Abstract
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 (PaO(2)). These phenomena have not yet been studied simultaneously. We examined the relationship between R/D and PaO(2) oscillations by contemporaneous measurement of lung-density changes and PaO(2). METHODS: Five anaesthetised pigs were studied after surfactant depletion via a saline-lavage model of R/D. The animals were ventilated with a mean fraction of inspired O(2) (FiO(2)) 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 manoeuvres were undertaken. Lung collapse and PaO(2) were recorded using dynamic computed tomography (dCT) and a rapid PaO(2) sensor. RESULTS: During tidal ventilation, the expiratory lung collapse increased when I:E <1 [mean (standard deviation) lung collapse=15.7 (8.7)%; P<0.05], but the amplitude of respiratory PaO(2) oscillations [2.2 (0.8) kPa] did not change during the respiratory cycle. The expected relationship between respiratory PaO(2) oscillation amplitude and R/D was therefore not clear. Lung collapse increased during breath-hold manoeuvres at end-expiration and end-inspiration (14% vs 0.9-2.1%; P<0.0001). The mean change in PaO(2) 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 PaO(2) oscillation amplitude and the degree of recruitment/derecruitment. The results suggest that changes in pulmonary ventilation are not the sole determinant of changes in PaO(2) during mechanical ventilation in lung injury.