THE URETHRAL PRESSURE PROFILE

By MALCOLM BROWN, B.Sc., and J. E. A. WICKHAM, M.S., B.Sc., F.R.C.S. From the Departments of Urology and Medical Electronics,

St Bartholomew’s Hospital, London, E.C.l

THE treatment of urinary incontinence by electrical stimulation of the external sphincter area has revealed an urgent need for the development of a simple method whereby the effectiveness of urethral occlusion produced by an implantable device may be assessed prior to operation.

Much recent work has been devoted to an examination of the characteristics of urethral urine flow and to the clinical assessment of resistance flow in patients with lower urinary tract obstruction. The physical methods required for the determination of mathematical values for urethral resistance (Smith, 1968) or effective cross-sectional area of the urethra (Rankin, 1967) are time-consuming and even when calculated provide little in the way of practical information as to the site, length or multiplicity of any areas restrictive to the free flow of urethral urine.

Other extant methods for recording urethral closure pressures have utilised devices such as intraluminal pressure sensitive balloons or strain gauges (Shelley and Warrell, 1965) but such systems can only measure pres- sure or force over a length which may be significant when compared to sphincter or stricture length. Also these measurements may only be made at a single site thus making identification of a localised constriction such as the external sphincter somewhat imprecise.

It appeared to us that it was necessary to develop some simple method of recording the effective pressure exerted by the urethral wall at every point in a way that would have direct clinical significance and would also be free from the criticisms that have been levelled at previous systems. The additional advantages of a method

A

B

C

I pw Pw>Pf

A

I Pf

I pw Pw<Pp

1 P W P w = P t

T pf FIG. 1

Urethral mucosal behaviour at the eye of a catheter. P, = wall (closure) pressure, Pf = catheter fluid pressure. Catheter wall

-shaded, mucosa-dotted.

providing direct visualisation of the recorded pressures coupled with rapid reproducibility would also be valuable. The following account is a report of our efforts to achieve these aims and describes a new method for urethral pressure recording.

Section I: Theoretical Considerations.-Consider the three possible situations that may exist at the eye of a fluid-filled urethral catheter (Fig. 1).

A. If the applied fluid pressure, P’, is less than the urethral wall pressure,P,, then the catheter is sealed.

B. If the applied fluid pressure is greater than the wall pressure, fluid will escape. It will move along the sides of the catheter, up into the bladder or out to the external urethral meatus. The rate of escape will depend on the excess pressure.

C . At an applied pressure such that Pf = P, the fluid neither escapes nor is the wall distorted. The wall pressure P, is the pressure which must be exceeded for urine to pass that point in

the urethra, and so is a most relevant parameter. The fluid pressure Pf can easily be measured at the distal end of the catheter and so we need only find some means of ascertaining when

21 1 P/ = P,.

212 BRITISH J O U R N A L O F U R O L O G Y

If P , is increased from zero, it just exceeds P , at the instant when a flow of water is first recorded. Alternatively, if P, is reduced from a known excess pressure, then P , is registered when the flow just ceases. This latter method was used by Lapides et al. in 1960, when a water mano- meter, filled to excess, was allowed to discharge through a 16 F whistle-tipped catheter opening into the urethra. When the water column fell to the wall pressure, flow ceased and the level was maintained and recorded. This procedure was repeated at several points along the urethra so that a crude pressure profile could be plotted.

Though ingenious, the procedure is lengthy, and many points are required for an adequate profile. More important, the system cannot register a pressure increase, since this would involve water being forced back up into the manometer.

Kleeman in 1966 described a slightly shorter procedure, but used air as his injection fluid. Surface tension effects of this method are questionable.

Section 11.-Now consider an apparatus which records Pf, and also provides a constant water flow to the catheter. This water will be forced into the urethra as in State B of Figure 1, and therefore P, will be greater than P,. However, it can be shown that at very slow rates the difference between P, and P, can be ignored. That is, the conditions in the urethra are little disturbed by a small water leak from the eye of the catheter. At large flow rates viscous effects in the escape routes will be significant.

