Abdominal compliance, linearity between abdominal pressure and ascitic fluid volume.

BACKGROUND: Drainage of ascitic fluid is a common practice in order to relief the respiratory discomfort of patients. AIM: To determine the relation between the intra-abdominal pressure (IAP) and extracted volume of the ascitic fluid, in order to calculate abdominal compliance (Cabd). SETTINGS AND
DESIGN: A study was designed at AHEPA University Hospital and analysed with prospectively collected data.
MATERIALS AND METHODS: Fifteen patients with tension ascites that had transcutaneous drainage with a wide catheter. The ascitic fluid removed was measured, while the IAP and a Visual Analogue Scale (VAS) score for dyspnea were recorded before and 15 min after the puncture. Cabd was calculated. STATISTICAL ANALYSIS: The data were analysed with descriptive statistics, paired Student's t-test and Pearson coefficiency.
RESULTS: The predrainage IAP was 18.26 mmHg (SD 1.67 mmHg), while the postdrainage was 14.46 mmHg (SD 1.34 mmHg) (P<0.001). The mean volume of ascitic fluid removed was 1624 mL (SD 861 mL). Cabd after drainage was 414.01 mL/mmHg (SD 139.15 mL/mmHg). A linear correlation was found between ascitic fluid removal and IAP variations. The dyspnea VAS score was 7.5 (SD=0.8) before the drainage and 4.3 (SD=1.0) after the drainage (P<0.001).
CONCLUSIONS: The drainage of ascitic fluid reduces IAP, facilitating in this way respiration. Moreover, IAP variation seems to be in linear relation with the volume of ascitic fluid removed. This linear relation between IAP and volume may probably predict the Cabd quite accurately and vice versa. However, larger studies are necessary to safely draw predicting ΔIAP - ΔV (Cabd) diagrams, and determine the optimal ascitic fluid removal to achieve best comforting of the patient and slower fluid reformation.

INTRODUCTION

Intra-abdominal pressure (IAP) is the pressure maintained in the abdominal cavity. The easiest and therefore widely used method for measuring IAP is the transvesical.[1-3] In healthy subjects calm and at supine position, IAP ranges from 1 to 6.5 mmHg.[2-4]

Abdominal paracentesis results in the removal of fluid from the peritoneal cavity. Total volume replacement has been shown to be an effective, safe, and widely used treatment for the management of tense ascites. Abdominal paracentesis is used to help diagnose the etiology and/or as a therapeutic measure whereby a large volume of ascitic fluid is removed (therapeutic paracentesis). The investigation and management of patients with ascites have been regularly reviewed in the literature.[5-8] However, no studies are available determining the real variations of abdominal compliance (Cabd) following paracentesis and abdominal cavity evacuation. Indirect evidence of Cabd variation following paracentesis exists by thoracic compliance variation.

The aim of this study was to determine the relation between the IAP and extracted volume of the ascitic fluid, in order to further determine Cabd in patients with tension ascites. We additionally attempted to explain the behavior of the abdominal cavity in order to adapt to the chronically increased IAP values.

MATERIALS AND METHODS

This prospective study lasted from January 1, 2009 to June 30, 2009. Fifteen patients with malignant tension ascites with normal renal function that were treated by transcutaneous drainage were included. The patients had no further radiation or chemotherapeutic treatment. The patients had no fever, abdominal pain, or encephalopathy. The mean age of the patients was 52.73 years (range, 39–65 years, SD=7.40 years). There were 11 males and 4 females with a Body Mass Index averaged 28.71 kg/m2 (range, 20.1–44.0 kg/m2, SD=6.38 kg/m2). All patients had no previous paracentesis and only their first drainage was included in this study. Only patients with short life expectancy were considered for this study. “Short life expectancy” was defined as patients who had a terminal illness (peritoneal carcinosis), refractory ascites and were expected to live less than 6 months. Four patients suffered from ovarian, four from pancreatic, six from colonic, and one from prostatic malignancy.

Abdominal paracentesis (transcutaneous drainage) technique

In order both to measure IAP and to empty the bladder, a Foley catheter was inserted into the urinary bladder, using a standard sterile technique, before the transcutaneous drainage of the ascitic fluid. The patient was positioned in the bed with the head elevated at 45°. This allows fluid to accumulate in lower abdomen. The point of aspiration was identified in the midline midway between the umbilicus and pubic bone. Under sterile conditions, 10 mL of local anesthesia was applied (Lidocaine, 20 mg/mL). An 18 gauge needle was slowly introduced into the abdominal cavity at a slightly oblique angle to the skin after pulling the skin down slightly. Then, the needle was attached to the tubing and stopcock. At the end of the drainage and the measurement, the needle was removed and an adhesive bandage was placed over the site to avoid leak. During the procedure, vitals were closely monitored.

Intra-abdominal pressure measures

The bladder was filled with 20 mL of sterile saline using a closed-system technique.[9-11] Hydrostatic pressure in the bladder was obtained by connecting the catheter to a pressure transducer through sterile tubing. Pressure measurements were made in cmH2O, and then converted into mmHg. The zero reference point for the measurements was set at the symphyse pubis. The median value of three separate measurements was recorded with the patient relaxed in the supine position. The measurements were taken before the drainage and 15 min after the drainage. The volume of the ascitic fluid removed was recorded.

Abdominal wall compliance (Cabd) was calculated using the formula: Cabd = ΔIntra-abdominal volume (ΔIAV)/ΔIAP.[12] The above formula was transformed to Cabd = -volume of fluid drained/postdrainage IAP - baseline IAP.[13]

Dyspnea was evaluated using a 10-grade VAS score (0 = no dyspnea, 10 = impossible to breath) before and 15 min after the drainage.

Data were analyzed using standard statistical methods. Descriptive statistics including means, ranges, and standard deviations were used to describe the IAP measurement, and the volumes of ascitic fluid removed for each subject. Paired Student's t-test was employed in comparing the pre- and postdrainage values, while Pearson coefficiency was calculated for IAP and the volume removed.

RESULTS

In order to determine the relation between IAP and drained intra-abdominal ascitic fluid, we have to separately consider the two parameters and then correlate them. Concerning the volume variation (fluid extracted), the mean volume of ascitic fluid removed was 1624 mL (ranging from 430 to 3200 mL, SD 861 mL). In order to homogenize the volumes removed by paracentesis, drainage was stopped when no spontaneous fluid flow was possible after a Valsalva maneuver. No aspiration of the fluid was employed at any moment, permitting in that way to have spontaneous fluid flow.

The second factor entering into the abdominal compliance calculation is IAP variation. This factor requires the measurement of both pre- and postdrainage IAP. The predrainage IAP was 18.26 mmHg (ranging from 16.18 to 20.59 mmHg, SD 1.67 mmHg), while the postdrainage was 14.46 mmHg (ranging from 12.50 to 16.18 mmHg, SD 1.34 mmHg). The mean ΔIAP was -3.80 mmHg (range, -1.47 to -6.62 mmHg, SD 1.29 mmHg). Paired Student's t-test demonstrated a statistically significant change of the IAP after the drainage (P<0.001), indicating that ascitic fluid paracentesis helps in IAP reduction in a statistically significant way.

After measuring both the volume of the drained ascitic fluid and the various IAPs, and calculating ΔIAP, abdominal wall compliance was calculated following the transformed formula as described by Papavramidis et al.[13] Cabd after drainage was 414.01 mL/mmHg (ranging from 167 to 670 mL/mmHg, SD 139.15 mL/mmHg). A linear correlation was found between ascitic fluid removed (in mL) and IAP variations, with a Pearson coefficient of r=0.7925. The linear relation between IAPs and the volumes of the ascitic fluid removed is displayed in Figure 1.

Figure 1

The relation between IAP variation and volume of the ascitic fluid removed

On a clinical basis, abdominal paracentesis for removal of ascitic fluid under tension aims to ameliorate the dyspnea sensed by the patient. Indeed, a statistical significant amelioration of the dyspnea was observed after the drainage (P<0.001). The dyspnea VAS score for the studied group was 7.5 (SD=0.8) before the drainage and 4.3 (SD=1.0) after the drainage, clearly indicating clinical amelioration of the patient.

DISCUSSION

Intra-abdominal pressure varies normally both in an acute and in a chronic manner. Concerning the acute IAP variations in healthy humans, it follows mainly the body position[14] and the patient's activity.[15-17] As far as chronic IAP changes is concerned, we know that in otherwise healthy subjects IAP is constantly increased in obesity,[1819] in visceromegaly,[20] and in pregnancy.[18] Additionally, IAP is chronically increased in various pathologic conditions such as ascites,[2122] large cysts (parasitic or not),[132324] and large neoplasmatic formations.[25] We believe that in all conditions altering in a chronic basis IAP there is a dynamic adaptive procedure aiming to alter the constitutional properties of the abdominal cavity in order to let the organism remain as close as possible to the normal functioning. In order to verify this hypothesis, we determined the relation between the IAP and extracted volume of the ascitic fluid. The slope of this ΔIAP versus extracted volume curve represents the abdominal compliance which is influenced by the constitutional properties of the abdominal cavity.

