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Pressure-to-Cross-Sectional Area Relationships in the Proximal Urethra of Men with Bladder Outlet Obstruction
0022-5347/96/1551-0267$03.00/0 T H E J O U R N A L OF. UXOLOCY
Vol. 155. 267-270, January 1996 Printed i n U.S.A
Copyright 0 1996 by A M E R I ~ AUROLOUICAI. N ASSOCIATION, INC.
PRESSURE-TO-CROSS-SECTIONAL AREA RELATIONSHIPS IN THE PROXIMAL URETHRA OF MEN WITH BLADDER OUTLET OBSTRUCTION PEDER H. GRAVERSEN, PER BAGI, HANS PETER TOFFT, HANS COLSTRUP J0RGEN KVIST KRISTENSEN
AND
From the Departments of Urology, Rigshospitalet and Gentofte Hospital, University of Copenhagen, Copenhagen, Denmark
ABSTRACT
Purpose: Related values of pressure and cross-sectional area in the proximal urethra were measured in patients with bladder outlet obstruction. Urethral opening pressure and elastance (the inverse of compliance) were estimated. Materials and Methods: We studied 15 men with standard urodynamic examinations. The pressure-to-cross-sectional area relationship in the prostatic urethra was determined using a special probe. Results: Elastance vaned significantly along the studied portion of the urethra, with higher values found in the sphincter area. The estimated urethral opening pressure appeared high compared to that in unobstructed cases and without variation along the prostatic urethra. Conclusions: The most important effect of prostatic obstruction appears to be the increased urethral opening pressure. KEY WORDS:urodynamics, prostatic hypertrophy, bladder neck obstruction, urethra, physiology
In the evaluation of bladder outlet obstruction conven- eter to allow for cross-sectional area measurement according tional urodynamic studies with objective quantitative meas- to the field-gradient principle. The balloon is inflated urements of pressure and flow have provided a new under- through a filling channel between the inner and outer cathstanding of lower urinary tract function and dysfunction. The eters. Pressure is measured in the bladder and balloon (range outlet function was characterized by Griffiths, who developed 0 to 250 cm. water), and cross-sectional area is measured a theory of flow through nonrigid tubes with a concept of a over a 2 mm. segment of the balloon between the measuring flow controlling zone in the proximal urethra.' This theory electrodes (range 11 to 102 mm.'). Measurements in a preswas further elaborated by Schafer, who described the basic sure chamber with the pressure sensors covered and uncovproperties of the flow controlling zone by the effective cross- ered are identical. sectional area and by the urethral opening pressure.2 These Investigations were performed with the patient in the suparameters of proximal urethral physiology have been esti- pine position with an empty bladder. The catheter was intromated previously during the micturition phase,3 yet the duced into the urethra, and placed with the balloon in the method was hard to standardize and measurements were bladder. The balloon was then connected to a pressure resdifficult to reproduce. We present measurements of cross- ervoir with a pressure of 10 to 15 cm. water above bladder sectional area and pressure in the proximal urethra during pressure, and slowly retracted until the sensing electrodes the storage phase in men with bladder outlet obstruction. entered the urethra as indicated by a decrease in crosssectional area. Measurements were initiated after the cathMATERIALS AND METHODS eter was retracted 5 mm. further from the bladder neck and We investigated 15 consecutive men after full informed repeated a t every 1 cm. until the high pressure zone was consent was obtained. Standard urodynamic examinations passed. At each measurement location the balloon was adincluded uroflowmetry with the patient standing and filling justed to a cross-sectional area of approximately 13 mm.' and by diuresis, resting urethral pressure profilometry with an thereafter it was inflated in steps of approximately 10 mm.' 8F, 2 side hole catheter at a retraction rate of 3 mm. per using a 1 ml. syringe. After each inflation pressure reached second a n d a perfusion rate of 2 ml. per minute, cystometry equilibrium, as indicated by a constant balloon pressure, the performed by transurethral filling of saline through a 5F inflation was continued until a cross-sectional area of 80 catheter at a filling rate of 50 ml. per minute with the patient mm.' or a balloon