Quality control in urodynamics: a review of urodynamic traces from one centre

The design and evaluation of a quick checklist for urodynamic quality control: A prospective single-center small sample study

PURPOSE

To design a quick checklist for urodynamic study (UDS), aiming to reduce the occurrence of errors in the process, which may help to increase the quality of UDS. And further to analyze the effectiveness of this quick checklist for UDS quality control.

METHODS

First, a quick checklist for uroflow study and pressure-flow study was developed, based on the International Continence Society-Good Urodynamic Practice standards, our previous studies, and recent literature, as well as expert suggestions. Then, patients who underwent UDS between January 2023 to February 2023 were randomly assigned to a study group or a control group. For the study group, the quick checklist was used throughout the UDS process, while the control group did not. The main artefacts were chosen to verify the effectiveness of the quick checklist for improving the UDS quality.

RESULTS

The quick checklist comprised three subtypes: checklist for patients, checklist for environment and device, and checklist for UDS test process. 38 UDS traces per group were included. The incidence of missing the standard cough test decreased significantly from 18.4% to 0 (p = 0.012), with the checklist implementation. The baseline drift frequency rate also declined significantly from 39.5% to 5.3% (p < 0.05). Volume < 150 mL on uroflow study occurred in 68.4% of cases and its frequency rate decreased significantly with checklist implementation (p < 0.05).

CONCLUSION

A quick checklist for quality control of UDS was developed. The quick checklist as a convenient, quick, and easy used urodynamic quality control method, may help to reduce the technical artefacts and improve fundamental urodynamic quality control. Future research with a larger sample size is needed to confirm the effectiveness of the checklist.

Statistical process control for the analysis of quality control in urodynamics: A potential new approach for quality review of urodynamics

PURPOSE

To analyze quality control in urodynamic studies, using a proportion control chart (p-chart) for statistical process control.

MATERIALS AND METHODS

This single-center study was conducted at the Urodynamic Center of West China Hospital, Sichuan University. We randomly selected 15 samples from each month in 2020, and 180 urodynamic traces were finally enrolled. We used the p-chart of statistical process control for analysis. We calculated the proportion of the incidence of a selected set of artefacts in the monthly urodynamic study process, including non-standard zero setting, no cough test, incomplete records of all measurements by urodynamicists, catheter displacement, and baseline drift. Through the specific calculation formula of statistical process control, we obtained the values of the center line, lower control limit, and upper control limit.

RESULTS

All data points of each artefact were within zone A. However, one outlier was found in the p-chart of all artefacts in October, which might have been caused by inexperienced operators.

CONCLUSIONS

Statistical process control may play an important role in the process control of urodynamic studies and guide us in identifying the cause of poor quality in process management.

Assessment of quality in urodynamics: Cough versus valsalva

INTRODUCTION

To ensure quality, the International Continence Society recommends asking the patient to cough every minute or 50 ml of infused volume. However, on occasions, it was noticed that if cough transmission was poor, Valsalva maneuvers could show pressure transmission was satisfactory. Hence, a study was designed to compare the two methods.

METHODS

Cough and Valsalva maneuver pressures were compared in 40 patients, 20 consecutive men and 20 consecutive women, undergoing urodynamics at three time points: baseline (20 ml filling), prevoid and postvoid.

RESULTS

At baseline, the percentage difference between p_abd and p_ves was significantly lower for Valsalva maneuvres compared with coughs for all patients (median 5.1% vs. 10.2%). This association was consistent in male patients, but not for female. At prevoid, this was reversed with coughs providing more consistent pressure spikes than Valsalva maneuvers (median 7.0% vs. 24.1%), for all patients. Postvoid, no difference was noted in either men or women.

CONCLUSIONS

Quality control during urodynamics is important to ensure diagnostic accuracy. This is the first study to compare two methods of quality control, coughs, and Valsalva maneuvers. Coughs are a useful indicator of pressure transmission quality at all time points throughout urodynamic studies. However, at low volumes, Valsalva maneuvers give a better indication of quality, with lower variability, whereas at cystometric capacity coughs performed better. Therefore, if poor pressure transmission occurs with cough during urodynamics, particularly at low volumes, it is recommended that a Valsalva maneuver is used to further assess whether pressure transmission and thus urodynamic quality is satisfactory.

