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Direct Recording of Urethral Resistance Using the “Urethroresistance”
0022-534 7/78/1195-0643$02. 00/0 Vol. 119, May Printed in U.SA.
THE JOURNAL OF UROLOGY
Copyright © 1978 by The Williams & Wilkins Co.
DIRECT RECORDING OF URETHRAL RESISTANCE USING THE "URETHRORESISTANCE" R. M. ANIKWE From the Department of Surgery, University of Aberdeen and the Urological Unit of Aberdeen, Royal Infirmary, Aberdeen, Scotland
ABSTRACT
Observations on urethral resistance during micturition in non-obstructed men and in patients with bladder outlet obstruction owing to benign prostatic hypertrophy are presented. The "Urethroresistance", a new instrument, was used for the direct recording of urethral resistance. The upper limit for the minimum urethral resistance during micturition for normal nonobstructed men was 0.5 units. Two patterns of urethral resistance were observed solely in nonobstructed subjects and 3 other patterns, which were found solely in patients with a moderate or severe degree of bladder outlet obstruction, were shown to be characteristic of bladder outlet obstruction. While direct recording of urethral resistance is a better test for bladder outlet obstruction than the recording of urinary flow rate the recording of urinary flow is a simple urodynamic test, which remains a useful screening test for bladder outlet obstruction. With the exception of severe obstructive symptoms none of the other clinical methods for the assessment of bladder outlet obstruction-the size of the prostate gland, the volume ofpost-micturition residual urine and the degree of bladder trabeculation - is reliable to determine the degree of bladder outlet obstruction. It has become increasingly obvious that clinical methods to evaluate bladder outlet obstruction are inadequate. 1 Isolated recordings of urinary flow rate or vesical pressures have been shown to be valuable but in some patients, particularly those with slight bladder outlet obstruction, a correct assessment of the bladder outlet drainage requires an objective quantitative measurement of urethral resistance. However, there has been some confusion concerning which factors to measure in determining urethral resistance and how these factors relate to one another. Thus, Gleason and Lattimer,2 Pierce and associates, 3 and Arbuckle and Paquin, 4 assuming stream-line urinary flow, used the equation R equals P/F, where R represents urethral resistance, P represents the intravesical pressure (pressure recorded within the bladder cavity in the course of micturition) and F represents the urinary flow rate. Ritter and associates thought that urinary flow was turbulent and questioned the validity of applying the formula R equals P/F to the determination of urethral resistance. 5 Smith allowed for frictional losses caused by eddies and undercurrents in turbulent flow and thought that the pressure decrease along the urethra (P - P 1) varied directly with the square of the urinary flow rate (R). 6 So P - P 1 a F2, therefore R equals P - P 1/F2, where R is a constant, the urethral resistance, P is the intravesical pressure and P 1 is the pressure of urine at the external urethral meatus (the urethral exit pressure). Measurement of P 1 has been tedious and unreliable. 6 - 8 Some investigators have assumed P 1 to be zero, thus using the formula R equals P/F2 for the calculation of urethral resistance. 3, 9 Huffman and Keitzer demonstrated mathematically that an accurate measurement of urethral resistance would require the measurements of urine velocity, viscosity and specific gravity, as well as urethral length and urethral diameter, at different levels. 10 However, there are serious doubts as to whether such a tedious urodynamic test can be applied clinically. Smith, 11 Claridge and Shuttleworth, 9 and Gleason and Lattimer" used the formula R equals P/F2 , finding it valuable for the diagnosis of bladder outlet obstruction. However, in the absence of any instrument for the direct recording of R most clinicians found calculations of urethral resistance at short intervals along the urinary flow and
intravesical pressure curves laborious and resorted to calculating solely the minimum urethral resistance (urethral resistance during peak urinary flow). After our studies on urethral resistance we devised a new instrument, the "Urethroresistance,"12 for the direct recording of urethral resistance during micturition using the formula R equals P/F 2 • Herein are presented observations with the new instrument. MATERIALS AND METHODS
Urinary flow and pressure recordings were done synchronously on 51 men, ranging in age from 50 to 82 years. The subjects were subdivided into 5 clinical groups on the basis of 4 clinical criteria: 1) the severity of obstructive symptoms, 2) the volume of post-micturition residual urine, 3) the size of the prostate gland on rectal-digital palpation and 4) the degree of bladder trabeculation on cystourethroscopy. Group 1 consists of 12 subjects without clinical evidence of bladder outlet obstruction. The subjects with urinary tract infection, diabetes mellitus, neurological conditions, detrusor dysfunction, previous operations on the lower urinary tract or in the pelvic cavity and patients on drugs known to affect the bladder were excluded from this control group. Group 2 consists of 10 individuals with benign prostatic hypertrophy but with a doubtful bladder outlet obstruction on clinical evaluation. Group 3 comprises 9 patients with mild bladder outlet obstruction owing to benign prostatic hypertrophy. Group 4 consists of 20 patients with either a moderate or a severe degree of bladder outlet obstruction owing to benign prostatic hypertrophy. Group 5 includes 10 patients who had had prostatectomy for bladder outlet obstruction owing to benign prostatic hypertrophy 6 to 10 weeks previously. Synchronous urinary flow and pressure recordings were done on each subject. With the subject supine a fine 6F ureteral catheter with 2 side holes near the tip was inserted aseptically per urethram into the bladder, thus serving as a fine urethral catheter. The side holes ensured that the catheter did not get blocked by the bladder urothelium. The catheter was coupled via a single-ended strain gauge pressure transducer to a Sanborn 6-channel recorder and was used to record the intravesical pressure. A uroflowmeter, which con-
Accepted for publication March 11, 1977. 643
644
ANIKWE
sisted of a graduated 500 ml. Perspex cylinder 4.3 cm. in internal diameter and 29 cm. long placed on a strain gauge transducer, was connected to the second channel of the recorder. The intravesical pressure and the urinary flow rate signals were fed into the Urethroresistance, which was connected to a third channel of the recorder for the direct recording of urethral resistance. With the subject in the sitting position the bladder was filled slowly with physiological saline through the urethral catheter until the patient had an urge to void. With the patient in the erect position the zero lines of the pressure transducers were adjusted to correspond to the upper border of the symphysis pubis. Before the onset of micturition the premicturition resting intravesical pressure was recorded for a few seconds. The subject was then requested to void alongside the indwelling fine urethral catheter into the uroflowmeter. Synchronous urinary flow, pressure and urethral resistance recordings were done throughout the course ofmicturition. At the end of micturition residual urine was determined by the evacuation of the bladder contents via the urethral catheter. The bladder was then refilled. At least 4 recordings were done on each subject. RESULTS
The lowest urethral resistance during micturition (minimum urethral resistance) occurred when urinary flow was at its peak (fig. 1). The minimum urethral resistance was not
A F
constant in a non-obstructed subject nor in a clinically obstructed patient. Therefore, a mean minimum urethral resistance was determined for each individual from the various recordings of urethral resistance. Table 1 shows that the range of the mean minimum urethral resistance for group 1 is 0.1 to 0.6 units and the group mean is 0.3 units. The group mean minimum urethral resistances for groups 2, 3, 4 and 5 are 0.8, 0.9, 2.4 and 0.3 units, respectively. There is a tendency for the mean minimum urethral resistance to increase with an increase in the degree of bladder outlet obstruction. However, there is no significant difference between the mean values for groups 2 and 3 (p > 0.05). Figure 2 shows an almost distinct separation between the mean minimum urethral resistance for groups 1 and 4. Table 2 shows a significant lowering in the minimum urethral resistance from the preoperative values for those patients who had undergone prostatectomy (p < 0.05). The mean minimum urethral resistance for those relieved of their obstruction after prostatectomy (group 5) is 0.3 units, which also is the figure obtained for the non-obstructed subjects (group 1). Five patterns of urethral resistance were observed (table 3 and fig. 3). The pattern 1 recording consisted of a sharp decrease in urethral resistance at the onset of micturition to a sustained lowest level less than 0.5 units and a sharp increase in resistance at the end of voiding. This pattern was observed in 42 of the 55 recordings (76 per cent) for subjects in group 1 and in 34 of the 44 recordings (77 per cent) for patients who had undergone prostatectomy. It also occurred in 10 and 14
PATIENT N0.1
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R URETHRAL 5~ RESISTANCE :
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R URETHRAL 5: RESISTANCE -
6
Clinicai Group
f
20
Time
in
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40
Subjects
Group 4 Subjects
O Group 5 Subjects
Jt
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4
x4 4
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Mean minimum urethral resistance for subjects in various clinical groups. Fm. 2.
