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Intra-abdominal pressure correlates with abdominal wall tension during clinical evaluation tests

Open AccessPublished:July 13, 2021DOI:https://doi.org/10.1016/j.clinbiomech.2021.105426

      Highlights

      • Pressure sensors measure abdominal wall tension
      • Pressure sensors indirectly evaluate intra-abdominal pressure.
      • Increases in abdominal wall tension correlate with intra-abdominal pressure changes.
      • Abdominal wall tension increases in five postural-respiratory scenarios.
      • Intra-abdominal pressure increases in five postural-respiratory scenarios.

      Abstract

      Background

      The abdominal muscles play an important respiratory and stabilization role, and in coordination with other muscles regulate the intra-abdominal pressure stabilizing the spine. The evaluation of postural trunk muscle function is critical in clinical assessments of patients with musculoskeletal pain and dysfunction. This study evaluates the relationship between intra-abdominal pressure measured as anorectal pressure with objective abdominal wall tension recorded by mechanical-pneumatic-electronic sensors.

      Methods

      In a cross-sectional observational study, thirty-one asymptomatic participants (mean age = 26.77 ± 3.01 years) underwent testing to measure intra-abdominal pressure via anorectal manometry, along with abdominal wall tension measured by sensors attached to a trunk brace (DNS Brace). They were evaluated in five different standing postural-respiratory situations: resting breathing, Valsalva maneuver, Müller's maneuver, instructed breathing, loaded breathing when holding a dumbbell.

      Findings

      Strong correlations were demonstrated between anorectal manometry and DNS Brace measurements in all scenarios; and DNS Brace values significantly predicted intra-abdominal pressure values for all scenarios: resting breathing (r = 0.735, r2 = 0.541, p < 0.001), Valsalva maneuver (r = 0.836, r2 = 0.699, p < 0.001), Müller's maneuver (r = 0.651, r2 = 0.423, p < 0.001), instructed breathing (r = 0.708, r2 = 0.501, p < 0.001), and loaded breathing (r = 0.921, r2 = 0.848, p < 0.001).

      Interpretation

      Intra-abdominal pressure is strongly correlated with, and predicted by abdominal wall tension monitored above the inguinal ligament and in the area of superior trigonum lumbale. This study demonstrates that intra-abdominal pressure can be evaluated indirectly by monitoring the abdominal wall tension.

