The Pressure Flow Study

This is the definitive way to diagnose bladder outlet obstruction.

There are 3 principal problems which can arise with the process of micturition.

  1. The urethra becomes obstructed usually due to hyperplasia of the prostate
  2. The detrusor-sphincter co-ordination is lost. This is called detrusor-sphincter dyssynergia.
  3. Underactive detrusor muscle.

As mentioned previously the micturition reflex initiates a contraction of the detrusor and a relaxing of the urethra. Because the urethra is highly distensible this enables a large flow for little rise in pressure in the normal case. The obstructed bladder leads directly to high pressure emptying. This derives from the bladder output relation. The obstructed urethra requires a very high detrusor pressure to initiate flow. This gradually decreases and the characteristic D shape is seen.

The high pressure detrusor in obstruction has been responsible for some misunderstanding among clinicians. It was first thought to be "compensation" by the detrusor whereby it hypertrophied in order to create these high pressures. However this concept is now discredited. Any normal detrusor would produce high pressures in the presence of obstruction. This is a result of the bladder output relation which derives from simple physiological measurements on bladder strips and geometry..

All of the above 3 abnormalities can be investigated by the pressure-flow study. Detrusor pressure and flow rate are measured simultaneously - often with a suprapubic catheter so that there is no obstruction to the flow as there would be with urethral catheterisation. For diagnostic purposes, the most informative way to present the data from a pressure flow study is to plot pressure against flow.

The seminal work on the dynamics of micturition has been done by Derek Griffiths and Paul Abrams (1979) whose papaer is used extensively to diagnose the obstructed bladder. Abrams and Griffiths studied 117 men who had been referred for investigation of possible obstruction. These men were classified clinically and then the Abrams Griffiths nomogram (the red quadrilateral on the plots) was devised as the best method for separating the pressure flow loops.

Although theoretical models of normal micturition have been evaluated (Griffiths & Rollema 1979 Med & Biol Eng and Comp 17 291-300) it is the abnormal micturition and in particular the obstructed urethra which is most commonly studied.

The figure below illustrates the characteristic patterns of pressure flow loops.

Low pressure high flow. The normal urethra is highly distensible and opens at low pressures
High pressure low flow; if the the normal detrusor is obstructed to give low flow rates it will produce high pressures. This has repercussions for the upper tracts

Normal pressure flow studies.

Time domain curves
Unobstructed pressure Flow loop with Abrams Griffiths nomogram. The tip of the loop is well into the unobstrcuted zone.

The loops are compared against the Abrams Griffiths nomogram in red. Abrams and Griffiths (Br Jour Urol. 1979 51 129) studied 117 males

In 2/3 of the cases obstruction could be assessed from the maximum flow together with the detrusor pressure at that point (the apex of the pressure flow curve). In the remaining third obstruction could be assessed only by studying the shape of the plot of Detrusor pressure against flow.

High pressure low flow - obstructed

Time domain curves
Pressure flow loop and Abrams Griffiths nomogram. Note that this is displayed on a different scale from the loop above and below because of the high detrusor pressure. The patient is highly obstructed.

Low pressure low flow

The curves below show the case where the flow information alone would lead one to diagnose obstruction, but the detrusor pressure is not in fact high. Cases like this are the reason uroflowmetry will never have a very high specificity.

Time domain curves
Pressure flow loop and Abrams Griffiths nomogram. The patient is unobstructed.

The principal treatment for proven obstruction is the transurethral prostatectomy