Background and goal of study: Mechanical ventilation in supine position generates dependent lung zones dorsally and non-dependent regions ventrally, these collectively determine global lung ventilation and mechanics. We sought to develop an experimental model suitable for the assessment of the ventilatory and mechanical properties of the dependent and non-dependent lung zones separately, thereby allowing the isolated examination of the effects of respiratory treatments.
Methods: In this prospective self-controlled study, anaesthetised pot-bellied pigs (n=7) were intubated with a right sided double-lumen endotracheal tube, and ventilated with 10 ml/kg tidal volume (VT) and zero positive end-expiratory pressure (PEEP) while VT was separately measured in each lung. The forced oscillation technique was used to determine airway resistance (Raw), inertance (Iaw), tissue damping (G) and elastance (H). At the beginning of the protocol, the pigs were placed in supine position, and baseline measurements (BL) were performed. Subsequently, the animals were turned to their left side, and measurements were repeated after 15 (P0-15) and 45 (P0-45) minutes. Further measurements were made with the PEEP increased to 4 (P4) and 8 cmH2O (P8).
Results: VT showed a tendency to decrease in the dependent lung and increase in the non-dependent lung, without reaching statistical significance. Increasing PEEP to 8 cmH2O lead to drops in Raw in the dependent lung compared to P0-15 (-35±16[SE]%). Iaw increased in the dependent side at P0-15 compared to BL, and remained unchanged to PEEP. G and H increased in the dependent (+55±13%) and decreased in the non-dependent lung (-29±12%). Elevated PEEP lead to reductions in G and H compared to the P0-45 in both the dependent ( 23±11% and  47±20%) and the non-dependent lung (-26±12% and -39±10% for G and H, respectively, p <0.05 for all).
Conclusion: This experimental model allows the separate assessment of lung ventilation and mechanics. Side-lying position results in decreased tidal volume with increased atelectases and tissue stiffness in the dependent lung, while converse changes can be observed in the non-dependent lung. Increasing PEEP to 8 cmH2O alleviates the adverse respiratory alterations in the dependent lung caused by lateral position without deteriorating lung mechanics in the non-dependent lung.
Grant: OTKA-NKFI K115253

Separate assessment of dependent and non-dependent lung regions by forced oscillation technique in a porcine model