      Keywords

      1. Introduction

      Spinal stability is secured by the bone structures, ligaments, and via coordinated activation between spinal extensors and flexors and all muscles regulating the intra-abdominal pressure (IAP) (
      • Cholewicki J.
      • McGill S.
      Mechanical stability of the in vivo lumbar spine: implications for injury and chronic low back pain.
      ;
      • Hodges P.
      • Martin Eriksson A.E.
      • Shirley D.C.
      • Gandevia S.
      Intra-abdominal pressure increases stiffness of the lumbar spine.
      ). The diaphragm and pelvic floor form two pistons which push against each other increasing the pressure in the abdominal cavity. Contraction of the abdominal muscles resists lateral movement of the contents within the abdominal cavity (
      • Chaitow L.
      • Bradley D.
      • Gilbert C.
      Recognizing and Treating Breathing Disorders E-Book.
      ;
      • Hodges P.W.
      Is there a role for transversus abdominis in lumbo-pelvic stability?.
      ). IAP is essentially a hydraulic pressure effective in all directions, stabilizing the torso and reducing axillary compression during activities that increase the demands on spinal stabilization, such as lifting heavy loads (
      • Cobb W.S.
      • Burns J.M.
      • Kercher K.W.
      • Matthews B.D.
      • James Norton H.
      • Todd Heniford B.
      Normal Intraabdominal pressure in healthy adults.
      ;
      • Grillner S.
      • Nilsson J.
      • Thorstensson A.
      Intra-abdominal pressure changes during natural movements in man.
      ). Hodges et al. has confirmed that an increase in IAP alone without activity of abdominal or back muscles still enhances the stability of the lumbar spine (
      • Hodges P.
      • Martin Eriksson A.E.
      • Shirley D.C.
      • Gandevia S.
      Intra-abdominal pressure increases stiffness of the lumbar spine.
      ).
      The amount of IAP can be measured by several different invasive and non-invasive methods. The most accurate is direct laparoscopic measurement using an intra-abdominal catheter (
      • Malbrain M.L.N.G.
      • Cheatham M.L.
      • Kirkpatrick A.
      • Sugrue M.
      • De Waele J.
      • Ivatury R.
      Abdominal compartment syndrome: it’s time to pay attention!.
      ). Indirect urethral measurement is considered to be the most frequent and reliable method to monitor IAP; however, this can result in urinary tract infections or urethral injury, therefore, it is not often used in postural function research (
      • Malbrain M.L.N.G.
      • De Laet I.E.
      • De Waele J.J.
      • Kirkpatrick A.W.
      Intra-abdominal hypertension: definitions, monitoring, interpretation and management.
      ;
      • Wise R.D.
      • Rodseth R.N.
      • Correa-Martin L.
      • Margallo F.S.
      • Becker P.
      • Castellanos G.
      • Malbrain M.L.N.G.
      Correlation between different methods of intraabdominal pressure monitoring in varying intraabdominal hypertension models.
      ).
      In rehabilitation medicine, instrumental IAP measurement via rectal or gastric probes are mainly used in experimental studies, and are not typically used in routine clinical assessment (
      • Malbrain M.L.N.G.
      • Cheatham M.L.
      • Kirkpatrick A.
      • Sugrue M.
      • De Waele J.
      • Ivatury R.
      Abdominal compartment syndrome: it’s time to pay attention!.
      ). Gastric or nasogastric tubes inserted into the stomach provide quite accurate IAP measurements, however, it is quite uncomfortable for patients and an expensive method requiring highly trained personnel (
      • Grillner S.
      • Nilsson J.
      • Thorstensson A.
      Intra-abdominal pressure changes during natural movements in man.
      ;
      • Hodges P.
      • Martin Eriksson A.E.
      • Shirley D.C.
      • Gandevia S.
      Intra-abdominal pressure increases stiffness of the lumbar spine.
      ;
      • Wauters J.
      • Spincemaille L.
      • Dieudonne A.-S.
      • Van Zwam K.
      • Wilmer A.
      • Malbrain M.L.N.G.
      A novel method (CiMON) for continuous intra-abdominal pressure monitoring: pilot test in a pig model.
      ). Special catheters or probes inserted into the rectum are used for anorectal measurements. Such pressure sensitive devices convert mechanical signals into electrical signals recorded and displayed on a computer monitor (
      • Pfeifer J.
      • Oliveira L.
      Anorectal Manometry and the Rectoanal inhibitory reflex.
      ). Recently, thin electric probes have become available. Smaller devices lead to fewer artifacts thus offering more exact display and measurement. Small probes are easy to install, temperature resistant, very sensitive to pressure changes and well tolerated by patients, with infrequent side effects (
      • Malbrain M.L.N.G.
      • Cheatham M.L.
      • Kirkpatrick A.
      • Sugrue M.
      • De Waele J.
      • Ivatury R.
      Abdominal compartment syndrome: it’s time to pay attention!.
      ;
      • Sugrue M.
      • De Waele J.
      • De Keulenaer B.L.
      • Roberts D.j.
      • Malbrain M.L.N.G.
      A user’s guide to intra-abdominal pressure measurement.
      ). The disadvantage is the high purchase price (
      • Pfeifer J.
      • Oliveira L.
      Anorectal Manometry and the Rectoanal inhibitory reflex.
      ). Such IAP recording has been reported in many studies exploring IAP changes in various postural situations (
      • Kawabata M.
      • Shima N.
      • Hamada H.
      • Nakamura I.
      • Nishizono H.
      Changes in intra-abdominal pressure and spontaneous breath volume by magnitude of lifting effort: highly trained athletes versus healthy men.
      ;
      • Sapsford R.R.
      • Clarke B.
      • Hodges P.W.
      The effect of abdominal and pelvic floor muscle activation patterns on urethral pressure.
      ).
      IAP measurement has also been combined with simultaneous electromyography or ultrasound assessments of core muscles. However, these methods do not evaluate the global coordination of the trunk muscles but rather local muscle activation. In addition, significant inaccuracies during such recording have been reported (
      • Henry S.M.
      • Westervelt K.C.
      The use of real-time ultrasound feedback in teaching abdominal hollowing exercises to healthy subjects.
      ;
      • Junginger B.
      • Baessler K.
      • Sapsford R.
      • Hodges P.W.
      Effect of abdominal and pelvic floor tasks on muscle activity, abdominal pressure and bladder neck.
      ).
      In clinical practice, palpation of the abdominal wall tension (AWT), especially in the area above the inguinal ligament and in the upper trigonum lumbale is used to evaluate an individual's ability to regulate their IAP (
      • Kobesova A.
      • Davidek P.
      • Morris C.E.
      • Andel R.
      • Maxwell M.
      • Oplatkova L.
      • Safarova M.
      • Kumagai K.
      • Kolar P.
      Functional postural-stabilization tests according to dynamic neuromuscular stabilization approach: proposal of novel examination protocol.
      ). Available studies suggest that the AWT occurs as a result of increased IAP (
      • Cresswell A.G.
      Responses of intra-abdominal pressure and abdominal muscle activity during dynamic trunk loading in man.
      ;
      • Kumar S.
      • Sharma V.P.
      • Aggarwal A.
      • Shukla R.
      • Dev R.
      Effect of dynamic muscular stabilization technique on low back pain of different durations.
      ;
      • Tayebi S.
      • Gutierrez A.
      • Mohout I.
      • Smets E.
      • Wise R.
      • Stiens J.
      • Malbrain M.L.N.G.
      A concise overview of non-invasive intra-abdominal pressure measurement techniques: from bench to bedside.
      ;
      • van Ramshorst G.H.
      • Salih M.
      • Hop W.C.J.
      • van Waes O.J.F.
      • Kleinrensink G.-J.
      • Goossens R.H.M.
      • Lange J.F.
      Noninvasive assessment of intra-abdominal pressure by measurement of Abdominal Wall tension.
      ). Different types of sensors have been used to measure the AWT during various postural tasks related to IAP changes (
      • Chen Yuan-zhuo
      • Yan S.
      • Chen Yan-qing
      • Zhuang Y.
      • Wei Z.
      • Zhou S.
      • Peng H.
      Noninvasive monitoring of intra-abdominal pressure by measuring abdominal wall tension.
      ;
      • Malátová R.
      • Rokytová J.
      • Stumbauer J.
      The use of muscle dynamometer for correction of muscle imbalances in the area of deep stabilising spine system.
      ,
      • Malátová R.
      • Pucelík J.
      • Rokytová J.
      • Kolár P.
      Technical means for objectification of medical treatments in the area of the deep stabilisation spinal system.
      ;
      • Novak J.
      • Busch A.
      • Kolar P.
      • Kobesova A.
      Postural and respiratory function of the abdominal muscles: a pilot study to measure abdominal wall activity using belt sensors.
      ;
      • van Ramshorst G.H.
      • Salih M.
      • Hop W.C.J.
      • van Waes O.J.F.
      • Kleinrensink G.-J.
      • Goossens R.H.M.
      • Lange J.F.
      Noninvasive assessment of intra-abdominal pressure by measurement of Abdominal Wall tension.
      ). This study presents simultaneous recording of IAP measured as anorectal pressure and AWT measured via four sensors attached to a trunk brace. In an attempt to further understand the relationship between IAP and outward tension of the abdominal wall, the purpose of this research was to compare anorectal manometry measurements, largely considered the gold standard in ambulatory patients, with abdominal wall outward tension measured by a trunk brace during clinical assessments.

