Abdominal compartment syndrome is a complication of torso trauma and abdominal catastrophe which frequently appears in CICM fellowship exam papers:
All requred a detailed knowledge of this condition. An international consensus group of critical care specialists got together in 2011 and published a mighty consensus statement, from which much of the below information is derived.
Much like the skull, the abdomen is a closed box, albeit with floppy flexible walls. The contents is predominantly non-compressible fluid. The pressure in this fluid increase with inspiration, and decreases with expiration.
A normal person typically has either a negative intra-abdominal pressure, or something in the range of 0-5mmHg. The morbidly obese and pregnant patient may suffer a chronic elevation of intra-abdominal pressure to as high as 10-15mmHg, but they compensate for it with various physiological mechanisms, and do not suffer organ underperfusion. In normal critically ill patients, this pressure is 5-7mmHg, and the upper limit of normal is 12mmHg.
The pressure of the blood which perfuses abdominal organs must compete with intra-abdominal pressure. Thus, abdominal perfusion pressure (APP) = MAP - IAP.
This parameter is important, because it can be used as a resuscitation endpoint. A target APP of over 60mmHg has been demonstrated to improve survival.
The abovementioned panel of experts have decreed that IAP is "measured at end-expiration in the complete supine position after ensuring that abdominal muscle contractions are absent and with the transducer zeroed at the level of the midaxillary line".
The contents of the abdomen is predominantly non-compressible fluid, and thus pressure in one part of the abdomen can be expected to represent the pressure in other parts of it. If you remove the constricting effects of the abdominal wall muscles by using a neuromuscular junction blocker, the pressure that remains represents the "true" abdominal compartment pressure.
We measure this by hooking a pressure transducer to the indwelling urinary catheter.
A transducer set is hooked up to a side-port of the IDC, and the bladder is emptied completely. The IDC is clamped. Then, 20ml of saline is introduced into the bladder. After giving the detrusor 60 seconds to relax, a reading is obtained from the transducer, in mmHg.
According to the abovementioned panel of experts, the definition of raised intra-abdominal pressure is a sustained or repeated pathological elevation in IAP≥12 mmHg.
According to the same panel, Abdominal Compartment Syndrome is deﬁned as a sustained IAP>20 mmHg that is associated with new organ dysfunction.
At an abdominal compartment pressure of over 35mmHg, capillary perfusion of the intestine becomes compromised, and bacterial translocation occurs, with predictably hideous consequences. Among those consequences is the continuing increase of intra-abdominal pressure.
This is an easy distinction. Primary abdominal compartment sydnrome originates form abdominal pathology; secondary ACS is due to some sort of extra-abdominal influence (for example, vigorous fluid resuscitation).
This paper offers a table of risk factors, which I have reproduced here in a substantially truncated form, in order to help the memory-deficient CICM exam candidate.
Primary abdominal compartment syndrome
Secondary abdominal compartment syndrome
These are numerous. I have attempted to arrange them in order of the ABCs of resuscitation.
There are a few major strategies: