Voiding Pressures in Boys: Pdetmax versus PdetQmax – Does it Make a Difference?

Abstract

Introduction:

Invasive urodynamics (UDS) is a standard investigation in children. Studies measuring voiding pressures in children use varied nomenclatures and quote a wide range of voiding pressures. Thus, voiding pressures in children are not considered reliable and they do not find any place in the pediatric diagnostic armamentarium. On the contrary, adult studies have well-defined nomograms and standard values which make voiding studies indispensable in the diagnosis of voiding dysfunctions in adults. The difference primarily lies in the uniformity of parameters assessed in adults and the contrasting heterogeneity in the pediatric literature.

Objective:

The objective of this study was to study the voiding parameters observed during UDS in boys.

Study Design:

We retrospectively reviewed the pressure flow data obtained during conventional invasive UDS in 106 neurologically normal boys (6 months–16 years) who had different indications for urodynamics. The values of Pdetmax and PdetQmax were analyzed and compared with the existing data of pressure flow studies in children.

Results:

Pdetmax decreased with age whereas PdetQmax was independent of age. The difference between the values of Pdetmax and PdetQmax was more in the younger kids. The wide range of voiding detrusor pressure (Pdet) in the existing pediatric literature is similar to the values of Pdetmax observed in our study, whereas the value of PdetQmax is much lower.

Discussion:

The values of Pdetmax observed in this study are similar to the values of “maximum Pdet during voiding” documented in previous studies and are determined by detrusor contractility and functional/dynamic contraction of outflow during voiding. PdetQmax has been documented in very few pediatric studies and is significantly less than Pdetmax. Further prospective studies are needed to corroborate UDS findings with radiologic/cystoscopic findings to create nomograms of voiding parameters in children.

Conclusion:

Existing literature on pediatric voiding studies mentions voiding pressures during variable phases of void (usually Pdetmax) and the values have been very heterogeneous, making voiding pressure-flow studies unreliable in children. PdetQmax values are much lower than values quoted as “standard” pressures and are age independent. The use of PdetQmax instead of PdetMax may make voiding pressures in children more reproducible and informative.

INTRODUCTION

Invasive urodynamics (UDS) in children primarily involves the assessment of filling phase parameters. The nomenclature of the voiding parameters to be assessed in children remains the same as in adult studies, but the standard values of normalcy are not well-defined. International Children Continence Society (ICCS) and International Continence Society (ICS) documents have enumerated the voiding parameters to be assessed of which the most important are flow rates and voiding pressures – Pdetmax and PdetQmax.[1,2]

Many studies have assessed voiding parameters in children– voiding pressures have been variedly described as maximum voiding pressure,[3,4,5,6,7] peak voiding pressure,[8] voiding pressure when the flow is actually occurring,[9] maximum detrusor pressure (Pdet) with and without pelvic floor activity,[10] and “true” maximum voiding Pdet.[11] There is considerable heterogeneity between the parameters studied and their observed values. The range of “maximum voiding pressure” extends from 70 to 160 cm H₂O and “maximum Pdet during flow” has been quoted to vary between 55 and 120 cm H₂O in various studies. The nomenclature used in ICCS and ICS guidelines are also descriptive-“usual voiding pressures” in neurologically normal boys in standard fill cystometry is quoted as 127 cm H₂O at a median age of 1 month and 118 cm H₂O in infants in the ICCS document,[1] whereas ICS guideline[2] considers “voiding Pdet” of >74 cm H₂O to be high.

Because of this heterogeneity in existing data, voiding pressures in children are considered unrepresentative and unreliable and do not get any relevance as having any diagnostic utility. The situation is contrasting in adults where voiding pressures/pressure flow studies are essential tools in the armamentarium of assessing voiding dysfunction. We believed that the observed variation in voiding pressures in children compared to adults is due to the nonuniformity in the parameter being assessed rather than due to inherent differences in voiding physiology.

We studied the voiding pressures in 106 neurologically normal boys who underwent invasive UDS for various indications. In all the children, we performed the study in an upright position (sitting/standing) and assessed the flow rate and voiding pressures simultaneously. We assessed flow rates and the values of Pdetmax and PdetQmax. We compared the values of voiding pressures in our group with that recorded in the pediatric literature.

MATERIALS AND METHODS

All neurologically normal boys (6 months–16 years) referred for urodynamics for varying indications from January 2017 to December 2021, who had complete UDS voiding phase data available, were included for assessment. Overall UDS data were available for 106 boys which were retrospectively analyzed. These children had undergone operative interventions/radiography at other centers under different surgeons and had been referred to us for urodynamic evaluation.

