Uroflowmetry

Uroflowmetry is the measurement of urine flow rate over time, and is the most commonly used urodynamic study. It is performed as an initial screening test when voiding dysfunction is suspected, and the results may prompt further investigations. Uroflowmetry is performed by having the patient void into a specially designed commode that allows the volume voided to be measured over time. Uroflowme-ters use various measurement techniques,such as the rotating disk method, electronic dip, weight transducers, and the gravimetric method. The rotating disk method is the most commonly used; it directs the voided urine onto a rotating disk, which produces a proportionate increase in the disk's inertia. The power required to keep the disk rotating at a constant rate is measured, thus allowing calculation of the flow rate. The patient is instructed to void normally in either a sitting or standing position with a comfortably full bladder, in as much privacy as possible to remove the inhibitory effect of the test environment. Privacy, voided volume, and patient's age and sex are factors affecting the parameters of uroflowmetry. Reported uroflowmetric parameters include total voided volume (V), maximum flow rate (Qmax), time to Qmax, voiding time (Vt), flow time, and average flow rate (Qave). The pattern of the flow curve is also observed. Voided volume represents the total voided volume as a single micturition event. The maximum flow rate is measured during a single void, and the time to reach this value is the time to Qmax. Flow time is the time over which measurable urine flow occurs. Average flow rate is calculated by dividing the voided volume over the flow time. Voiding time is the total duration of micturition including interruptions. When voiding is completed without interruption, voiding time is equal to flow time.1 The voided volume should be at least 150mL and preferably 200 mL (50mL for a child), because Qmax increases by 5.6mL/s with each 100-mL increase of voided volume, but only up to a voided volume of 200 mL. In young males, Qmax should be 15 to 25mL/s. Women typically void with a slightly higher Qmax (5-10mL/s) for a given volume than men.2 A normal flow pattern is a continuous bell-shaped smooth curve with rapidly increasing flow rate reaching Qmax within one-third of the total voiding time (Figure 3-2.1). The first 45% of voided volume is reached before Qmax is reached. The overall appearance of the flow pattern may disclose abnormalities. An irregular pattern or curve with multiple peaks represents intermittent flow in which downward deflections do not reach 2mL/s. This is often attributable to abnormal straining superimposed on a detrusor contraction. An interrupted pattern is consistent with intermittent flow in which downward deflection reaches 2 mL/s or less, leading to several micturitions separated by 2 to 20 seconds, often caused by straining in the absence of a detrusor contraction. Obstructed flow is characterized by prolonged flow time, sustained low flow rate, low Qmax and Qave, and a plateau-shaped curve (Figure 3-2.2). There is slow initial rise with an increased time to achieve Qmax,but because the voiding time is longer, Qmax may be seen relatively earlier.3 The flow pattern of a patient with urethral obstruction is a flat-topped curve with decreased Qmax, which is reached quickly, but remains at the same level for most of micturi-

Figure 3-2.1. Normal flow pattern.

100mL warrants surveillance or treatment. In normal children, residual volume is less than 10% of bladder capacity.6 Residual volume may be overestimated in patients with vesicoureteral reflux, hydroureteronephrosis, or bladder diverticulum.

When a normal Qmax and normal voided volume without residual urine are present, infravesical obstruction or reduced contractility is unlikely. Several nomograms have been used to interpret measured flow rates. Siroky's nomogram is used for males; the Liverpool nomograms have scales for males and females. A peak flow more than the 90th percentile on the Liverpool nomogram may indicate detrusor instability.7 A low Qmax in the presence of significant residual volume indicates obstruction. A low Qmax without significant residual volume indicates reduced contractility. However, a pressure flow study can provide more accurate diagnosis and differentiation of the two conditions.

tion. The curve is flat and unbroken, with a large part of the volume voided at a constant Qmax.4 Urinary flow rate provides useful information about whether there is outflow obstruction, especially in males. A flow rate greater than 40 mL/s is considered superflow. It may be attributable to decreased outlet resistance,5 and is common in women, particularly those with genuine stress urinary incontinence (SUI) in which outlet resistance is reduced and in those with marked bladder activity. Although low Qmax may indicate urinary outlet obstruction, measurement of the flow rate alone has limited value when determining whether obstruction is present in a particular patient. To provide detailed information, uroflowmetry can be combined with a measurement of the postvoid residual volume (PVR). Postvoid residual volume is the bladder volume immediately after voiding. It reflects bladder contractility, and is an excellent assessment of bladder emptying. One method for measuring PVR is by ultrasound, with a large stationary machine, or, more commonly, using a small portable ultrasound unit. Transurethral catheterization immediately after a voluntary void is the most accurate method of measuring PVR; however, it is invasive and associated with a small risk of infection. Residual volume in an adult can be up to 25 mL; a residual volume of more than

Uroflowmetry Patterns
Figure 3-2.2. Obstructed flow pattern on uroflowmetry.
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Responses

  • albino
    What causes peak flow of 25ml in uroflowmetry?
    7 months ago

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