Effects of Respiratory Rate, Plateau Pressure, and PEEP on PaO2 Oscillations after Saline Lavage

One of the proposed mechanisms of ventilator-associated lung injury is cyclical recruitment of atelectasis. Collapse of dependent lung regions with every breath should lead to large oscillations in arterial PO2 (PaO2) as shunt varies throughout the respiratory cycle. We placed a fluorescent-quenching PO2 probe in the brachiocephalic artery of 6 anesthetized rabbits after saline lavage. Using pressure-controlled ventilation with oxygen, ventilator settings were varied in random order over 3 levels of PEEP, respiratory rate (RR), and plateau pressure minus PEEP (delta). Dependence of the amplitude of PaO2 oscillations on PEEP, RR, and delta was modeled by multiple linear regression. Before lavage, arterial PO2 oscillations varied from 3 to 22 Torr. After lavage, arterial PO2 oscillations varied from 5 to 439 Torr. Response surfaces showed markedly non-linear dependence of amplitude on PEEP, RR, and delta. The large PaO2 oscillations observed provide evidence for cyclical recruitment in this model of lung injury. The important effect of respiratory rate on the magnitude of PaO2 oscillations suggests that the static behavior of atelectasis cannot be accurately extrapolated to predict dynamic behavior at realistic breathing frequencies. Number of words in abstract: 181