Abdominal compartment syndrome

Abdominal compartment syndrome is a complication of torso trauma and abdominal catastrophe which frequently appears in CICM fellowship exam papers:

• Question 1 from the second paper of 2020
• Question 8 from the first paper of 2013
• Question 21 from the second paper of 2006
• Question 10 from the second paper of 2001

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.

Intra-abdominal pressure and abdominal perfusion pressure

Intra-abdominal pressure is the steady state pressure concealed within the abdominal cavity.

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.

Abdominal perfusion pressure is MAP minus the intra-abdominal pressure

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.

Measuring the intra-abdominal pressure

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.

Raised intra-abdominal pressure and the abdominal compartment syndrome

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.

Primary vs secondary abdominal compartment syndrome

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).

Risk factors and causes of abdominal compartment syndrome

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 Intra-abdominal infection Abdominal trauma Ascites Ileus Intra-abdominal hematoma Acute pancreatitis Laparoscopy with excessive inflation pressures Peritoneal dialysis

Secondary abdominal compartment syndrome Massive transfusion Massive fluid resuscitation Sepsis Mechanical ventilation Abdominal wall burns High body mass index (>30) Prone positioning

Consequences of abdominal compartment syndrome

These are numerous. I have attempted to arrange them in order of the ABCs of resuscitation.

Management of abdominal compartment syndrome

There are a few major strategies:

Prevention

• Control yourself when it comes to fluid resuscitation.
• Consider leaving the abdomen open, for staged closure.

Management

• Reopen the abdomen. Of course, sometimes you arent able to do that. In that case:
• Decompress the ascites
• Maintain an abdominal perfusion pressure of around 60mmHg
• Titrate PEEP to maintain recruitment
• Use neuromuscular blockade to remove the abdominal muscles from the equation