Once the abdominal wall can no longer expand, any further fluid leaking into the tissue results in fairly rapid rises in the pressure within the closed space. Initially this increase in pressure does not cause organ failure but does prevent organs from working properly – this is called intra-abdominal hypertension and is defined as a pressure over 12 mmHg in adults. ACS is defined by a sustained IAP(intra-abdominal pressure) above 20 mmHg with new-onset or progressive organ failure. Severe organ dysfunctionent syndrome. These pressure measurements are relative. Small children get into trouble and develop compartment syndromes at much lower pressures while young previously healthy athletic individuals may tolerate an abdominal pressure of 20 mmHg very well. The underlying cause of the disease process is capillary permeability caused by the systemic inflammatory response syndrome (SIRS) that occurs in every critically ill patient. SIRS leads to leakage of fluid out of the capillary beds into the interstitial space in the entire body with a profound amount of this fluid leaking into the gut wall, mesentery and retroperitoneal tissue. Abdominal compartment syndrome follows a destructive pathway similar to compartment syndrome of the extremities. When increased compression occurs in such a hollow space, organs will begin to collapse under the pressure. As the pressure increases and reaches a point where the abdomen can no longer be distended it starts to affect the cardiovascular and pulmonary systems. When abdominal compartment syndrome reaches this point without surgery and help of a silo the patient will most likely die.
Abdominal compartment syndrome (ACS) occurs when the abdomen becomes subject to increased pressure reaching past the point of intra-abdominal hypertension (IAH). ACS is present when intra-abdominal pressure rises and is sustained at > 20 mmHg and there is new organ dysfunction or failure. ACS is classified into three groups: Primary, secondary and recurrent ACS. It is not a disease and as such it occurs in conjunction with many disease processes, either due to the primary illness or in association with treatment interventions. Specific cause of abdominal compartment syndrome is not known, although some causes can be sepsis and severe abdominal trauma. Increasing pressure reduces blood flow to abdominal organs and impairs pulmonary, cardiovascular, renal, and gastro-intestinal (GI) function, causing obstructive shock, multiple organ dysfunction syndrome and death. Causes
Peritoneal tissue edema secondary to diffuse peritonitis, abdominal trauma
Fluid therapy due to massive volume resuscitation
Retroperitoneal hematoma secondary to trauma and aortic rupture
Peritoneal trauma secondary to emergency abdominal operations
Reperfusion injury following bowel ischemia due to any cause
Retroperitoneal and mesenteric inflammatory edema secondary to acute pancreatitis
Ileus and bowel obstruction
Intra-abdominal masses of any cause
Abdominal packing for control of bleeding
Closure of the abdomen under undue tension
Ascites (intra-abdominal fluid accumulation)
Acute pancreatitis with abscesses formation

Pathophysiology
Abdominal compartment syndrome occurs when tissue fluid within the peritoneal and retroperitoneal space (either edema, retroperitoneal blood or free fluid in the abdomen) accumulates in such large volumes that the abdominal wall compliance threshold is crossed and the abdomen can no longer stretch.