Troubleshooting in Urodynamics (technical)

Rebecca Hall MSc, MRes, BSc Hons
Rebecca Hall MSc, MRes, BSc HonsClinical Scientist (Gastrointestinal Physiology and Urodynamic Science) Warrel Unit St Mary's Hospital Manchester University NHS Trust UK
Leah Carman MSc
Leah Carman MScClinical Scientist (Gastrointestinal Physiology and Urodynamic Science) Warrel Unit St Mary's Hospital Manchester University NHS Trust UK

The term urodynamics refers to tests that are used to investigate lower urinary tract (LUT) symptoms by measurement of bladder and urethral function both directly and indirectly. Direct assessment of LUT function requires transurethral catheterization and placement of an abdominal pressure catheter in the vagina or rectum. Physiological pressure measurements can be transduced using either a column of fluid (fluid-filled system), or a column of air (air-filled system). Troubleshooting refers to the methods used to correct artefacts which are affecting pressure measurements and may lead to misinterpretation of the test findings.

Pressure transmission quality should be continually checked throughout a urodynamics study by asking the patient to cough. The transmitted transient increase in intra-abdominal pressure should be reflected equally on both the vesical and abdominal pressure lines and no or little variation be observed on the detrusor line.

The presence of air bubbles in fluid-filled systems/catheters is one of the commonest causes of reduced pressure transmission on a urodynamics trace. To rectify this dampened response, the system (including the dome covering the external transducer and the catheter) must be re-flushed with sterile water to eradicate air bubbles. A subsequent cough check should be performed to determine whether normal pressure transmission has resumed.

In both air-filled and water-filled systems, the connections need to be air-tight and water-tight, respectively. Any leaks in the system will dampen pressure transmission. If pressure readings are dampened on the vesical or abdominal line, all connections between the catheters, tubing, and transducer should be checked.

Erroneous pressure readings can arise from the abdominal pressure catheter being positioned in the rectum. Rectal contractions may be visualized as phasic increases in abdominal pressure. The clinician/operator must not mistake rectal contractions for phasic changes in pressure on the detrusor line, which may appear similar to a pattern of phasic detrusor overactivity.

Abdominal pressure readings can be dampened in patients who have poor anal sphincter tone and if the catheter is misplaced into the sphincteric complex. The catheter should be re-positioned in this instance. Conversely, in patients with good anal tone where the catheter has slipped into the anal canal, the abdominal pressure reading can be abnormally high, which causes a corresponding negative detrusor pressure. Again, the catheter position should be assessed and repositioned if necessary.

Pelvic floor muscle relaxation during the voiding phase of urodynamics can produce an artefactual increase in detrusor pressure. This must be manually corrected by subtracting the decrease in abdominal pressure (during relaxation of the voiding phase) from the detrusor pressure to obtain a true detrusor pressure at the maximum flow rate.

Occasionally, the vesical or abdominal catheter may be extruded during the voiding phase. The detrusor pressure at maximum flow rate cannot be reliably ascertained in these circumstances. The catheter should be re-inserted, the bladder re-filled, and the patient attempts a second void for reliable analysis.