TEALE FENNING

Nick Raine-Fenning · Posted: Mon Oct 23, 2006 8:49 pm Post subject: Structured Viva: Urodyanamics

Terminology

Compliance refers to the the elastic property of the detrusor muscle. A normal bladder should "stretch" to "normal" capacity while maintaining low pressures.

Bladder stability refers to the detrusor pressure during filling to normal capacity.

Uroflowmetry

This refers to the plotting of urine flow rate against time.

Uroflowmetry is a cheap and non-invasive test that offers an acceptable sensitivity and specificity for the detection of abnormal flow patterms which relate to outflow obstruction or poor bladder contraction.

The plots are self-explanatory.

Cystometry

Cystometry involves the measurement of rectal and intravesical pressure thropugh the insertion of two individual pressure catheters. The rectal transducer measures abdominal pressure (Pabd) which increases with straining or stress as induced by coughing or a Valsalva. Because increases in abdominal pressure are translated through to the bladder, the pressure within the bladder (vesicle pressure or Pves) also increases. If the abdominal pressure increase (Pabd) is subtracted from the intravesical pressure (Pves) the final reading is a reflection of the true pressure reading from the detrusor muscle (detrusor pressure or Pdet). This explains why these studies are often reffered to as subtraction cystogram.

Other important measures include bladder compliance and their desire to void which is recorded when first noted ("first desire to void") and when when the patient can not hold on for much longer ("strong desire to void").

Urethral pressure may also be measured allowing calculation of a subtracted urethral closure pressure.

Detrusor Overactivity

The normal bladder should be stable duirng filling and when stressed. The normal detrusor if filled slowly accepts 300 - 600 ml with a small rise in pressure only. Click here to see a stable filling cystogram.

If there is detrusor overactivity, the bladder undergoes phasic contractions while the patient is trying to inhibit voiding. Bladder capacity is generally reduced. Click here to see a typical pattern.

If the detrusor pressure exceeds the maximum urethral pressure then incontinence ensues. This is exacerbated by the fact that the urethral pressure is lowered.

Low Compliance

A low compliance bladder is one which demonstrates a marked increase in pressure during filling that falls when filling is stopped, a process known as accommodation. Urodymanic profiles tend to show a more gradual change in detrusor pressure as a result.

There is great debate about the significance of this finding. It is seen in patients with spinal injuries. Low compliance is rarely, if ever, seen with ambulatory urodynamics and many cases of 'low compliance' diagnosed with standard urodynamic testing are subsequently shown to have detrusor overactivity on ambulatory testing. This has been explained by the lower, more physiological, filling pressures used in the latter and that low compliance is actually caused by too rapid filling of the bladder so that it is unable to 'accommodate'.

Urodynamic Stress Incontinence

This refers to demonstrable urinary leakage during cystometry when there is no increase in the detrusor pressure. Incontinence is usually induced by asking the patient to cough or strain.

Continence is actually maintained by the "closure pressure" which reflects the urethral pressure minus the intravesical pressure. Incontinence may occur when the intravesical pressure exceeds the urethral pressure. When a patient coughs pressure is usually equally transmitted to the bladder and urethra so that the closure pressure is maintained.

The relative anatomical position of the 'bladder neck' and therefore the bladder and urethra may be disturbed following childbirth, the menopause, pelvic surgery, and in obese women. Under these circumstances the pressure delivered to the urethra is less than to the bladder which lowers the closure pressure and results in urinary leakage.

It is important to realise that detrusor overactivity may mimic USI as it can also be induced through coughing. This is the main reason behind the introduction of ambulatory urodynamics which is intended to mimic normal daily activity more.

Other important points

The International Continence Society (ICS) has made several recommendations to ensure there is a degree of standardisation in the comparison and interpretation of Uroflow test results. It is important therefore that you mention the following if you get a urodynamic question in the OSCE:

Patient's name
Date of the test
Maximum Flow Rate
Volume Voided
Voiding Time
Flow Time (if intermittent)
Average Flow Rate > Total Voided Volume Total Voiding Time (cc/sec)
Patient's opinion of normality of voiding
Residual urine, if measured

Normal Values

A bladder with normal compliance will demonstrate < 15cm H2O increase in detrusor pressure during filling.

A bladder with low compliance will demonstrate an increase in detrusor pressure > 15cm H2O as it fills and will have a low capacity. This is not overactivity.

A bladder with high compliance will demonstrate a minimal increase in detrusor pressure with a capacity above normal limits.

The pressure in an empty bladder is usually called the resting pressure and this is usually between 8 and 40 cmH2O.

The average urethral closure pressure is 60 cmH20.

Abnormally high voiding pressures indicate outflow tract obstruction when associated with a low initial flow rate. High flow rates, in excess of 40 cc/sec may be associated with exceptionally powerful detrusor contractions and higher than normal voiding pressure. This is seen in patients with long standing bladder overactivity and detrusor hypertrophy, but no outflow obstruction.

In women, voiding commonly occurs with a low voiding pressure. The detrusor may be proven to be contracting by measuring the isometric pressure on interruption of flow.

A poorly sustained voiding pressure may be related to a failing detrusor and any unsustained contraction is likely to lead to residual urine.

A bladder with instability often contracts before the patient is ready to void. The detrusor pressure usually falls as the sphincter is relaxed and voiding begins

Fluctuating detrusor contractions result in an interrupted flow and are seen in patients with neurological problems such as multiple sclerosis.
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