Good urodynamic practice: Pressure signal quality immediately after catheter insertion for cystometry with a water-filled pressure transducer system and its relevance for the ICS zero procedure

Aim

This study aims to evaluate the intracorporeal pressures immediately after the insertion of the catheters for urodynamic testing with a water‐filled urodynamic pressure transducer system to determine the relevance of the International Continence Society (ICS) zeroing principles.

Methods

Here, a retrospective analysis of a random series of urodynamic recordings is performed. The initial pressures, immediately after the insertion of the catheters, have been compared with the pressures after some milliliters of filling and flushing away of the gel, used with insertion, and/or the mucus and debris from the inserted catheters. Differences of initially recorded intravesical and intrarectal pressures from those after flushing and filling are analyzed and associated with the ICS standard practice of zeroing.

Results

Statistically and clinically significant differences between the initial pressures and the pressures after filling and flushing are observed, with nonphysiological initial pressures in 62% of the studies. Some filling (20 ml or more in the bladder) and flushing of the pressure channels resulted in the registration of physiological pressures and synchronous response from both lines on abdominal pressure increases.

Conclusions

The pressure signal quality of a water‐filled urodynamic system immediately after catheter insertion is low with inaccurately displayed pressure values, but it changes to normal after flushing the pressure channels and some filling. Rezeroing of the intracorporeal pressures immediately after catheter insertion for cystometry is the inappropriate correction procedure that misleadingly modifies the false initial pressures, resulting in ongoing unrealistic urodynamic study pressures.

Comparison of air-filled and water-filled catheters for use in cystometric assessment

INTRODUCTION AND HYPOTHESIS

To determine whether pressure readings measured with air-filled catheter (AFC) and water-filled catheter (WFC) systems are equivalent during cystometric assessment, especially in case of pressure measurements at Valsalva manoeuvres and coughs.

METHODS

Twenty-five subjects were recruited. The commercially available 7-Fr TDOC AFC, which simultaneously reads water and air pressures in the bladder and rectum, was used to compare filling and voiding data recordings. Data were compared using paired t-tests, Bland-Altman plots and linear correlation methods, respectively.

RESULTS

Pressure readings measured by the two systems showed a good correlation at Valsalva manoeuvres [R² = 0.988, 0.968 for vesical pressure (Pves) and abdominal pressure (Pabd), respectively] and at coughs (R² = 0.972, 0.943 for Pves and Pabd, respectively). There was a statistically significant difference between the two different measurement modalities at coughs (p < 0.01), initial resting pressure (p < 0.01) and the maximum pressure at detrusor overactivity (p < 0.01). This indicated that the difference between the two measurement modalities during Valsalva manoeuvres could reach up to 5.2 cmH₂O and 8.1 cmH₂O in Pves and Pabd measurements, respectively. During coughs, the difference could reach up to 20 cmH₂O and 19.5 cmH₂O in Pves and Pabd measurements, respectively.

CONCLUSIONS

Pressure recordings from AFC and WFC systems appear to be interchangeable for some urodynamics parameters such as Pves at Valsalva manoeuvres if the baseline pressure is compensated, but not for fast-changing pressure signals such as coughs. This has to be considered when pressures are being taken with the AFC.

Correlation of Xiphopubic Distance, Body Weight, Height and Body Mass Index with Intravesical and Abdominal Initial Resting Pressures in Urodynamic Testing in the Sitting Position

OBJECTIVES

To compare the initial resting intravesical pressure (p_ves ) and abdominal pressure (p_abd ) in the sitting position with the typical range of values, to assess the correlation between such pressures and patient xipho-pubic distance, body weight, height and body mass index (BMI), and to estimate if xipho-pubic distance can be a guide to interpret initial resting pressures in urodynamic testing.

METHODS

Women with lower urinary tract symptomatology referred for urodynamic testing were consecutively enrolled in a prospective study. Conventional cystometry was done following "good urodynamic practices". The correlation between initial resting p_ves and p_abd and xipho-pubic distance, body weight, height and BMI were studied using simple and multiple linear regression analysis. Statistical significance was defined as P < 0.05.