Some typical error values (P, -Pw) are presented in Figure 2 showing variation with flow rate in a No. 12 French catheter. To find these errors, Pf was noted at a known flow rate and then noted again immediately on stopping the flow (State C , Fig. 1). A typical urethral pressure might be 40 cm. water pressure and so the errors encountered at flow rates below, say, 2 ml./ ininute will be unimportant. The error will always be positive and so will cancel out in any comparative derivations from the results.

It is important when recording coughing pressures or continuous pressure profiles (Section I11 below) that any change in pressure be registered quickly. Delay occurs because of the time taken to inflate or deflate the catheter and manometer tubes to the new pressure, and depends on the elasticity of the tubes, their capacity and the flow rate. With the water manometer system of Lapides and the preferred order of flow rates (from Fig. 2) this could take many minutes or even hours, but using an all-electrical system it will take only a fraction of a second.

Figure 3 shows typical response times for the system used at St Bartholomew’s Hospital. The times given are those required to register an increase of 50 cm. water pressure. They are by no means the best possible but they are more than adequate in the considered situation. From Figures 2 and 3 a compromise flow rate can be chosen.

Figure 4 shows the apparatus, excluding the pen recorder. I t was chosen for universal availability rather than for optimum performance. A standard drip set provides the flow. This should be held at about 8 feet or pressurised, to minimise back-pressure effects, and the regulating tap should be moved to the lower end of the tube since this affects the response time. With this system, flow rate may be translated into drip rate and for most sets 1 ml. is about 15 drops. The pressure transducer is connected to the flow line through a remote injection catheter. The other small standard parts are for evacuating air from the tubes during preparation.

Section III: Practical Recording of the Pressure Proiiles.-The catheter is introduced into the bladder, the drip and transducer lines connected to the recorder, and the flow regulated to the chosen rate-usually about one drop every two seconds. The recorder now registers the bladder pressure. The paper is set running at 1 cm. per second and the catheter is drawn slowly down the urethra, the recorder automatically plotting the pressure profile. Finally, the catheter is with- drawn until the eye is clear of the external meatus but the tip is not allowed to fall until a zero value is registered by the recorder.

Figure 5 shows the resulting profiles from a patient before and after urethral dilatation. There was stricture of the distal bulbar urethra. After dilatation to 24 F the effective

T H E U R E T H R A L PRESSURE P R O F I L E 213

5.0

W W I X I X an

1.0

0

0.2sec

W z 0 a v) U IX

m O.lsec

I

I

CATHETER = 12F

I

. I I I I

1 1 I I i

8.04.0 2.0 1.0 0.5 FLOW RATE (mI/min)

FIG. 2 Graph of the error in the measured pressure plotted against flow rate.

14F I

. I

1 1 I I I 8.04.0 2.0 1.0 0.5

FLOW RATE (ml/min) FIG. 3

Graph of response time against flow rate for a range of catheters.

FIG. 4

The apparatus (less the pen recorder). (1,) ,A bottle and drip set containing distilled water. (2) The urethral catheter. (3) The remote injection catheter (used as a side arm to the pressure

transducer) Portex. Ref.A.110). (4) A strain gauge pressure transducer.

70

60

50

0 40

5 w

v) v)

DL 30 a

!$ 20 n

10

URETHRAL PRESSURE PROFILES

BEFORE DILATATION

I I I I I I I I I I I 1 60 -

AFTER DILATATION TO 20-24 F

Catheter - 10 F 5o

1" 40 - 0

5 w

0 1 2 3 4 5 6 7 8 9 10 11 12 DISTANCE IN cm

FIG. 5

Pressure profile of male urethra, (1) showing distal bulbar stricture, (2) the same urethra after dilatation to 20 to 24 F. Note the area of low pressure in the prostatic urethra between the bladder neck and the external sphincter.

T H E U R E T H R A L PRESSURE P R O F I L E 215

URETHRAL PRESSURE PROFILES

27.5.68 BEFORE DILATATION AFTER DILATATION 26-30

Stimulation - 0 Patient attitude - Lithotomy

Catheter - 12 F Catheter - 1 2 F

Stimulation - 0 Patient attitude - Litliotomy

60

c

-

70

6 0

50 0

I N

E, 40 W lY 3

W K P

30

20

10

0 1 2 3 4 5 DISTANCE IN cm DISTANCE IN cm

Pressure profile of female urethra, (1) before dilatation to 26 to 30 F and (2) afterwards. Her clinical FIG. 6

condition was unchanged.

