Prone position ameliorates lung elastance and increases functional residual capacity independently from lung recruitment

BACKGROUND: Prone position is used to recruit collapsed dependent lung regions during severe acute respiratory distress syndrome, improving lung elastance and lung gas content. We hypothesised that, in the absence of recruitment, prone position would not result in any improvement in lung mechanical properties or gas content compared to supine position. METHODS: Ten healthy pigs under general anaesthesia and paralysis underwent a pressure–volume curve of the respiratory system, chest wall and lung in supine and prone positions; the respective elastances were measured. A lung computed tomography (CT) scan was performed in the two positions to compute gas content (i.e. functional residual capacity (FRC)) and the distribution of aeration. Recruitment was defined as a percentage change in non-aerated lung tissue compared to the total lung weight. RESULTS: Non-aerated (recruitable) lung tissue was a small percentage of the total lung tissue weight in both positions (4 ± 3 vs 1 ± 1 %, supine vs prone, p = 0.004). Lung elastance decreased (20.5 ± 1.8 vs 15.5 ± 1.6 cmH(2)O/l, supine vs prone, p < 0.001) and functional residual capacity increased (380 ± 82 vs 459 ± 60 ml, supine vs prone, p = 0.025) in prone position; specific lung elastance did not change (7.0 ± 0.5 vs 6.5 ± 0.5 cmH(2)O, supine vs prone, p = 0.24). Lung recruitment was low (3 ± 2 %) and was not correlated to increases in functional residual capacity (R(2) 0.2, p = 0.19). A higher amount of well-aerated and a lower amount of poorly aerated lung tissue were found in prone position. CONCLUSIONS: In healthy pigs, prone position ameliorates lung mechanical properties and increases functional residual capacity independently from lung recruitment, through a redistribution of lung aeration.