Intra-Abdominal Hypertension

Intra-abdominal hypertension is a sustained or repeated intra-abdominal pressure (IAP) of 12 mmHg or more. Abdominal compartment syndrome is a sustained IAP of over 20 mmHg, with or without an abdominal perfusion pressure under 60 mmHg, together with new organ dysfunction.

Epidemiology. Intra-abdominal hypertension occurs in about 30 to 50% of critically ill patients, and abdominal compartment syndrome in under 10%. Mortality in abdominal compartment syndrome is high (about 25 to 60%).

Grades. Grade I 12 to 15 mmHg, grade II 16 to 20, grade III 21 to 25 and grade IV over 25.

Causes and associations. Massive fluid resuscitation, major burns, abdominal trauma, damage control surgery with packing, severe pancreatitis, ileus, bowel obstruction, retroperitoneal or intraperitoneal haemorrhage, ascites, ruptured aortic aneurysm, peritonitis, obesity and tight abdominal closure.

Pathophysiology. Raised IAP reduces venous return, compresses the vena cava, lowers cardiac output, pushes the diaphragm up (reducing compliance, raising airway pressures and intrathoracic pressure), reduces renal perfusion and mesenteric flow, and raises intracranial pressure. Bowel ischaemia, hepatic dysfunction, acute kidney injury and lactic acidosis follow.

Presentation is a tense distended abdomen with oliguria, rising airway pressures, hypoxia and hypercapnia, hypotension needing fluids, and rising lactate. The abdominal examination is a poor guide, so measure pressure.

Red flags.

• Oliguria (under 0.5 mL/kg/h) despite fluids.
• Rising peak airway pressure, falling compliance or falling PF ratio.
• IAP over 20 mmHg, or abdominal perfusion pressure under 60 mmHg.

Measure IAP by the bladder: patient supine, end expiration, zero at the mid-axillary line, instil no more than 25 mL sterile saline into the bladder, and read in mmHg. Repeat every 4 to 6 hours in patients at risk. Calculate abdominal perfusion pressure as mean arterial pressure minus IAP, aiming for 60 mmHg or more. Measure lactate, creatinine, urine output, blood gas and airway pressures. CT shows cause.

Differential diagnosis. Bladder pressure artefacts (neuromuscular blockade absent, pelvic haematoma, neurogenic bladder), tension pneumothorax and cardiac tamponade.

Treat in stages, escalating if IAP stays over 12 mmHg.

1. Evacuate intraluminal contents. Nasogastric and rectal decompression, stopping enteral feed, prokinetics (metoclopramide 10 mg IV every 8 hours, erythromycin 250 mg IV every 8 hours, or neostigmine 2.5 mg IV over 3 to 5 minutes for colonic pseudo obstruction with ECG monitoring and avoidance in mechanical obstruction, bradycardia and asthma), and colonic decompression.

2. Evacuate fluid. Percutaneous drainage of ascites or collections.

3. Improve abdominal wall compliance. Adequate analgesia and sedation, short course of neuromuscular blockade (cisatracurium 0.15 mg/kg then 1 to 3 microgram/kg/min), head-of-bed elevation limited to 20 to 30 degrees, and removal of tight dressings.

4. Optimise fluids. Avoid positive balance. Use diuretics (furosemide 20 to 80 mg IV) or ultrafiltration once resuscitated, and use hypertonic fluid or colloid (albumin 20% 100 mL) as needed.

5. Maintain perfusion. Support blood pressure to an abdominal perfusion pressure of 60 mmHg or more with vasopressors, and avoid excess volume.

6. Decompressive laparotomy. For abdominal compartment syndrome refractory to medical measures: midline laparotomy with temporary abdominal closure (negative pressure dressing). Plan staged closure, ideally within 7 days.

Monitoring. IAP every 4 hours until under 12 mmHg for 24 hours, abdominal perfusion pressure, urine output, airway pressures and lactate.

Referral and escalation. Surgical and intensive care teams at once for IAP over 20 mmHg with organ dysfunction.

Special groups. In burns, escharotomy of the abdominal wall and fluid reduction. In pregnancy, normal IAP is higher, so use clinical context.