RESULTS

One hundred women aged (mean ± SD) 59.2 ± 13.3 were studied. There was a significant correlation between both p_ves and p_abd , and xipho-pubic distance, body weight and BMI and no correlation with height. Over the multiple analysis, xipho-pubic distance and body weight correlated independently with p_ves ; body weight correlated independently with p_abd . Three patients had p_ves and/or p_abd out of typical range, with BMIs values of 18.48, 18.72 and 35.81 kg/m² . There was a wide range of difference between xipho-pubic distance and initial resting pressures.

CONCLUSIONS

Out of range initial resting pressures occur in underweight (or close to) or severely obese patients. Xipho-pubic distance and body weight correlated independently with p_ves . Body weight correlated independently with p_abd . Xipho-pubic distance cannot be a guide to interpret the initial resting pressures.

Typical Value Ranges and Typical Signal Patterns in the Initial Cough in Patients With Neurogenic Bladder: Quality Control in Urodynamic Studies

Purpose

The goal of this study was to establish typical value ranges (TVRs) and to analyze typical signal patterns (TSPs) of the initial cough (cough before bladder filling) for quality control in urodynamic studies.

Methods

A total of 539 urodynamic traces from patients with neurogenic bladder obtained over the course of a year were retrospectively reviewed. The TVRs for cough amplitude in measurements of the intravesical pressure (P_ves), abdominal pressure (P_abd), and detrusor pressure (P_det) during the initial cough were established. We used the 95% range as a reference range for all parameters. Cough spikes in P_det were described and classified, and the reasons for different patterns of cough spikes were established. The quality of all the cystometry traces was checked, and we also present remedial actions for inappropriate cough spikes.

Results

The cough amplitudes in the measurements of P_ves and P_abd were similar, with 95% of measurements falling within the following ranges: 4–62 cm H₂O and 3–70 cm H₂O, respectively, in supine position and 9–95 cm H₂O and 8–98 cm H₂O, respectively, in sitting position. For P_det, the cough amplitude ranged from −38 to 25 cm H₂O in supine position and from −44 to 41 cm H₂O in sitting position. The cough spikes for P_det were classified as follows: type I, P_det pressure exhibited a minimal change (<5 cm H₂O) during the cough; type II, a monophasic spike (>5 cm H₂O) was observed for P_det; and type III, biphasic spikes were observed for P_det. Type I coughs were found to have more high-quality traces (P<0.01).

Conclusions

TVRs for the initial cough test among neurogenic patients were established in order to provide guidelines for quantitative quality control. The TSPs for the initial cough signal were described, and the presence of a high-quality cough signal may be recommended as a component of quality control in urodynamic measurements.

Quantitative quality control during urodynamic studies with TVRs for cystometry in men with lower urinary tract symptoms suggestive of benign prostatic hyperplasia

OBJECTIVE

To establish typical value ranges (TVRs) and to outline their role in urodynamic quality control.

METHODS

Five hundred and eighty-two data sets of free flow, filling cystometry, and voiding from 181 males in a strictly quality-controlled study were analyzed for intravesical pressure (Pves), abdominal pressure (Pabd), and detrusor pressure (Pdet) before, at the beginning and end of filling, and after voiding. Cystometric capacity, maximum flow rate (Qmax), voided volume (Vvoid), and compliance were determined. Mean value, standard deviation, median, and various TVRs were calculated. Technical errors related to the TVRs are classified and examples given.

RESULTS

TVRs for initial resting Pves, Pabd, and Pdet were 31-42, 28-39, and 0-4 cmH₂O, respectively. Various technical errors are classified as type I: normal initial Pdet, both Pves and Pabd are incorrect; type II: negative initial Pdet; and type III: initial Pdet too high. The incidences of I, II, and III errors were 9.8, 4.5, and 1.4%, respectively. The TVRs for maximum cystometric capacity and compliance were 157-345 mL and 26.7-70.8 mL/cmH₂O; Qmax, Pdet, and Vvoid were 5.5-9 mL/s, 57-92 cmH₂O, and 167-315 mL, respectively; Qmax and Vvoid in free flow were 8-9.2 mL/s and 167-301 mL, respectively. After voiding, two errors were found: type V (Pves and Pdet after voiding still high) and type VI (Pves and Pdet negative).

CONCLUSIONS

TVRs in urodynamics are indispensable and effective tools for quantitative plausibility checks and quality control. They are sensitive and reliable indicators for correct measurement and a relevant contribution to a collection of normal values.