1. Mean minimum urethral resistance in the various clinical groups for voided urinary volume of200 ml. and more No. Pts.
1
12 10
3
9 20 10
5
£
.A
1-0
0,5
2 4
•
A
.A
Fm. 1. Direct recording of urinary flow rate and urethral resistance. A, patient without bladder outlet obstruction. B, patient with bladder outlet obstruction.
TABLE
1.5
z < w
b
Group 2 Subjects
.A
z ::E
Group 1 Subjects
)C
A Group 3
A
...J
<
in seconds
•
Range of Mean Resistance (units) 0.10-0.60 0.42-1.64 0.41-1.73 0.50-6.60 0.11-0.50
Group Mean Resistance (units) 0.3 0.8 0.9 2.4 0.3
S.D.
S.E.
0.17 0.44 0.48 1.56 0.16
0.05 0.15 0.18 0.42 0.06
Sign_ificance of Differertce From Group 1 p< p< p< p>
0.05 0.05 0.05 0.05
645
DIRECT RECORDING OF URETHRAL RESISTANCE USING "URETHRORESISTANCE"
per cent of SUQjects in groups 2 and 3, respectively. The pattern 2 curve consisted of a slow decrease in urethral resistance at the onset of micturition to a lowest level less than 0.5 which was sustained for the major part of and a increase in resistance towards the end of This pattern was obtained in 13 of 55 recordings (24 per cent) for subjects in group 1 and in 10 of 44 recordings (23 per cent) for patients who had undergone prostatectomy. It also was observed in 13 and 11 per cent of subjects in groups 2 and respectively. The pattern 3 resistance curve consisted of a decrease in urethral resistance at the commencement of micturition to a sustained lowest level more than 0.5 units for the major part of voiding. This resistance curve was observed in 32 of the 72 recordings (44 per cent) for patients vvith a moderate or a severe degree of bladder outlet obstruction. It also occmTed in 68 and 39 per cent of subjects in groups 2 and 3, respectively. The pattern 4 curve consisted of a sharp decrease in urethral resistance at the onset of micturition to a lowest level less than 0.5 units, which lasted 1 to 3 seconds. Subsequently, the resistance increased sharply to a value more than 0.5 units. This resistance curve was observed in 12 of the 72 recordings (17 per cent) for patients in group 4, in 10 per cent of the recordings for patients in group 2 and in 11 per cent of the recordings for patients in group 3. The pattern 5 consisted of alternate increases and decreases in resistance throughout micturition, the lowest resistmore than 0.5 units. This resistance observed 28 of the 72 resistance curves (39 patients in group 4 and in 11 of the 36 recordings for pa1;1e11i;s in group 3. TABLE 2. Mean difference (X - Y) for 7 subjects who had prostatectomy (group 5) between preoperative (X) and postoperative (Y) mean minimum urethral resistance
Intervals of Mean Resistance Difference*
(X - YJ 5 (50)
resistance difference eqnal.s + 1.48, with S.D. of difference + 1.25 and p < 0.05.
3. Frequency of the patterns of urethral resistance during micturition in the various clinical groups of subjects
TABLE
Pattern
l 2
l No. (%J
2 No.(%)
55
42 (76)
13 (24)
40
4 5 0 34
36 4 5
72
44
TABLE 4. Relationship between the mean minimum urethral resistance and the severity of obstructive symptoms for subjects in clinical groups 3 and 4
Severity of Obstructive Symptoms
No obstructive symp-
toms Slight Moderate Severe
No.
5 (13)
(10) (14) (0)
4 (11)
0 (OJ 10 (23)
(77)
~100
4 No. 0 27 14 32
(OJ (68) (39) (44)
0 (0)
No.(%) 0 (0) 4 (10) 4 (11)
12 (17)
0 (OJ
Range of Mean Minimum Urethral Resistance
Group Mean Minimum Urethral Resistance
S.E.
0.26-4.0
1.3
+0.31
6
0.50-2.5 0.38-3. 7 0.'18-24.3
1.1
+0.21 +0.43 +2.16
5 10
2.1 7.8
5 No.(%) 0 0 9 28
(0) (0) (25) (39)
0 (OJ
Size of Prostate
No.
Range of Mean Minin1um Urethral Resistance
Not enlarged-LO cm.
Group Mean Minimum Urethral Resistance
S.E.
p < 0.05
ance
4
0.63-24.3
7.3
+3.90
10
0.26-10.2
1.8
+0.71
7
0.71-4.0
2,0
+0,39
8
0.35-15.0
4.6
+l.75
protrusion into rectum
Slightly enlarged-1 to 2 cm. protrusion into rectnm
Moderately enlarged 2 to 3 cm. protrusion into rectum
Markedly enlarged-3 to 4 cm. protrnsion into rectum
.P4 rnml t-U.Ui!