      2. Methods

      2.1 Participants

      Thirty-one asymptomatic volunteers were recruited for the study. Written informed consent was obtained from each participant, and demographic characteristics of the sample including age, weight, height and BMI are shown in Table 1. Exclusion criteria were any symptomatic neurologic, orthopedic, respiratory, internal or musculoskeletal disorder, spine or abdominal surgery, severe trauma during the last year, pregnancy, and history of therapy focusing on IAP training. The study conforms with The Code of Ethics of the World Medical Association and was approved by an Institutional Ethics Committee (Ethics Committee of the University Hospital Motol and 2nd Faculty of Medicine, Charles University in Prague. No.1263.1.15/19; approval date: November 6, 2019). This study adhered to the Helsinki declaration.
      Table 1Participant's anthropometric characteristics. N = 31, 15 males, 16 females.
      Age (years)Height (cm)Weight (kg)BMI
      Mean21.3170.563.224.1
      SD1.66.57.93
      Min191604717.3
      Max251858027.6

      2.2 DNS Brace

      To monitor AWT, a special new device called DNS Brace was used (Fig. 1 – A,C). The DNS abbreviation is derived from the rehabilitation concept called Dynamic Neuromuscular Stabilization (DNS) (
      • Kobesova A.
      • Ulm R.
      • Kolar P.
      Dynamic Neuromuscular Stabilization. In: Liebenson C. Ed. Rehabilitation of the Spine. A Patient-Centered Approach, 3rd.
      ,
      • Kobesova A.
      • Safarova M.
      • Kolar P.
      Dynamic neuromuscular stabilization: Exercise in developmental positions to achieve spinal stability and functional joint centration.
      ). DNS emphasizes the importance of IAP in spinal stabilization and treatment. The diaphragm, pelvic floor and abdominal wall muscles regulate the IAP (
      • Hodges P.W.
      • Sapsford R.
      • Pengel L.H.M.
      Postural and respiratory functions of the pelvic floor muscles.
      ). IAP increases during postural activity (
      • Hodges P.W.
      • Gandevia S.C.
      Changes in intra-abdominal pressure during postural and respiratory activation of the human diaphragm.
      ), resulting in a contraction and expansion of the abdominal wall due to muscle activity. Abdominal wall expansion and contraction result in pressure that compresses the DNS Brace sensors. The Brace is an original device produced by Ortotika, FN Motol V Úvalu 84, Praha. Four mechanical-pneumatic-electronic sensors are placed on the inner wall of plastic trunk orthosis. Two ventral sensors are located bilaterally above the groin and two sensors are located on the brace parts adhering to latero-dorsal sections of the abdominal wall (trigonum lumbale superius). Silicon brace sensors contain the inner air-chamber that is deformed by the abdominal wall pressure. The values recorded in kilopascals (kPa) are transferred via Bluetooth, stored and graphically displayed in a smart-phone device. More details about the brace can be found elsewhere (
      • Jacisko J.
      • Stribrny M.
      • Novak J.
      • Busch A.
      • Cerny P.
      • Kolar P.
      • Kobesova A.
      Correlation between palpatory assessment and pressure sensors in response to postural trunk tests.
      ). The brace sensors measure the pressure exerted by the abdominal wall in kilopascals (kPa) (Figs. 2. B, 3. B, 4. B) and transfer the data via Bluetooth to a smart-phone or computer so the data can be statistically processed and graphically displayed.
      Fig. 1
      Fig. 1A: DNS Brace, B: Anorectal probe, C: Participant equipped with DNS Brace and anorectal probe during assessment.
      Fig. 2
      Fig. 2Example of graphical visualization of the measured pressures (A: anorectal manometry, B: DNS Brace) during Valsalva maneuver scenario. A minor delay in DNS Brace measurement relative to ARM is caused by a minimal delay of AWT relative to IAP and by the fact, that DNS Brace measures AWT in 0.5 s. intervals while ARM measures IAP in 0.1 s intervals. Additionally, brace sensors identify only pressure changes over 1 kPa. These factors may cause negligible inaccuracy. The starting pressure before the maneuver is around 5 mmHg for ARM whereas the DNS system starts from zero and returns to zero after the maneuver. DNS Brace automatically reset to zero starting pressure for user friendly reasons. This has no impact on the results because all indirect measurement techniques are able to monitor the IAP changes rather than estimating the absolute IAP value (
      • Tayebi S.
      • Gutierrez A.
      • Mohout I.
      • Smets E.
      • Wise R.
      • Stiens J.
      • Malbrain M.L.N.G.
      A concise overview of non-invasive intra-abdominal pressure measurement techniques: from bench to bedside.
      ).
      Fig. 3
      Fig. 3Example of graphical visualization of the measured pressures (A: anorectal manometry, B: DNS Brace) during Müller maneuver scenario.
      Fig. 4
      Fig. 4Example of graphical visualization of the measured pressures (A: anorectal manometry, B: DNS Brace) during resting breathing scenario.