pressure of 150 cm. water was reached. supine a n d pressure-flow measurements. All patients were The balloon was then deflated before retraction to the next proved to have bladder outlet obstruction according to the site of measurement. Cross-sectional area in the balloon, and criteria of Abrams and Griffiths.4 The relationship between pressures in the balloon and bladder were registered simulintraurethral pressure and cross-sectional area was deter- taneously on a Dantec DISA UROsystem 21F16 2100 (fig. 1) mined with a special probe that enables dilation of a short or on a Dantec Menuet instrument. Diagrams of related urethral segment, and simultaneous registration of urethral values of balloon pressure at equilibrium and cross-sectional area at each measurement site were constructed, and the pressure and cross-sectional area in this segment. The technique has been described in detail p r e v i o u s l ~ . ~formula, regression line of balloon pressure at equilibrium = Briefly, the probe consists of a n outer 14F polyvinylchloride urethral opening pressure + change in pressurelchange in catheter and a n inner catheter used for pressure measure- cross-sectional area x cross-sectional area through the alment. A small polyvinylchloride balloon is mounted at the most linear portion of the curve, was determined (fig. 2). Elastance indicates the change in pressure for a change in end of the outer catheter and covers 4 (2 generating and 2 measuring) ring electrodes that are glued to the inner cath- volume and is the inverse of compliance. For a urethral segment between the measuring electrodes with a distance of 2 mm. the change in volume is 2 times the change in crossAccepted for publication June 23, 1995. 267
268
PRESSURE-TO-CROSS-SECTIONAL AREA MEASUREMENTS IN PROSTATIC URETHRA
FIG. 1. Sidtaneous measurement of pressure and cross-8ectional area in balloon, bladder pressure and anal surface eleetromyography during inflation with small fluid volumes in balloon.
lntraurethral pressure (cm H,O)
100
10
20
30
RESULTS
Median patient age was 74 years (range 57 to 93). Results of urinalysis (dipstick) and blood tests (hemoglobin, sodium, potassium and creatinine levels) were all within the normal range. Table 1 shows the urodynamic findings. The distance from the bladder neck to the distal sphincter peak ranged from 4.0 to 7.5 cm. (median 5.5). The pressure-tocross-sectional area relationship at the individual measurements showed an almost linear course for the lower dilations (fig. 2). Beyond the range o f linearity dilation was still possible but with a marked increase in elastance. From each measurement the regression line through the linear portion of the pressure-to-cross-sectionalarea curve was determined, and the regression constant (urethral opening pressure) and slope (change in pressure divided by change in crosssectional area) were calculated. The estimated urethral opening pressure tended to be lowest at the bladder neck (fig. 3, A), yet it did not differ significantly along the studied portion of the urethra. However, the elastance showed a significant variation along the urethra with steeper slopes in the sphincter area (fig. 3, B and table 2).
140
0
the bladder neck by the length of the posterior urethra and multiplying by 100, thus expressing the measurement location in percent of the distance From the bladder neck to the site of the maximum pressure determined by urethral pressure profilometry with 0%at the bladder neck. In light of this finding, the posterior urethra was further divided into 5 segments each with a length of 30% (segment 1-0 to 306, segment 2 3 1 to 60%, segment 3-61 to 90%, segment 4-91 to 120% and segment 5-121 to 150%)for comparison between individuals. Methods, definitions, and units are in accordance with the standards proposed by the International Continence Society.6 The number of measurement sites vaned between patients. The observations from each individual patient were subjected to a cubic spline regression procedure using the Box-Cox power transformation to allow for comparison between individuals.7 Homogeneity of variance was obtained by logarithmic transformation. The variation along the urethra was studied using Friedman's test. If this test demonstrated a significant difference between urethral segments, the analysis was extended with a multiple test procedure to identify the deviating segment(d.8 Significance was defined as p <0.05.