60

50

0 40

5 N I

k? 30 3 v) v) W CY a

20

1 0

0

11.5.68

Sling Round

URETHRAL CALIBRATION

Situation - See below Rate - 20 pps Pulse length - 2 nis Electrodes - Needles Position - Perineum Patient attitude - Lithotomy Catheter - 8F

1 2 3 4 5 6 7 8 9 10 11 12 DISTANCE IN cm FIG. I

Three superimposed pressure profiles of the (abnormal) urethra of an 8-year-old boy,, at different levels of electrical stimulation. The stimulation was applied to needles in the perineum.

216 B R I T I S H J O U R N A L O F UROLOGY

resistance of the stricture is seen to have been abolished, the clinical results confirming this. Figure 6 shows a similar procedure carried out in a female patient. Here the dilatation appeared to make little or no difference to the pressure profile, and in fact her retention was not influenced by the treatment.

Figure 7 shows successive pressure profiles in a case of incontinence. Here the external sphincter region was subjected to electrical stimulation at various levels through perineal needle electrodes.

DISCUSSION

A full clinical evaluation of this method of recording urethral pressure profiles is outside the scope of this paper, which only reports on the method. The advantages of the technique are that it is simple, the results are presented immediately in an easily appreciated visual form and more

FIG. 8 The special catheter used at St Bartholo- mew’s Hospital. Note the holes all around at one point, the length of unused catheter distal to the holes and the centimetre

graduations.

importantly it is entirely reproducible, each urethra exhibiting its characteristic pressure profile. Anatomical localisation of points of high or low pressure is made easy by calibration of the catheter or by radiographic techniques. Point pressure localisation is good and accurate to the diameter of the catheter eye unlike the diffuse pressure zone recorded by balloon or strain gauges. This means the bladder neck and other sudden pressure steps in the urethra are recorded accurately.

Moreover, the recorded pressure has quantitative significance in that it is an absolute and not a comparative value.

Any urethral deformation is due only to the diameter of the catheter and there is no theoretical objection to using minute catheters, and since the pressures required to be measured are often those of the closed urethra, this would be a further advantage.

In stricture assessment it may be worth while performing the same procedure twice, once with a very small catheter, then again with a large catheter. Elastic and muscular sections will increase pressure Slightly, fibrous and hard sections will increase pressure markedly.

T H E U R E T H R A L P R E S S U R E P R O F I L E 217

The rapid response time allows the complete profile to be plotted leaving out no information about the urethral pressures. The pressure changes which can occur in 2 or 3 cm. show that a single measurement at an unknown point in the urethra is of little use.

A small and recent refinement of the system has been the use of a special catheter, calibrated in centimetres and with several eyes around the circumference of the tip. This ensures that the lowest wall pressure at any point is always measured and also prevents any rotational errors in the orientation of the catheter. Figure 8 shows such a catheter. There is 5 cm. of uncalibrated tube distal to the eyes to enable the eyes to be brought out of the external meatus and record zero pressure without the whole catheter falling free. These catheters are available on request from the Department of Urology, St Bartholomew’s Hospital, London.

SUMMARY

Continuous pressure profiles would seem to include all possible advantages over other methods of urethral assessment. The method applies equally to electro-stimulation testing and to the evaluation of stricture. A wide range of variations and refinements to the method may be suggested, for instance, the profile catheter should preferably be withdrawn at a known steady rate, and this could perhaps be achieved by a mechanical device. It might also be useful if subsequent profiles could be superimposed on the initial trace to allow of a more accurate comparison between sequential examinations.

Perfectly adequate results may however be obtained with the simple apparatus described above.

REFERENCES

KLEEMAN, FRANK J. (1966). J. Urol., 95, 222. LAPIDES, J., AJEMIAN, E. P., STEWART, B. H., BREAKEY, B. A., and LICHTWARDT, J. R . (1960).

RANKIN, J. T. (1967). Br. J. Urol., 39, 594. SHELLEY, T., and WARRELL, D. W. (1965). J. Obstet. Cynaec. Br. Commonw., 72, 926. S m , J. C. (1968). Br. J. Urol., 40, 125.

J. Urol., 84, 86.