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urodynamics tracing Pves Pabd Pdet Volume Flow EMG cystometry interpretation
Reading File
This is a urodynamic (cystometry) tracing — a multi-channel study used to assess bladder and urethral function. Here's a channel-by-channel breakdown of what's shown:
Channels Recorded
| Channel | Unit | What It Measures |
|---|---|---|
| Pves (red/orange) | cmH₂O | Intravesical pressure — total pressure inside the bladder |
| Pabd (green) | cmH₂O | Intra-abdominal pressure — measured via rectal/vaginal catheter |
| Pdet (dark blue) | cmH₂O | Detrusor pressure = Pves − Pabd (calculated, reflects true bladder muscle contraction) |
| Volume (ml) | ml | Volume of urine in the bladder |
| Flow (red) | ml/s | Urine flow rate (flat here — no voiding recorded in this segment) |
| Infusion (ml) | ml | Volume of saline being infused into the bladder during filling |
| EMG (red) | µV | Pelvic floor / sphincter electromyography |
What the Trace Is Showing
Filling (Storage) Phase
The infusion channel rises steadily, confirming the bladder is being filled with saline. The volume channel mirrors this.
Annotations
- LPP markers = Leak Point Pressures — the pressure at which leakage occurs with increased abdominal stress
- Cough events are clearly marked, showing sharp simultaneous spikes in Pves and Pabd — this is the stress test
- 100 cc / 200 cc markers indicate volumes at which sensations/events are recorded
Key Findings Visible
- Pves and Pabd spike together on coughs — as expected (transmitted abdominal pressure)
- Pdet shows rises independent of Pabd — these represent detrusor contractions (involuntary detrusor activity during filling = detrusor overactivity)
- EMG is active throughout — baseline pelvic floor activity with bursts during coughs
- Pabd goes flat after ~12:00 — the rectal catheter likely fell out or lost signal, which is a quality/artifact issue
- No voiding flow is seen (Flow channel remains near zero) — this is purely the filling/storage phase
Clinical Significance
This tracing is consistent with a patient being evaluated for lower urinary tract symptoms (LUTS), likely urinary incontinence. The prominent detrusor pressure rises during filling suggest detrusor overactivity (DO), and the cough-provoked LPP measurements are used to assess stress urinary incontinence (SUI).
The Pdet formula — Pdet = Pves − Pabd — is the fundamental principle: by subtracting abdominal pressure from total vesical pressure, you isolate the true detrusor (bladder muscle) contribution.
(Bailey and Love's Short Practice of Surgery, 28th Ed., p. 1519)
Normal values?
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Here are the normal urodynamic values for each channel in the tracing:
Normal Urodynamic Values
Pressures
| Parameter | Normal Value | Abnormal |
|---|---|---|
| Pves (intravesical) | Reflects Pabd during filling; rises with voiding | Abnormal if rises independently of Pabd |
| Pabd (abdominal) | Baseline ~0–10 cmH₂O; spikes only with cough/Valsalva | Artifactually flat = catheter displacement |
| Pdet (detrusor) | <10 cmH₂O throughout filling (should remain essentially flat) | Phasic rises = detrusor overactivity (DO) |
| Pdet during voiding | 20–60 cmH₂O in women; 40–80 cmH₂O in men | High Pdet + low flow = bladder outlet obstruction |
Bladder Compliance
| Value | |
|---|---|
| Normal compliance | ≥20 mL/cmH₂O (large volume change with minimal pressure rise) |
| Abnormal (low compliance) | <20 mL/cmH₂O — upper tract at risk |
| Danger threshold | Sustained Pdet >40 cmH₂O — high risk for upper tract damage |
Bladder Capacity
| Sensation | Normal Volume |
|---|---|
| First desire to void | ~150–250 mL |
| Normal desire | ~250–350 mL |
| Maximum cystometric capacity (MCC) | 300–600 mL (typically 400–500 mL in adults) |
Leak Point Pressures (LPP)
| Type | Definition | Cutoffs |
|---|---|---|
| ALPP (abdominal/Valsalva LPP) | Pves at which leakage occurs with Valsalva/cough | >90 cmH₂O = normal sphincter; 60–90 = equivocal; <60 cmH₂O = intrinsic sphincter deficiency (ISD) |
| DLPP (detrusor LPP) | Pdet at which leakage occurs without abdominal rise or contraction | <40 cmH₂O = safe; >40 cmH₂O = upper tract at risk |
Maximum Urethral Closure Pressure (MUCP)
| Value | |
|---|---|
| Normal | >40 cmH₂O |
| Intrinsic sphincter deficiency | <20 cmH₂O |
EMG
| Phase | Normal |
|---|---|
| Filling | Increasing EMG activity (guarding reflex) |
| Voiding | EMG goes silent before/during voiding |
| Abnormal | EMG active during voiding = detrusor-sphincter dyssynergia (DSD) |
Flow Rate
| Parameter | Normal |
|---|---|
| Qmax (peak flow) | >15 mL/s in women; >10–15 mL/s in men |
| Flow pattern | Bell-shaped curve |
| Post-void residual (PVR) | <100 mL (or <33% of voided volume) |
Key rule of thumb: During normal bladder filling, Pdet should stay flat (compliance preserved), there should be no involuntary contractions, and leakage should only occur voluntarily or at high abdominal pressures.
(Campbell-Walsh-Wein Urology; Berek & Novak's Gynecology)
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