40
50
80
70
80
Cross sectional area (mm2) FIG. 2. Related values of urethd pressure and cross-sectional area at equilibrium. Line indicates regression line through almost linear rtion of dilation m e . Estimated pressure in uninstrumenðra (Pm)and elastance (changein pressure [dP]divided by change in cross-sectional area [dCAl) are indicated.
sectional area (mm?) provided that the walls have a negligible slope. Thus, for unity of tube length (1mm.) the slope of the regression line (change in pressure divided by change in cross-sectional area) is numerically identical with elastance. Consequently, the slope of the regression line equals urethral elastance and the inverse of this slope ([change in pressure divided by change in cross-sectionalareal-') equals the compliance. The intercept of the regression line with the pressure axis denotes the estimated theoretical pressure in the uninstrumented urethra, that is the estimated urethral opening pressure. Due to significant variations in the length of the posterior urethra, defined as the distance from the bladder neck to the site of the maximum urethral pressure determined during urethral pressure profilometry, the locations of the measurement sites were standardized by dividing the distance from
DISCUSSION
According to the theory of Schafer, the balance of voiding is defined on 1side by the detrusor muscle, which provides the energy for micturition, and on the other side by the properties of the flow controlling zone, which determine the relationship of pressure to flow rate.9 In a simplified model the pressure-flow relationship depends on the effective crosssectional area of the flow controlling zone and on the urethral elasticity, represented by urethral opening pressure, with these parameters being characteristic of the bladder outlet function. Consequently, assessment of these parameters is important to determine objectively prostatic obstruction further.
TABLE1. Urodvnamic findings Urodynamic Parameters
Max. flow rate (ml./sec.)
Median (range)
9.9 (6.2-19.2) 174 (50-360) Voided vol. (rn1.J Detrusor opening pressure (em. water) 77.5 (52-125) Detrusor pressure at maximum flow (cm. water) 98 146-158) Max. detrusor pressure (ern. water) 103 (54-242) Max. cystometric capacity (ml.) 268 (61-381) Six patients had a stable and 9 had an unstable detrusor.
PRESSURE-TO-CROSS-SECTIONALAREA MEASUREMENTS IN PROSTATIC URETHRA
269
(cm H,0/mrn2)
60
1 Range
'-
- Quartiles
+ Median
35 r
I
32.5 -
50 40
2-
~
30 -
1,5 -
20 -
1-
- 1
10 -
0'
A
1
2
3
4
I
0.5 -
5
Segments
01
B
1
2
i
3
4
5
Segments
FIG. 3. A, estimated urethral opening pressures ( P U Jrelated to urethral segments. B, urethral elastance related to urethral segments. Values are determined from Box-Cox power transformation for each of 5 urethral segments. 2 . Comparison of elaslance and urethral opening pressure among individual segments according to the multiple test orocedure
TARLk:
~
Urethral Segment
Elastance (D value)
1 Versus: 2
3 4
5 2 Versus: 3 4
5 3 Versus:
Not significant Not significant <0.01 10.01 Not significant 10.01 10.01
Not significant Not significant 5 Not sienificant 4 Versus 5 The p value for urethra opening pressure was not significant. 4
Our method combines simultaneous measurements of urethral pressure at equilibrium following dilation and crosssectional area. This relationship (fig. 2 ) follows an almost straight line with the equation balloon pressure at equilibrium = urethral opening pressure + change in pressure/ change in cross-sectional area x cross-sectional area, makmg calculation of the constants, urethral opening pressure and change in pressure, divided by change in cross-sectional area (elastance) possible. However, it is important to realize that while the course of the line at smaller dilations is experimentally determined, permitting calculation of the slope (change in pressure divided by change in cross-sectional area), the regression constant (urethral opening pressure) remains an estimated parameter. This constant has been termed the estimated theoretical pressure in the uninstrumented urethra"' and reflects the pressure needed to unfold the urethral lumen provided that any mucosal adhesive effects are disregarded and that the elastance remains the same at dilations of less than 12 to 13 mm.'. Whether these prerequisites hold true is controversial and, therefore, the numeric value of urethral opening pressure may not be identical to the actual physiological opening pressure. All measurements were performed under standardized circumstances at rest during the storage phase. The pressureto-cross-sectional area relationships consequently seem to reflect the responses of the peri-luminal tissues along the proximal urethra with segment 1corresponding to the bladder neck, segments 2 and 3 corresponding to the prostatic urethra, segment 4 Corresponding to the distal sphincter area and segment 5 corresponding to the urethra just distal to the sphincter. The elastance proved to be significantly lower in