p 5
u, 0
~ 80
~
a: ~ 60 ~
ill
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O 40 ~
C
~
a.nee
8
4 (40) 1 (10)
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Groups
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TABLE 5. Relationship between the mean minimum urethral resistance and the size of the prostate gland. Some of the patients voided less than 200 ml. fluid
Freqnency (%)
+0.2-+ 1.3 +1.4-+2.5 +2.6-+3.7
In relation to each of the clinical findings the F~'"'''""u with a clinically proved bladder outlet obstruction 3 and were classified into 4 grades (I to IV). Thus, there were 4 grades of obstructive symptoms based on the presentation of this clinical finding. An analysis of variance showed that the mean minimum urethral resistance for various grades of obstructive symptoms differed u•,,,_,... ~~u from one another (p < 0.05) (table 4). The mean resistance increased with an increase in the severity of obstructive symptoms. There also were 4 g-rndes for the size of the prostate gland as determined by rectal-digital .... ~.,'"~'"""'"· An of variance showed that the mean urethral ance for the various grades of prostatic size differed cantly from one another (p < 0.05) (table 5). Patients no prostatic enlargement had a group mean minimum urethral resistance of 7.3 units, while patients with a larged prostate gland had a lower value of 4.6 units. and 7 show that the mean values of the minimum urethraI resistance for the 4 grades of residual urine and bladder
20
ro
Cl,
Fm. 3. Patterns of urethral resistance (percentage) for subjects in various clinical groups
p < 0,05
646
ANIKWE TABLE
6. Relationship among the volume of residual urine, the degree of bladder trabeculation and the minimum urethral resistance Volume of Residual Urine
- - - - - - - - - - - - - - - - - - - - - - - - - - -Grade - -of-Clinical Findings II
No. subjects Group mean mm1mum urethral resistance (units) Analysis of variance
TABLE
f
IV
III
4
7
8
1.3
0.92
2.0
Degree of Bladder Trabeculation
3
10 2.4
1.4
II
III
7 2.0
11 4.8
p > 0.05
IV 8
4.1
p > 0.05
7. Mean peak fT,ow rates (ml. per second) in the various clinical groups for voided volumes of200 ml. and more
Clinical Group
No. Subjects
Range (ml./sec.)
Group Mean (ml./sec.)
1 2 3 4 5
12 8 7 14 8
11.8-35.0 9.3-17.5 8.8-14.5 5.0-12.3 11.6-32.3
20.1 12.4 11.3 8.2 19.0
trabeculation, respectively, are not significantly different from one another (p > 0.05). Peak urinary flow rate was determined from each urinary flow recording and a mean peak flow rate was calculated for each individual. Table 7 shows no significant difference between subjects in the normal group and patients who had undergone prostatectomy (p > 0.05). Therefore, the 2 groups are pooled as the non-obstructed group. Mean peak flow rates for subjects in groups 2, 3 and 4 are significantly lower than the value for subjects in group 1 (p < 0.05). Figure 4 shows that 15 of 20 non-obstructed subjects (75 per cent) in groups 1 and 5 have mean peak flow rates of 15.0 ml. per second and higher, while the remaining 25 per cent have values of 10 to 14.9 ml. per second. Since no patient with a moderate or severe obstruction (group 4) has a mean peak flow rate of more than 15.0 ml. per second a mean peak flow rate of 15 ml. per second can be regarded as the lower limit of adequate urinary flow. 13 Of 14 subjects in group 4, 11 (79 per cent) had mean peak flow rates less than 10 ml. per second, while the remaining 21 per cent had values of 10 to 14.9 ml. per second. It is, thus, apparent that a value of 10.0 ml. per second and less is evidence of impaired drainage of urine while for individuals with mean peak flow rates oflO to 14.9 ml. per second furtherurodynamic studies are required for the correct assessment of bladder drainage. DISCUSSION
The formula R equals P/F2 , currently in use for the calculation of urethral resistance, may be over-simplified. It takes no account of urethral length and diameter, as well as the viscosity and coefficient of friction of the fluid used. However, this formula, although inaccurate in absolute values, is valuable for the determination of relative resistances. Cardus and associates reported a minimum urethral resistance of 0.48 units in their only normal subject, 14 Smith reported that the minimum urethral resistance was less than 0.4 units in 13 non-obstructed men11 and Claridge reported an upper limit of mean minimum urethral resistance of 0.4 units in 8 non-obstructed men. 15 In the study of Claridge, however, the detrusor pressure (the pressure generated solely by the contraction of the detrusor muscle) was used for P instead of the whole intravesical pressure, thereby obtaining a lower value for urethral resistance than would have been the case. It is shown in the present study that the mean minimum urethral resistance of 0.8 units for group 2 is not significantly different from 0.9 units for group 3. Consequently, patients with a doubtful bladder outlet obstruction on clinical evaluation are not distinguishable from those with a slight obstruction on the basis of mean minimum urethral resistance. The
S.D.
S.E.
7.3 3.3 2.9 2.2 6.7
2.1 1.2 1.1 0.6 2.4
Significance of Difference from Group 1 p< p< p< p>
0.05 0.05 0.05 0.05
40
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w ::iE
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GROUP 1 SUBJECTS X GROUP 2 SUBJECTS A GROUP 3 SUBJECTS A GROUP 4 SUBJECTS O GROUP 5 SUBJECTS
X
A
!,
A
...
A
:... ... ......
X
xXX !AX
••• ...
0 Fm. 4. Mean peak flow rates (ml. per second) of subjects in various clinical groups for voided volumes of 200 ml. and more.
mean minimum urethral resistance for non-obstructed subjects is 0.3 units, the value also obtained for subjects who had undergone prostatectomy. Pooling groups 1 and 5, figure 2 shows that 1 of the 22 subjects (5 per cent) has a mean minimum urethral resistance of more than 0.5 units. Furthermore, 1 of 20 moderately and severely obstructed patients (5 per cent) has a mean minimum urethral resistance of 0.5 units. It appears reasonable to suggest a mean minimum urethral resistance of 0.5 units as the upper limit of this urodynamic parameter for non-obstructed subjects. In the study of urethral resistance previous investigators dwelt solely on the calculation of the minimum urethral resistance. The direct recording of urethral resistance in the
DIRECT RECORDING OF URETHRAL RESISTANCE USING "URETHRORESISTANCE"
present study has shown that a patient could have bladder outlet obstruction even with a minimum urethral resistance less than 0.5 units. In these patients the low resistance occurs for a brief period, usually at the commencement of micturition. Therefore, for the correct evaluation of urethral resistance the minimum resistance and the curve of resistance are essential. Five patterns of urethral resistance have been observed in the present study. Patterns 1 and 2 alone are observed in the non-obstructed patients (groups 1 and 5) but they are not observed in patients with a moderate or a severe degree of bladder outlet obstruction. Although these patterns of urethral resistance also are found in some patients in groups 2 and 3 these subjects have a sustained minimum urethral resistance less than 0.5 units and are, therefore, nonobstructed. It appears reasonable to conclude that patterns 1 and 2 are characteristic of unimpaired drainage of the bladder outlet. Urethral resistance patterns 3, 4 and 5 are observed in all the patients in group 4 and in some of the patients in groups 2 and 3. Although the values of the minimum urethral resistance for individuals with pattern 4 urethral resistance are less than 0.5 units, which is the upper limit for nonobstructed patients, the duration of the minimum resistance is brief. Furthermore, no subject without bladder outlet obstruction has this pattern of urethral resistance. It seems reasonable