      2.3 High resolution anorectal manometry

      The intra-abdominal pressure was measured using the ManoScan™ AR HRM system (Given imaging, 15 Hampshire Street, Mansfield, MA 02048 US). It allows for complex assessment of anorectal pressures (Fig. 1 – B,C). The anorectal probe is equipped with 12 channels each measuring 12 circumferentially located spots thus recording pressures from 144 points simultaneously. The diameter of the probe is 10 mm. The pressure values are measured in mmHg (Figs. 2. A, 3. A, 4. A) and transferred at 0.1 s intervals to a computer, where the data can be further processed. The ManoView™ software color-visualizes the measured pressures. Two distal sensors located behind the anal sphincters in the ampulla of rectum monitor the IAP. The remaining 10 probe sensors record the pressures produced by the sphincters. Before starting the measurement, the probe must always be calibrated to 0 atmospheric pressure and a ManoShield rubber protection must be fitted. The probe records pressure in real time.

      2.4 Assessments

      The assessment of all participants was performed by the same examiners under similar conditions (time of day, assessment room, temperature). All participants were first informed about the procedure in detail. After calibration, the anorectal probe was inserted into the participant's anus in a side lying position. Then, the participant stood up and the correct location of the probe was ensured. By activating the sphincters, it was verified that the 2 distal sensors are located in the rectal ampulla monitoring the IAP but not the activity of the sphincters (
      • McCarthy T.A.
      Validity of rectal pressure measurements as indication of intra-abdominal pressure changes during urodynamic evaluation.
      ;
      • Pfeifer J.
      • Oliveira L.
      Anorectal Manometry and the Rectoanal inhibitory reflex.
      ;
      • Shafik A.
      • El-Sharkawy A.
      • Sharaf W.M.
      Direct measurement of intra-abdominal pressure in various conditions.
      ). Then, DNS Brace was fixed to the participant's trunk and the sensors were calibrated to 0 kPa during the tidal exhalation prior to each measurement. The dorsal sensors were adjusted to be placed bilaterally in the superior trigonum lumbale, bellow the floating ribs, and the ventral sensors were placed bilaterally above the groin at the intersection of the mammilar and bispinal connecting line. Then, the participants were instructed to maintain the upright standing position throughout the whole measurement, avoiding increased spinal kyphosis, lordosis or extremity movements. Five postural tests were performed by each subject and evaluated by DNS Brace and Anorectal manometry simultaneously in the same order. The anorectal pressure and AWT values were both collected for 10 s during each of the five scenarios, and the average value of each measurement was used for statistical analysis.
      The measured scenarios:

      2.5 Statistical analysis

      Data analyses were conducted using the Statistical Package for the Social Sciences v27.0 for Mac (IMBCorp, Armonk, NY).. Pearson's correlations and linear regression tests were used to assess the relationship between the 10-s mean anorectal manometry values and DNS Brace values under all five scenarios. Statistical significance was determined a priori at p < 0.05, and power analyses revealed in order to achieve a power of 0.80, 29 subjects were needed to identify a large effect size of 0.50 for Pearson's correlations, and 26 subjects were needed to achieve a large effect size of 0.35 for linear regression analyses. The strength of correlations were interpreted as weak (< 0.30), moderate (0.30–0.50), or strong (> 0.50), and the strength of regression predictions were interpreted as weak (< 0.02), moderate (0.15–0.35) or strong (> 0.35) as reported by
      • Cohen J.
      Statistical power analysis for the social sciences (2nd. Edition).
      (
      • Cohen J.
      Statistical power analysis for the social sciences (2nd. Edition).
      ).