segments 1 , 2 and 3 compared to the sphincter area (table 2 ) , implying that the prostatic urethra is more readily distensible as soon as the opening pressure is overcome. Furthermore, the numeric values and characteristic variation in elastance along the proximal urethra in these men with bladder outlet obstruction were almost the same as those in a prior similar study of healthy men performed by Bagi et a1.10 On the other hand, urethral opening pressure demonstrated no significant variation along the studied portion of the urethra, even though the values tended to be lowest at the bladder neck (table 2 and fig. 3, A). These findings are in contrast with the results of a study of healthy men in whom urethral opening pressure proved to be significantly highest in the sphincteric region compared to all proximal segments,lO and the values in the prostatic urethra seemed higher in our patients with obstruction. However, as also noted in the previous study, urethral opening pressure in the proximal 3 segments tended to increase with increasing age. Thus, it appears that in men with bladder outlet obstruction the pressure level a t which unfolding of the prostatic urethral lumen begins is higher but as soon as it occurs the slope of the curve (elastance) remains the same. These findings appear to agree with the statement of Schafer that the most important consequence of prostatic obstruction on outflow conditions is the increased urethral opening pressure.9 As noted by Schafer, this demand for an elevated pressure to open the flow rate controlling zone and to maintain the dilation during micturition means that the detrusor work necessary to empty equivalent volumes of urine is increased significantly in men with bladder outlet obstruction. CONCLUSIONS
This method of measuring bladder outlet obstruction is simple and standardizabie. Measurements represent a direct estimation of the urodynamic consequences of bladder outlet obstruction and reproducibility is fairly good.s.10 The results of the measurements confirm the statement of Schafer regarding the urodynamic effects of prostatic obstruction. However further studies are needed to define normal and abnormal findings, and the method is still far from being used routinely in clinical practice. REFERENCES
1. Griffiths, D. J.: A physical approach to flow through the urethra: uniform tubes. In: Urodynamics. The Mechanics and Hydrodynamics of the Lower Urinary Tract. Bristol: Adam Hilger, Ltd., chapt. 3,pp. 25-65, 1980. 2. Schafer, W.:The contribution of the bladder outlet to the relation between pressure and flow rate during micturition. In:
270
PRESSURE-TO-CROSS-SECTIONAL AREA MEASUREMENTS IN PROSTATIC URETHRA Benign Prostatic Hypertrophy. Edited by F. Hinman, Jr. and S. Boyarsky. New York: Springer-Verlag, chapt. 44,pp. 470-
496, 1983. 3. Mortensen, S. 0.: Cross-Sectional Areas in the Normal Male Urethra During Voiding. Copenhagen: Edition Mortensen, pp.
1-133. 1989. D. J.: The assessment of prostatic obstruction from urodynamic measurements and from residual urine. Brit. J. Urol.. 51: 129,1979. 5 . Bagi, P., Vejborg, I., Colstrup, H. and Kristensen. J. K.: A technique for measurement of related values of pressure and crosssectional area in the male urethra. Urol. Res., 21: 245, 1993. 6. Abrams. P..Blaivas. J. G., Stanton, S. L. and Andersen, J. T.:
4. Abrams, P. H. and Griffiths.
Standardisation of terminology of lower urinary tract function. Neurourol. Urodynam., 7:403, 1988. 7. Box, C . E. P. and Cox, D. R.: An analysis of transformations. J. Roy. Stat. Sac., B26 211, 1964. 8. Siegel, S.and Castellan, N. J.: Nonparametrie Statistics for the Behavioral Sciences. New York: McGraw-Hill Book Co., 1988. 9. Schafer, W.:Urethral resistance? Urodynamic concepts of physiological and pathological bladder outlet function during voiding. Neurourol. Urodynam., 4 161,1985. 10. Bagi, P.,Vejborg, I., Colstrup, H. and Kristensen, J. K.: Pressurekross-sectional area relations in the proximal urethra of healthy males. Part 1: elastance and estimated opening pressure in the uninstrumented urethra. Eur. Urol., 2 8 51,1995.