to suggest that patterns 3, 4 and 5 are characteristic of bladder outlet obstruction. The present study has revealed that for patients with a clinically proved bladder outlet obstruction (groups 3 and 4) neither the volume of residual urine nor the degree of bladder trabeculation is related to the mean minimum urethral resistance. The severity of obstructive symptoms is shown to be related to the mean urethral resistance. However, 8 of the 19 subjects in groups 2 and 3 with no obstructive symptoms have a mean minimum urethral resistance of more than the upper limit of 0.5 units. Thus, while severe obstructive symptoms generally reflect a high minimum urethral resistance, the absence of obstructive symptoms does not exclude an increase in minimum urethral resistance. However, severe obstructive symptoms also occur in patients with motor paralytic bladder. It is through the direct recording of urethral resistance and detrusor pressure that the correct objective diagnosis can be made. We contribute to the common urologic view that the size of the prostate is no clinical indication of significant obstruction. The present investigation is not concerned with the reappraisal of this established view, which would require an evaluation of the relationship between the various sizes of the prostate gland in obstructed and non-obstructed subjects (groups 1 to 5) and the minimum urethral resistance. The relationship was determined between the various sizes of the prostate in clinically obstructed patients (groups 3 and 4) and the minimum urethral resistance in an attempt to evaluate whether the size of the prostate is a reliable clinical indication of the degree of obstruction. Although there is a general tendency for the minimum urethral resistance to be higher in obstructed patients with an enlarged prostate gland there is a considerable overlap between the values for the 4 subdivisions of prostatic size. Thus, 1 patient with no enlargement of the prostate had a mean minimum urethral resistance of 24.3 units, while 1 patient with a markedly enlarged prostate gland had a value of 0.35 units. Furthermore, all the obstructed subjects without an enlarged prostate had minimum urethral resistance more than 0.6 units, while 4 of the 8 patients (50 per cent) with a markedly enlarged prostate had minimum urethral resistance less than 0.6 units. Consequently, in evaluating the degree of obstruction of any patient with clinical obstruction the size of the prostate gland cannot be relied on as a measure of urethral resistance. The present study has revealed that the direct recording of urethral resistance in the course of micturition distinguishes
647
the non-obstructed subject from patients with bladder outlet obstruction in all cases. On the other hand, figure 4 shows that the direct recording of urinary flow distinguishes the normal from the obstructed in 34 of 48 subjects (71 per cent), the rest of the individuals having their flow rates within the zone of overlap of 10 to 14.9 ml. per second. It is evident from the present study that the direct recording of urethral resistance is a better test for the drainage of the bladder outlet than the recording of urinary flow. However, direct recording of urinary flow is a simple urodynamic test and remains a useful screening test for bladder outlet obstruction. Intravesical pressure (P) consists of the pressure generated by the contraction of the detrusor muscle (detrusor pressure) and the intra-abdominal pressure owing to the contraction of abdominal muscles and the diaphragm. Measurements of resistance in patients with poor detrusor contraction and in paraplegics have shown that resistance P/F2 is not elevated. Our studies of intravesical pressure on patients with bladder outlet obstruction showed that detrusor contraction was either within normal range or elevated. No case with poor detrusor contraction was observed, even among patients who had retention of urine with overflow incontinence. 16 Patients with poor detrusor contraction have a compensatory elevation of intra-abdominal pressure during micturition as a result of straining. When there is no detrusor contraction as in a patient with a motor paralytic bladder the pressure compensation may be inadequate, resulting in a decrease of P, which leads to poor urinary flow as seen from P equals RF 2 , R remaining unchanged. In these patients P and F are not sustained during micturition but increase and decrease with straining. Furthermore, contraction of the detrusor muscle has been shown to be chiefly responsible for the opening of the bladder neck during micturition. 11- 19 Thus, in a patient with detrusor paralysis, including men with adequate pressure compensation as a result of straining, the mechanism for the opening of the bladder neck is defective and urinary flow is impaired as a result. REFERENCES
1. Anikwe, R. M.: Correlations between clinical findings and
urinary flow rate in benign prostatic hypertrophy. Int. Surg., 61: 392, 1976. 2. Gleason, D. M. and Lattimer, J. K.: The pressure-flow study: a method of measuring bladder neck resistance. J. Urol., 87: 844, 1962. 3. Pierce, J. M., Jr., Braun, E., Sniderman, A. J. and Lewis, H.
Y.: The concept of resistance to flow applied to the lower urinary tract. Surg., Gynec. & Obst., 116: 217, 1963. 4. Arbuckle, L. D., Jr. and Paquin, A. J., Jr.: Urinary outflow tract resistance in normal human females. Invest. Urol., 1: 216, 1963.
5. Ritter, R. C., Zinner, N. R. and Paquin, A. J., Jr.: Clinical 6. 7. 8.
9. 10. 11.
urodynamics II. Analysis of pressure-flow relations in the normal female urethra. J. Urol., 91: 161, 1964. Smith, J. C.: Some theoretical aspects of urethral resistance. Invest. Urol., 1: 477, 1964. Schwarz, 0. and Brenner, A.: Undersuchungen iiber die Physiologie und Pathologie der Blasenfunktion. VIII. Die Dynamik der Blase. Z. Urol. Chir., 8: 32, 1922. Bryndorf, J. and Sandoe, E.: The hydrodynamics ofmicturition. Dan. Med. Bull., 7: 65, 1960. Claridge, M. and Shuttleworth, K. E. D.: The dynamics of obstructed micturition. Invest. Urol., 2: 188, 1964. Huffman, G. C. and Keitzer, W. A.: Urodynamics of the lower urinary tract. Invest. Urol., 3: 1, 1965. Smith, J. C.: Urethral resistance to micturition. Brit. J. Urol.,
40: 125, 1968. 12. Smith, D. B. and Anikwe, R. M.: A new instrument for comput-
ing the urethral resistance in urodynamic studies. Med. Biol. Eng., 13: 472, 1975.
648
ANIKWE
13. Anikwe, R. M.: Urinary flow rate in benign prostatic hypertrophy. Int. Surg., 61: 109, 1976. 14. Cardus, D., Quesada, E. M. and Scott, F. B.: Studies on the dynamics of the bladder. J. Urol., 90: 425, 1963. 15. Claridge, M.: Hydrodynamics of micturition. Ch.M. Thesis, Oxford University, 1964. 16. Anikwe, R. M.: Intravesical pressure in benign prostatic hypertrophy. Int. Surg., 62: 528, 1977. 17. Lapides, J.: Structure and function of the internal vesical sphincter. J. Urol., 80: 341, 1958. 18. Woodburne, R. T.: Structure and function of the urinary bladder. J. Urol., 84: 79, 1960. 19. Anikwe, R. M.: Bladder wall tension in benign prostatic hypertrophy. Invest Urol., 14: 452, 1976.
EDITORIAL COMMENT The author again has emphasized the fact that resistance varies throughout micturition and, therefore, a single minimum resistance may be misleading. His recording (urethroresistance) provides a pattern of resistance variation. Interpretation of this pattern will improve our understanding ofmicturitional disorders. However, the reader should realize 2 important points: 1) the degree of relaxation or contraction of the urethral sphincter was not part of this investigation and 2) R equals P/F2 is a different value, depending upon whether one uses detrusor, intravesical or intra-abdominal pressure forP. F. Brantley Scott St. Lukes Episcopal Hospital Houston, Texas
THE JOURNAL OF UROLOGY
Copyright © 1978 by The Williams & Wilkins Co.
