• FUNDAMENTALS
  • PATHOLOGY & AETIOLOGY
  • DIAGNOSIS & INVESTIGATIONS
  • MANAGEMENT

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Introduction & Definition

What is normal intra-abdominal pressure (IAP)?

 

  • IAP is approximately 5–7 mmHg in critically ill adults
  • It is not static and varies with respiration

What is Intra-abdominal hypertension (IAH) and how is it graded?

IAH is defined by the World Society of Abdominal Compartment Syndrome as:

A sustained or repeated pathological elevation in IAP >12 mmHg

It is graded according according to pressure:

Grade I

Grade II

Grade III

Grade IV

12-15 mmHg
16-20 mmHg
21-25 mmHg
>25 mmHg

What is abdominal compartment syndrome?

ACS is defined by the World Society of Abdominal Compartment Syndrome as:

A sustained IAP >20 mmHg that is associated with new organ dysfunction/failure

Epidemiology, Clinical Course & Prognosis

How common are IAH and ACS?

  • Commonly seen in ventilated critically ill patients
  • A large prospective study of at-risk critical care patients found an incidence of:
    • 33% of patients developing intrabdominal hypertension
    • 3.6% of patients developing abdominal compartment syndrome

What is the course and prognosis of abdominal compartment syndrome?

  • Severity of organ failure related to the duration of intra-abdominal hypertension
  • Abdominal compartment syndrome carries a poor prognosis:
    • Without treatment the mortality is 100%
    • Studies have shown mortality of 35-50% despite treatment

Aetiology

What are primary and secondary ACS?

Primary ACS or IAH

Secondary ACS or IAH

  • Due to a condition associated with injury or disease in the abdominopelvic region
  • Frequently requires early surgical or interventional radiological intervention
  • Due to a conditions that do not originate from the abdominopelvic region

What are the risk factors for and causes of abdominal compartment syndrome?

Primary IAH/ACS

Decreased Abdominal Wall Compliance

  • Abdominal surgery
  • Prone positioning
  • Major trauma
  • Major burns

Increased Intra-luminal Contents

  • Gastroparesis / gastric distension
  • Ileus / colonic pseudo-obstruction
  • Volvulus

Increased Intra-luminal Contents

  • Acute pancreatitis
  • Haemoperitoneum / pneumoperitoneum / intrabdominal fluid collection
  • Intra-abdominal infection / abscess
  • Liver dysfunction with ascites
  • Intra-abdominal /retroperitoneal tumour
  • Peritoneal dialysis
Secondary IAH/ACS

Capillary Leak / Fluid Resuscitation

  • Sepsis
  • Acidosis
  • Hypothermia
  • Increased APACHE score
  • Massive fluid resuscitation
  • Polytransfusion
  • Major trauma / burns

Capillary Leak / Fluid Resuscitation

  • Obesity
  • High PEEP
  • Pneumonia
  • Coagulopathy

Pathophysiology

How does raised intra-abdominal pressure affect other organ systems in abdominal compartment syndrome?

Respiratory

Cardiovascular

Neurological

Renal

Gastrointestinal

Effects

  • Diaphragmatic splinting and extrinsic compression of lung tissue
  • Leads to:
    • Reduced compliance and increased airway pressures
    • Increased ventilation/perfusion (V/Q) mismatch
    • Basal atelectasis and collapse, hypoxemia and hypercapnia
  • Cardiac output reduced due to:
    • Decreased venous return due to venous compression
    • Increased afterload due to aortic compression
  • Increased intra-thoracic pressure due to diaphragmatic splinting may compromise CO further:
    • Decreased ventricular compliance and contractility
  • Raised ICP due to:
    • Impaired CSF absorption in the lumbar plexus
    • Impaired jugular venous return
  • Increased further due to cerebral vasodilatation caused by concomitant hypoxaemia and hypercapnia
  • Renal failure due to:
    • Reduced renal blood flow
    • Increased pressure within the tubules and reducing the filtration gradient
  • Compensatory activation of the renin-angiotensin-aldosterone (RAA) worsening the renal insult
  • Bowel wall venous obstruction and hypertension due to compression effect resulting in oedema and further reduced compliance
  • Bowel ischaemia and bacterial translocation increases the risk of sepsis
  • Reduced hepatic artery, vein and portal system flow leading to liver dysfunction
  • Biliary stasis due to increased pressure within the biliary tree

Abdominal Pressure Measurement

Who should have intrabdominal pressures (IAP) measured?

  • The WSACS has recommended that all critically ill patients with any risk factor for the development of IAH/ACS should have IAP measured
  • If pressures are elevated, serial measurements should be performed every 4-6 hours

     

Which methods can be used to measure intrabdominal pressure?

