How do you interpret a pressure flow study in urology?

Interpreting Pressure Flow Studies in Urology

Pressure flow studies are the only direct method to measure the relative contributions of the bladder and bladder outlet to lower urinary tract function, providing essential diagnostic information about urethral resistance/obstruction and detrusor contraction/contractility. 1

Key Components of Pressure Flow Studies

• Pressure flow studies involve simultaneous measurement of urine flow rate and the detrusor pressure required to achieve this flow rate 2
• The study plots detrusor pressure against flow rate during voiding, offering helpful visualization alongside conventional urodynamic curves 3
• 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 1

Clinical Indications

• 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 1
• 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) 1
• Indicated in patients with voiding disorders of undetermined etiology, especially when there's discordance between clinical history, examination, and uroflowmetry 2
• Essential for evaluating patients with relevant neurological conditions (e.g., spinal cord injury, myelomeningocele) during initial urological evaluation and follow-up 1
• Valuable when invasive, potentially morbid, or irreversible treatments are considered for patients with urgency incontinence 1

Interpretation Guidelines

For Bladder Outlet Obstruction (BOO)

• 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 1
• The Abrams-Griffiths nomogram is commonly used to classify obstruction 4
• Different methods for analyzing BOO (like the Abrams-Griffiths nomogram and linPURR) have different aims but give broadly consistent results 5

For Detrusor Function

• Detrusor underactivity can be distinguished from BOO by relating detrusor pressure at maximum flow rate to the maximum flow rate 1
• 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 6
• Detrusor contractility has been less extensively studied than obstruction, but assessment methods are available 3

For Female Patients

• In women, there is no standard urodynamic definition for obstruction like the classic "high pressure-low flow" pattern seen in men 1
• Elevated detrusor voiding pressure with low flow may suggest obstruction in women, particularly with new onset symptoms after surgery 1
• Women with detrusor overactivity typically have higher opening detrusor pressure, closing detrusor pressure, and detrusor pressure at peak flow than women with urethral sphincter incompetence 7

Important Considerations

• EMG testing is a technically challenging, nonspecific component that should be interpreted in context with fluoroscopy, cystometry, and flow rate 1
• Pressure flow studies should be interpreted in the context of global assessment, including examination, diaries, and residual urine measurements 6
• Test-retest reliability shows that detrusor pressure may decrease in sequential voids, which can affect classification of obstruction 4
• Detrusor pressure shows modest correlation with prostate volume, supporting the hypothesis that prostate size contributes to symptomatic BPH 4
• Pressure flow parameters are useful in evaluating women with LUTS as they help assess urethral and detrusor function 7

Limitations and Caveats

• Pressure flow studies are invasive and carry risks including infection and, in patients with spinal cord injury, autonomic dysreflexia 1
• 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 1
• Urodynamic findings should be interpreted in the context of the global assessment, as UDS may not diagnose DO even in very symptomatic patients 1