DIRECT RECORDING OF URETHRAL RESISTANCE USING THE "URETHRORESISTANCE" R. M. ANIKWE From the Department of Surgery, University of Aberdeen and the Urological Unit of Aberdeen, Royal Infirmary, Aberdeen, Scotland
ABSTRACT
Observations on urethral resistance during micturition in non-obstructed men and in patients with bladder outlet obstruction owing to benign prostatic hypertrophy are presented. The "Urethroresistance", a new instrument, was used for the direct recording of urethral resistance. The upper limit for the minimum urethral resistance during micturition for normal nonobstructed men was 0.5 units. Two patterns of urethral resistance were observed solely in nonobstructed subjects and 3 other patterns, which were found solely in patients with a moderate or severe degree of bladder outlet obstruction, were shown to be characteristic of bladder outlet obstruction. While direct recording of urethral resistance is a better test for bladder outlet obstruction than the recording of urinary flow rate the recording of urinary flow is a simple urodynamic test, which remains a useful screening test for bladder outlet obstruction. With the exception of severe obstructive symptoms none of the other clinical methods for the assessment of bladder outlet obstruction-the size of the prostate gland, the volume ofpost-micturition residual urine and the degree of bladder trabeculation - is reliable to determine the degree of bladder outlet obstruction. It has become increasingly obvious that clinical methods to evaluate bladder outlet obstruction are inadequate. 1 Isolated recordings of urinary flow rate or vesical pressures have been shown to be valuable but in some patients, particularly those with slight bladder outlet obstruction, a correct assessment of the bladder outlet drainage requires an objective quantitative measurement of urethral resistance. However, there has been some confusion concerning which factors to measure in determining urethral resistance and how these factors relate to one another. Thus, Gleason and Lattimer,2 Pierce and associates, 3 and Arbuckle and Paquin, 4 assuming stream-line urinary flow, used the equation R equals P/F, where R represents urethral resistance, P represents the intravesical pressure (pressure recorded within the bladder cavity in the course of micturition) and F represents the urinary flow rate. Ritter and associates thought that urinary flow was turbulent and questioned the validity of applying the formula R equals P/F to the determination of urethral resistance. 5 Smith allowed for frictional losses caused by eddies and undercurrents in turbulent flow and thought that the pressure decrease along the urethra (P - P 1) varied directly with the square of the urinary flow rate (R). 6 So P - P 1 a F2, therefore R equals P - P 1/F2, where R is a constant, the urethral resistance, P is the intravesical pressure and P 1 is the pressure of urine at the external urethral meatus (the urethral exit pressure). Measurement of P 1 has been tedious and unreliable. 6 - 8 Some investigators have assumed P 1 to be zero, thus using the formula R equals P/F2 for the calculation of urethral resistance. 3, 9 Huffman and Keitzer demonstrated mathematically that an accurate measurement of urethral resistance would require the measurements of urine velocity, viscosity and specific gravity, as well as urethral length and urethral diameter, at different levels. 10 However, there are serious doubts as to whether such a tedious urodynamic test can be applied clinically. Smith, 11 Claridge and Shuttleworth, 9 and Gleason and Lattimer" used the formula R equals P/F2 , finding it valuable for the diagnosis of bladder outlet obstruction. However, in the absence of any instrument for the direct recording of R most clinicians found calculations of urethral resistance at short intervals along the urinary flow and
intravesical pressure curves laborious and resorted to calculating solely the minimum urethral resistance (urethral resistance during peak urinary flow). After our studies on urethral resistance we devised a new instrument, the "Urethroresistance,"12 for the direct recording of urethral resistance during micturition using the formula R equals P/F 2 • Herein are presented observations with the new instrument. MATERIALS AND METHODS
Urinary flow and pressure recordings were done synchronously on 51 men, ranging in age from 50 to 82 years. The subjects were subdivided into 5 clinical groups on the basis of 4 clinical criteria: 1) the severity of obstructive symptoms, 2) the volume of post-micturition residual urine, 3) the size of the prostate gland on rectal-digital palpation and 4) the degree of bladder trabeculation on cystourethroscopy. Group 1 consists of 12 subjects without clinical evidence of bladder outlet obstruction. The subjects with urinary tract infection, diabetes mellitus, neurological conditions, detrusor dysfunction, previous operations on the lower urinary tract or in the pelvic cavity and patients on drugs known to affect the bladder were excluded from this control group. Group 2 consists of 10 individuals with benign prostatic hypertrophy but with a doubtful bladder outlet obstruction on clinical evaluation. Group 3 comprises 9 patients with mild bladder outlet obstruction owing to benign prostatic hypertrophy. Group 4 consists of 20 patients with either a moderate or a severe degree of bladder outlet obstruction owing to benign prostatic hypertrophy. Group 5 includes 10 patients who had had prostatectomy for bladder outlet obstruction owing to benign prostatic hypertrophy 6 to 10 weeks previously. Synchronous urinary flow and pressure recordings were done on each subject. With the subject supine a fine 6F ureteral catheter with 2 side holes near the tip was inserted aseptically per urethram into the bladder, thus serving as a fine urethral catheter. The side holes ensured that the catheter did not get blocked by the bladder urothelium. The catheter was coupled via a single-ended strain gauge pressure transducer to a Sanborn 6-channel recorder and was used to record the intravesical pressure. A uroflowmeter, which con-
Accepted for publication March 11, 1977. 643
644
ANIKWE
sisted of a graduated 500 ml. Perspex cylinder 4.3 cm. in internal diameter and 29 cm. long placed on a strain gauge transducer, was connected to the second channel of the recorder. The intravesical pressure and the urinary flow rate signals were fed into the Urethroresistance, which was connected to a third channel of the recorder for the direct recording of urethral resistance. With the subject in the sitting position the bladder was filled slowly with physiological saline through the urethral catheter until the patient had an urge to void. With the patient in the erect position the zero lines of the pressure transducers were adjusted to correspond to the upper border of the symphysis pubis. Before the onset of micturition the premicturition resting intravesical pressure was recorded for a few seconds. The subject was then requested to void alongside the indwelling fine urethral catheter into the uroflowmeter. Synchronous urinary flow, pressure and urethral resistance recordings were done throughout the course ofmicturition. At the end of micturition residual urine was determined by the evacuation of the bladder contents via the urethral catheter. The bladder was then refilled. At least 4 recordings were done on each subject. RESULTS
The lowest urethral resistance during micturition (minimum urethral resistance) occurred when urinary flow was at its peak (fig. 1). The minimum urethral resistance was not
A F
constant in a non-obstructed subject nor in a clinically obstructed patient. Therefore, a mean minimum urethral resistance was determined for each individual from the various recordings of urethral resistance. Table 1 shows that the range of the mean minimum urethral resistance for group 1 is 0.1 to 0.6 units and the group mean is 0.3 units. The group mean minimum urethral resistances for groups 2, 3, 4 and 5 are 0.8, 0.9, 2.4 and 0.3 units, respectively. There is a tendency for the mean minimum urethral resistance to increase with an increase in the degree of bladder outlet obstruction. However, there is no significant difference between the mean values for groups 2 and 3 (p > 0.05). Figure 2 shows an almost distinct separation between the mean minimum urethral resistance for groups 1 and 4. Table 2 shows a significant lowering in the minimum urethral resistance from the preoperative values for those patients who had undergone prostatectomy (p < 0.05). The mean minimum urethral resistance for those relieved of their obstruction after prostatectomy (group 5) is 0.3 units, which also is the figure obtained for the non-obstructed subjects (group 1). Five patterns of urethral resistance were observed (table 3 and fig. 3). The pattern 1 recording consisted of a sharp decrease in urethral resistance at the onset of micturition to a sustained lowest level less than 0.5 units and a sharp increase in resistance at the end of voiding. This pattern was observed in 42 of the 55 recordings (76 per cent) for subjects in group 1 and in 34 of the 44 recordings (77 per cent) for patients who had undergone prostatectomy. It also occurred in 10 and 14
PATIENT N0.1
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R URETHRAL 5: RESISTANCE -
6
Clinicai Group
f
20
Time
in
seconds
40
Subjects
Group 4 Subjects
O Group 5 Subjects
Jt
X
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4
x4 4
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Mean minimum urethral resistance for subjects in various clinical groups. Fm. 2.
