Effects of respiratory rate, plateau pressure, and positive end-expiratory pressure on Pao2 oscillations after saline lavage

One of the proposed mechanisms of ventilator-associated lung injury is cyclic recruitment of atelectasis. Collapse of dependent lung regions with every breath should lead to large oscillations in Pa-O2 as shunt varies throughout the respiratory cycle. We placed a fluorescence-quenching Pot probe in the brachiocephalic artery of six anesthetized rabbits after saline lavage. Using pressure-controlled ventilation with oxygen, ventilator settings were varied in random order over three levels of positive end-expiratory pressure (PEEP), respiratory rate (RR), and plateau pressure minus PEEP (Delta). Dependence of the amplitude of Pa-O2 oscillations on PEEP, RR, and Delta was modeled by multiple linear regression. Before lavage, arterial Pa-O2 oscillations varied from 3 to 22 mm Hg. After lavage, arterial Pot oscillations varied from 5 to 439 mm Hg. Response surfaces showed markedly nonlinear dependence of amplitude on PEEP, RR, and Delta. The large Pa-O2 oscillations observed provide evidence for cyclic recruitment in this model of lung injury. The important effect of RR on the magnitude of Pa-O2 oscillations suggests that the static behavior of atelectasis cannot be accurately extrapolated to predict dynamic behavior at realistic breathing frequencies.