Vol. 155. 267-270, January 1996 Printed i n U.S.A
Copyright 0 1996 by A M E R I ~ AUROLOUICAI. N ASSOCIATION, INC.
PRESSURE-TO-CROSS-SECTIONAL AREA RELATIONSHIPS IN THE PROXIMAL URETHRA OF MEN WITH BLADDER OUTLET OBSTRUCTION PEDER H. GRAVERSEN, PER BAGI, HANS PETER TOFFT, HANS COLSTRUP J0RGEN KVIST KRISTENSEN
AND
From the Departments of Urology, Rigshospitalet and Gentofte Hospital, University of Copenhagen, Copenhagen, Denmark
ABSTRACT
Purpose: Related values of pressure and cross-sectional area in the proximal urethra were measured in patients with bladder outlet obstruction. Urethral opening pressure and elastance (the inverse of compliance) were estimated. Materials and Methods: We studied 15 men with standard urodynamic examinations. The pressure-to-cross-sectional area relationship in the prostatic urethra was determined using a special probe. Results: Elastance vaned significantly along the studied portion of the urethra, with higher values found in the sphincter area. The estimated urethral opening pressure appeared high compared to that in unobstructed cases and without variation along the prostatic urethra. Conclusions: The most important effect of prostatic obstruction appears to be the increased urethral opening pressure. KEY WORDS:urodynamics, prostatic hypertrophy, bladder neck obstruction, urethra, physiology
In the evaluation of bladder outlet obstruction conven- eter to allow for cross-sectional area measurement according tional urodynamic studies with objective quantitative meas- to the field-gradient principle. The balloon is inflated urements of pressure and flow have provided a new under- through a filling channel between the inner and outer cathstanding of lower urinary tract function and dysfunction. The eters. Pressure is measured in the bladder and balloon (range outlet function was characterized by Griffiths, who developed 0 to 250 cm. water), and cross-sectional area is measured a theory of flow through nonrigid tubes with a concept of a over a 2 mm. segment of the balloon between the measuring flow controlling zone in the proximal urethra.' This theory electrodes (range 11 to 102 mm.'). Measurements in a preswas further elaborated by Schafer, who described the basic sure chamber with the pressure sensors covered and uncovproperties of the flow controlling zone by the effective cross- ered are identical. sectional area and by the urethral opening pressure.2 These Investigations were performed with the patient in the suparameters of proximal urethral physiology have been esti- pine position with an empty bladder. The catheter was intromated previously during the micturition phase,3 yet the duced into the urethra, and placed with the balloon in the method was hard to standardize and measurements were bladder. The balloon was then connected to a pressure resdifficult to reproduce. We present measurements of cross- ervoir with a pressure of 10 to 15 cm. water above bladder sectional area and pressure in the proximal urethra during pressure, and slowly retracted until the sensing electrodes the storage phase in men with bladder outlet obstruction. entered the urethra as indicated by a decrease in crosssectional area. Measurements were initiated after the cathMATERIALS AND METHODS eter was retracted 5 mm. further from the bladder neck and We investigated 15 consecutive men after full informed repeated a t every 1 cm. until the high pressure zone was consent was obtained. Standard urodynamic examinations passed. At each measurement location the balloon was adincluded uroflowmetry with the patient standing and filling justed to a cross-sectional area of approximately 13 mm.' and by diuresis, resting urethral pressure profilometry with an thereafter it was inflated in steps of approximately 10 mm.' 8F, 2 side hole catheter at a retraction rate of 3 mm. per using a 1 ml. syringe. After each inflation pressure reached second a n d a perfusion rate of 2 ml. per minute, cystometry equilibrium, as indicated by a constant balloon pressure, the performed by transurethral filling of saline through a 5F inflation was continued until a cross-sectional area of 80 catheter at a filling rate of 50 ml. per minute with the patient mm.' or a balloon pressure of 150 cm. water was reached. supine a n d pressure-flow measurements. All patients were The balloon