      3. Results

      Preliminary analyses showed linear relationships, with no outliers as assessed by scatterplots, but not all variables were normally distributed, as assessed by Shapiro-Wilk's test (p < 0.05). Data are mean ± standard deviation unless otherwise stated. Pearson's correlations demonstrated strong statistically significant positive relationships between anorectal manometry pressures and DNS Brace pressures, under all five scenarios: resting breathing: r(31) = 0.735, p < 0.001; Valsalva maneuver: r(31) = 0.836, p < 0.001; Müller's maneuver: r(31) = 0.651, p < 0.001; instructed breathing: r(31) = 0.708, p < 0.001; and loaded breathing: r(31) = 0.921, p < 0.001 (Table 2). Simple linear regression models established that anorectal manometry pressure could significantly be predicted from the DNS Brace values under all five scenarios: resting breathing: F(1, 29) = 34.14, p < 0.001; Valsalva maneuver: F(1, 29) = 67.42, p < 0.001; Müller's maneuver: F(1, 29) = 21.29, p < 0.001; instructed breathing: F(1, 29) = 29.14, p < 0.001; and loaded breathing: F(1, 29) = 161.2, p < 0.001 (Fig. 5, Fig. 6, Fig. 7, Fig. 8, Fig. 9) . Table 3 depicts all results from regression analyses.
      Table 2Correlations between Intra-Anal Manometer and DNS Brace Pressures. Values are Mean [Standard Deviation].
      ConditionManometric probe pressureDNS Brace pressurePearson rSig
      1-Resting Breathing22.73 [12.38]20.34 [11.68]0.735< 0.001
      Statistically significant correlation (P < 0.01).
      2-Valsalva Maneuver47.20 [27.09]35.93 [20.19]0.836< 0.001
      Statistically significant correlation (P < 0.01).
      3-Müller's Maneuver35.92 [24.96]20.87 [10.45]0.651< 0.001
      Statistically significant correlation (P < 0.01).
      4-Instructed Breathing34.72 [17.45]26.57 [15.05]0.708< 0.001
      Statistically significant correlation (P < 0.01).
      5-Loaded Breathing36.35 [21.46]30.97 [25.86]0.921< 0.001
      Statistically significant correlation (P < 0.01).
      Note: DNS = Dynamic neuromuscular stabilization.
      low asterisk Statistically significant correlation (P < 0.01).
      Fig. 5
      Fig. 5Simple linear regression analysis of anorectal manometry values (mmHg) and DNS Brace values (kPa) measured during resting breathing.
      Fig. 6
      Fig. 6Simple linear regression analysis of anorectal manometry values (mmHg) and DNS Brace values (kPa) measured during Valsalva maneuver.
      Fig. 7
      Fig. 7Simple linear regression analysis of anorectal manometry values (mmHg) and DNS Brace values (kPa) measured during Müller maneuver.
      Fig. 8
      Fig. 8Simple linear regression analysis of anorectal manometry values (mmHg) and DNS Brace values (kPa) measured during instructed breathing.
      Fig. 9
      Fig. 9Simple linear regression analysis of anorectal manometry values (mmHg) and DNS Brace values (kPa) measured during loaded breathing.
      Table 3Summary of Simple Regression Analyses for Predicting Intra-Anal Manometer Pressure using DNS Brace Pressure (n = 31).
      ConditionBSE BR2Adjusted R295% CIEffect Size (f2)
      1-Resting Breathing0.780.130.540.530.51, 1.051.18
      2-Valsalva Maneuver1.120.140.700.690.84, 1.402.32
      3-Müller's Maneuver1.550.340.420.400.87, 2.240.73
      4-Instructed Breathing0.820.150.500.480.51, 1.131.00
      5-Loaded Breathing0.760.060.850.840.64, 0.895.58
      Note: DNS = Dynamic neuromuscular stabilization.
      R2 = R square: proportion of variance explained by model in sample.
      Adjusted R2: proportion of variance explained by model in population.

