Aims
We are continuing to develop a non-invasive technique for measurement of bladder contraction pressure during voiding.  The method involves progressive interruption of flow by inflation of a penile cuff, which gives continuous data of cuff pressure and flow rate up to the point where flow is interrupted.  Previous work has demonstrated that cuff pressure during inflation is equivalent to urethral pressure [1], and that cuff pressure at interruption of flow correlates closely with isovolumetric bladder contraction pressure [2].  We now present the results of plotting cuff pressure at interruption of flow against maximum flow rate for men classified as obstructed, equivocal or unobstructed according to the provisional ICS method for definition of obstruction [3].

Methods

Data were obtained from 32 men with lower urinary tract symptoms and from 7 asymptomatic volunteers.  In each case a conventional medium fill cystometrogram (CMG) was first performed using an 8 Fr double lumen catheter (MediPlus Ltd, UK).  For each subject, values for Q_max and p_det,Qmax were obtained from the CMG and plotted on the provisional ICS nomogram.  The bladder was then refilled and the previously described non-invasive bladder pressure test carried out [1, 2].  Briefly, a cuff was fitted round the penis: once voiding had commenced, the cuff was inflated in steps of 10 cm H₂O at intervals of 0.75 s.  The test was terminated when flow was interrupted or a pressure of 200 cm H₂O reached.  Cuff pressure was plotted against flow rate (allowing for the delays in the flow meter), in order that the cuff pressure at which flow ceased (p_cuff,int) could be determined.  For each subject, the value of p_cuff,int was then plotted against the corresponding value of Q_max, the maximum flow rate recorded during the cuff test.

 

Results
Data from 6 subjects were excluded from the analysis: 1 failed to void, 2 strained excessively, and 3 were studied using a narrow cuff which we now know to be unreliable [1].  Using the conventional CMG data, the ICS nomogram classified 15 of the remaining subjects as obstructed, 11 (including 4 volunteers!) as equivocal, and 7 as unobstructed.  A graph of p_cuff,int versus Q_max for these 33 subjects is given in the figure.

Conclusions
The graph shows a clear differentiation between obstructed men lying to the upper left, with equivocal or unobstructed men to the lower right.  The line drawn emphasises this distribution.  The intersect of the line of division at 80 cm H₂O compares with a value of 40 cm H₂O in the ICS nomogram.  We believe this difference is accounted for by the following components: abdominal pressure, height difference between cuff and bladder, and the fact that the stepped cuff inflation will overshoot by an average of 5 cm H₂O.  The increased gradient of the line is consistent with the expected greater difference between isovolumetric pressure (p_ves,iso) and p_ves,Qmax for higher flow rates [4, 5].  We conclude that non-invasive voiding studies using the cuff inflation technique can provide useful information on obstruction.

References
1.                   A new method for non-invasive measurement of voiding pressure? Assessment of penile cuff occlusion. . Neurourol Urodyn 1999; 18: 256-257.
2.                   A new method for non-invasive assessment of bladder pressure during voiding compared with simultaneous invasive urodynamics.  Neurourol Urodyn 1999; 18: 253-254.
3.                   Standardization of terminology of lower urinary tract function: pressure-flow studies of voiding, urethral resistance, and urethral obstruction. Neurourol Urodyn 1997; 16: 1-18.
4.                   Urodynamics: the mechanics and hydrodynamics of the lower urinary tract. Medical Physics Handbooks 4. Bristol: Adam Hilger Ltd 1980.
5.         Urethral resistance?  Urodynamic concepts of physiological and pathological bladder outlet function during voiding. Neurourol Urodyn 1985; 4: 161-201.