was then deflated before retraction to the next proved to have bladder outlet obstruction according to the site of measurement. Cross-sectional area in the balloon, and criteria of Abrams and Griffiths.4 The relationship between pressures in the balloon and bladder were registered simulintraurethral pressure and cross-sectional area was deter- taneously on a Dantec DISA UROsystem 21F16 2100 (fig. 1) mined with a special probe that enables dilation of a short or on a Dantec Menuet instrument. Diagrams of related urethral segment, and simultaneous registration of urethral values of balloon pressure at equilibrium and cross-sectional area at each measurement site were constructed, and the pressure and cross-sectional area in this segment. The technique has been described in detail p r e v i o u s l ~ . ~formula, regression line of balloon pressure at equilibrium = Briefly, the probe consists of a n outer 14F polyvinylchloride urethral opening pressure + change in pressurelchange in catheter and a n inner catheter used for pressure measure- cross-sectional area x cross-sectional area through the alment. A small polyvinylchloride balloon is mounted at the most linear portion of the curve, was determined (fig. 2). Elastance indicates the change in pressure for a change in end of the outer catheter and covers 4 (2 generating and 2 measuring) ring electrodes that are glued to the inner cath- volume and is the inverse of compliance. For a urethral segment between the measuring electrodes with a distance of 2 mm. the change in volume is 2 times the change in crossAccepted for publication June 23, 1995. 267
268
PRESSURE-TO-CROSS-SECTIONAL AREA MEASUREMENTS IN PROSTATIC URETHRA
FIG. 1. Sidtaneous measurement of pressure and cross-8ectional area in balloon, bladder pressure and anal surface eleetromyography during inflation with small fluid volumes in balloon.
lntraurethral pressure (cm H,O)
100
10
20
30
RESULTS
Median patient age was 74 years (range 57 to 93). Results of urinalysis (dipstick) and blood tests (hemoglobin, sodium, potassium and creatinine levels) were all within the normal range. Table 1 shows the urodynamic findings. The distance from the bladder neck to the distal sphincter peak ranged from 4.0 to 7.5 cm. (median 5.5). The pressure-tocross-sectional area relationship at the individual measurements showed an almost linear course for the lower dilations (fig. 2). Beyond the range o f linearity dilation was still possible but with a marked increase in elastance. From each measurement the regression line through the linear portion of the pressure-to-cross-sectionalarea curve was determined, and the regression constant (urethral opening pressure) and slope (change in pressure divided by change in crosssectional area) were calculated. The estimated urethral opening pressure tended to be lowest at the bladder neck (fig. 3, A), yet it did not differ significantly along the studied portion of the urethra. However, the elastance showed a significant variation along the urethra with steeper slopes in the sphincter area (fig. 3, B and table 2).
140
0
the bladder neck by the length of the posterior urethra and multiplying by 100, thus expressing the measurement location in percent of the distance From the bladder neck to the site of the maximum pressure determined by urethral pressure profilometry with 0%at the bladder neck. In light of this finding, the posterior urethra was further divided into 5 segments each with a length of 30% (segment 1-0 to 306, segment 2 3 1 to 60%, segment 3-61 to 90%, segment 4-91 to 120% and segment 5-121 to 150%)for comparison between individuals. Methods, definitions, and units are in accordance with the standards proposed by the International Continence Society.6 The number of measurement sites vaned between patients. The observations from each individual patient were subjected to a cubic spline regression procedure using the Box-Cox power transformation to allow for comparison between individuals.7 Homogeneity of variance was obtained by logarithmic transformation. The variation along the urethra was studied using Friedman's test. If this test demonstrated a significant difference between urethral segments, the analysis was extended with a multiple test procedure to identify the deviating segment(d.8 Significance was defined as p <0.05.