Urodynamics was performed on SmartDyn Urodynamics System, Albyn Medical. Catheterization was done using 6 Fr double lumen urodynamics catheter and filling was performed with normal saline at room temperature infused at a rate of 10% of expected bladder capacity or functional bladder capacity as per the voiding diary. Eight Fr balloon rectal catheter was used for abdominal pressure measurements. The tests were performed in a sitting position or standing position so that voiding parameters could be assessed. Two filling and voiding cycles were performed wherever feasible and the most representative data were taken into consideration. No sedation was used during the procedure.

The nomenclatures of Pdetmax and PdetQmax were used as per the ICS guidelines.[12]

1. Pdetmax – Maximum registered Pdet during voiding

2. PdetQmax – Pdet recorded at maximum urine flow rate.

Statistics

All statistical calculations were performed on MedCalc statistical software, MedCalc Software Ltd., Belgium. Data were expressed as mean ± standard deviation (range) or n (%) and compared among the groups by Student's t-test. Pearson's correlation coefficient with P value was calculated for assessing the correlation between different parameters. Statistical significance was accepted at a P < 0.05.

RESULTS

Complete UDS data were available for 106 boys with an age range of 6 months to 16 years (6.5 ± 4).

Seventeen children were <2 years and 33 more than 10 years age. Seventy patients were follow-up cases of posterior urethral valve (PUV) and 28 had presented with lower urinary tract symptoms/voiding dysfunction [Table 1].

Table 1.

Distribution of patients according to age groups and diagnoses

Age groups (years)	Diagnosis						Total
	ARM	HDN	LUTS	Preoperative PUV	PUV	UTI
≤2	0	1	1	0	14	1	17 (16.0)
>10	0	0	3	1	17	0	21 (19.8)
2–6	0	2	10	0	22	1	35 (33.0)
6–10	1	1	14	0	17	0	33 (31.1)
Total	1 (0.9)	4 (3.8)	28 (26.4)	1 (0.9)	70 (66.0)	2 (1.9)	106

LUTS: Lower urinary tract symptoms, PUV: Posterior urethral valve, UTI: Urinary tract infection, ARM: Anorectal malformation, HDN: Hydronephrosis

Pdetmax was observed to be 62.1 ± 33.2 cm H₂O. PdetQmax ranged from 44.4 ± 19.7 cm H₂O. The values of PdetQmax were significantly lower than that of Pdetmax (P < 0.0001) with the mean difference being 18.6 cm H₂O [Figure 1].

Figure 1.

Comparison between values of Pdetmax and PdetQmax. PdetQmax was significantly lower than Pdetmax

Overall, Pdetmax had a negative correlation with age (P < 0.01; r = −0.288), i.e., the maximum voiding pressures were reduced with age. Average Pdetmax in children <3 years of age was 81 ± 32 cm H₂O, whereas Pdetmax in children ≥3 years was 55 ± 30 cm H₂O (P < 0.0005) [Figure 2]. This correlation with age disappeared when children <3 years were excluded from analysis, i.e., beyond 3 years of age, Pdetmax did not vary with age.

Figure 2.

Comparison between the values of Pdetmax in children younger than and older than 3 years

PdetQmax did not correlate with age (P = 0.16).

The difference between PdetQmax and Pdetmax reduced with age; the mean difference being 24 cm H₂O in children <6 years whereas it was 12.9 cm H₂O in children >6 years (P = 0.02) [Figure 3].

Figure 3.

Difference between corresponding values of Pdetmax and PdetQmax in children younger than and older than 6 years

We compared the children with PUV with those with other diagnoses in terms of age, Pdetmax, and PdetQmax. The mean age in the groups was 6.5 ± 4.4 and 6.4 ± 3.1 years, respectively (P > 0.05). PdetMax and PdetQmax were more in the children with PUV, but the difference was not statistically significant.

PdetQmax was found to be statistically higher in children with detrusor overactivity (43 ± 18 cm H₂O vs. 37 ± 12 cm H₂O; P = 0.04) and those with increased bladder wall thickness (49.6 ± 20.4 cm H₂O vs. 41.3 ± 16.4 cm H₂O; P = 0.043) compared to those without overactivity and those with normal bladder wall thickness, respectively.

