Physiology

• Contents of the abdomen are predominantly non-compressible fluid
• The pressure increases with inspiration and decreases with expiration
• A normal person has either a negative intra-abdominal pressure, or something in the range of 0-5 mmHg
  • The morbidly obese and pregnant patient may have pressures as high as 10-15 mmHg
• In normal critically ill patients the pressure is 5-7 mmHg and the upper limit of normal is 12 mmHg
• Abdominal perfusion pressure is given by:

• Abdominal pressures > 35 mmHg result in compromised capillary perfusion of the intestine and bacterial translocation
• A target abdominal perfusion pressure ≥ 60 mmHg has been demonstrated to improve survival

Measuring Intra-Abdominal Pressure

• The intra-abdominal pressure equals the bladder pressure when the following criteria are fulfilled:
  • End of expiration
  • Supine
  • Zeroed at the iliac crest in the mid-axillary line
  • Instilation of 25 mL of saline into the bladder (1 mL/kg for children up to 25 kg, minimum of 3 mL)
  • Measured 60 seconds after instillation to allow detrusor muscle relaxation
  • In absence of active abdominal muscle contraction
• Usually measured every 4 hours

• A transducer set is attached to a side-port of the IDC
• The bladder is emptied completely then clamped
• 20 mL of saline is introduced into the bladder and after giving the detrusor 60 seconds to relax, the reading in the transducer is the intra-abdominal pressure
• Caveats
  • Pressures may vary across individuals (with obese patients having higher baseline pressure values)
  • As with all physiological phenomena, pressure gradients are more important than absolute pressure values
    • Consequently, the intra-abdominal pressure value in isolation is inadequate to define abdominal compartment syndrome
  • Pressure measurements may be inaccurate in the context of pelvic pathology (e.g., hematoma directly compressing the bladder)
  • Interpretation among patients who are not intubated and breathing passively on mechanical ventilation is challenging
    • If analgesia/sedation causes normalization of the intra-abdominal pressure, this makes abdominal compartment syndrome less likely

Definitions

• Raised intra-abdominal pressure (intra-abdominal hypertension) is defined by a sustained or repeated pathological elevation in intra-abdominal pressure ≥ 12 mmHg
  • Normal = IAP 5-7 mmHg
  • Grade I = IAP 12–15 mmHg
  • Grade II = IAP 16–20 mmHg
  • Grade III = IAP 21–25 mmHg
  • Grade IV = IAP >25 mmHg
• Abdominal compartment syndrome is defined as a sustained intra-abdominal pressure > 20 mmHg and new organ dysfunction that is attributable to elevated intra-abdominal pressure
  • However, figuring out if organ failure is caused by abdominal compartment syndrome as opposed to other causes is subjective; the kidneys are quite sensitive to increased abdominal pressure (i.e. if urine output is adequate it’s unlikely the patient has abdominal compartment syndrome)
• Polycompartment syndrome is a condition where two or more anatomical compartments have elevated compartmental pressures

Risk Factors

• Decreased abdominal wall compliance
  • Acute respiratory failure with elevated intrathoracic pressure
  • Abdominal surgery with fascial closure
  • Major trauma/burns
  • Prone positioning
  • Head > 30°
  • High BMI/central obesity
• Increased intra-luminal contents
  • Gastroparesis
  • Ileus
  • Colonic pseudo-obstruction
• Increased abdominal contents
  • Haemo/pneumoperitoneum
  • Ascites
  • Liver dysfunction
• Capillary leak or fluid resuscitation
  • Acidosis (pH < 7.2)
  • Hypotension
  • Hypothermia (T < 33°C)
  • Massive transfusion (> 10 U in 24 hrs)
  • Coagulopathy
  • Massive fluid resuscitation
  • Pancreatitis
  • Sepsis
  • Oliguria
  • Damage control laparotomy

Aetiologies

Primary	Secondary
Intra-abdominal infection	Massive transfusion
Abdominal trauma	Massive fluid resuscitation
Ascites	Sepsis
Ileus	Mechanical ventilation
Intra-abdominal haematoma	Abdominal wall burns
Laparoscopy with excessive inflation pressures	High body mass index
Acute Pancreatitis	Prone positioning
Peritoneal dialysis
Dilated viscera (e.g. severe ileus, SBO, toxic megacolon)

