How is intra-abdominal pressure calculated?

Measurement of Intra-Abdominal Pressure

The standard method for measuring intra-abdominal pressure (IAP) is via the trans-bladder technique using a maximum of 25 mL of sterile saline instilled into the bladder, measured at end-expiration with the patient in the supine position, ensuring no abdominal muscle contractions, and with the transducer zeroed at the level of the midaxillary line. 1

Proper Technique for IAP Measurement

Patient Positioning and Preparation

• Position patient in complete supine position
• Ensure absence of abdominal muscle contractions (may require adequate sedation)
• Zero the pressure transducer at the level where the mid-axillary line crosses the iliac crest 1, 2
• Mark this reference point on the patient to increase reproducibility 2

Bladder Measurement Technique

1. Connect urinary catheter to the measurement system
2. Empty the bladder completely
3. Instill sterile saline into the bladder:
   • Adults: Maximum 25 mL 1, 3
   • Children: 1 mL/kg with minimum 3 mL and maximum 25 mL 1, 4
4. Measure pressure at end-expiration
5. Record IAP in mmHg

Important Technical Considerations

• Smaller instillation volumes (≤25 mL) are preferred as larger volumes can cause overestimation of IAP 5
• Even 25 mL may cause slight overestimation compared to zero volume measurements 5
• Measurement should be taken when the patient is relaxed without abdominal muscle activity 1, 2
• Ensure the measurement system is properly flushed to remove air bubbles 2

Interpretation of IAP Values

Normal and Abnormal Values

• Normal IAP in adults: 5-7 mmHg 1, 3
• Normal IAP in children: 4-10 mmHg 1, 4
• Intra-abdominal hypertension (IAH) grading in adults 1, 3:
  • Grade I: 12-15 mmHg
  • Grade II: 16-20 mmHg
  • Grade III: 21-25 mmHg
  • Grade IV: >25 mmHg
• IAH in children: >10 mmHg 1, 4
• Abdominal compartment syndrome (ACS): IAP ≥20 mmHg with new organ dysfunction 1, 3

Clinical Application

When to Measure IAP

• Measure IAP when any known risk factor for IAH/ACS is present in critically ill patients 1
• Risk factors include 1, 3:
  • Diminished abdominal wall compliance (abdominal surgery, major trauma, burns)
  • Increased intra-luminal contents (gastroparesis, ileus)
  • Increased intra-abdominal contents (hemoperitoneum, ascites, intra-abdominal infection)
  • Capillary leak/fluid resuscitation (massive fluid resuscitation, acidosis)

Protocol Implementation

• Develop standardized protocols for IAP measurement in each ICU 2
• Use protocolized monitoring and management of IAP 1
• Consider continuous IAP monitoring in high-risk situations 2

Common Pitfalls and How to Avoid Them

• Excessive instillation volume: Use no more than 25 mL in adults and appropriate weight-based volumes in children 1, 5
• Improper patient positioning: Ensure complete supine position 1, 6
• Abdominal muscle contractions: Provide adequate sedation if needed 1, 6
• Improper transducer level: Zero at the level of mid-axillary line 1, 2
• Air bubbles in the system: Properly flush the measurement system 2
• Measurement during inspiration: Always measure at end-expiration 1, 6

The trans-bladder technique remains the gold standard for IAP measurement due to its simplicity, low cost, and reliability 1, 2. When properly performed, it provides accurate measurements that are essential for diagnosing and managing IAH and ACS in critically ill patients.