Plateau and Transpulmonary Pressure With Elevated Intra-Abdominal Pressure or Atelectasis

Background

ARDSnet standards limit plateau pressure (Pplat) to reduce ventilator induced lung injury (VILI). Transpulmonary pressure (Ptp) [Pplat –pleural pressure (Ppl)], not Pplat, is the distending pressure of the lung. Lung distention can be affected by increased intra-abdominal pressure (IAP) and atelectasis. We hypothesized that the changes in distention caused by increases in IAP and atelectasis would be reflected by Ptp but independent of Pplat.

Methods

In Yorkshire pigs, esophageal pressure (Pes) was measured with a balloon catheter as a surrogate for Ppl under two experimental conditions: (1) high IAP group (n=5), where IAP was elevated by CO₂ insufflation in 5mm Hg steps from 0 to 30mm Hg; and (2) Atelectasis group (n=5), where a double lumen endotracheal tube allowed clamping and degassing of either lung by O₂ absorption. Lung collapse was estimated by increases in pulmonary shunt fraction.

Results

High IAP: Sequential increments in IAP caused a linear increase in Pplat (r²=0.754, P<0.0001). Ptp did not increase (r²=0.014, P=0.404) with IAP due to the concomitant increase in Pes (r²=0.726, P<0.0001). Partial Lung Collapse: There was no significant difference in Pplat between the atelectatic (21.83±0.63cm H₂O) and inflated lung (22.06±0.61 cmH₂O, P<0.05). Partial lung collapse caused a significant decrease in Pes (11.32±1.11mm Hg) compared with inflation (15.89±0.72mm Hg, P<0.05) resulting in a significant increase in Ptp (inflated=5.97±0.72mm Hg; collapsed=10.55±1.53mm Hg, P<0.05).

Conclusions

Use of Pplat to set ventilation may under-ventilate patients with intra-abdominal hypertension and over-distend the lungs of patients with atelectasis. Thus, Ptp must be used to accurately set mechanical ventilation in the critically ill.

Key Words: transpulmonary pressure, intra-abdominal pressure, atelectasis, ventilator induced lung injury