US measurements are used to normalize pressure volume data for resting volume, calculation of bulk modulus, stress strain relationships and the adapted Young's modulus associated with tangential wall stress.
This means that the resting volume of the chest wall, i.e., the volume where the chest wall does not strive outwards any more, is around 3 l above FRC, also in respiratory failure patients.
Our hypothesis was that, in the absence of potential for lung recruitment, prone position would not result in any change in lung mechanical behaviour and lung resting volume compared to supine position.
As long as this first expiration lung volume increase does not cause the end-expiratory lung volume to exceed the chest wall resting volume at 70 80 % of TLC, the end-expiratory pleural pressure will be negative when the expiratory valve closes (Figs. 3 and 7), indicating that the rib cage spring-out force is active.
Thus, instead of compressing, squeezing, or leaning at the lung at end-expiration, the chest wall tends to expand the lung at end-expiration as long as the end-expiratory lung volume, irrespective of the reason for the increased volume, emphysema or PEEP inflation, is below the resting volume of the chest wall at 70 90 % of total lung capacity (TLC) [10, 30, 31, 33 36].
The determinants of VILI are excessive pressures acting on lung parenchyma (i.e. transpulmonary pressure or stress) and lung deformation over lung resting volume (i.e. tidal volume/functional residual capacity or strain), closely linked to each other by lung intrinsic mechanical properties (i.e. specific lung elastance) [16, 17].
At end-expiration at any PEEP/EELV level below the resting volume of the chest wall, which is more than 3 l above FRC [30, 33], the spring-out force of the resilient rib cage makes the chest wall strive to a higher volume, while the end-expiratory airway pressure keeps the lung distended.
The protective effect of prone position could accordingly be due either to an increase in lung resting volume (i.e. 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).
The change in the mechanical properties of the lung is usually attributed to lung recruitment with an increase in lung resting volume [3, 9] and to a lower vertical pleural pressure gradient in prone position, with consequent more homogenous distribution of transpulmonary pressure, lung inflation and thus ventilation [5, 6, 10].
As the mean pleural pressure at FRC is around −5 and 0 cmH2O at the chest wall resting volume, the end-expiratory chest wall compliance is 3000/5 ≈ 600 ml/cmH2O estimated from the chest wall relaxation P/V curve as described by Rahn and coworkers in 1946 [10].
