Plateau and transpulmonary pressure with elevated intra-abdominal pressure or atelectasis.

ABSTRACT

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 5 mm Hg steps from 0 to 30 mm 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+/-0.63 cm H2O) and inflated lung (22.06+/-0.61 cmH2O, P<0.05). Partial lung collapse caused a significant decrease in Pes (11.32+/-1.11 mm Hg) compared with inflation (15.89+/-0.72 mm Hg, P<0.05) resulting in a significant increase in Ptp (inflated=5.97+/-0.72 mm Hg; collapsed=10.55+/-1.53 mm 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.