Interpreting Pressure Flow Studies in Urology

Key Components of Pressure Flow Studies

Multichannel subtracted pressure measurement is preferred over single channel cystometrogram, which is subject to significant artifacts of abdominal pressure 1
At least two flow rates should be obtained, ideally both with volumes greater than 150 ml of voided urine, to account for individual variability 2

Pressure flow studies are recommended before invasive therapy in men with a maximum flow rate (Qmax) greater than 10 ml/second, as men with higher flow rates are less likely to be obstructed and therefore less likely to benefit from surgical therapy 3
Pressure flow studies are useful in evaluating men with LUTS who have failed prior invasive therapy or who have concomitant neurologic disease affecting bladder function (e.g., stroke, Parkinson's disease, neuropathy) 3
Pressure flow studies are essential for evaluating patients with relevant neurological conditions (e.g., spinal cord injury, myelomeningocele) during initial urological evaluation and follow-up 4
Pressure flow studies are valuable when invasive, potentially morbid, or irreversible treatments are considered for patients with urgency incontinence 1

A classic "high pressure-low flow" pattern characterizes male BOO 1
In men, obstruction is likely if Qmax is less than 10 ml/second; pressure flow studies may not be necessary in these cases 2

Detrusor underactivity can be distinguished from BOO by relating detrusor pressure at maximum flow rate to the maximum flow rate 2
Detrusor overactivity (DO) may be identified during filling cystometry, though its absence on a single study does not exclude it as a cause of symptoms 5

In women, elevated detrusor voiding pressure with low flow may suggest obstruction, particularly with new onset symptoms after surgery 1

EMG testing is a technically challenging, nonspecific component that should be interpreted in context with fluoroscopy, cystometry, and flow rate 6
Pressure flow studies should be interpreted in the context of global assessment, including examination, diaries, and residual urine measurements 5

Pressure flow studies are invasive and carry risks including infection and, in patients with spinal cord injury, autonomic dysreflexia 4
The absence of DO on a single urodynamic study does not exclude it as a causative agent for symptoms 1
Artifacts are common in EMG testing, requiring clear understanding of history and physical findings for proper interpretation 6
Urodynamic findings should be interpreted in the context of the global assessment, as UDS may not diagnose DO even in very symptomatic patients 1