1. Mean minimum urethral resistance in the various clinical groups for voided urinary volume of200 ml. and more No. Pts.
1
12 10
3
9 20 10
5
£
.A
1-0
0,5
2 4
•
A
.A
Fm. 1. Direct recording of urinary flow rate and urethral resistance. A, patient without bladder outlet obstruction. B, patient with bladder outlet obstruction.
TABLE
1.5
z < w
b
Group 2 Subjects
.A
z ::E
Group 1 Subjects
)C
A Group 3
A
...J
<
in seconds
•
Range of Mean Resistance (units) 0.10-0.60 0.42-1.64 0.41-1.73 0.50-6.60 0.11-0.50
Group Mean Resistance (units) 0.3 0.8 0.9 2.4 0.3
S.D.
S.E.
0.17 0.44 0.48 1.56 0.16
0.05 0.15 0.18 0.42 0.06
Sign_ificance of Differertce From Group 1 p< p< p< p>
0.05 0.05 0.05 0.05
645
DIRECT RECORDING OF URETHRAL RESISTANCE USING "URETHRORESISTANCE"
per cent of SUQjects in groups 2 and 3, respectively. The pattern 2 curve consisted of a slow decrease in urethral resistance at the onset of micturition to a lowest level less than 0.5 which was sustained for the major part of and a increase in resistance towards the end of This pattern was obtained in 13 of 55 recordings (24 per cent) for subjects in group 1 and in 10 of 44 recordings (23 per cent) for patients who had undergone prostatectomy. It also was observed in 13 and 11 per cent of subjects in groups 2 and respectively. The pattern 3 resistance curve consisted of a decrease in urethral resistance at the commencement of micturition to a sustained lowest level more than 0.5 units for the major part of voiding. This resistance curve was observed in 32 of the 72 recordings (44 per cent) for patients vvith a moderate or a severe degree of bladder outlet obstruction. It also occmTed in 68 and 39 per cent of subjects in groups 2 and 3, respectively. The pattern 4 curve consisted of a sharp decrease in urethral resistance at the onset of micturition to a lowest level less than 0.5 units, which lasted 1 to 3 seconds. Subsequently, the resistance increased sharply to a value more than 0.5 units. This resistance curve was observed in 12 of the 72 recordings (17 per cent) for patients in group 4, in 10 per cent of the recordings for patients in group 2 and in 11 per cent of the recordings for patients in group 3. The pattern 5 consisted of alternate increases and decreases in resistance throughout micturition, the lowest resistmore than 0.5 units. This resistance observed 28 of the 72 resistance curves (39 patients in group 4 and in 11 of the 36 recordings for pa1;1e11i;s in group 3. TABLE 2. Mean difference (X - Y) for 7 subjects who had prostatectomy (group 5) between preoperative (X) and postoperative (Y) mean minimum urethral resistance
Intervals of Mean Resistance Difference*
(X - YJ 5 (50)
resistance difference eqnal.s + 1.48, with S.D. of difference + 1.25 and p < 0.05.
3. Frequency of the patterns of urethral resistance during micturition in the various clinical groups of subjects
TABLE
Pattern
l 2
l No. (%J
2 No.(%)
55
42 (76)
13 (24)
40
4 5 0 34
36 4 5
72
44
TABLE 4. Relationship between the mean minimum urethral resistance and the severity of obstructive symptoms for subjects in clinical groups 3 and 4
Severity of Obstructive Symptoms
No obstructive symp-
toms Slight Moderate Severe
No.
5 (13)
(10) (14) (0)
4 (11)
0 (OJ 10 (23)
(77)
~100
4 No. 0 27 14 32
(OJ (68) (39) (44)
0 (0)
No.(%) 0 (0) 4 (10) 4 (11)
12 (17)
0 (OJ
Range of Mean Minimum Urethral Resistance
Group Mean Minimum Urethral Resistance
S.E.
0.26-4.0
1.3
+0.31
6
0.50-2.5 0.38-3. 7 0.'18-24.3
1.1
+0.21 +0.43 +2.16
5 10
2.1 7.8
5 No.(%) 0 0 9 28
(0) (0) (25) (39)
0 (OJ
Size of Prostate
No.
Range of Mean Minin1um Urethral Resistance
Not enlarged-LO cm.
Group Mean Minimum Urethral Resistance
S.E.
p < 0.05
ance
4
0.63-24.3
7.3
+3.90
10
0.26-10.2
1.8
+0.71
7
0.71-4.0
2,0
+0,39
8
0.35-15.0
4.6
+l.75
protrusion into rectum
Slightly enlarged-1 to 2 cm. protrusion into rectnm
Moderately enlarged 2 to 3 cm. protrusion into rectum
Markedly enlarged-3 to 4 cm. protrnsion into rectum
.P4 rnml t-U.Ui!