      4. Discussion

      4.1 IAP measurement methods

      Currently, various methods to measure the IAP are available. It can be monitored directly via sensors located intraperitoneally or in the inferior caval vein. Intra-vesical, intra-gastric intra-anal or intra-vaginal recording allow to measure the IAP indirectly (
      • Malbrain M.L.N.G.
      • Cheatham M.L.
      • Kirkpatrick A.
      • Sugrue M.
      • De Waele J.
      • Ivatury R.
      Abdominal compartment syndrome: it’s time to pay attention!.
      ;
      • Wise R.D.
      • Rodseth R.N.
      • Correa-Martin L.
      • Margallo F.S.
      • Becker P.
      • Castellanos G.
      • Malbrain M.L.N.G.
      Correlation between different methods of intraabdominal pressure monitoring in varying intraabdominal hypertension models.
      ). This study utilized intra-anal, i.e. measurement using anorectal manometry, which has been determined the safest and easiest method of assessment (
      • Malbrain M.L.N.G.
      • De Laet I.E.
      • De Waele J.J.
      • Kirkpatrick A.W.
      Intra-abdominal hypertension: definitions, monitoring, interpretation and management.
      ,
      • Malbrain M.L.N.G.
      • Cheatham M.L.
      • Kirkpatrick A.
      • Sugrue M.
      • De Waele J.
      • Ivatury R.
      Abdominal compartment syndrome: it’s time to pay attention!.
      ). Other methods posed different challenges, such as intra-vesical catheters may cause urinal infection and urethral trauma, intra-gastric measurement is uncomfortable for participants, and intra-vaginal measurement would exclude male participants. The intra-anal pressure measurement is a reliable way to monitor the IAP, although it does not match with the IAP as accurately as the intra-vesical pressure (
      • Wise R.D.
      • Rodseth R.N.
      • Correa-Martin L.
      • Margallo F.S.
      • Becker P.
      • Castellanos G.
      • Malbrain M.L.N.G.
      Correlation between different methods of intraabdominal pressure monitoring in varying intraabdominal hypertension models.
      ). There are only a few inconveniences of intra-anal pressure monitoring such as the presence of residual faeces, incorrect insertion of the probe and participant's embarrassment (
      • Bhatia N.N.
      • Bergman A.
      Urodynamic appraisal of vaginal versus rectal pressure recordings as indication of intra-abdominal pressure changes.
      ;
      • Pfeifer J.
      • Oliveira L.
      Anorectal Manometry and the Rectoanal inhibitory reflex.
      ).
      In a clinical practice, practitioners often palpate the abdominal wall assuming it to be a non-invasive and indirect way of IAP evaluation. The abdominal wall expands with the IAP increase (
      • van Ramshorst G.H.
      • Salih M.
      • Hop W.C.J.
      • van Waes O.J.F.
      • Kleinrensink G.-J.
      • Goossens R.H.M.
      • Lange J.F.
      Noninvasive assessment of intra-abdominal pressure by measurement of Abdominal Wall tension.
      ). Palpation can be performed in the area above the inguinal ligament and in the superior lumbar triangle (
      • Kobesova A.
      • Davidek P.
      • Morris C.E.
      • Andel R.
      • Maxwell M.
      • Oplatkova L.
      • Safarova M.
      • Kumagai K.
      • Kolar P.
      Functional postural-stabilization tests according to dynamic neuromuscular stabilization approach: proposal of novel examination protocol.
      ). Poor activation in these specific areas of the abdominal wall are commonly found in individuals with low back pain (LBP) (
      • Frank C.
      • Kobesova A.
      • Kolar P.
      Dynamic neuromuscular stabilization & sports rehabilitation.
      ;
      • Kobesova A.
      • Safarova M.
      • Kolar P.
      Dynamic neuromuscular stabilization: Exercise in developmental positions to achieve spinal stability and functional joint centration.
      ). The same trunk sections were previously assessed by other researchers when evaluating abdominal wall activity in relation to IAP regulation (
      • Kumar S.
      • Sharma V.P.
      • Aggarwal A.
      • Shukla R.
      • Dev R.
      Effect of dynamic muscular stabilization technique on low back pain of different durations.
      ;
      • Malátová R.
      • Rokytová J.
      • Stumbauer J.
      The use of muscle dynamometer for correction of muscle imbalances in the area of deep stabilising spine system.
      ;
      • Novak J.
      • Busch A.
      • Kolar P.
      • Kobesova A.
      Postural and respiratory function of the abdominal muscles: a pilot study to measure abdominal wall activity using belt sensors.
      ). Therefore, the sensors are placed on the DNS Brace in the parts adhering to the abdominal wall above the inguinal ligaments and in the superior lumbar triangles. Here, only the attachments of the flat abdominal muscles are located and therefore the abdominal wall is easily accessible (
      • Grevious M.A.
      • Cohen M.
      • Shah S.R.
      • Rodriguez P.
      Structural and functional anatomy of the Abdominal Wall.
      ).
      Our in vivo correlations between IAP and AWT in asymptomatic individuals are in line with the study by Ramshorst et al. previously performed on corpses. Ramshorst used a special dynamometer to monitor AWT resulting from IAP changes in corpses, in which the IAP was changed artificially by insufflation (
      • van Ramshorst G.H.
      • Salih M.
      • Hop W.C.J.
      • van Waes O.J.F.
      • Kleinrensink G.-J.
      • Goossens R.H.M.
      • Lange J.F.
      Noninvasive assessment of intra-abdominal pressure by measurement of Abdominal Wall tension.
      ). Ramshorst's study reports that AWT reflects the IAP. The findings from this study demonstrate significant correlations between the natural IAP regulation and AWT in all five measured scenarios with Pearson's coefficient ranging 0.651 to 0.921 which indicates strong correlations, with the ability to predict the IAP from the measured tension values.

