Urodynamic testing is the best way to diagnose disorders of the lower urinary tract. However, many urodynamic technicians and doctors have never received formal training regarding the multiple tests and data interpretation. In a 2002 survey1 of 192 North American urodynamics services, less than 20% of respondents reportedly received formal training. While the techniques themselves are easy to master, urodynamics interpretation can be more difficult. How to do it the right way will be described here.
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Free Flow Rate Test
Before your patient begins the invasive portion of urodynamics testing, have him or her take a free flow rate test2, and also perform a dipstick urinalysis. The free flow rate test will serve as a standard to look back on in case you question the presence of artifacts on the urodynamic tracings. The urinalysis will indicate whether a urinary tract infection is present. If so, the patient may not be able to undergo testing.
During the free flow rate test, the computer will automatically calculate voided volume and maximum flow rate. It is important to look at the tracing and rule out the existence of artifacts, such as large spikes.
Observe the pattern of flow and ask yourself whether it is indicative of normal urinary flow. If not, check the patient’s bladder diary and see if it is consistent with reported symptoms. One explanation for a large spike in the flow tracing is a male who inadvertently partially obstructs the urethra, causing pressure build up and an artificially high flow rate when urine is released.
Multichannel Filling and Voiding Cystometry
For the invasive portion of the exam, the computer will make all necessary calculations. It is your job to accurately interpret these values. It is imperative that the machine is zeroed correctly. Check to make sure the pressures zero to atmospheric pressure, and when the lines are opened to the patient, the vesical and abdominal pressures3 are above zero, which indicates resting pressure. If this is not the case, the machine was not properly zeroed and results could be inaccurate.
A cough test should be performed prior to the start of filling to ensure that both vesical and abdominal pressures are recording properly. If one of the pressures is not recording, you should troubleshoot that sensor before proceeding with the test. You should also equalize Pabd to Pves which will make the calculated detrusor pressure zero. Then, if detrusor pressure is negative, a catheter has likely moved.
When diagnosing detrusor over activity, the clinician must verify there is no drop in abdominal pressure; but a rise in vesical pressure. Changes in abdominal pressure in conjunction with signs of detrusor over activity could be a sign of catheter migration or it could be a sign of the patient straining, squeezing, or moving, which might lead to a misdiagnosis. It is important to observe both pressures along with the detrusor pressure and the patient’s activity to truly ascertain whether detrusor over activity is occurring.
Additionally, when interpreting urodynamics4 results it is important to label the tracings correctly. For instance, detrusor over activity without permission to void can appear identical on the tracing to permission to void. Without proper labeling, detrusor over activity cannot be diagnosed with confidence. Also, when checking for abdominal leak point pressures, the leak point should be marked on the event (Valsalva or cough) that produced leakage.
When diagnosing bladder outlet obstruction, the bladder outlet obstruction index is calculated. This equation is maximum detrusor pressure minus twice the maximum flow rate (PdetQmax – 2Qmax). If the difference is greater than 40, bladder outlet obstruction can be diagnosed.
However, clinicians must look at the tracing to rule out artificial results. Spikes in the flow rate and detrusor pressures from artifacts are common, which can result in artificially high values. Therefore the maximum values of the curve, as opposed to the spike, must be calculated for accurate interpretation.
Finally, if irregularities are observed during the test, such as interrupted urine flow or an abnormally long flow, ask the patient if what he or she is experiencing is normal. Also compare what you are observing to the free flow rate study and the patient’s bladder journal. If the urodynamics testing does not reproduce the patient’s symptoms, the testing and interpretation is useless.
Overall, when interpreting a test that was performed by yourself or another clinician, you must look for:
- Good tracings with few artifacts
- Pressures zeroed to atmospheric pressure
- Resting pressures above zero atmospheric
- Cough tests before filling to ensure proper functionality of abdominal and vesical pressure sensors
- Accurate labeling of all events, including leak point pressures and permission to void
- Symptoms reported by patient are reproduced during test
- Appropriate peak flow and bladder capacity are observed
When these criteria are met, accurate interpretations can be made the right way.
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- K. M., & G. M. (2002). Characteristics of North American urodynamic centers and clinicians. Urol Nurs, 22(3), 179-82. Retrieved February 21, 2017, from https://www.ncbi.nlm.nih.gov/pubmed/12087791. Link
- Persu, C., Lavelle, J., Nita, G., & Geavlete, P. (2011). C152 Free Uroflowmetry Vs. Flow Rate During Pressure Flow Test – A Comparative Study. European Urology Supplements, 10(9), 650. doi:10.1016/s1569-9056(11)61732-6 Link
- Salinas, J., Virseda, M., Méndez, S., Menéndez, P., Esteban, M., & Moreno, J. (2015). Abdominal strength in voiding cystometry: a risk factor for recurrent urinary tract infections in women. International Urogynecology Journal, 26(12), 1861-1865. doi:10.1007/s00192-015-2737-2 Link
- Smith, P. P., Hurtado, E. A., & Appell, R. A. (2009). Post hoc interpretation of urodynamic evaluation is qualitatively different than interpretation at the time of urodynamic study. Neurourology and Urodynamics, 28(8), 998-1002. doi:10.1002/nau.20730 Link