Tidal changes in PaO₂ and their relationship to cyclical lung recruitment/derecruitment in a porcine lung injury model.

Abstract

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. 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 O₂ (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 manoeuvres were undertaken. Lung collapse and PaO₂ were recorded using dynamic computed tomography (dCT) and a rapid PaO₂ sensor. 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 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 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 PaO₂ from beginning to end of breath-hold manoeuvres was significantly different with each type of breath-hold manoeuvre (P<0.0001). This study in a porcine model of collapse-prone lungs did not demonstrate the expected association between PaO₂ oscillation amplitude and the degree of recruitment/derecruitment. The results suggest that changes in pulmonary ventilation are not the sole determinant of changes in PaO₂ during mechanical ventilation in lung injury.