Fig. 1
Uroflowmeter (right), complete voiding equipment with funnel and chair (left). During uroflowmetry patient must void as usually does
Indications
Prior to invasive UDS, it is important to do non-invasive UFM. Without a catheter, evaluation of flow is more accurate than during the pressure-flow study [7]. Indications for UFM include initial evaluation of patients with benign prostatic hypertrophy, urinary incontinence, urethral strictures, recurrent urinary tract infections and neurogenic bladder dysfunction. In patients with LUTS, UFM may suggest an abnormality of voiding/emptying [8]. UFM has also been very helpful in follow up of patient status post urethroplasty in determining stricture recurrence.
Equipment
Types of Uroflowmeters
1.
Weight: A load cell transducer measures voided weight and differentiates it with respect to time to determine the flow rate.
2.
Electrical capacitance: A dipstick mounted in a collecting chamber measures the electrical capacitance . The output of the signal is proportional to the accumulated volume, and the volumetric flow rate is determined by differentiation.
3.
Rotating disc: The urine stream is directed onto a rotating disc which has the power necessary to keep it rotating at a constant rate is measured with the power (in unit of measure) proportional to the flow rate (in ml/s).
Preparation
The preparation of the patient and the room are very important to reproduce a normal void. The patient should know he/she will void into an uroflowmeter (per their usual habit) usually standing up or sitting down. The patient should also be made aware of the importance of the exam to evaluate voiding symptoms. Ideally, the exam would be done with normal desire to void (preferably first desire to void), not under urgency. Bladder overdistention could alter normal flow and increase post void residual (PVR.) Theoretically you need three UFM measurements to confirm a reliable result.
The UDS room must be comfortable and silent. Unintended inability to void or artifacts can occur because of an unfamiliar environment. Sometimes it helps to leave the patient alone, turn off the lights and turn on the water.
Uroflowmetry Technique
Paper speed of 0.25 cm/s is the standard, as it allows an accurate and systematic curve construction. Different paper speeds change curve shapes, and thus can alter the interpretation of the test. Urinary flow and PRV depends on bladder urinary volume. For accuracy of testing, voided volume must be over 150 ml and ideally less than 400–500 cc as the detrusor muscle may become overstretched and contractility may decrease, consequently creating a false result.
Uroflowmetry Interpretation
A UFM report must include the curve description, voided volume, maximum flow rate (Q Max), average flow rate (Q Ave) and postvoid residual (PVR) to be complete.
Standard Terminology Related to UFM [9]
1.
Urine Flow is described either as continuous, intermittent or fluctuant. The continuous flow curve is represented as a smooth arc shaped curve without interruptions. Intermittent flow is when the flow stops and starts in a repeated pattern. The curve is fluctuating when there are multiple peaks during a period of continuous urine flow without dropping off completely as in the intermittent curve pattern. While the precise shape of the flow curve may be affected by detrusor contractility, possible bladder outlet obstruction, urethral obstruction, and or the presence of any abdominal straining, one cannot make these diagnoses without the pressure flow study on the shape of the curve alone.
2.
Flow rate is defined as the volume of fluid expelled via the urethra per unit time expressed in ml/s.
3.
Voided volume is the total volume expelled via the urethra.
4.
Maximum flow rate is the maximum measured value of the flow rate after correction for artifacts (ml/s).
5.
Voiding time is the total duration of micturition, including interruptions. When a void is completed without interruption, voiding time is equal to flow time.
6.
Flow time is the time over which measurable flow actually occurs. Men average approximately 30 s and women average approximately 20 s to void.
7.
Average flow rate is volume divided by flow time. The average flow should be interpreted with caution if flow is interrupted or if there is a terminal dribble.
8.
Time to maximum flow is the elapsed time from onset of flow to maximum flow.
9.
Post void residual (PVR) is defined as the volume of urine left in the bladder at the end of micturition determined by either ultrasound or catheterization. If there is no demonstrated PVR after repeated free flowmetry, then the finding of residual urine during the UDS should be considered an artifact.
10.
Bladder outlet obstruction is the general term for obstruction during micturition characterized by increased detrusor pressure and reduced urine flow rate. It is usually diagnosed by studying the synchronous values of flow rate and detrusor pressure. BOO has been defined for men, but as yet, not adequately quantified in women and children (see chapter “The Pressure Flow Study” for more information).
11.
Dysfunctional voiding is an intermittent and/or fluctuating flow rate due to involuntary intermittent contractions of the periurethral striated muscle during micturition in neurologically normal individuals.
Normal Values
In adult males Q max ≤ 10 ml/s, UFM has a specificity of 70–90 %, with a PPV of 70 % and a sensitivity of 39–47 % to diagnosis obstruction [10, 11].
Common causes of BOO in woman include pelvic organ prolapse, iatrogenic obstruction after incontinence surgery and pelvic masses [12]. Women BOO values are not standardized. UFM excludes BOO in women with Q max > 15 ml/s, voided volume > 100 ml, normal curve and no significant postvoid residual [13], but pressure-flow nomogram defined obstruction if Q max < 12 ml/s and Pdet Q max > 20 cm H2O [14].
Curves
Urine flow results from a detrusor contraction and urethral resistance, thus changes in flow curves imply an imbalance between these factors. While a noninvasive flow test is a good screening test for voiding dysfunction, one must remember the only way to definitively diagnose these issues is through pressure flow studies (see chapter “The Pressure Flow Study”).
1.
Normal Curve: Is continuous and has a bell shape, Q max is reached in the first third of the tracing and within 3–10 s from the start of flow. The final phase shows a rapid fall from high flow together with a sharp cutoff at the end of flow (Fig. 2).
Fig. 2
Illustration of normal curve, continuous and bell shape, Q max is reached in the first third and final phase shows a rapid fall from high flow together with a sharp cutoff at the end
2.
Continuous Flow Curves (Fig. 3):
Fig. 3
(a) Compressive flow curve (benign prostatic obstruction); (b) Constrictive flow curve (urethral stricture); (c) Idiopathic detrusor overactivity; (d) Detrusor underactivity
Bladder outlet obstruction : This type of curve has an elongated shape in benign prostatic obstruction (compressive). It appears normal in the first third but has a reduced Qmax with the latter part of the curve elongated with a terminal dribble indicating a reduction in flow rate. In the presence of urethral stricture (constrictive), the curve is plateau-shaped with little, change between Qmax and Qave.
With idiopathic detrusor overactivity a supranormal curve for high detrusor contraction velocities is observed. The curve is of normal shape with a very high Q max, within 1–3 s of the initial flow.
With detrusor underactivity the diagnosis is cystometric. UFM shows a symmetrical tracing with low Q max. The time to reach Q max is variable, and Q max may occur in the second half of the curve. If these features appear in UFM, a pressure-flow study must be done.
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