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ill
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O 40 ~
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Groups
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TABLE 5. Relationship between the mean minimum urethral resistance and the size of the prostate gland. Some of the patients voided less than 200 ml. fluid
Freqnency (%)
+0.2-+ 1.3 +1.4-+2.5 +2.6-+3.7
In relation to each of the clinical findings the F~'"'''""u with a clinically proved bladder outlet obstruction 3 and were classified into 4 grades (I to IV). Thus, there were 4 grades of obstructive symptoms based on the presentation of this clinical finding. An analysis of variance showed that the mean minimum urethral resistance for various grades of obstructive symptoms differed u•,,,_,... ~~u from one another (p < 0.05) (table 4). The mean resistance increased with an increase in the severity of obstructive symptoms. There also were 4 g-rndes for the size of the prostate gland as determined by rectal-digital .... ~.,'"~'"""'"· An of variance showed that the mean urethral ance for the various grades of prostatic size differed cantly from one another (p < 0.05) (table 5). Patients no prostatic enlargement had a group mean minimum urethral resistance of 7.3 units, while patients with a larged prostate gland had a lower value of 4.6 units. and 7 show that the mean values of the minimum urethraI resistance for the 4 grades of residual urine and bladder
20
ro
Cl,
Fm. 3. Patterns of urethral resistance (percentage) for subjects in various clinical groups
p < 0,05
646
ANIKWE TABLE
6. Relationship among the volume of residual urine, the degree of bladder trabeculation and the minimum urethral resistance Volume of Residual Urine
- - - - - - - - - - - - - - - - - - - - - - - - - - -Grade - -of-Clinical Findings II
No. subjects Group mean mm1mum urethral resistance (units) Analysis of variance
TABLE
f
IV
III
4
7
8
1.3
0.92
2.0
Degree of Bladder Trabeculation
3
10 2.4
1.4
II
III
7 2.0
11 4.8
p > 0.05
IV 8
4.1
p > 0.05
7. Mean peak fT,ow rates (ml. per second) in the various clinical groups for voided volumes of200 ml. and more
Clinical Group
No. Subjects
Range (ml./sec.)
Group Mean (ml./sec.)
1 2 3 4 5
12 8 7 14 8
11.8-35.0 9.3-17.5 8.8-14.5 5.0-12.3 11.6-32.3
20.1 12.4 11.3 8.2 19.0
trabeculation, respectively, are not significantly different from one another (p > 0.05). Peak urinary flow rate was determined from each urinary flow recording and a mean peak flow rate was calculated for each individual. Table 7 shows no significant difference between subjects in the normal group and patients who had undergone prostatectomy (p > 0.05). Therefore, the 2 groups are pooled as the non-obstructed group. Mean peak flow rates for subjects in groups 2, 3 and 4 are significantly lower than the value for subjects in group 1 (p < 0.05). Figure 4 shows that 15 of 20 non-obstructed subjects (75 per cent) in groups 1 and 5 have mean peak flow rates of 15.0 ml. per second and higher, while the remaining 25 per cent have values of 10 to 14.9 ml. per second. Since no patient with a moderate or severe obstruction (group 4) has a mean peak flow rate of more than 15.0 ml. per second a mean peak flow rate of 15 ml. per second can be regarded as the lower limit of adequate urinary flow. 13 Of 14 subjects in group 4, 11 (79 per cent) had mean peak flow rates less than 10 ml. per second, while the remaining 21 per cent had values of 10 to 14.9 ml. per second. It is, thus, apparent that a value of 10.0 ml. per second and less is evidence of impaired drainage of urine while for individuals with mean peak flow rates oflO to 14.9 ml. per second furtherurodynamic studies are required for the correct assessment of bladder drainage. DISCUSSION
The formula R equals P/F2 , currently in use for the calculation of urethral resistance, may be over-simplified. It takes no account of urethral length and diameter, as well as the viscosity and coefficient of friction of the fluid used. However, this formula, although inaccurate in absolute values, is valuable for the determination of relative resistances. Cardus and associates reported a minimum urethral resistance of 0.48 units in their only normal subject, 14 Smith reported that the minimum urethral resistance was less than 0.4 units in 13 non-obstructed men11 and Claridge reported an upper limit of mean minimum urethral resistance of 0.4 units in 8 non-obstructed men. 15 In the study of Claridge, however, the detrusor pressure (the pressure generated solely by the contraction of the detrusor muscle) was used for P instead of the whole intravesical pressure, thereby obtaining a lower value for urethral resistance than would have been the case. It is shown in the present study that the mean minimum urethral resistance of 0.8 units for group 2 is not significantly different from 0.9 units for group 3. Consequently, patients with a doubtful bladder outlet obstruction on clinical evaluation are not distinguishable from those with a slight obstruction on the basis of mean minimum urethral resistance. The
S.D.
S.E.
7.3 3.3 2.9 2.2 6.7
2.1 1.2 1.1 0.6 2.4
Significance of Difference from Group 1 p< p< p< p>
0.05 0.05 0.05 0.05
40
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GROUP 1 SUBJECTS X GROUP 2 SUBJECTS A GROUP 3 SUBJECTS A GROUP 4 SUBJECTS O GROUP 5 SUBJECTS
X
A
!,
A
...
A
:... ... ......
X
xXX !AX
••• ...
0 Fm. 4. Mean peak flow rates (ml. per second) of subjects in various clinical groups for voided volumes of 200 ml. and more.
mean minimum urethral resistance for non-obstructed subjects is 0.3 units, the value also obtained for subjects who had undergone prostatectomy. Pooling groups 1 and 5, figure 2 shows that 1 of the 22 subjects (5 per cent) has a mean minimum urethral resistance of more than 0.5 units. Furthermore, 1 of 20 moderately and severely obstructed patients (5 per cent) has a mean minimum urethral resistance of 0.5 units. It appears reasonable to suggest a mean minimum urethral resistance of 0.5 units as the upper limit of this urodynamic parameter for non-obstructed subjects. In the study of urethral resistance previous investigators dwelt solely on the calculation of the minimum urethral resistance. The direct recording of urethral resistance in the
DIRECT RECORDING OF URETHRAL RESISTANCE USING "URETHRORESISTANCE"
present study has shown that a patient could have bladder outlet obstruction even with a minimum urethral resistance less than 0.5 units. In these patients the low resistance occurs for a brief period, usually at the commencement of micturition. Therefore, for the correct evaluation of urethral resistance the minimum resistance and the curve of resistance are essential. Five patterns of urethral resistance have been observed in the present study. Patterns 1 and 2 alone are observed in the non-obstructed patients (groups 1 and 5) but they are not observed in patients with a moderate or a severe degree of bladder outlet obstruction. Although these patterns of urethral resistance also are found in some patients in groups 2 and 3 these subjects have a sustained minimum urethral resistance less than 0.5 units and are, therefore, nonobstructed. It appears reasonable to conclude that patterns 1 and 2 are characteristic of unimpaired drainage of the bladder outlet. Urethral resistance patterns 3, 4 and 5 are observed in all the patients in group 4 and in some of the patients in groups 2 and 3. Although the values of the minimum urethral resistance for individuals with pattern 4 urethral resistance are less than 0.5 units, which is the upper limit for nonobstructed patients, the duration of the minimum resistance is brief. Furthermore, no subject without bladder outlet obstruction has this pattern of urethral resistance. It seems reasonable