DISCUSSION

Voiding parameters which are commonly assessed are – Pdetmax – Maximum Pdet recorded during voiding, PdetQmax – Pdet recorded at the point of maximum flow, PdetOpening – Pdet when flow begins, Qmax and Qavg. Adult studies assessing pressure-flow dynamics involve the assessment of Qmax and PdetQmax and the determination of several indices to predict outflow tract obstruction or abnormal contractility of bladder.[13]

Standardization of urodynamic voiding parameters in children lags behind that in adults. Pediatric literature is more descriptive and provides vague reference ranges. Different studies have assessed voiding parameters variedly such as “maximum voiding pressure” or “voiding pressure when flow is actually occurring.” Table 2 enlists the voiding parameters assessed in a few landmark studies.

Table 2.

Different voiding parameters assessed in various studies and their values

Author	Study population		Voiding parameters assessed and their observed values (cm H2O)
Hjalmas et al[4 5]	1 – 3 years		Maximum voiding pressure	70cm H2O
Yeung et al[6]	Infants; mean age 6 months; after upper tract surgery		Maximum voiding pressure	117
Bachelard et al[3]	Asymptomatic infants - siblings of children with vesicoureteral reflux; mean age 1 month; VUR present in 25%		Maximum voiding pressure	127 (range – 84 – 211)
Bachelard et al[8]	Infants, diagnosis of UTI 11 days to 46 days ago		Peak voiding pressure	122 +/- 45
Taskinen et al[7]	Infants with PUV – preoperative		Maximum voiding pressure	112 (40 – 331)
	Infants with history of febrile UTI – control group		Maximum voiding pressure	91 (48 – 191)
	1 year after fulguration		Maximum voiding pressure	100 (60 – 193)
Wen et al[9]	Boys (3 days– 12 years) – no neurological or lower urinary tract pathology		Maximum voiding pressure	73.9 (16.6)	Detrusor pressure during voiding	66.1 (13.1)
Wen et al[10]	Infants (mean age 6 months) – history of renal dilatation or episodes of UTI		Maximum voiding detrusor pressure with pelvic floor activity	110+/-44	Maximum voiding detrusor pressure without pelvic floor activity	71+/-19
Ichino[11]	Children <3 years with congenital renal abnormalities or history of UTI	4Fr Catheter	PdetMax	102.0+/-22.5	True maximum voiding detrusor pressure (T-Pdetmax)	86.9+/-30.3
		7.7Fr Catheter		94.7+/-25.6		89.0+/-31.7
Guha Vaze[14]	11 months – 15 years age, follow up patients of PUV		Pdet Max	70+/- 38	Pdet Qmax	48+/-20
Present study	6 months to 16 years; patients with LUTS, VUR or follow up patients of PUV		PdetMax	62.1+/-32	Pdet Qmax	44+/-19.7
			<3 years	81+/-32
			>=3 years	55+/-30

It is important to understand the basic concept underlying pressure – Flow studies. Voiding force is generated by the detrusor, which is reflected as Pdet and flow rate. If the outflow resistance is low, Pdet does not rise and flow rates are good. On the other hand, if outflow resistance is high, most of the detrusor contractile force is manifested as a rise in Pdet and the flow rate is low. The outflow resistance may be of two types:

• Anatomical/Fixed – Resistance due to stricture/PUV etc; provides a fixed resistance to flow

• Dynamic – Resistance due to intermittent contraction of sphincter/bladder neck; varies during different phases of voiding– the Pdet and flow rate will vary accordingly.

The instance when dynamic outflow resistance is least, i.e., the sphincters are optimally relaxed, the flow rate will be maximum; the corresponding Pdet will represent true “anatomic” resistance offered by outflow – PdetQmax. At other instances during the flow, when the sphincter is contracting, there will be a transient rise in Pdet – The degree of which will depend on the severity of sphincteric contraction. Pdetmax – Maximum Pdet observed during voiding – Will be observed when the dynamic obstruction is highest during a flow. Thus, the parameters Pdetmax, “maximum voiding pressure” and “maximum Pdet when the flow is actually occurring” will be strongly influenced by the dynamic state of outflow, may be different from PdetQmax, and will be more indicative of detrusor contractility. It can be surmised from the above that whereas PdetQmax may give an appropriate representation of the anatomical state of outflow, the currently used parameters are giving us an idea about detrusor contractility and not about the outflow resistance. In children, especially young ones, physiological dynamic outflow obstruction is common – Caused by sphincteric contraction, which is exaggerated by irritation by the catheter. This can explain the wide range of values of normalcy observed in different studies.