The immediate postdrainage IAP values were statistically significantly lower in comparison to the predrainage ones, due to the decompression of the abdominal cavity achieved by removing the ascitic fluid. A similar phenomenon was also observed by our team concerning IAP alterations after draining the fluid from large pancreatic pseudocysts.[13] Similarly to our previous study, the explanation of the reduction of IAP is given following the Pascal principle. The decrease of the IAP is due to alteration of the physical characteristics of the abdominal cavity. The pressure (IAP in this case) is initially related to the total volume of the intra-abdominal content (VIAC).VIAC is the result of the sum of the volume of the intra-abdominal structures and the volume of the free ascitic fluid. When ascitic fluid is drained then VIAC is reduced. We can easily understand that when VIAC decreases then the pressure is reduced and therefore the IAP measured decreases. Of course, the decrease of the IAP after drainage is a well-known phenomenon,[2627] but what is new here is that the immediate decrease of the IAP seems to be due to physical/constitutional characteristics of the abdominal cavity.

The difference between pre- and postdrainage IAP is in relation to the elastic/constitutional properties of the abdominal cavity. In this study, higher values of Cabd were observed in comparison to our previous studies[1328] that included patients drained for large volumes due to pancreatic pseudocysts. The important difference, however, resides in the fact that pancreatic pseudocysts were drained within 6–8 weeks after the onset of the disease, while ascites was drained several months after the onset of the disease. To our opinion, this time-related difference between the two groups is the key point in explaining the differences observed in Cabd. In detail, the abdominal cavity's walls are composed the bones and the muscles. The bones do not have elastic abilities while the muscles do. The Cabd alterations are mainly due to muscular adaptations. Indeed, several studies in animals in the past showed that muscular components of the abdominal cavity, as well as the diaphragm, adapt when subjected to conditions increasing IAP.[29-34] Additionally, all the above studies[29-34] tend to state that the muscular components of the abdominal cavity require at least 6 weeks in order to adapt. Taking all these into consideration, we could formulate that the Cabd alterations induced by chronically increased IAP due to ascitic fluid collection are mainly resulting from muscular adaptation of the abdominal cavity's muscles (abdominal wall, diaphragm, pelvic, and lumbar muscles).

Another point that it is of interest is the fact that it seems that the equation describing the relation between ΔIAP and the extracted volume is linear. Normally, when describing the elastic properties of a spring by using the Hooke's law of elasticity, we can observe two sections on the stress–strain curve. The first section is linear and corresponds to the elastic properties of the spring, while the second section (beyond the yield point) is variable and represents the plastical deformity of the material. We believe that in our series we have only a part of the curve representing the Cabd of the abdominal cavity (inverse of elasticity), because to our opinion the yield point of the abdominal cavity is not compatible with life.

In acute and subacute conditions (lasting less than 6 weeks), the increased IAP is a very good parameter to consider to evaluate the force exercised by abdominal contents to the thoracic cavity, influencing in that way respiration. However, more chronic conditions (lasting more than 6 weeks) that introduce chronically increased IAP measuring this parameter leads to dead-ends. This is actually the reason why IAP has to be co-evaluated with the existence or not of organ dysfunction prior to decompression when intra-abdominal hypertension exists.[9] Abdominal compliance seems to be a better parameter to evaluating the force exercised by the abdominal organs in these situations. However, this parameter is very difficult to obtain, when percutaneous decompression is not performed. We believe that accurate determination of the linear relation between IAP and additional volume included in the abdomen (either this is fluid (ascites, abscess, or blood), edema, or neoplastic tissue) may probably predict the Cabd quite accurately and vice versa. Concerning ascites, large studies are necessaries in order to safely draw predicting ΔIAP ΔV (Cabd) diagrams, and determine the optimal ascitic fluid removal in order to achieve best comforting of the patient and slower fluid reformation. The recent development and validation of an experimental model may help in this direction.[35]

This study was able to demonstrate the linear relation between IAP and the volume of the extracted ascitic fluid. Cabd was determined by the use of those two parameters and was found to be significantly higher than in other subjects. Tension ascites increases IAP in a chronic basis, which in its turn induces constitutional changes to the abdominal cavity's characteristics.

CONCLUSIONS

The results of this prospective study show that chronic Cabd changes induced by the IAP seem to be of prime importance after drainage, facilitating the respiratory function. The linear relation between IAP and volume may probably predict the Cabd quite accurately and vice versa. However, larger studies are necessary to safely draw predicting ΔIAP – ΔV (Cabd) diagrams, and determine the optimal ascitic fluid removal to achieve best comforting of the patient and slower fluid reformation.