40
50
80
70
80
Cross sectional area (mm2) FIG. 2. Related values of urethd pressure and cross-sectional area at equilibrium. Line indicates regression line through almost linear rtion of dilation m e . Estimated pressure in uninstrumenðra (Pm)and elastance (changein pressure [dP]divided by change in cross-sectional area [dCAl) are indicated.
sectional area (mm?) provided that the walls have a negligible slope. Thus, for unity of tube length (1mm.) the slope of the regression line (change in pressure divided by change in cross-sectional area) is numerically identical with elastance. Consequently, the slope of the regression line equals urethral elastance and the inverse of this slope ([change in pressure divided by change in cross-sectionalareal-') equals the compliance. The intercept of the regression line with the pressure axis denotes the estimated theoretical pressure in the uninstrumented urethra, that is the estimated urethral opening pressure. Due to significant variations in the length of the posterior urethra, defined as the distance from the bladder neck to the site of the maximum urethral pressure determined during urethral pressure profilometry, the locations of the measurement sites were standardized by dividing the distance from
DISCUSSION
According to the theory of Schafer, the balance of voiding is defined on 1side by the detrusor muscle, which provides the energy for micturition, and on the other side by the properties of the flow controlling zone, which determine the relationship of pressure to flow rate.9 In a simplified model the pressure-flow relationship depends on the effective crosssectional area of the flow controlling zone and on the urethral elasticity, represented by urethral opening pressure, with these parameters being characteristic of the bladder outlet function. Consequently, assessment of these parameters is important to determine objectively prostatic obstruction further.
TABLE1. Urodvnamic findings Urodynamic Parameters
Max. flow rate (ml./sec.)
Median (range)
9.9 (6.2-19.2) 174 (50-360) Voided vol. (rn1.J Detrusor opening pressure (em. water) 77.5 (52-125) Detrusor pressure at maximum flow (cm. water) 98 146-158) Max. detrusor pressure (ern. water) 103 (54-242) Max. cystometric capacity (ml.) 268 (61-381) Six patients had a stable and 9 had an unstable detrusor.
PRESSURE-TO-CROSS-SECTIONALAREA MEASUREMENTS IN PROSTATIC URETHRA
269
(cm H,0/mrn2)
60
1 Range
'-
- Quartiles
+ Median
35 r
I
32.5 -
50 40
2-
~
30 -
1,5 -
20 -
1-
- 1
10 -
0'
A
1
2
3
4
I
0.5 -
5
Segments
01
B
1
2
i
3
4
5
Segments
FIG. 3. A, estimated urethral opening pressures ( P U Jrelated to urethral segments. B, urethral elastance related to urethral segments. Values are determined from Box-Cox power transformation for each of 5 urethral segments. 2 . Comparison of elaslance and urethral opening pressure among individual segments according to the multiple test orocedure
TARLk:
~
Urethral Segment
Elastance (D value)
1 Versus: 2
3 4
5 2 Versus: 3 4
5 3 Versus:
Not significant Not significant <0.01 10.01 Not significant 10.01 10.01
Not significant Not significant 5 Not sienificant 4 Versus 5 The p value for urethra opening pressure was not significant. 4
Our method combines simultaneous measurements of urethral pressure at equilibrium following dilation and crosssectional area. This relationship (fig. 2 ) follows an almost straight line with the equation balloon pressure at equilibrium = urethral opening pressure + change in pressure/ change in cross-sectional area x cross-sectional area, makmg calculation of the constants, urethral opening pressure and change in pressure, divided by change in cross-sectional area (elastance) possible. However, it is important to realize that while the course of the line at smaller dilations is experimentally determined, permitting calculation of the slope (change in pressure divided by change in cross-sectional area), the regression constant (urethral opening pressure) remains an estimated parameter. This constant has been termed the estimated theoretical pressure in the uninstrumented urethra"' and reflects the pressure needed to unfold the urethral lumen provided that any mucosal adhesive effects are disregarded and that the elastance remains the same at dilations of less than 12 to 13 mm.'