      4.2 Changes in IAP in response to respiration and postural load

      The findings of the current study support prior experiments reported by Davis (
      • Davis PeterR
      The causation of HERNIÆ by weight-lifting.
      ) and Cholewicki (
      • Cholewicki J.
      • Juluru K.
      • Radebold A.
      • Panjabi M.M.
      • McGill S.M.
      Lumbar spine stability can be augmented with an abdominal belt and/or increased intra-abdominal pressure.
      ), confirming that IAP increases with progressing demands on postural stability. The IAP increase results in the proportional activation of the abdominal wall which can be objectively monitored by the sensors or subjectively palpated in the area above the inguinal ligament and in the superior lumbar triangle. In other words, these results confirm that subjective palpation of the abdominal wall is an indirect evaluation of IAP.
      Breathing has been shown to considerably influence IAP, trunk stability and movement (
      • Bradley H.
      • Esformes J.
      Breathing pattern disorders and functional movement.
      ). In this study, inhalation during resting breathing caused only slight increases in the IAP. During exhalation, the AWT and the IAP returned to the basic value. This physiological fluctuation of IAP is normal within the respiratory cycle. Permanent excessive resting IAP would cause organ function failure (
      • Cobb W.S.
      • Burns J.M.
      • Kercher K.W.
      • Matthews B.D.
      • James Norton H.
      • Todd Heniford B.
      Normal Intraabdominal pressure in healthy adults.
      ;
      • De Waele J.J.
      • De Laet I.
      • Kirkpatrick A.W.
      • Hoste E.
      Intra-abdominal hypertension and abdominal compartment syndrome.
      ;
      • Smit M.
      • Werner M.J.M.
      • Lansink-Hartgring A.O.
      • Dieperink W.
      • Zijlstra J.G.
      • van Meurs M.
      How central obesity influences intra-abdominal pressure: a prospective, observational study in cardiothoracic surgical patients.
      ). In this study, the largest increase in the IAP was noted during the Valsalva maneuver. Perhaps this is due to the fact that the muscles of the torso do not have to perform a respiratory function during the Valsalva when the air is not flowing out of the body, the intra-thoracic pressure increases and the cranial displacement of the diaphragm is smaller than with a normal exhalation (
      • Talasz H.
      • Kremser C.
      • Kofler M.
      • Kalchschmid E.
      • Lechleitner M.
      • Rudisch A.
      Proof of concept: differential effects of Valsalva and straining maneuvers on the pelvic floor.
      ,
      • Talasz H.
      • Kofler M.
      • Lechleitner M.
      Misconception of the Valsalva maneuver.
      ). During the Müller maneuver, the intra-thoracic pressure is significantly reduced, the diaphragm descends towards the abdominal cavity but no air flows into the lungs (
      • Kushida C.A.
      Encyclopedia of Sleep.
      ). In our study, Pearson's correlation coefficient was the smallest in this scenario (0.651) which was also the most difficult task for the participants to understand and perform. The instructed breathing represents the Diaphragm Test according to DNS concept. The participants voluntarily expand the abdominal wall towards all four sensors, keeping the abdominal cavity pressurized during the entire respiratory cycle (
      • Kobesova A.
      • Davidek P.
      • Morris C.E.
      • Andel R.
      • Maxwell M.
      • Oplatkova L.
      • Safarova M.
      • Kumagai K.
      • Kolar P.
      Functional postural-stabilization tests according to dynamic neuromuscular stabilization approach: proposal of novel examination protocol.
      ). With this scenario, the participant must be able to combine the respiratory and postural functions of the diaphragm, which is a frequent problem in clinical practice (
      • Kawabata M.
      • Shima N.
      • Hamada H.
      • Nakamura I.
      • Nishizono H.
      Changes in intra-abdominal pressure and spontaneous breath volume by magnitude of lifting effort: highly trained athletes versus healthy men.
      ;
      • Shirley D.
      • Hodges P.W.
      • Eriksson A.E.M.
      • Gandevia S.C.
      Spinal stiffness changes throughout the respiratory cycle.
      ). It is speculated that individuals unable to do so maybe in a greater risk of developing LBP in the future (
      • Ostwal P.P.
      • Wani S.K.
      Breathing patterns in patients with low back pain.
      ;
      • O’Sullivan P.B.
      • Beales D.J.
      Changes in pelvic floor and diaphragm kinematics and respiratory patterns in subjects with sacroiliac joint pain following a motor learning intervention: a case series.
      ). During the last scenario, the participants were holding a barbell of a weight corresponding with 20% of body weight. This situation caused less IAP increase than the Valsalva maneuver but more than resting and instructed breathing and Müller maneuver. Other studies also report significant increases in abdominal muscle activity monitored by EMG (
      • Ershad N.
      • Kahrizi S.
      • Abadi M.F.
      • Zadeh S.F.
      Evaluation of trunk muscle activity in chronic low back pain patients and healthy individuals during holding loads.
      ;
      • Mesquita Montes A.
      • Gouveia S.
      • Crasto C.
      • de Melo C.A.
      • Carvalho P.
      • Santos R.
      • Vilas-Boas J.P.
      Abdominal muscle activity during breathing in different postural sets in healthy subjects.
      ) and in the IAP monitored by anorectal probe (
      • Hodges P.
      • Martin Eriksson A.E.
      • Shirley D.C.
      • Gandevia S.
      Intra-abdominal pressure increases stiffness of the lumbar spine.
      ;
      • Tayashiki K.
      • Takai Y.
      • Maeo S.
      • Kanehisa H.
      Intra-abdominal pressure and trunk muscular activities during abdominal bracing and hollowing.
      ) during posturally challenging situations. With normal resting breathing, a decrease in the IAP during exhalation occurs. However, there is only slight pressure fluctuation within the respiratory cycle with postural loading when the IAP must be reflexively maintained on a higher level throughout the whole respiratory cycle. In this test, the correlation between the values obtained from the manometry and from the DNS Brace sensors was the strongest (Pearson r = 0.921). When holding a load, the stabilization strategy is purely reflexive, i.e. involuntary, and therefore diagnostically valuable in determining possible risks associated with poor trunk stabilization.

