Bladder wall compliance estimation via the U-tube

Introduction

Finding a bladder wall compliance estimation method that would be simple, accurate, standardized and reproducible may present a true clinical interest to measure IAP via the U-Tube whatever urinary volume is. Our hypothesis was that integrating bladder wall compliance in the U-Tube IAP measurement method would increase the accuracy of the measurements, especially considering high urinary volumes.

Methodology

Bladder wall compliance was estimated during a preliminary part of the study, by observing the shape of volume-height curves drawn from ICU patients with preserved hourly urinary output. Only patients with 12 mmHg IAP were considered, this value corresponding to the threshold for a “grade I” intra-abdominal hypertension [1, 2]; IAP measurements were made according to the ‘Gold-standard’ closed-system repeated measurement technique with bladder pressure transducer [1, 2]. Following criteria were also required: (1) intubated, sedated and mechanically ventilated,in the absence of active abdominal muscle contraction (identified by examination of the airway pressure curve); (2) urinary output ≥ 0.5 ml/kg/h; (3) no contra-indication for intra-bladder pressure measurement (past medical history of bladder pathology, bladder and pelvic trauma, neurogenic bladder, post-radic bladder, haematuria, prostate and lower urologic surgery); (4) age ≥ 18 year-old. As for the main part of the study, the protocol was approved by the local ethic committee of our institution for human subjects. In view of the nature of the study, which did not demand a deviation from standard ICU clinical care, informed consent from the patient or next of kin was not required.

Nine patients were included in this preliminary step of the study (four women). Patients were admitted in ICU because of severe sepsis (n = 4), acute pancreatitis (n = 3), and abdominal trauma (n = 2). Mean age was 51±9 years, mean body mass index was 22±3 kg/m2, mean simplified acute physiology II (SAPS II) score was 56±9, mean sequential organe failure assessment (SOFA) score was 9±2 and mean acute physiology and chronic health evaluation (APACHE) II score was 29±5 [3-5].

The volume-height curves were drawn after a U-Tube clamping procedure, i.e. bladder filled naturally [6]. During this procedure, bladder urinary volume (BUV) was evaluated every thirty minutes by ultrasonography. The procedure ended when BUV reached approximately 400 ml or after six hours clamping; the U-tube was then declamped, opening the measurement system to the atmosphere. Urine column height (h) measurements were made at end-expiration by using a graduated measuring feature (rigid rod), vertically from the mid-axillary line level, with 25 ml decrements until bladder vacuity. Each decrement was followed by a one minute equilibration period. Bladder vacuity was again confirmed by ultrasonography. One set of measurements was performed per patient. All h measurements were made by the same observer; this observer was different than the one who made the U-Tube clamping procedure. Results are presented in Table 4. Bladder wall compliance was estimated by solving the equationdefined in SDC Text 1 using mean values of h, and considering a non-linear bladder wall compliance modelization (Fig. 6). Following values of the β and γcoefficients, related to bladder wall compliance, were found:γ = 6.5 and β = 5.5*1011.

Bladder wall compliance integration

By using the equivalences (III) and (IV) defined in SDC Text 1, it is possible to integrate bladder wall compliance in the IAP measurement method via the U-Tube as following:

Where h = urine column height (in meter), BUV = bladder urinary volume (in cubic meter), D = U-Tube diameter (in meter). By convenience, constant of gravity (g = 9.81 N/m2) was used and urinary density, commonly ranged between 1000 and 1020, was replaced by water density (e = 1000 kg/m3).

Clinical perspective

From the biomechanical model of the IAP measurement method via the U-Tube integrating bladder wall compliance, a graphical representation has been made (Fig. 7). Bladder wall compliance has been represented according to the following principles: (1) the lower bladder urinary volume, the higher bladder wall compliance; (2) the higher IAP, the lower bladder wall compliance. The linear part of the curves is observed as long as h ≤ h0. Over h0, bladder cavity starts expanding and bladder wall compliance starts applying. On the graph, h0 can be read at the lower-inflexion position of each of the isobaric curves. The V0 volume reached at h0 corresponds to the urinary volume inside of the U-Tube, before urine leak in the bladder. V0 variations induced by possible U-tube diameter’s variations (e.g. when using ‘urine drainage bags’ from different manufacturers) are low; consequences on IAP measurements could therefore be negligible. Note that if VV0, then IAP will be underestimated, leading to the emergence of false negative subjects when screening IAH at bedside via the U-Tube.

The objective of such a graphical representation is the inclusion of IAP as a routine parameter of ICU patients monitoring. It may constitute a useful tool in ICU, especially for nurses or anyone who aim at screening IAH at bedside. The IAP value (in mmHg) can be read after reporting of urine column height (h, in cm) and bladder urinary volume (BUV, in ml) on the graph.

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3.Knaus, W.A., et al., APACHE II: a severity of disease classification system. Crit Care Med, 1985. 13(10): p. 818-29.

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5.Vincent, J.L., et al., The SOFA (Sepsis-related Organ Failure Assessment) score to describe organ dysfunction/failure. On behalf of the Working Group on Sepsis-Related Problems of the European Society of Intensive Care Medicine. Intensive Care Med, 1996. 22(7): p. 707-10.

6.Klevmark, B., Natural pressure-volume curves and conventional cystometry. Scand J Urol Nephrol Suppl, 1999. 201: p. 1-4.

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