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.
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).
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.