      4.3 Methods to measure abdominal wall tension and abdominal wall activation

      The DNS Brace helps to assess both voluntary control and reflex postural activation. It can be used as a feedback tool to train abdominal wall activation and the IAP fluctuations. The DNS Brace can be fixed to the trunk keeping all four sensors in stable contact with the abdominal wall thus allowing evaluation in various body positions. Future studies need to identify the AWT in other postural situations. Other devices like a pressure Biofeedback Unit (
      • Lima P.O.
      • De P.
      • de Oliveira R.R.
      • Costa L.O.P.
      • Laurentino G.E.C.
      Measurement properties of the pressure biofeedback unit in the evaluation of transversus abdominis muscle activity: a systematic review.
      ) and muscle dynamometry (
      • Malátová R.
      • Rokytová J.
      • Stumbauer J.
      The use of muscle dynamometer for correction of muscle imbalances in the area of deep stabilising spine system.
      ), designed to measure or train trunk muscles and lumbopelvic stability may not allow such positional variability. Electromyography (
      • Marshall P.
      • Murphy B.
      Delayed abdominal muscle onsets and self-report measures of pain and disability in chronic low back pain.
      ) or ultrasound (
      • Amerijckx C.
      • Goossens N.
      • Pijnenburg M.
      • Musarra F.
      • van Leeuwen D.M.
      • Schmitz M.
      • Janssens L.
      Influence of phase of respiratory cycle on ultrasound imaging of deep abdominal muscle thickness.
      ) analyze mainly local activation of the abdominal muscles. The information from the four DNS Brace sensors monitor more global co-activation of all abdominal muscles. Based on the strong correlations identified with the DNS Brace and anorectal manometry it can be concluded that the DNS Brace presents a new simple and non-invasive method to evaluate IAP indirectly. The DNS Brace may prove to be useful in physical rehabilitation medicine and research to monitor AWT in response to postural-respiratory demands, and may help to objectivize therapeutic effects, while also providing biofeedback during self-treatment. In the ideal condition, the DNS system is able to track IAP fluctuations and not measure absolute values of IAP, and therefore would not be suitable for IAP monitoring at intensive care units.

      4.4 Study limitations

      This study has several limitations. An average value from the four DNS Brace sensors was calculated and used for statistical analysis. Therefore, possible asymmetric tension of the abdominal could not be taken into account. The current version of the DNS Brace is not commercially available, but sensors working on a similar principle called Ohm Track (
      • Novak J.
      • Busch A.
      • Kolar P.
      • Kobesova A.
      Postural and respiratory function of the abdominal muscles: a pilot study to measure abdominal wall activity using belt sensors.
      ) can be purchased and used in a similar way. DNS Brace cannot be applied to any participants with very narrow or extremely wide waistlines, therefore a different version of the DNS Brace is needed to increase the variability in testing individuals with different corset circumferences. While BMI seems to have no impact on indirect IAP measurements (
      • Chen Yuan-zhuo
      • Yan S.
      • Chen Yan-qing
      • Zhuang Y.
      • Wei Z.
      • Zhou S.
      • Peng H.
      Noninvasive monitoring of intra-abdominal pressure by measuring abdominal wall tension.
      ), the thickness of the abdominal fat layer may play a role. The relationship between the AWT changes measured by the brace sensors and subcutaneous fat thickness measured by a caliper can be explored in future studies. The research was performed on 31 asymptomatic and rather young individuals. Further studies should investigate larger cohorts of individuals comprised of both asymptomatic and LBP or other musculoskeletal problems.

      5. Conclusions

      This study established strong correlations between IAP measured as the anorectal pressure through high resolution manometry with AWT measured by the DNS Brace. Such manometry values could be predicted through the measurement of AWT. Strong correlations were identified during various breathing modifications and also during postural stabilization situations when holding a load. It was confirmed that with progressing demands on postural stability, the IAP increases in a direct correlation with proportional tension of the abdominal wall. The AWT was identified by four DNS Brace sensors located above inguinal ligaments and in the upper lumbar triangle bilaterally. For clinical applications, subjective palpation may be an effective indirect evaluation of intra-abdominal pressure.

      Credit authorship contribution statement

      Jakub Novak: Conceptualization, Project administration, Methodology, Investigation, Data curation, Writing - original draft. Jakub Jacisko: Conceptualization, Methodology, Investigation. Andrew Busch: Data curation, Software, Writing - review & editing. Pavel Cerny: Conceptualization, Interpretation and analysis of data. Martin Stribrny: Conceptualization, Methodology, Investigation. Martina Kovari: Supervision, Writing - review & editing. Patricie Podskalska: Project administration, Data curation. Pavel Kolar: Conceptualization. Alena Kobesova: Conceptualization, Supervision, Writing - review & editing, Funding acquisition.

      Funding

      This study was supported by The Charles University Grant Agency (GAUK No. 340220 ), and by Institutional research program Progres Q41.

      Declarations of Competing Interest

      None.

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