FRCA Notes

Abdominal Compartment Syndrome

Intra-abdominal hypertension and abdominal compartment syndrome form part of the FFICM curriculum, though they're absent from the FRCA curricula.

The time-poor candidate may deem this topic expendable; there aren't any recent editorials in the BJA on the topic.

Resources

  • Intra-abdominal pressure (IAP) is the steady-state pressure concealed within the abdominal cavity
  • It is typically negative, or certainly <5mmHg
  • It may be higher in the morbidly obese and pregnant patients, but they have physiological adaptations to prevent organ ischaemia

Measuring

  • Direct measurement of IAP is performed using an intraperitoneal surgical technique

  • Indirect measurement is much more common, typically by measuring intra-vesical pressure
    • Intra-gastric, intra-uterine or rectal routes can also be used
    • Readings should take place at end-expiration

Abdominal perfusion pressure

  • In a similar to fashion to other organ perfusion:

    • Abdominal perfusion pressure = Mean arterial pressure - intra-abdominal pressure

  • Abdominal perfusion pressure is an important parameter; a pressure >60mmHg is a resuscitation endpoint

  • The definition of intra-abdominal hypertension is a sustained or repeated measurement of IAP >12mmHg

  • Abdominal compartment syndrome is defined by an intra-abdominal pressure consistently >20mmHg + new organ failure

Grading of intra-abdominal hypertension

Grade IAP (mmHg) Management
1 10-15 Monitor IAP, conservative
2 16-25 Consider early decompression
3 26-35 Surgical decompression
Organ dysfunction once IAP >25mmHg
4 >35 Repeat decompression

Primary intra-abdominal hypertension

  • Arises due to increased intra-abdominal volume:

  • Blood (haemorrhage) e.g. trauma, ruptured AAA
  • Fluid
    • E.g. ascites, abscess, peritoneal dialysis
    • Capillary leak e.g. pancreatitis, sepsis, massive transfusion
  • Gas
    • Bowel dilation e.g. ileus, bowel obstruction
    • Surgical: laparoscopy with high pressures

Secondary intra-abdominal hypertension

  • Arises due to decreased abdominal wall compliance:

  • Prone positioning
  • Morbid obesity
  • Burns
  • PPV

Respiratory

  • Increased intra-thoracic pressure causes atelectasis and reduced FRC
  • Decreased pulmonary and chest wall compliance

  • Net effects are:
    • Increased V/Q mismatch and enlarged A-a gradient
    • Patients are prone to hypoxia
    • Requirements for high ventilatory pressures and high PEEP

Cardiac

  • IVC obstruction causes reduced venous return
  • Compounded by hypovolaemia and increased intra-thoracic pressure
  • Compensatory increases in SVR
  • Elevation of the diaphragm lowers PVR and reduces ventricular compliance

  • Net effects are:
    • Increased blood pressure due to increased SVR
    • Falsely high CVP as raised IAP is transmitted to central vessels (true CVP = CVP - IAP)
    • Increased risk of lower limb DVT due to venous stasis

Neurological

  • Reciprocal increases in ICP

Renal

  • Renal blood flow and GFR decrease - associated oligura
  • RAAS activation

Gastrointestinal

  • Compression and congestion of mesenteric vasculature results in reduced oxygen delivery and mesenteric ischaemia
  • Histamine and serotonin release from ischaemic tissues causes oedema, further increasing IAP
  • Impaired hepatic blood flow reduces lactate clearance
  • Bacterial translocation can occur

Decrease abdominal volume

  • Nasogastric drainage
  • Prokinetics
  • Enemas and bowel management systems
  • Drain intra-peritoneal abscesses/collections

Improve abdominal wall compliance

  • Adequate analgesia
  • Positioning e.g. avoid pronation
  • Remove restrictive dressings
  • Neuromuscular blockade

Optimise haemodynamics and organ perfusion

  • Aim for abdominal perfusion pressure >60mmHg
  • Goal-directed fluid therapy inc. colloid
  • Vasopressors as required

Surgical decompression

  • Improves morbidity and mortality, especially if performed early
  • Reduces mortality to 20% if decompression occurs before organ dysfunction develops
  • Warranted if IAP >25mmHg or APP <50mmHg despite optimal management

  • During a laparotomy for abdominal compartment syndrome there are a number of pertinent physiological changes:
    • Sudden increase in lung compliance after IAP released may cause volutrauma if pressure-controlled ventilation
    • Sudden reduction in SVR as IAP released may cause hypotension if suitable fluids/vasoactive drugs aren't available
    • Reperfusion injury with increased free-radicals, lactate and potassium
      • May lead to myocardial depression/arrhythmia/vasodilation or even cardiac arrest
    • Prone to hypothermia, especially if abdomen left open
    • Prone to high insensible fluid losses with open abdomen
    • Need ongoing monitoring in case of recurrence of intra-abdominal hypertension