. Whether these prerequisites hold true is controversial and, therefore, the numeric value of urethral opening pressure may not be identical to the actual physiological opening pressure. All measurements were performed under standardized circumstances at rest during the storage phase. The pressureto-cross-sectional area relationships consequently seem to reflect the responses of the peri-luminal tissues along the proximal urethra with segment 1corresponding to the bladder neck, segments 2 and 3 corresponding to the prostatic urethra, segment 4 Corresponding to the distal sphincter area and segment 5 corresponding to the urethra just distal to the sphincter. The elastance proved to be significantly lower in
segments 1 , 2 and 3 compared to the sphincter area (table 2 ) , implying that the prostatic urethra is more readily distensible as soon as the opening pressure is overcome. Furthermore, the numeric values and characteristic variation in elastance along the proximal urethra in these men with bladder outlet obstruction were almost the same as those in a prior similar study of healthy men performed by Bagi et a1.10 On the other hand, urethral opening pressure demonstrated no significant variation along the studied portion of the urethra, even though the values tended to be lowest at the bladder neck (table 2 and fig. 3, A). These findings are in contrast with the results of a study of healthy men in whom urethral opening pressure proved to be significantly highest in the sphincteric region compared to all proximal segments,lO and the values in the prostatic urethra seemed higher in our patients with obstruction. However, as also noted in the previous study, urethral opening pressure in the proximal 3 segments tended to increase with increasing age. Thus, it appears that in men with bladder outlet obstruction the pressure level a t which unfolding of the prostatic urethral lumen begins is higher but as soon as it occurs the slope of the curve (elastance) remains the same. These findings appear to agree with the statement of Schafer that the most important consequence of prostatic obstruction on outflow conditions is the increased urethral opening pressure.9 As noted by Schafer, this demand for an elevated pressure to open the flow rate controlling zone and to maintain the dilation during micturition means that the detrusor work necessary to empty equivalent volumes of urine is increased significantly in men with bladder outlet obstruction. CONCLUSIONS
This method of measuring bladder outlet obstruction is simple and standardizabie. Measurements represent a direct estimation of the urodynamic consequences of bladder outlet obstruction and reproducibility is fairly good.s.10 The results of the measurements confirm the statement of Schafer regarding the urodynamic effects of prostatic obstruction. However further studies are needed to define normal and abnormal findings, and the method is still far from being used routinely in clinical practice. REFERENCES
1. Griffiths, D. J.: A physical approach to flow through the urethra: uniform tubes. In: Urodynamics. The Mechanics and Hydrodynamics of the Lower Urinary Tract. Bristol: Adam Hilger, Ltd., chapt. 3,pp. 25-65, 1980. 2. Schafer, W.:The contribution of the bladder outlet to the relation between pressure and flow rate during micturition. In:
270
PRESSURE-TO-CROSS-SECTIONAL AREA MEASUREMENTS IN PROSTATIC URETHRA Benign Prostatic Hypertrophy. Edited by F. Hinman, Jr. and S. Boyarsky. New York: Springer-Verlag, chapt. 44,pp. 470-
496, 1983. 3. Mortensen, S. 0.: Cross-Sectional Areas in the Normal Male Urethra During Voiding. Copenhagen: Edition Mortensen, pp.
1-133. 1989. D. J.: The assessment of prostatic obstruction from urodynamic measurements and from residual urine. Brit. J. Urol.. 51: 129,1979. 5 . Bagi, P., Vejborg, I., Colstrup, H. and Kristensen. J. K.: A technique for measurement of related values of pressure and crosssectional area in the male urethra. Urol. Res., 21: 245, 1993. 6. Abrams. P..Blaivas. J. G., Stanton, S. L. and Andersen, J. T.:
4. Abrams, P. H. and Griffiths.
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