The curves in Figure 4 effectively elucidate that Pdetmax, “Pdetmax during flow,” and PdetQmax are three different entities and may be markedly different.

Figure 4.

Four representative curves showing variation in Pdetmax, “maximum Pdet when flow is actually occurring” and PdetQmax

Basic problem of pressure flow evaluation in children stems from the ergonomic difficulty in assessing flow simultaneously with the voiding pressure, in the absence of which determination of PdetQmax becomes impossible. To overcome this shortcoming descriptive terms such as “maximum voiding pressure, when the flow is actually occurring” of “voiding pressure with or without sphincteric activity” have been used.

Inspired from the adult data, we performed pressure flow studies in children (6 months–16 years) in the upright position to assess Pdetmax and PdetQmax and tried to assess whether the values mentioned in pediatric literature are representative.

The overall Pdetmax values observed in our study (62.1 ± −33.2) matches with the values of “maximum Pdet during flow” observed in previous studies. We found the values of PdetQmax (Pdet recorded at maximum urine flow rate) to be significantly lower than the values of Pdetmax. The overall values of Pdetmax correlated with values of PdetQmax, but Pdetmax was consistently more than the corresponding value of PdetQmax (difference between Pdetmax and PdetQmax of 17 ± 20.8 cm H₂O; P < 0.0001).

The high Pdetmax observed in younger children was probably due to higher incidence of dysfunctional voiding in younger subjects which was heightened by irritation due to the presence of catheter in the urethra and more synchronized voiding in older patients. This also explains the reduction of the difference between the values of Pdetmax and Pdetmax with increasing age.

There was no statistical difference in the values of PdetQmax and Pdetmax in children with PUV and those with other diagnoses. This can be explained by the fact that all the children with PUV had undergone some or the other procedure to alleviate outflow obstruction reducing the outflow resistance to some extent; whereas many children with other diagnoses also had features of functional outflow obstruction in the form of detrusor hypertrophy or upper tract changes on Ultrasonography (USG) suggesting raised outflow resistance.

The values of Pdetmax and PdetQmax observed in the present study are similar to that observed in our previous study (70 ± 38 cm H₂O and 48 ± 20 cm H₂O, respectively) in follow-up patients of PUV.

The reasons for the heterogeneity in pediatric literature may be multitude:

1. Very few studies have documented PdetQmax and used more descriptive and subjective terms instead

2. Ergonomic and practical difficulty in assessing flow parameters simultaneously with voiding pressures in small children – Thus, only Pdetmax and “Pdetmax when flow is actually occurring” are evaluated

3. High rate of sphincteric contraction due to irritation by the catheter in young children raising Pdets – Falsely elevating “Pdetmax when flow is occurring“

4. Effect of catheter size on flow parameters – Different catheter sizes may have been used in different studies.

To our knowledge, this is the first attempt to quantify PdetQmax as voiding pressures in children. Taking cues from adult studies, assessment of PdetQmax instead of using vague terms can result in paradigm shift in the way we perform pediatric voiding studies and make it far more meaningful. As the cystoscopic/micturating cystourethrogram (MCU) details of the patients were unavailable, we did not aim to stratify the voiding pressures as obstructed/unobstructed. Comparison of values of PdetQmax in patients with known outflow obstruction and after its relief can provide standard values for predicting the presence of obstruction in undiagnosed cases. Corroboration of voiding pressures and corresponding flow rates with respective MCU or cystoscopic findings for the presence or absence of obstruction can allow the development of nomograms of voiding pressures and flow rates in children.

Limitations: Our study group is a heterogeneous population with a wide age range of 6 months to 16 years and varied indications for invasive UDS. It is difficult and unethical to subject children with normal lower urinary tracts to UDS. In spite of the heterogeneity in our study population, we could find a common trend of pressure parameters– the value of PdetQmax is much lower than the value of PdetMax.

CONCLUSION

The heterogeneity in voiding pressures in children in the existing literature stems from the nonuniformity of the parameters assessed (most of the studies have assessed PdetMax) and possible difficulty in assessing voiding pressures and flow simultaneously. Measurement of PdetQmax, although challenging, is feasible in children. PdetQmax is age-independent and not as variable as the existing parameters. The average values of PdetQmax in this heterogeneous group of children with varied diagnoses, where voiding pressures would otherwise be expected to be higher than in normal children, are still much lower than the “standard voiding pressures.” Assessment of PdetQmax rather than Pdetmax can reduce the heterogeneity and make pediatric voiding pressures more reliable.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.