Qualitative quality control during urodynamic studies with TSPs for cystometry in men with lower urinary tract symptoms suggestive of benign prostatic hyperplasia

The aim of this study was to describe typical signal patterns (TSPs) by amplitude and pressure gradients and indicate the role and significance of them in quality control. A total of 582 measurements from a multicenter urodynamic study on males (mean age, 65.3 years) were re-analyzed. Using manual graphical analysis, we identified signal patterns by typical amplitude (A) and typical pressure gradient (PG). TSPs were classified into four types: I: fine structure (A < 3 cm H2O); II: minor dynamic changes (A < 5 cm H2O); III: major changes due to cough tests (A > 50 cm H2O, PG > 100 cm H2O/s); IV: typical major changes due to muscular activity: detrusor overactivity (A > 3 cm H2O, PG > 1-5 cm H2O/s), rectal contractions (A = 5-10 cm H2O, PG = 5-10 cm H2O/s), and straining (A > 5 cm H2O, PG ≥ 30 cm H2O/s). At beginning of and during filling, 91.8 and 98.3 % of traces showed the identical fine structure and minor changes between P ves and P abd tracings, and P det tracing was quiet. 92.3 % of P ves and P abd traces had equal pressure changes at test coughs. During filling, 8.3 % traces showed straining, 17.4 % showed rectal contractions, and 33.7 % showed detrusor overactivity. Before voiding, 94 % of P ves and P abd traces had equal cough response. During voiding, 91.2 % of traces showed the same fine structure, 53.3 % of traces showed straining, and 15.3 % showed relaxation of the pelvic floor. After voiding, 91.2 % of traces had the same fine structure, and 87.5 % had an equal cough response. TSPs are a powerful tool for qualitative plausibility and quality control and are an indispensable pre-condition for good urodynamic practice.

Availability of a newly devised ambulatory urodynamics monitoring system based on personal device assistance in patients with spinal cord injury

Conventional urodynamics systems have been widely used for the assessment of bladder functions. However, they have some drawbacks due to the unfamiliar circumstances for the patient, restrictive position during the test, expense and immovability of the instrument as well as the unphysiological filling of the bladder. To mitigate these problems, we developed a fully ambulatory urodynamics monitoring system, which enables the abdominal pressure to be measured in a non-invasive manner, as well as the manual recording of various events such as the bladder sensations or leakage of urine. Conventional (CMG) and furosemide-stimulated filling cystometry (FCMG) were performed for 28 patients with neurogenic bladders caused by spinal cord injury (24 males and 4 females, age: 49.4 ± 13.9 years, BMI: 23.5 ± 2.4). There were high correlation coefficients (r=0.97 ± 0.02) between the clinical parameters measured by the conventional rectal catheter and those measured by our non-invasive algorithm in the FCMG studies. Also, 10 of the patients (36%) were diagnosed as having different reflexibility of the bladder between conventional CMG and FCMG (p<0.05). In the patients with detrusor overactivity, the average volume and detrusor pressure at bladder sensation in FCMG were lower than those in CMG, while the average compliance was higher (p<0.05). In the patients with areflexic bladders, the number of patients with detrusor overactivity was higher in FCMG and leakage was observed more frequently. These results showed that our system could be a useful additional tool in the clinical assessment of patients in which conventional cystometry failed to explain their symptoms.

Perineal surface electromyography does not typically demonstrate expected relaxation during normal voiding

AIMS

To describe perineal surface patch electromyography (EMG) activity during urodynamics (UDS) and compare activity between filling and voiding phases and to assess for a relationship between preoperative EMG activity and postoperative voiding symptoms.

METHODS

655 women underwent standardized preoperative UDS that included perineal surface EMG prior to undergoing surgery for stress urinary incontinence. Pressure-flow studies were evaluated for abdominal straining and interrupted flow. Quantitative EMG values were extracted from 10 predetermined time-points and compared between fill and void. Qualitative EMG activity was assessed for the percent of time EMG was active during fill and void and for the average amplitude of EMG during fill compared to void. Postoperative voiding dysfunction was defined as surgical revision or catheterization more than 6 weeks after surgery. Fisher's exact test with a 5% two-sided significance level was used to assess differences in EMG activity and postoperative voiding dysfunction.

