The Implications of Arterial Po2 Oscillations for Conventional Arterial Blood Gas Analysis

In a surfactant-depletion model of lung injury, tidal recruitment of atelectasis and changes in shunt fraction lead to large Pao 2 oscillations. We investigated the effect of these oscillations on conventional arterial blood gas (ABG) results using different sampling techniques in ventilated rabbits. In each rabbit, 5 different ventilator settings were studied, 2 before saline lavage injury and 3 after lavage injury. Ventilator settings were altered according to 5 different goals for the amplitude and mean value of brachiocephalic Pao 2 oscillations, as guided by a fast responding intraarterial probe. ABG collection was timed to obtain the sample at the peak or trough of the Pao 2 oscillations, or over several respiratory cycles. Before lung injury, oscillations were small and sample timing did not influence Pao 2 . After saline lavage, when Po 2 fluctuations measured by the indwelling arterial Po 2 probe confirmed tidal recruitment, Pao 2 by ABG was significantly higher at peak (295 ± 130 mm Hg) compared with trough (74 ± 15 mm Hg) or mean (125 ± 75 mm Hg). In early, mild lung injury after saline lavage, Pao 2 can vary markedly during the respiratory cycle. When atelectasis is recruited with each breath, interpretation of changes in shunt fraction, based on conventional ABG analysis, should account for potentially large respiratory variations in arterial Po 2 .