How to correct plateau pressure in the presence of abdominal hypertension?

Correction of Plateau Pressure in the Presence of Abdominal Hypertension

To correct plateau pressure when abdominal hypertension is present, the transmural plateau pressure should be calculated by subtracting approximately 50% of the intra-abdominal pressure (IAP) from the measured plateau pressure. This adjustment accounts for the pressure transmitted from the abdomen to the thoracic cavity 1, 2.

Understanding the Problem

Intra-abdominal hypertension (IAH) significantly impacts respiratory mechanics through several mechanisms:

• Elevation of the diaphragm causing restrictive lung physiology
• Transmission of abdominal pressure to the thoracic cavity
• Decreased chest wall and total respiratory system compliance
• Increased risk of atelectasis and extravascular lung water

Correction Formula and Rationale

The abdominal-thoracic pressure transmission is approximately 50% 2. Therefore:

Corrected Plateau Pressure = Measured Plateau Pressure - (IAP × 0.5)

This correction is essential because:

• Uncorrected plateau pressures may appear falsely elevated in IAH
• Using uncorrected values may lead to inappropriate ventilator adjustments
• The actual transpulmonary pressure (stress on lung tissue) may be lower than suggested by raw plateau measurements

Clinical Application Algorithm

1. Measure IAP using the transbladder technique (standard method) 3

   • Ensure proper zeroing at the mid-axillary line
   • Measure with patient in supine position
   • Use no more than 25 mL of sterile saline in the bladder
   • Measure at end-expiration

2. Calculate the correction factor

   • Multiply the measured IAP by 0.5

3. Apply the correction

   • Subtract the correction factor from the measured plateau pressure
   • Use this corrected value for clinical decision-making

4. Adjust ventilator settings based on corrected values

   • Target lung-protective ventilation with low tidal volumes (6 mL/kg PBW)
   • Consider that higher PEEP levels may be needed with IAH 4
   • Accept higher plateau pressures than normally considered if driving pressure remains within safe limits 2

Important Considerations

• Monitor IAP regularly (every 4-6 hours) in patients with IAH 1, 3
• Target maintaining IAP <15 mmHg through medical management 1, 3
• Consider the driving pressure (plateau pressure minus PEEP) as a key parameter, as it better reflects the actual stress on the lungs 5
• Implement medical therapies to reduce IAP when possible, as even small reductions in intra-abdominal volume can significantly reduce airway pressures 2

Pitfalls to Avoid

• Do not rely solely on uncorrected plateau pressures in patients with IAH, as this may lead to inappropriate ventilator adjustments
• Avoid using intragastric pressure measurements for IAP assessment, as these tend to underestimate true IAP values and may lead to missed diagnosis of IAH 6
• Do not assume a fixed relationship between IAP and thoracic pressure in severe lung injury, as the transmission ratio may vary 5
• Avoid excessive fluid resuscitation which can worsen IAH 1, 3

By properly correcting plateau pressures in the presence of abdominal hypertension, clinicians can more accurately assess lung stress and optimize ventilator settings to minimize ventilator-induced lung injury while maintaining adequate gas exchange.