Direct

Pressure measured from the peritoneum:

  • At laparoscopy
  • Peritoneal pressure transducer
  • Peritoneal drain
Indirect

Pressure measured via:

  • Bladder (Reference Standard):
    • Foley catheter pressure transducer (Modified Kron method) - Most common
    • Foley catheter manometer (Harrahill method)
    • T-Doc air charged catheter
  • Stomach
    • Nasogastric tube pressure transducer
    • GastroManometer
    • CiMon
  • Rectum
  • Vagina
  • Inferior Vena Cava

Which method is recommended for measuring intrabdominal pressure?

  • It is recommended that intravesicular pressure is measured via foley catheter
  • The ‘Modified Kron’ method is the most popular method due to its simplicity and low cost:

  • Wash hands and follow universal antiseptic precautions
  • Insert a foley catheter and connect a urinary drainage system
  • Using a sterile field and gloves, the drainage tubing is cut (with sterile scissors) 40 cm after the culture aspiration port after disinfection.
  • Set up a pressure transducer set:
    • Connect to a bag of 500 mL of normal saline and ensure system is flushed
    • Connect a 20ml syringe to the 3 way tap
    • Select a scale from 0 to 20 or 40 mm Hg on the monitor
  • Patient should in the supine position for measurement:
  • If not clinically feasible:
    • Recognize head elevation will result in a higher pressure
    • Ensure all subsequent readings are taken in the same position.
  • Adjust the height of the transducers and ensure it is zeroed level with the mid-axillary line
  • Clamp the drainage tube to the urine bag
  • Connect the needle to the rigid tubing of the pressure transducer
  • Insert the needle into the sampling port of the catheter
  • Fill the bladder with 1ml/kg (maximum 25mls) of 0.9% sodium chloride using the syringe
  • Close the stopcock of the syringe and allow 30 seconds for equilibrium to occur
  • Obtain the mean pressure reading upon end-expiration to minimize the effects of pulmonary pressures
  • Fluctuations in the pressure waveform should be seen with pulsations in abdominal blood flow.

Management Summary

How do you manage the patient with abdominal compartment syndrome (ACS)?

Key Principles

  • Resuscitation and management of underlying condition
  • Treatments to improve wall compliance and evacuate intra-abdominal contents
  • Optimise tissue perfusion
Initial Resuscitation & Supportive Care
  • ABCDE approach:
    • Intubation and ventilation if respiratory distress
    • Optimize ventilatory support
    • Fluid and vasopressor resuscitation if compromised hemodynamics
  • Treat the underlying cause
Specific Management

1. Improve abdominal wall compliance:

  • Adequate sedation and analgesia
  • Ensure no external constriction e.g. dressings/eschars
  • Appropriate positioning:
    • Avoid proning/head up>20°
    • Consider reverse Trendelenburg positioning
    • Neuromuscular blockade

2. Treatments to evacuate intra-luminal content:

  • Nasogastric/rectal decompression via aspiration/free drainage of nasogastric/rectal tubes;
  • Administration of gastric/colonic prokinetics - caution after surgery
  • Reduce enteral nutrition volume
  • Enemas
  • Colonoscopic decompression

3. Treatments to identify and evacuate intra-abdominal collections:

  • Abdominal imaging
  • Drainage:
    • Percutaneous drainage or paracentesis
    • Surgical evacuation

4. Treatments to optimize fluid balance:

  • Optimal, not excessive, fluid resuscitation
  • Hypertonic solutions and colloids; diuretics to drive negative fluid balance if haemodynamically stable
  • Renal replacement therapy

5. Treatments to optimize tissue perfusion to maintain an abdominal perfusion pressure:

  • Use goal directed fluid resuscitation
  • Vasoactive drugs

If refractory to medical management consider surgical management with abdominal decompression

Referral, Monitoring & Deposition
  • Monitor IAP every 4-6 hours if elevated
  • The WSACS approach an algorithm with a 4 step approach for each treatment arm
  • Treatment arms should be addressed simultaneously and tailored to the individual patient

Supportive Care

What is the abdominal perfusion pressure (APP)?

  • APP abdominal perfusion pressure (APP) is the blood pressure perfusing abdominal viscera
  • It can be considered the abdominal analogue to cerebral perfusion pressure
  • It is calculated using the formula:

Abdominal Perfusion Pressure = Mean Arterial Pressure – Intra-abdominal Pressure

What APP should be targeted during resuscitation in ACS?

  • Previously, the WSACS recommended that the APP be maintained above 60mmHg in an attempt to provide adequate visceral perfusion
  • Still widely practised but recommendation no longer given due to lack of evidence

Author

The Guidewire
Trainee in ICM & Anaesthesia

Reviewer

The Guidewire
Trainee in ICM & Anaesthesia