RESULTS

321 UDS had interpretable EMG studies, of which 131 (41%) had EMG values at all 10 predetermined and annotated time-points. Quantitative and qualitative EMG signals during flow were usually greater than during fill. The prevalence of postoperative voiding dysfunction in subjects with higher preoperative EMG activity during void was not significantly different. Results were similar in the 42 subjects who had neither abdominal straining during void nor interrupted flow.

CONCLUSIONS

Perineal surface patch EMG did not measure expected pelvic floor and urethral sphincter relaxation during voiding. Preoperative EMG did not predict patients at risk for postoperative voiding dysfunction.

Preoperative voiding detrusor pressures do not predict stress incontinence surgery outcomes

Introduction and hypothesis

The aim of this study was to determine whether preoperative voiding detrusor pressures were associated with postoperative outcomes after stress incontinence surgery.

Methods

Opening detrusor pressure, detrusor pressure at maximum flow (p_det Q_max), and closing detrusor pressure were assessed from 280 valid preoperative urodynamic studies in subjects without advanced prolapse from a multicenter randomized trial comparing Burch and autologous fascia sling procedures. These pressures were compared between subjects with and without overall success, stress-specific success, postoperative detrusor overactivity, and postoperative urge incontinence using independent sample t tests.

Results

There were no clinically or statistically significant differences in mean preoperative voiding detrusor pressures in any comparison of postoperative outcomes.

Conclusions

We found no evidence that preoperative voiding detrusor pressures predict outcomes in women with stress predominant urinary incontinence undergoing Burch or autologous fascial sling procedures.

Reference urodynamic values for stress incontinent women

OBJECTIVE

To determine reference urodynamic values for preoperative urodynamic studies in women undergoing surgery for pure or predominant stress urinary incontinence (SUI).

MATERIALS AND METHODS

Six hundred fifty-five women with pure or predominant SUI were enrolled in a multicenter surgical trial and were randomized to undergo a Burch or autologous fascia sling procedure as part of the Urinary Incontinence Treatment Network (UITN) Stress Incontinence Surgical Treatment Efficacy Trial (SISTEr). Preoperative free uroflowmetry, filling cystometry, and pressure flow studies were performed in all women using a standardized research protocol and standardized urodynamic interpretation guidelines. We define the normal range of urodynamic values in this population as the values that encompass 95% of the results.

RESULTS

In 655 women undergoing filling cystometry in the standing position, baseline vesical and abdominal pressures were between 12 and 60 cm H(2)O. The upper limit of detrusor pressure increase during bladder filling to maximum cystometric capacity was 16 cm H(2)O. Ten percent of women who qualified for stress incontinence surgery with a positive cough stress test on physical exam did not demonstrate urodynamic stress incontinence (USI) and less than 10% of subjects in this study demonstrated detrusor overactivity.

CONCLUSIONS

Results from a large cohort of women with SUI are now available for quantitative plausibility assessments or as reference values when interpreting urodynamic studies.

Process for development of multicenter urodynamic studies

OBJECTIVES

To describe our experiences with a continuous quality improvement process for the standardization of multicenter urodynamic studies (UDSs) in a multi-institutional network. Multicenter UDSs can have considerable variations in testing procedures, training, equipment, and reviewer biases.

METHODS

A quality control process was developed that included protocol development, certification of urodynamic testers, central review to assess compliance with protocol and quality, protocol modifications, standardization of equipment and signal configuration, development of an electronic signal repository, and the development of UDS Interpretation Guidelines.

RESULTS

We describe our experience and process in the development and implementation of a standardized UDS protocol in a multicenter surgical trial for stress urinary incontinence. The process included our protocol development, quality control measures, standardization processes, electronic signal repository, and the need for UDS Interpretation Guidelines. A urodynamic testing procedures protocol was implemented successfully by 20 urodynamic testers at nine continence treatment centers. The protocol provides explicit and detailed guidelines for equipment, calibration, patient position, specific annotations, lay language bladder sensation parameters, visual leak point pressure techniques, modifications for prolapse, and data recording. A UDS Interpretation Guidelines document provides specific suggestions for validity and plausibility determination, expected ranges of urodynamic variables, and reasonable agreement of measuring systems. Both documents are available to urodynamic investigators on the Urinary Incontinence Treatment Network website (http://www.uitn.net/resourcesforphysicians.htm).

CONCLUSIONS

Multicenter UDSs require a continuous quality improvement process and the development of UDS testing procedures and interpretation guidelines.

