Examqo › Examqo library › Urology › Voiding dysfunction and urodynamics · IMLE
Urodynamics: flow, cystometry and pressure-flow study
Urodynamics measures bladder and abdominal pressure and calculates detrusor pressure (Pdet = Pves minus Pabd) to tell obstruction from muscle weakness, and detrusor overactivity from stress leakage.
Why it matters in the exam
The exam asks about indices and calculations: maximum flow (Qmax) is valid only when the voided volume is 150 mL or more, and a Qmax below 10 mL per second points to obstruction or to detrusor weakness — but does not distinguish between them. Only a pressure-flow study distinguishes: the obstruction index BOOI equals PdetQmax minus 2 times Qmax — above 40 obstructed, below 20 unobstructed; and the contractility index BCI equals PdetQmax plus 5 times Qmax — above 150 strong contraction, below 100 weak contraction. A flattened ("plateau") flow curve fits a urethral stricture.
In the filling phase (cystometry) one looks for detrusor overactivity — involuntary contractions — and checks compliance, which is the change in volume divided by the change in pressure; low compliance below 20 mL per cm water is dangerous for the kidneys. A detrusor leak point pressure above 40 cm water in a neurogenic bladder predicts upper tract damage. An abdominal leak point pressure (VLPP) below 60 cm water supports intrinsic sphincter deficiency (ISD).
Urodynamics is not needed before a first operation for simple stress incontinence in a woman. It is recommended in a neurogenic bladder (preferably video-urodynamics), before invasive surgery when the diagnosis is unclear, after surgical failure, and in a young man with LUTS. "Eyeball" urodynamics is simple bedside cystometry: the bladder is filled through a catheter and an open syringe by gravity, and the rise of the water column is watched to detect contractions and estimate sensation and volume.
Key points
- Pdet = Pves minus Pabd.
- BOOI = PdetQmax minus 2 Qmax; above 40 obstructed.
- BCI = PdetQmax plus 5 Qmax; below 100 weak.
- DLPP above 40 — renal risk.
- Qmax is valid from a voided volume of 150 mL.
Summary table
| Test | What it measures | Key value |
|---|---|---|
| Uroflowmetry | Qmax and curve shape | Volume 150 mL or more; Qmax below 10 suspicious |
| Post-void residual (PVR) | Emptying | Measured by ultrasound |
| Cystometry | Overactivity, sensation, compliance | Compliance below 20 is dangerous |
| Pressure-flow study | Obstruction versus weakness | BOOI above 40 obstructed; BCI below 100 weak |
| Leak point pressure | Stress leakage and renal risk | VLPP below 60; DLPP above 40 |
In the full version for subscribers
- A simple algorithm
- Memory aid
- Exam traps
- 5 review cards
- Enough for IMLE
The full version is open to subscribers of: Urology, Medical licensing exam (IMLE), Geriatrics.
Subscriber? Read the full topic Plans and prices 25 free questions
Sources
- AUA/SUFU Guideline: Adult Urodynamics, 2012, amended 2018 — Indications and interpretation
- International Continence Society: Good Urodynamic Practices and Terms, 2016 — Standardisation of urodynamic terms
- EAU Guidelines on Management of Non-neurogenic Male LUTS, 2025 edition — Pressure-flow studies
Sources are given for further reading; the text is written afresh and not copied from them.
Reviewed by the Examqo AI Professor · 2026-11-06. The text may contain errors; when in doubt, check the source.