Cystometry

Clinical Introduction

Cystometry is carried out to categorise the bladder. The main issue is the stability of the detrusor. The normal bladder should be stable under all conditions of filling or stress. The compliance of the bladder (DVol / DP) is also of interest as is its capacity and the patient's sensations of strength of desire to void. Two pressure Channels are typically measured, rectal pressure and bladder(intravesical). The rectal pressure responds to any changes of the abdominal cavity due to straining or stress. These can then be subtracted from the intravesical to give the true intrinsic bladder pressure from the detrusor muscle (detrusor pressure).
Quality control is important during cystometry. The cancellation from the rectal catheter must be accurate.
Click here for an example of perfect rectal cancellation

NB All the images in this column are clickable
	The diagram shows a stable bladder. The traces are from top to bottom rectal (abdominal) pressure, Intravesical pressure, detrusor pressure, and infused volume.

The diagram shows an unstable bladder with low capacity. In such a case the height of the contractions are of interest as they may exceed the maximum urethral pressure leading to incontinence. The unstable bladder is a sign that the micturition reflex is triggered and the reflex lowers urethral pressure. For leakage to take place the detrusor pressure just has to exceed this lowered urethral pressure. The patient in the diagram had a static urethral pressure of 100 cm water when the bladder was stable. The contraction has almost reached this height, so leakage had almost certainly taken place. Patients with unstable bladders may also carry the risk of urine refluxing up the ureters causing kidney damage. This would be tested for by x ray contrast cystometry. Although a large pressure rise has taken place, this type of bladder would not be termed low compliance. This term is reserved for the stable bladder with a large pressure rise.

An example of a low compliance bladder is shown. Note how the pressure drops after filling is stopped. This is termed accommodation. This type of behaviour is a consequence of the viscoelastic nature of the bladder. It has been observed for years using standard urodynamics techniques and much debate has been had over what it means. Some would assert that the low compliance bladder is unstable. In ambulatory urodynamics filling rates are physiological i.e. a few ml per minute instead of a few tens of ml. Bladders are never found to be low compliance with ambulatory urodynamics. Low compliance behaviour is a consequence of filling the bladder faster than it can accommodate. At present work still needs to be done into this phenomenon.

The hyperreflexic detrusor is one which is unstable in the presence of confirmed neuropathy

Cystometry Instrumentation

Pressure measurements in the Bladder and Urethra

Pressure is measured with strain gauge transducers. In these the pressure sensitive elements form the arms of a Wheatstone bridge. Pressure raises the resistance of one arm and lowers the resistance of the other.

The pressure is typically conveyed to the sensors via fluid filled lines.
These lines will damp the pressure waveform, removing high frequencies.
In diagnostic cystometry, the characteristic pressures of interest - e.g. detrusor instability are low frequency.
Some damping from the lines is acceptable.
The transducers can be zeroed to atmospheric pressure by balancing the bridge.
For most urodynamic measurements a sampling rate of 30 Hz is more than adequate. Although the transducer will satisfy this, care must be taken interpreting results from a fluid filled system as air in the lines can cause damping which is unacceptable. In cases where fast frequency response is required, catheter tip transducers are used - see ambulatory urodynamics.

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