Dynamic transurethral sonography and 3-dimensional reconstruction of the rhabdosphincter and urethra: initial experience in continent and incontinent women

OBJECTIVE

The purpose of this study was to evaluate the female urethra and the striated urinary sphincter, the rhabdosphincter (RS), by means of dynamic transurethral sonography and sonographic 3-dimensional (3D) reconstructions.

METHODS

In 15 female patients with urinary stress incontinence (mean age, 67.5 years) and 5 continent women (mean age, 48.3 years), morphologic characteristics and function of the RS and urethra were examined with a 10-MHz transurethral ultrasound transducer. With the help of a mechanical pullback system, the transducer was slowly retracted to scan the whole urethra and the RS from the bladder neck to the urethral orifice. Subsequently, 3D reconstructions of the urethra using an integrated computer system were performed. The RS as well as the length of the urethra were investigated under contracted and noncontracted conditions to measure contractility of the RS and dynamic changes of the lower urinary tract.

RESULTS

Partial or complete loss of RS function was detected in patients with stress incontinence. The findings on sonography were found to correlate well with the grade of incontinence. Furthermore, under contraction of the RS, a median increase in urethral length was observed. In incontinent patients, the increase in the urethral length was statistically significantly less (P = .04), which was related to the reduced contractility of the RS.

CONCLUSIONS

Dynamic transurethral sonography with subsequent 3D reconstructions allows for assessment of function and morphologic characteristics of the RS and urethra. Normal contraction of the RS results in an elongation of the urethra.

Trying to predict 'dangerous' bladders in children: the area under the curve concept

OBJECTIVE

Children with neurological and non-neurological lower urinary tract dysfunction normally undergo video urodynamics. One reason is to try to diagnose 'dangerous' bladders. Currently, bladder compliance is used to predict 'dangerous' bladders; however, in children there are no standardized methods of measurement and thus no 'cut-off' values. Compliance may also be normal even though high-pressure detrusor overactivity waves may exist during the filling phase of urodynamics. We tried to determine whether measuring the area under the detrusor pressure curve (AUC) during the filling phase of urodynamics would be a useful parameter in predicting 'dangerous' bladders.

PATIENTS AND METHODS

Children referred to the urodynamics unit at Southmead Hospital, a tertiary referral centre, from 2000 to 2004 were investigated. Although 130 patients were identified, only 15 patients had raw data which were analysable using the available computer software.

RESULTS

There was no correlation between the AUC and predicting 'dangerous' bladders possibly due to limitations in the computer software.

CONCLUSION

Although the study did not reveal any correlation between the AUC and 'dangerous' bladders, it revealed the limitations of the available computer software in determining AUC, and highlighted the need for new standardized software and multinational, multi-centre trials to look into the concept of AUC. There is also a need for the International Continence Society to standardize methods and terminology in predicting 'dangerous' bladders.

Randomized comparison of vaginal and rectal measurement of intra-abdominal pressure during subtracted dual-channel cystometry

OBJECTIVES

To compare the rectal (Prec) and vaginal (Pvag) measurement of intra-abdominal pressure during cystometry in women by assessment of patient acceptability and cystometrogram quality control.

METHODS

Of 136 consecutive women undergoing cystometrography, 100 were randomized to measurement of Prec or Pvag. The patients' experience was measured using visual analogue scales of anticipated and actual discomfort and embarrassment before and after the procedure. The setup time was recorded, and quality control was assessed by the number of catheter events (pressure lines falling out, requiring flushing, or repositioning), and quality of the cough signal.

RESULTS

A total of 136 consecutive patients were approached. Of those eligible, 50 were randomized to Prec and 50 to Pvag; 23 women declined randomization because of preference for a vaginal line. Overall, 81 line events occurred in 29 women with a vaginal line and 117 occurred in 34 women with a rectal line. No difference was found in patient acceptability nor in trace quality for the two routes. Despite catheter repositioning and flushing, only 13% of traces showed optimum trace quality throughout the entire investigation; the cough signal quality was worse after provocation, during voiding, and in women with prolapse.

CONCLUSIONS

A significant proportion of patients undergoing cystometry expressed a preference for the vaginal line. The quality of output from cystometry was limited by the conditions of the investigation rather than the method of intra-abdominal pressure measurement.