to suggest that patterns 3, 4 and 5 are characteristic of bladder outlet obstruction. The present study has revealed that for patients with a clinically proved bladder outlet obstruction (groups 3 and 4) neither the volume of residual urine nor the degree of bladder trabeculation is related to the mean minimum urethral resistance. The severity of obstructive symptoms is shown to be related to the mean urethral resistance. However, 8 of the 19 subjects in groups 2 and 3 with no obstructive symptoms have a mean minimum urethral resistance of more than the upper limit of 0.5 units. Thus, while severe obstructive symptoms generally reflect a high minimum urethral resistance, the absence of obstructive symptoms does not exclude an increase in minimum urethral resistance. However, severe obstructive symptoms also occur in patients with motor paralytic bladder. It is through the direct recording of urethral resistance and detrusor pressure that the correct objective diagnosis can be made. We contribute to the common urologic view that the size of the prostate is no clinical indication of significant obstruction. The present investigation is not concerned with the reappraisal of this established view, which would require an evaluation of the relationship between the various sizes of the prostate gland in obstructed and non-obstructed subjects (groups 1 to 5) and the minimum urethral resistance. The relationship was determined between the various sizes of the prostate in clinically obstructed patients (groups 3 and 4) and the minimum urethral resistance in an attempt to evaluate whether the size of the prostate is a reliable clinical indication of the degree of obstruction. Although there is a general tendency for the minimum urethral resistance to be higher in obstructed patients with an enlarged prostate gland there is a considerable overlap between the values for the 4 subdivisions of prostatic size. Thus, 1 patient with no enlargement of the prostate had a mean minimum urethral resistance of 24.3 units, while 1 patient with a markedly enlarged prostate gland had a value of 0.35 units. Furthermore, all the obstructed subjects without an enlarged prostate had minimum urethral resistance more than 0.6 units, while 4 of the 8 patients (50 per cent) with a markedly enlarged prostate had minimum urethral resistance less than 0.6 units. Consequently, in evaluating the degree of obstruction of any patient with clinical obstruction the size of the prostate gland cannot be relied on as a measure of urethral resistance. The present study has revealed that the direct recording of urethral resistance in the course of micturition distinguishes
647
the non-obstructed subject from patients with bladder outlet obstruction in all cases. On the other hand, figure 4 shows that the direct recording of urinary flow distinguishes the normal from the obstructed in 34 of 48 subjects (71 per cent), the rest of the individuals having their flow rates within the zone of overlap of 10 to 14.9 ml. per second. It is evident from the present study that the direct recording of urethral resistance is a better test for the drainage of the bladder outlet than the recording of urinary flow. However, direct recording of urinary flow is a simple urodynamic test and remains a useful screening test for bladder outlet obstruction. Intravesical pressure (P) consists of the pressure generated by the contraction of the detrusor muscle (detrusor pressure) and the intra-abdominal pressure owing to the contraction of abdominal muscles and the diaphragm. Measurements of resistance in patients with poor detrusor contraction and in paraplegics have shown that resistance P/F2 is not elevated. Our studies of intravesical pressure on patients with bladder outlet obstruction showed that detrusor contraction was either within normal range or elevated. No case with poor detrusor contraction was observed, even among patients who had retention of urine with overflow incontinence. 16 Patients with poor detrusor contraction have a compensatory elevation of intra-abdominal pressure during micturition as a result of straining. When there is no detrusor contraction as in a patient with a motor paralytic bladder the pressure compensation may be inadequate, resulting in a decrease of P, which leads to poor urinary flow as seen from P equals RF 2 , R remaining unchanged. In these patients P and F are not sustained during micturition but increase and decrease with straining. Furthermore, contraction of the detrusor muscle has been shown to be chiefly responsible for the opening of the bladder neck during micturition. 11- 19 Thus, in a patient with detrusor paralysis, including men with adequate pressure compensation as a result of straining, the mechanism for the opening of the bladder neck is defective and urinary flow is impaired as a result. REFERENCES
1. Anikwe, R. M.: Correlations between clinical findings and
urinary flow rate in benign prostatic hypertrophy. Int. Surg., 61: 392, 1976. 2. Gleason, D. M. and Lattimer, J. K.: The pressure-flow study: a method of measuring bladder neck resistance. J. Urol., 87: 844, 1962. 3. Pierce, J. M., Jr., Braun, E., Sniderman, A. J. and Lewis, H.
Y.: The concept of resistance to flow applied to the lower urinary tract. Surg., Gynec. & Obst., 116: 217, 1963. 4. Arbuckle, L. D., Jr. and Paquin, A. J., Jr.: Urinary outflow tract resistance in normal human females. Invest. Urol., 1: 216, 1963.
5. Ritter, R. C., Zinner, N. R. and Paquin, A. J., Jr.: Clinical 6. 7. 8.
9. 10. 11.
urodynamics II. Analysis of pressure-flow relations in the normal female urethra. J. Urol., 91: 161, 1964. Smith, J. C.: Some theoretical aspects of urethral resistance. Invest. Urol., 1: 477, 1964. Schwarz, 0. and Brenner, A.: Undersuchungen iiber die Physiologie und Pathologie der Blasenfunktion. VIII. Die Dynamik der Blase. Z. Urol. Chir., 8: 32, 1922. Bryndorf, J. and Sandoe, E.: The hydrodynamics ofmicturition. Dan. Med. Bull., 7: 65, 1960. Claridge, M. and Shuttleworth, K. E. D.: The dynamics of obstructed micturition. Invest. Urol., 2: 188, 1964. Huffman, G. C. and Keitzer, W. A.: Urodynamics of the lower urinary tract. Invest. Urol., 3: 1, 1965. Smith, J. C.: Urethral resistance to micturition. Brit. J. Urol.,
40: 125, 1968. 12. Smith, D. B. and Anikwe, R. M.: A new instrument for comput-
ing the urethral resistance in urodynamic studies. Med. Biol. Eng., 13: 472, 1975.
648
ANIKWE
13. Anikwe, R. M.: Urinary flow rate in benign prostatic hypertrophy. Int. Surg., 61: 109, 1976. 14. Cardus, D., Quesada, E. M. and Scott, F. B.: Studies on the dynamics of the bladder. J. Urol., 90: 425, 1963. 15. Claridge, M.: Hydrodynamics of micturition. Ch.M. Thesis, Oxford University, 1964. 16. Anikwe, R. M.: Intravesical pressure in benign prostatic hypertrophy. Int. Surg., 62: 528, 1977. 17. Lapides, J.: Structure and function of the internal vesical sphincter. J. Urol., 80: 341, 1958. 18. Woodburne, R. T.: Structure and function of the urinary bladder. J. Urol., 84: 79, 1960. 19. Anikwe, R. M.: Bladder wall tension in benign prostatic hypertrophy. Invest Urol., 14: 452, 1976.
EDITORIAL COMMENT The author again has emphasized the fact that resistance varies throughout micturition and, therefore, a single minimum resistance may be misleading. His recording (urethroresistance) provides a pattern of resistance variation. Interpretation of this pattern will improve our understanding ofmicturitional disorders. However, the reader should realize 2 important points: 1) the degree of relaxation or contraction of the urethral sphincter was not part of this investigation and 2) R equals P/F2 is a different value, depending upon whether one uses detrusor, intravesical or intra-abdominal pressure forP. F. Brantley Scott St. Lukes Episcopal Hospital Houston, Texas