functional residual capacity (FRC)) with a consequent decrease in strain and transpulmonary pressure for the same tidal volume applied or to the prevention of opening and closing of lung units during tidal ventilation (atelectrauma).īoth mechanisms of protection (increase in FRC with decrease in transpulmonary pressure and reduction of atelectrauma) are usually thought to depend on the presence of recruitable lung tissue, i.e. The protective effect of prone position could accordingly be due either to an increase in lung resting volume (i.e. tidal volume/functional residual capacity or strain), closely linked to each other by lung intrinsic mechanical properties (i.e. transpulmonary pressure or stress) and lung deformation over lung resting volume (i.e. The determinants of VILI are excessive pressures acting on lung parenchyma (i.e. Many animal studies either on healthy or diseased lungs have reported the role of prone position in delaying VILI appearance or in reducing its severity. Since the first description of ARDS, it became evident that mechanical ventilation per se can worsen lung damage, spread systemic inflammation and affect outcome and a new nosologic entity was defined, namely ventilator-induced lung injury (VILI). The change in the mechanical properties of the lung is usually attributed to lung recruitment with an increase in lung resting volume and to a lower vertical pleural pressure gradient in prone position, with consequent more homogenous distribution of transpulmonary pressure, lung inflation and thus ventilation. The oxygenation benefit is due to a better ventilation–perfusion matching and/or to recruitment of dorsal lung parenchyma with a decrease in shunt fraction. Prone position is used as a rescue therapy during acute respiratory distress syndrome (ARDS) in severely hypoxic patients in whom it usually improves oxygenation and lung mechanics. In healthy pigs, prone position ameliorates lung mechanical properties and increases functional residual capacity independently from lung recruitment, through a redistribution of lung aeration. A higher amount of well-aerated and a lower amount of poorly aerated lung tissue were found in prone position. Lung recruitment was low (3 ± 2 %) and was not correlated to increases in functional residual capacity ( R 2 0.2, p = 0.19). 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). Recruitment was defined as a percentage change in non-aerated lung tissue compared to the total lung weight. functional residual capacity (FRC)) and the distribution of aeration. A lung computed tomography (CT) scan was performed in the two positions to compute gas content (i.e. 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. 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. Prone position is used to recruit collapsed dependent lung regions during severe acute respiratory distress syndrome, improving lung elastance and lung gas content.
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