Complications

• Respiratory
  • Decreased respiratory compliance ⇒ ↑ peak airway pressures → ↑ risk of atelactasis and pneumonia
  • Decreased respiratory compliance ⇒ ↓ tidal volume
• Cardiovascular
  • Decreased preload → decreased cardiac output
    • Peripheral vascular resistance is elevated due to renal compression resultining in maintained systolic blood pressure
• Gastrointestinal
  • Direct compression
  • Portal vein thrombosis
  • Decreased gastric emptying
  • Bowel wall ischaemia and oedema
    • Translocation of bacterial products
• Increased intracranial pressure
• Renal failure
• Ileus
• Lower limb venous thrombosis

Clinical Features

• Initial manifestations vary depending on patient’s underlying physiology, for example:
  • A patient with tenuous renal function could manifest with renal failure
  • A patient with severe, chronic COPD might present with respiratory distress
• Palpation is only ~50% sensitive for abdominal compartment syndrome (i.e. abdomen can still be soft)
  • However a tense abdomen is ~80% specific for abdominal compartment syndrome
• Abdominal pain and/or bloating are the commonly reported symptoms
• Other symptoms/signs are often a result of complications
  • Oliguric acute renal failure is generally one of the earliest manifestations of abdominal compartment syndrome
  • Increased work of breathing or increased airway pressures on the ventilator
  • Raised ICP can also cause delirium or coma (occurs due to elevations of central venous pressure)

Management

• Monitor intra-abdominal pressure when a patient has ≥2 risk factors or in the presence of new or progressive organ failure
• Prevention
  • Controlled fluid resuscitation
  • Consider leaving the abdomen open, for staged closure

Management

Haemodynamics

• Maintain a abdominal perfusion pressure > 60 mmHg with vasopressors (as additional crystalloids may merely aggravate abdominal tissue oedema)
  • Use a $\text{Target MAP}>60\text{mmHg}+P_{\text{intra-abdominal}}$ to achieve this
• In established abdominal compartment syndrome, the patient is often intravascularly volume-depleted (despite tissue oedema), making it difficult to remove fluids without worsening haemodynamics
  • Abdominal compartment syndrome may compress the inferior vena cava making it look empty
  • Avoid fluid administration as while this may help temporarily, fluid rapidly transudates into the tissues which worsens swelling and increases intra-abdominal pressure

Management

Decompress the Abdomen

• Evacuate Intra-Luminal Contents
  • Nasogastric decompression
  • Rectal decompression
  • Prokinetics
  • Minimise enteral nutrition
  • Enemas
• Evacuate Abdominal Fluid Collections (e.g. paracentesis, percutaneous drainage, surgical evaluation)
• Surgical decompression with delayed closure
  • On release be aware of hypotension (PEA arrest from reperfusion and a sudden decrease in systemic vascular resistance)
  • Following fascial release, patients may experience severe ischaemia-reperfusion event, requiring haemodynamic support
  • Post-operatively, patients will be left with an open abdomen which may be closed after 1-2 weeks following the resolution of the primary process

Management

Decompress the Thorax

• Reduction of PEEP and plateau pressure on the ventilator may decrease intra-abdominal pressure
• Drainage of a large pleural effusion if present
• Intubation converts the thorax from a negative pressure system to a positive pressure system which may increase abdominal pressures, precipitating multi-organ failure with haemodynamic collapse

Management

Sedation and Paralysis

• Sedation and paralysis may relax the muscles of the abdominal wall and improve abdominal compliance thereby reducing abdominal pressure
  • However, in patients who are spontaneously breathing, their work of breathing decreases the intrathoracic pressures; eliminating this will transition to purely, positive pressure ventilation increasing their intrathoracic pressure and increase the intra-abdominal pressure

Management

Other Measures

• Avoid head elevation > 30 degrees
• Remove constrictive dressing
• Eschars

Sources

• Abdominal Compartment Syndrome • LITFL • CCC
• Intra-Abdominal Pressure Monitor • LITFL • CCC Equipment
• Abdominal compartment syndrome - EMCrit Project
• Abdominal Compartment Syndrome - Deranged Physiology