Background:Tidal recruitment/derecruitment (R/D) of collapsed regions in lung injury has been presumed to cause respiratory oscillations in the partial pressure of arterial oxygen (PaO2). These phenomena have not yet been studied simultaneously. We examined the relationship between R/D and PaO2 oscillations by contemporaneous measurement of lung-density changes and PaO2. Methods: Five anaesthetised pigs were studied after surfactant depletion via a saline lavage model of R/D. Animals were ventilated with mean FiO2 of 0.7 and a tidal volume of 10 ml kg-1. Protocolised changes in pressure- and volume-controlled modes, inspiratory:expiratory ratio (I:E) and three types of breath hold manoeuvre were undertaken. Lung collapse and PaO2 were recorded using dynamic computed tomography (dCT) and a rapid PaO2 sensor. Results: During tidal ventilation, expiratory lung collapse increased when I:E < 1 (mean (SD) lung collapse = 15.7 (8.7)%, p<0.05), but the amplitude of respiratory PaO2 oscillations (2.2 (0.8) kPa) did not change during the respiratory cycle. The expected relationship between respiratory PaO2 oscillation amplitude and R/D was not clear. Lung collapse increased during breath-hold manoeuvres (p<0.005) at end-expiration and end-inspiration (14% vs. 0.9-2.1%, p<0.0001). The mean change in PaO2 from beginning to end of breath-hold manoeuvres was significantly different with each type of breath-hold manoeuvre (p<0.0001). Conclusions: This study in a porcine model of collapse-prone lungs did not demonstrate the expected association between PaO2 oscillation amplitude and the degree of R/D. The results suggest changes in pulmonary ventilation are not the sole determinant of changes in PaO2 during mechanical ventilation in lung injury
