistical transformations of the rough data of flow with the aim to perform a proper analysis in a wide range of voided volumes (15–600 ml).
The most common are:
