Abstract
According to the International Continence Society (ICS) (2016), cystometry is the continuous fluid filling of the bladder via a transurethral catheter (or other route, e.g. suprapubic or mitrofanoff), with at least intravesical and abdominal pressure measurements and display of detrusor pressure, including cough (stress) testing. Cystometry ends with ‘permission to void’ or with incontinence of the total bladder content [1].
Flow rate testing is a simple, non-invasive test which can provide useful clinical information, although with important limitations. It is an assessment of the volume passed in unit time and is often undertaken in conjunction with other measurements, most notably post-void residual urine volume (PVR) measurement. This chapter covers flow rate testing in females and males but many of the examples relate to conditions found only in men. The principles of interpretation remain the same irrespective of gender.
5.1 The Role of Flow Rate Testing
Many professional urological advisory bodies recommend urine flow rates as a baseline assessment for individuals experiencing lower urinary tract symptoms (LUTS) [1,2]. Given their non-invasive nature, flow rates are a useful tool, both pre- and post-administration of medication, and provide information on patient’s urinary stream as well as ability to empty the bladders fully.
5.2 How to Perform a Flow Rate Test
The set-up for uroflowmetry has been described in Chapter 4. The flow machine should be set up in a private area and all efforts should be made to make sure the environment makes the patient as comfortable as possible.
The two most common types of flow meter are:
Gravimetric (weight transducer): the weight of urine voided is measured over time; the flow rate is calculated from the rate of change in weight of urine.
Rotating disc: as the urinary stream falls onto a spinning disc, it increases the weight of the disc, so the motor has to increase power to keep the disc spinning. The flow rate is proportionate to the power needed to keep the disc spinning at the same rate.
Both approaches are widely employed in current commercial systems. They are prone to variations in reliability and need to be calibrated before use. Regular checks are needed at intervals specified by the manufacturer and should also be done if the equipment is moved or disturbed.
5.2.1 Preparation for the Test
Patients should bring with them a completed pre-test frequency volume chart/bladder diary, which will show the patients’ typical and maximum voided volume. It is important that the flows recorded are representative of a patient’s normal void and therefore the bladder diary is an essential tool to compare with clinical voids; a bladder diary enables comparison of the flow test volume against larger volume voided in day-to-day life. Patients should be well hydrated on arrival and prepared to wait for as long as needed to obtain an adequate result. The patient should be warned that the process can take some time. If they are in a rush to get somewhere else, representative flow rate results will be difficult to obtain.
5.2.2 Performing the Test
Instruct patient a to void normally and in their normal voiding position. Men should aim at one point on the funnel and avoid squeezing the urethra or letting the stream ‘wander’. Post-void residual volume (PVR) should ideally be measured within 10 minutes of voiding. The patient may be required to void two to three times due to intra-individual variability, which could affect the conclusions drawn [3]. Adequacy of voided volume should be assessed at the time, referring back to the patient’s bladder diary to determine whether it is representative of a normal volume. Voided volumes of over 150 ml are ideal for producing reproducible flow curves [4].
5.3 What Is Assessed and How It Is Interpreted
Several parameters are evaluated with flow rate testing:
Voided volume – the total volume passed via the patient’s urethra into the flowmeter.
Maximum flow rate – maximum value on the flow rate, excluding any artefacts.
Voiding time – total time the patient is voiding for.
Flow time – total time there is measurable flow being recorded on the flowmeter.
Time to maximum flow – time between onset of flow and maximum rate of flow.
PVR – volume of urine left in the bladder after voiding (ultrasound or catheter).
Urine flow curve shape – the pattern of flow presented on a graph.
5.3.1 Voided Volume
When combined with the PVR, the volume passed provides information on the storage capacity and is also one of the parameters presented on a flow rate nomogram. Flow rate nomograms clearly show an effect of voided volume on flow rate [5,6]. At low voided volume, Qmax may be artefactually reduced. Most clinicians recommend evaluating flow rate only above a voided volume of 150 ml.
Above 550 ml, the bladder starts to overfill, causing artefactual reduction in Qmax. Some people can only void at high volumes; this information can be obtained by analysing the frequency volume chart and PVR is often elevated in such individuals.
5.3.2 Maximum Flow Rate
Normal ranges of maximum flow rate (Qmax) have been worked out by screening large populations of asymptomatic individuals. They tend to vary based on age and gender. In women, the normal Qmax is 20–36 ml/s, increasing by 5.6 ml/s/100 ml voided volume [7].
Qmax is affected by voided volume, so nomograms (Liverpool nomograms [5] and Siroky nomograms [6]) have been developed to aid interpretation. Where Qmax is significantly below normal (usually taken as two standard deviations below the expected value on a nomogram), there are several possible explanations:
5.3.3 Voiding Time and Flow Time
In patients with a ‘normal’ flow rate and flow pattern, it is likely that the voiding and flow times will be the same. Discrepancies occur when patient’s voiding is not continuous and thus produces an intermittent flow curve with periods of non-measureable flow. Patients with terminal dribbling, whose final period of voiding may not meet the threshold for measureable flow, may also exhibit different voiding and flow times.
5.3.4 Urine Flow Rate Curve
The shape of the flow curve provides some hints as to the nature of a person’s diagnosis. The normal curve should have a rapid upstroke, a curve with a clear Qmax and decline quickly to end cleanly (see Case 5.1 at the end of this chapter). This is often described as ‘bell shaped’, although the trace is rarely symmetrical.
There are a number of characteristic flow patterns that have been described and although they are not diagnostic, they are often thought to act as an indicators of the individual’s underlying pathology. It is, however, key to remember that a description of flow pattern should refer to just the pattern of flow and not the potential cause. Further work is needed to better clarify the terms frequently used, but until then the most common ones are:
- Urethral stricture
Rapid upstroke, constant low flow rate giving a ‘plateau appearance’ and downstroke ends cleanly (see Case 5.1 on page 32).
- Fluctuating
Continuous flow with several peaks in Qmax often caused by repeated Valsalva manoeuvres such as during straining.
- Intermittent
A non-sustained flow rate which stops and starts within the voiding period (see Figure 5.3).
- Supervoider
Very high Qmax and rapid upstroke and downstroke. Not diagnostic, but people with detrusor overactivity generally have flow rates at the top end of the range (see Case 5.4) [8].
- Artefacts
Overestimates of Qmax owing to a high-speed squirt of urine from release of compression (Figure 5.1) or the stream ‘wandering’ on the spinning disc (Figure 5.2). Kicking or knocking the flow meter will also produce an artefactual spike in flow rate.
