The abdomen can be defined as a closed space or compartment enclosed by the spine, pelvis, diaphragm and abdominal wall. The elasticity of the walls and character of the contents of the abdomen, determine the pressure within it at a given time. The Intra-abdominal pressure is defined as the steady state pressure concealed within the abdominal cavity[1].
Abdominal pressure therefore varies depending on the physiological status of the patient. Abdominal pressure increases with inspiration, use of abdominal muscles, and increasing volume of fluid (eg. ascites, blood) in the abdominal space. It also increases with visceral expansion or collection within the viscera whether that is air, fluid or faeces. Intra-abdominal pressure is also affected by conditions which limit abdominal expansion such as 3rd space oedema or burn eschars.
Normal intra abdominal pressure may range from subatmospheric to 0mmHg. In the critically ill, IAP is frequently elevated above normal. In critically ill adults normal IAP is defined as 5-7mmHg [1].
In one group of children where IAP was measured directly via peritoneal dialysis catheter post cardiac surgery, median IAP was found to be 4mmHg with a range of 1-8mmHg [3].